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Techny Chemy

74 The Masterbuilder - June 2012 • www.masterbuilder.co.in

Geosynthetics are a construction material made from polymers. These materials are manufactured as textiles, grids, nets, solid membranes or as

a combination of one or more of the above. The type of geosynthetic selected for a particular project depends on the intended application, which can include drainage, separation of different materials, filtration of soil particles from draining water, reinforcement, confinement and containment. Geosynthetic usage has steadily increased in both public and private construction projects and innovative uses and new products continue to appear on the market. Geosynthetics reinforce the shear stresses of the fill material leading to an increase of the foundation bearing capacity. High strength geosynthetics provide a cost effective solution to achieve a greater and quicker stability of embankments constructed on soft foundations (Refer Figure 1). Geosynthetics allow:

- Optimum embankment height over a minimum area- Steeper side slopes- Increase in construction speed with no loss of stability- Resistance to outward movement of the embankment

Types Geosynthetics

Geosynthetics have been increasingly used in geotechnical and environmental engineering for the last 4 decades. Over the years, these products have helped designers and contractors to solve several types of engineering problems where the use of conventional construction materials would be restricted or considerably more expensive. There is a significant number of geosynthetic types and geosynthetic applications in geotechnical and environmental engineering. Due to space limitations, this paper will examine the advances on the use of these materials in

Sonjoy DebAssociate Editor, B.Tech,’Civil’, Research Scholar, Indian Institute of Technology

Use of Geosynthetics to Assist Construction Over Soft Soils

Ground Engineering Geosysnthetics in Soft Soil

www.masterbuilder.co.in • The Masterbuilder - June 2012 75

reinforcement and in environmental protection. Common types of geosynthetics used for soil reinforcement include geotextiles (particularly woven geotextiles), geogrids and geocells. Geotextiles (Refer Figure 2) are continuous sheets of woven, nonwoven, knitted or stitch-bonded fibers or yarns. The sheets are flexible and permeable and generally have the appearance of a fabric. Geogrids have a uniformly distributed array of apertures between their longitudinal and transverse elements. These apertures allow direct contact between soil particles on either side of the sheet. Geocells are relatively thick, three-dimensional networks constructed from strips of polymeric sheet. The strips are joined together to form interconnected cells that are infilled with soil and sometimes concrete. In some cases 0.5 m to 1 m wide strips of polyolefin geogrids have been linked together with vertical polymeric rods used to form deep geocell layers called geomattresses.

A wide variety of geosynthetics products can be used in environmental protection projects, including geomembranes, geosynthetic clay liners (GCL), geonets, geocomposites and geopipes. Geomembranes are continuous flexible sheets manufactured from one or more synthetic materials. They are relatively impermeable and are used as liners for fluid or gas containment and as vapour barriersWhen hydrated they are effective as a barrier for liquid or gas and are commonly used in landfill liner applications often in conjunction with a geomembrane. Geonets are open grid-like materials formed by two sets of coarse, parallel, extruded polymeric strands intersecting at a constant acute angle. The network forms a sheet with

in-plane porosity that is used to carry relatively large fluid or gas flows. Geocomposites are geosynthetics made from a combination of two or more geosynthetic types. Examples include: geotextile-geonet; geotextile-geogrid; geonetgeomembrane; or a geosynthetic clay liner (GCL). Geopipes are perforated or solid-wall polymeric pipes used for drainage of liquids or gas (including leachate or gas collection in landfill applications). In some cases, the perforated pipe is wrapped with a geotextile filter. Figure 3 presents schematically these products.

Figure 1. Geosynthetics reinforce the embankment foundation

Figure 2. Commonly used geosynthetics for soil reinforcement

Figure 3. Schematic view of some typical geosynthetics used in environmental protection works

Construction in Soft Soil

Construction across soft soils creates a dilemma for the engineer. The construction proceeds at a slow pace because much time is spent recovering equipment mired in muck and hauling large quantities of fill to provide adequate bearing strength. Traditionally, the following options may be considered:

- Bypass the area: This course of action is often negated by the tactical situation or other physical boundaries.

- Remove and replace the soil: Commonly referred to as “mucking,” this option is sometimes a very difficult and time-consuming procedure. It can only be used if the area has good, stable soil underneath the poor soil. Furthermore, a suitable fill material must be found nearby.

- Build On Directly: Base course construction material is often placed directly on the weak soil; however, the base course layer is usually very thick and the solution is temporary. A “pumping” action causes fines to intrude into the base course, which causes the base course to sink into the weak soil (see Figure 4). As a result, the base course itself becomes weak. The remedy is to dump more material on the site.

- Stabilize: By using geofabrics, the poor soils can be separated and confined to prevent intrusion or loss of soil finally leads to stabilization of the soft soil. The process is very fast and cost effective too, there is no



need to bring good soil for foundation or replace entire foundation soil which is soft. For low lying areas or areas where good soil in scarcely available, the method of stabilization using geosynthetic is very effective.

of these products. One claim is that bitumen coatings provide a superior bond to other polymers, enhancing grid performance in preventing crack propagation.

Figure 4. Effect of pumping action on a base course

Functions of Geosynthetics

Geotextiles/Geosynthetics serve four primary functions:

- Reinforcement.- Separation.- Drainage.- Filtration.

(A) There are many developments in mechanically stabilized earth (MSE) walls and slopes and in basal stabilization. The MSE concept is essentially a uniaxial force problem and is served by the insertion of tensile members whose principal strength is uniaxial and that property is oriented to the expected forces of failure in the design. In 1993 a textile geogrid was employed using an ultra high strength polymer (the aramid known as Kevlar) to construct a road over karst terrain, as schematically shown in Figure 5.

Figure 5. Reinforced embankment on unstable foundation soil (Reinforcing function of geosynthetic)

(B) Rigid grids have also experienced innovation with the development of new punching patterns that yield triangular shaped apertures after the stretching process. The new shape has several benefits in the product profile, rib thickness and in plane stiffness and this three dimensional structure is expected to offer improvement in confinement which will yield improved rut resistance and better load distribution. Geogrids have been employed to resist or remediate reflective cracking in asphalt for many years (Refer Figure 6). Nonetheless, innovation is present here in the continuing study and analysis of performance

Figure 6. Geogrids to avoid reflective cracking in pavements (Separation function of geosynthetic)

(C) The most important and critical of all the problems is to counter the difficulties in soft soil foundations. Soft soil is characterized by huge pore pressure and very long consolidation period, which makes construction of foundation over it a challenging task. Electrokinetics and electroosmosis are techniques employed in manipulating pore pressure and plasticity indices of soils. Formerly hampered by difficulty in establishing suitable electrodes in soil structures, electrokinetics and eleectroosmosis are becoming viable technologies for soil reinforcement and environmental rehabilitation and geosynthetics are one of the means of introducing anodes and cathodes into a soil structure (Refer Figure 7), soil nailing is another. The concept of electrokinetics is the use of current to induce water flow. The technique can be used in environmental remediation wherein contaminants are recovered or removed from soil by causing groundwater to flow to a collection point. Anodes and cathodes are created from geosynthetics by using conductive materials such as carbon fiber, or by interlacing conductors (wire) in the textile. Other geosynthetic applications are mine tailing dewatering and sewage (perhaps contained in geotextile tubes) dewatering. Sports turf is managed by using current to draw off excess water, or by reversing polarity, delivering water to plant roots. The concepts of electrokinetics are applicable to slope stability, mechanically stabilized earth (walls), drainage and can result in cementation wherein ions precipitated from solution cement clays and the result is stiffer clays.

Geocells have been used in innovative ways to stabilize aggregate while providing high volume drainage and working platform support. In an airport de-icing compound, the geocell confines the aggregate, improves the load capacity of the aggregate and the subgrade, contains large volumes of fluid in high volume events and drains fluid from the structure in a controlled manner. Another innovative use of geocells is as the facia on avalanche protection earthen mounds in Iceland (Bygness 2007). Five mile long barriers were raised 15 to 20 feet using multi layers of geocells with compacted soil filling as the facia resulting in an aesthetically pleasing alternative to conventional technique of concrete retaining walls.


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Design Principles Of Geosynthetic For Roads

During construction of roads on soft soils a certain bearing capacity of the subbase is required to prevent unnecessary differential settlements of the road structure. For subsoil with insufficient bearing capacity, stabilization is necessary. The bearing capacity can be increased by excavation of the soft material, chemical stabilization by using chalk or by using geosynthetics. When using geosynthetics in paved road structures (surface layer existing of asphalt or concrete) the long term behavior has to be taken into account. The measures bearing capacity on top of the base should be maintained during the total service life of the road.

Existing design methods for flexible pavements reinforced with a geosynthetic in the unbound base aggregate layer are largely empirically based (Berg et al., 2000). These existing design methods have been limited in use by many state departments of transportation due to several factors, namely:

- Design methods are not part of a nationally recognized pavement design procedure.

- Design methods are often times applicable to a narrow range of design conditions.

- Design methods are often times proprietary, making it difficult to directly compare the cost-benefit of several reinforcement products from different manufacturers

Efficiency Factor of Geosynthetic as Reinforcement

The efficiency of the geosynthetics as reinforcement in a pavement (Palmeira, IGS) can be estimated by the efficiency factor (E):

Nr = number of load repetitions up to failure for the reinforced pavement.

Nu = number of load repetitions up to failure for the unreinforced pavement.

Construction sequence using Geosynthetic As Reinforcement

The steps followed are

Step1 - Ground preparation

Step2 - Laying of geosynthetic

Step3 - laying and spreading aggregate over the geosynthetic

Step4 - Compaction of the aggregate

The entire sequence is shown in Figure 8 below.Figure 7. Electrokinetics geosynthetics for soft soil stabilization (Drainage function of geosynthetic)

Figure 8. Construction Sequence using Geosynthetic

Conclusion

Geosynthetics have great potential to be used as cost-effective solutions for several engineering problems. This paper presented type and functions of geosynthetic products, on the utilization of these materials in reinforced soil structures. Manufacturing of geosynthetics products allows incorporating recent advances in material sciences. Therefore, the expectation is that innovations in products, types and properties will continue to take place, adding to the already vast range of applications of these materials. Geosynthetic reinforced soft soil present better performance than soil strengthening options. Thus, this type of structure can be cost effective. New construction methodologies have also broaden the applications of geosynthetic reinforced soil retaining wall, which include new facing units and that reduces the construction time, costs and allow better aesthetic conditions for the final structure.The use of geosynthetics has also led to major advances


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Pennar Engineered Building Systems Ltd


in environmental applications. While geosynthetics has been used in a number of applications in environmental project, application of use of geosynthetics in landfills are also increasing. Specifically, simple yet accurate formulations are now available for the design of liquid collection systems, which involve proper quantification of the thickness of liquid within drainage composites. Also, significant advances have taken place regarding the use of reinforcements for stabilization of steep cover systems. Approach include the use of geosynthetic reinforcements parallel to the cover slope, horizontal reinforcements embedded into solid waste, and fiber reinforcement of the cover soils. Overall, the use of geosynthetics has led to major advances towards the construction environmental systems that are cost effective but that provide enhanced environmental protection.

Reference

1. An overview on the use of geosynthetics in pavement structures, Mounes et al., Scientific Research and Essays Vol. 6(11), pp. 2251-2258, 4 June, 2011

2. Advances in Geosynthetics Materials and Applications for Soil Reinforcement and Environmental Protection Works, Palmeira et al.

3. Adams, M. (2008) “The GRS bridges of Defiance County”. Geosynthetics, 26(1): 14-21.

4. Allen, T.M. and Bathurst, R.J. (2006) “Design and performance of an 11-m high blockfaced

geogrid wall”. 8th International Conference on Geosynthetics, Yokohama, Japan, September 2006, 953-956.

5. Allen, T.M. and Holtz, R.D. (1991) “Design of retaining walls reinforced with geosynthetics“. In Geotechnical Engineering Congress 1991, McLean, F., Campbell, D.A., and Harris, D.W., Editors, ASCE Geotechnical Special Publication No. 27, Vol. 2, In proceedings of a congress held in Boulder, Colorado, USA, June 1991, 970-987.

6. Allen, T.M., Bathurst, R.J., Holtz, R.D., Walters, D.L. and Lee, W.F. (2003) “A New Working Stress Method for Prediction of Reinforcement Loads in Geosynthetic Walls”. Canadian Geotechnical Journal, 40(5): 976-994.

7. Allen, T.M., Nowak, A.S. and Bathurst, R.J. (2005) “Calibration to Determine Load and Resistance Factors for Geotechnical and Structural Design”, Transportation Research Board Circular E-C079, Washington, DC, 93 p.

8. Alexiew, D. (2008) “Ultimate bearing capacity tests on an experimental geogridreinforced vertical bridge abutment without stiffening facing”, New Horizons in Earth Reinforcement, Taylor & Francis Group, London, 507-511.

9. www.tencategeosynthetics.com


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Maccaferri


Geosynthetic Lining Systems in Engineered Landfills - An Indian Perspective

Geosynthetics are extensively used in modern landfills to perform all the five classic functions, viz., separation, filtration, drainage, reinforcement

and protection. An important constituent of all these, is a geomembrane, which is a relatively impermeable sheet of polymeric formulation used as a barrier to liquids and/or vapours. According to ISO, a Geomembrane is a low permeability material in the form of a factory made synthetic polymeric or bituminous sheet, used in geotechnical engineering and civil engineering applications with the purpose of reducing or preventing the flow of fluid and /or vapour through the construction.

With the regulatory, economic and activist climate making replacement or expansion of existing landfills as well as new siting it is becoming difficult and expensive. Management of the given volume of the landfill has become quite critical. The goal of the landfill manager is to obtain maximum fees for the waste collections made. It is no longer possible to continue practices that began when landfilling was just what the word implies-an effort to fill in areas considered undesirable or to create buildable space in gullies, swamps or marshes. Today, squeezing the last Rupee out of that space requires close attention to many factors. The primary objective is clear- to fully compact the maximum amount of material into every cubic inch of air space.

Though the statutory organizations in each country specify in detail, the quality of the geomembrane and also detail the method of application, in India , more often than not, it is found that the workmanship is far from satisfactory and also the specifications are strictly not being followed. So much so that in one case the landfill had to be abandoned.

Keeping this in view, this paper deals with the system prevalent in India, the methodology of installation and the fool proof seaming methodology.

Lining Systems in Engineered Landfills

The Central Pollution Control Board, Ministry of Environment and Forests, Government of India (CPCB) has brought out the Criteria for Hazardous Waste Landfill (HASWAMS/17/2000-01) in February 2001. They include:

- Site Selection - Site investigation criteria - Planning and design criteria - Landfill liner criteria and cover criteria - Construction and operation criteria - Inspection, Monitoring - Post - closure criteria

Subsequently, the Manual for Design, Construction and Quality Control of Liners and Covers for Hazardous Waste Landfills (Hazardous Waste Management Series: HAZWAMS/20/2001-02) was published by the CPCB, in December 2002. It deals with the principles of design, construction and quality control of various components of liners and covers used for the secured landfill for disposal of hazardous landfills.

Hazardous Waste Landfill liner and cover criteria

The liner system must include the following components. However, depending on the design requirements, the number of components as well as the specifications of the components can exceed the minimum specifications listed

Dr. G. Venkatappa Rao1 and Dr. R. S. Sasidhar2

1Distinguished Professor, KL University, Vijayawada, 2Managing Director, Saimaster Geoenvironmental Services Pvt. Ltd., Hyderabad

The paper presents the existing provisions of the Central Pollution Board, Government of India for Geomembranes in Hazardous Waste Landfills and by the Ministry of Environment of and Forests, Government of India, for Municipal Solid Waste Landfills. To ensure adequate performance the method of installation, particularly the seaming method is of utmost concern. This has been described adequately, along with the necessary field and laboratory testing methods.

Ground Engineering Lining Systems


below. The components listed below are waste downwards (Fig 1).

a) A primary leachate collection layer of thickness 30 cm or more and coefficient of permeability in excess of 10-2 cm/sec

b) A primary composite liner comprising of

i) A HDPE geomembrane of thickness 1.5 mm or more (see specification), and

ii) A compacted clay (or compacted amended soil) layer of thickness 45 cm or more having a coefficient or permeability of 1 x 10-7 cm/sec or less.

c) A secondary leachate collection layer (also called leak detection layer) of thickness 45 cm or more and coefficient of permeability in excess of 10-3 cm/sec.

d) A secondary composite liner comprising of

i) A HDPE geomembrane of thickness 1.5 mm or more, and

ii) A compacted clay (or compacted amended soil) layer of thickness 45 cm or more having a coefficient or permeability of 10-7 cm/sec or less.

For extremely hazardous waste, the number of composite liner layers shall, if necessary, exceed two and these will be finalized by the design engineer.

The cover system is typically shown in Fig. 3.

a) A leachate collection layer of thickness 30 cm or more and coefficient of permeability in excess of 10-7 cm/sec

b) A single composite liner comprising of

i) A HDPE geomembrane of thickness 1.5 mm or more, and

ii) A compacted clay (or compacted amended soil) layer of thickness 150 cm or more having a coefficient of permeability of 10-7 cm/sec (10-9 m/sec).

At locations where availability of clay is limited, amended soil will constitute by mixing bentonite or any other suitable clay to locally available soil to achieve the desired permeability.

In regions where rainfall is high and/or subsoil is highly permeable (e.g. gravel, sand, silty sand) and/or the water table is within 2.0 m to 6.0 m beneath the base of the landfill, the linear system shall be a double composite liner and shall include the following components, waste downwards (Fig.2).

Figure 1. Single composite liner system (CPCB, 2001)

Figure 2. Double composite liner system (CPCB, 2001)

Figure 3. Cover system (CPCB, 2001)

Figure 4 depicts the arrangement of various components of landfill lining system and cover, along with the details of the junction.

As per CPCB 2001, geomembranes are required to be of HDPE and the other recommended values along with their test methods are presented in Table 1. The other desirable parameters are also mentioned in the same table. The carbon black should be of group 3 category or lower as defined in ASTM D-1765.

Use of materials other than HDPE like VLDPE, PVC and



CSPE is governed by fulfilling the conditions stated in Sections 7.1.1 and 7.2.1 of CPCB (2001) for use as equivalent materials.

Use of materials other than HDPE like VLDPE, PVC and CSPE is governed by fulfilling the conditions stated in Sections 7.1.1 and 7.2.1 of CPCB (2001) for use as equivalent materials.

The anchorage details of Geomembranes are presented in Fig 5.

Figure 4. Landfill cover and liner components (CPCB, 2001)

Property Test Method* Specified Values

Thickness ASTM D5199, D5994, D1593

1.5 mm

Tensile strength & properties

ASTM D638 18kN/m at yield 30 kN/m at break

Tear Resistance ASTM D1004 150 N

Puncture Resistance ASTM D5494 or FTMS 101B (method 2065)

250N

Typical Desirable Value

Density ASTM D1505 0.94 g/cc

Melt Flow Index ASTM D1238 <1 g /10min

Carbon Black Content ASTM D1603 2%

Carbon Black Dispersion

ASTM D3015

Low Temperature Brittleness

ASTM D746

Environmental Stress Cracking Resistance

ASTM D1693

Seam Strength (Shear) ASTM D4437 90 % of parent material

Seam Strength (Peel) ASTM D4437, D413 60 % of parent material

Dimensional Stability ASTM D1204 2%

Interface Shear Resistance(Soil - Geomembrane)(Clay - Geomembrane)

ASTM D5321 or Modified Direct Shear Test (Standard size)

Waste Geomembrane Compatability

EPA 9090

* Other equivalent codal methods can be used for testing Table 1. Specifications for Geomembranes (CPCB, 2001)

As per CPCB 2001, geomembranes are required to be of HDPE and the other recommended values along with their test methods are presented in Table 1. The other desirable parameters are also mentioned in the same table. The carbon black should be of group 3 category or lower as defined in ASTM D-1765.

Figure 5. Geomembrane anchorage details (CPCB, 2001)

Geomembrane for base and side lining of Municipal Solid Waste

An impermeable lining system is required to be constructed at the base and wall of the waste disposal area for MSW landfills. According to MoEF (2000), the low permeability lining system must have barrier soil layer (clay/amended soil) of minimum 60 cm thickness with permeability not greater than 1 x 10-7 cm per second if, waste reaching the landfill is non-biodegradable and inert. For landfill receiving residues of waste processing facilities or mixed waste 01 waste having contamination of hazardous materials (such as aerosols, bleaches, polishes, batteries, waste oils, paint products and pesticides) minimum liner specifications shall be a composite barrier having 1.5 mm thick High Density Polyethylene (HDPE) geomembranes (or equivalent) overlying 90 cm of soil (clay/amended soil) having permeability not greater than 1 x 10-7 cm/sec. The highest level of water table shall be at least 2.0 m below the base of clay/amended soil barrier layer.

Integrity of Geomembrane Seams

As was brought out earlier, the performance of the geomembrane lining system will be predominantly dependent the seaming methodology, which is detailed in the subsequent section. Whereas the methods of Test for these the specific Test Method may be referred, in the


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following a highlight of the principle of the seam testing is brought forth. Further details may also be referred to in Venkatappa Rao and Pothal (2008) and Venkatappa Rao and Sasidhar (2009).

The shear and T- peel methods are destructive quality control and quality assurance tests used to determine the integrity of geomembrane seams. These test procedures are intended for non-reinforced geomembranes only.

to accommodate field conditions. Each panel used for the installation will be given a number that will be correlated with a batch or roll number.

Figure 6. Shear and T-peel specimen

For the shear and peel tests specimens are seamed as shown in Fig.6 and tested for maximum load and maximum strain, according to ASTM D 4337 - 99 and ASTM D 6392 - 99 (2006). The locus of the break for both these tests is interpreted with the help of Figs. 7 and 8.

Important Considerations During Construction

One of the key factors in construction of landfills is seaming of geomembranes. This has to be done diligently using the right kind of equipment by trained operators. Great care has to be taken in preparation of the subgrade and the proper placement of the cut geomembrane panels. Checks are done during seaming and also some tests are destructive. All these aspects are presented in detailed herein.

Preparation for Geomembrane Deployment

Preparation of panels

Prior to commencement of HDPE geomembrane deployment, layout drawings shall be prepared to indicate the panel configuration and general location of field seams for the project. The actual panel layout may vary in order

Figure 7. Locus-of-Break Codes for Dual Hot Wedge Seams in Un-reinforced Geomembranes Tested for Seam Strength in Shear and Peel Modes (as per ASTM)

Figure 8. Locus-of-Break Codes for Fillet Extrusion Weld Seams in Unreinforced Geomembranes Tested for Seam Strength in Shear and Peel Modes (as per ASTM)



Overlap the panels of geo-membrane approximately 10 to 15 cm prior to welding. Clean the seal area prior to seaming to assure the area is clean and free of moisture, dirt or debris of any kind. No grinding is required for fusion welding.

Adjust the panels so that the seams are aligned with the fewest possible number of wrinkles and “fish mouth”.

Grind seams overlap prior to welding within one hour of the welding operation in a manner that does not damage the geo-membrane. Grind marks should be covered with extrude whenever possible. In all cases, grinding should not extend more than 6 mm past the edge of the area covered by the extrude during welding.

The minimum roll length and width of HDPE sheet shall be 20 m and 7 m respectively.

Sub-base Preparation

The sub-base must be properly prepared and compacted for installation of HDPE liner. The sub-base must not contain any particles. The sub-base must be checked for footprints or similar depressions before laying the liner. It should not have any cracks or evidence of swelling. The seaming equipment tends to get caught in such small depressions, causing burnout and subsequent repair. A small piece of the synthetic membrane placed below the membranes that are being seamed (this piece is moved forward along with the seaming equipment) may reduce burnout due to small depressions.

Field Panel Placement

HDPE deployment will generally not done during any precipitation, in the presence of excessive moisture, in an area of standing water, or during high winds.

Installation of field panels shall be done as indicated on the layout drawing..

Adequate temporary loading (i.e. sandbags, tyres) which will not damage the HDPE, will be placed to prevent uplift of the HDPE by wind. The method and equipment used to deploy the panels must not damage the HDPE

HDPE Field Seaming

In general, seams shall be oriented parallel to the slope, i.e. oriented along, not across the slope. Whenever possible, horizontal seams should be located on the base of the cell, not less than 2 m from the toe of the slope. Each seam made in the field shall be numbered. Seaming information shall include seam number, welder ID, machine number, temperature setting and weather conditions.

Equipment

(1) Fusion Welding: Fusion Welding consists of placing a heated wedge, mounted on a self propelled vehicular unit, between two (2) over-lapped sheets such that the surface of both sheets are heated above the polyethylene’s melting point. After being heated by the wedge, the overlapped panels pass through a set of pre-set pressure wheels which compress the two (2) panels together to form the weld. The fusion welder is equipped with a device, which continuously monitors the temperature of the wedge.

(2) Extrusion Fillet Welding: Extrusion fillet welding consists of introducing a ribbon of molten resin along the edge of the overlap of the two (2) HDPE sheets to be welded. A hot air preheat and the addition of molten polymer causes some of the material of each sheet to be liquefied resulting in a homogeneous bond between the molten weld bead and the surfaces of the overlapped sheets. The extrusion welder is equipped with gauges giving the temperature in the apparatus and a numerical setting for the pre-heating unit.

Seaming of HDPE

- Seams shall be made by overlapping adjacent sheets approximately 2.5 cm for extrusion welding and approximately 10 cm for hot wedge welding. Seam strength is not a function of the overlap. The overlap simply needs to be wide enough to weld and test properly. In general, seams shall be oriented parallel to the line of maximum slope, i.e., oriented along and not across the slope.

- ”Fishmouths” or wrinkles at the seam overlaps, shall be cut along the ridge of the wrinkle in order to achieve a flat overlap. The cut “fishmouths” or wrinkles shall be seamed.

- Seaming shall extend along panel edges into the anchor trench.

Seam Testing

All field seams shall be non-destructively tested over their full length using test equipment and procedures described herein. Seam testing shall be performed as the seaming work progresses, not at the completion of the field seaming.

Air Pressure Testing

The welded seam is composed of a primary seam and a secondary track that creates an un-welded channel. The presence of an un-welded channel permits fusion seams to be tested by inflating the sealed channel with air to a



predetermined pressure and observing the stability of the pressurized channel over time.

Equipment for Air Testing

The equipment required for air testing consists of following components:

1. An air pump (manual or motor driven) capable of generating and sustaining a pressure between 1.5 to 4 kg/cm2

2. A rubber hose with fittings and connections.

3. A sharp hollow needle or other approved pressure feed device with a pressure gauge capable of reading and sustaining a pressure between 0 to 4 kg/cm2.

Procedure for Air Testing

Both the ends of the seam to be tested should be sealed. Needle or other approved pressure feed device should be inserted into the sealed channel created by the fusion weld.

Test channel should be inflated to a pressure of approximately 2 kg/cm2, and the pressure should be maintained within the range listed in Initial Pressure Schedule given below. Valve should be closed and the initial pressure should be observed and recorded.

Procedure for Non-Complying Test

In the event of a Non-complying Air Pressure Test, the following procedure shall be followed:

1) Seam end seals should be checked and seams should be retested.

2) If a seam does not maintain the specified pressure, the seam should be visually inspected to localize the flaw.

3) If no flaw is found, area to be vaccum tested should be marked. Entire length of the seam should be vaccum tested as explained later in this section.

a) If leak is located by the vaccum test, it should be repaired by extrusion fillet welding. Repair should be tested by vaccum testing.

b) If no leak is discovered by vaccum testing, the seam will be considered to have passed non-destructive testing.

Vaccum Testing

This test is used on extrusion welds, or when the geometry of a fusion well makes air pressure testing impossible or impractical, or when attempting to locate the precise location of a defect believed to exist after air pressure testing.

Equipment for Vaccum Testing

The equipment required for vaccum testing shall consist of following components:

1. Vaccum box assembly consisting of rigid housing with a soft neoprene gasket attached to the open bottom, a transparent viewing window, port hole or valve assembly, and a vaccum gauge.

2. Vaccum pump or Ventura assembly equipped with a pressure controller and pipe connection.

3. A rubber pressure/vaccum hose with fittings and connections.

4. A bucket and means to apply a soapy solution.

5. A soapy solution.

Procedure for Vaccum Testing

1. Excess overlap from the seam should be trimmed, if any.

2. Vaccum pump/compressor should be turned on to reduce the vaccum Box to approximately 25 cm of mercury, i.e., 0.5 kg/cm2 gauge.

3. A strong solution of liquid detergent and water should be applied to the area to be tested.

4. Vaccum box should be placed over the area to be tested and sufficient downward pressure should be applied to “seat” the seal strip against the liner.

5. Bleed valve should be closed and vaccum valve should be opened.

6. A minimum of 0.3 kg/cm2 vaccum should be applied to the area as indicated by the gauge on the vaccum box.

7. It should be ensured that a leak tight seal is created.

8. The suction should be held for an adequate time to thoroughly examine the HDPE through the viewing window for the presence of soap bubbles.

9. After this period vaccum valve should be closed and bleed valve should be opened, the box should be moved over the next adjoining area with a minimum 7.5 cm overlap, and the process should be repeated.

Procedure for Non-Complying Test

1. All the areas where soap bubbles appear should be marked and repaired.

2. The repaired areas should be retested.

Destructive Testing

The purpose of destructive testing is to determine and evaluate seam strength. These tests require direct sampling and thus subsequent patching. Therefore, destructive


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testing should be held to a minimum to reduce the amount of repairs to the HDPE.

Procedure for Destructive Testing

1. Destructive test samples shall be marked and cut out randomly at a minimum average frequency of one test location every 150 m of seam length, unless otherwise specified or agreed.

2. Destructive samples should be taken and tested as soon as possible after the means are welded (the same day), in order to receive test results in a timely manner.

3. Qualified personnel will observe all field destructive testing and record date, time, seam number, location, and test results on Destructive Testing Form.

4. Sample Size

(a) The sample should be 30 cm wide with a seam 40 cm long centered length-wise in the sample. The sample may be increased in size to accommodate independent laboratory testing by the Owner at the Owner’s request or by specific project specifications.

(b) A 2.5 cm specimen shall be cut from each end of the test seam for field testing.

(c) The two 2.5 cm wide specimens shall be tested on a field tensiometer for peel strength. If either field specimen does not pass, it will be assumed the sample would also not pass laboratory destructive testing.

for each test method. Four out of the five specimens must exhibit for each round of peel and shear testing. In addition, four of the five individual specimens must meet or exceed the strength requirements as listed in Material specification sheet in order for the seam to pass the destructive test.

Defects and Repairs

Repair Procedures

Any portion of the HDPE or HDPE seam shown a flaw, or having a destructive or non-destructive test in non-compliance shall be repaired. Procedures for repair include the following

Patching

Patching shall be used to repair large holes, tears and destructive sample locations. All patches shall extend at least 7.5 cm beyond the edges of the defects and all corners of patches shall be rounded.

Grinding and Welding

Grinding and welding shall be used to repair sections of extruded fillet seams.

Spot Welding or Seaming

Spot welding or seaming shall be used to repair small tears, pinholes or other minor localized flaws.

Capping

Capping shall be used to repair lengths of extrusion or fusion welded seams.

Operation Of Landfilling

Landfill managers emphasize the importance of regular operational planning involving the entire staff. The equipments needs are to be clearly identified.

Compacting equipment

The compacting equipment can be as heavy as 40 t and deliver that weight to the working face through specialized wheels. While it is generally agreed that compactors increase short term face densities and stability, some believe that in the long term it is the overburden which is effective. Important factors to remember while selecting a compactor are maneuverability, ground clearance, durability and ease of maintenance. Drums can be smooth or textured and some are fitted with replaceable teeth to help shredding and compress the waster.

Dozers

To help spread the waste evenly before compaction, dozers on tracks are very much required.

Initial Pressure Schedule

Material (Mil) Min. Psi Max. Psi

40 24 30

60 27 35

80 30 35

100 30 35

Maximum permissible pressure differential after 5 minutes for HDPE

Material (Mil) Pressure DIFF (PSI)

40 4

60 3

80 2

100 2

Laboratory Testing of Destructive Seam Samples

1. Seam destructive samples may be sent to laboratory or tested on site when permitted by a site specific quality control plan or in the event that third partly laboratory destructive testing is not being performed.

2. Destructive samples will be tested for “Shear Strength” and “Peel Adhesion”. Five specimens shall be tested


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Alternative Daily Cover

At the end of each traditionally soil cover is required over the working face. This ultimately eats into effective volume of the landfill apart from the cost involved bringing such large quantities daily. The alternatives may include use of foam or covering with tarpaulin to minimize the environmental influences. Alternately waste materials like foundry sand, distressed soils, construction and demolition waste or ash from different sources are worth considering. Where conventional soil is used, it is becoming a common practice in the U.S. to scrape it back up again for reuse the next day.

Concluding Remarks

It is clear that without effective construction and operation practices, the fundamental objective of Land Filling, i.e.,

minimum impact to the environment will not be fulfilled.

References

- CPCB (2001), Criteria for Hazardous Waste Landfill (HASWAMS/17/2000-01)

- CPCB (2002), the Manual for Design, Construction and Quality Control of Liners and Covers for Hazardous Waste Landfills (Hazardous Waste Management Series: HAZWAMS/20/2001-02)

- CPCB website: envfor.nic.in/cpcb

- MoEF (2000) Municipal Waste Management & Handling Rules, Ministry of Environment and Forests, Government of India.

- Venkatappa Rao, G. and Pothal, Goutam K. (2008). “Geosynthetics Testing - A Laboratory Manual,” Saimaster Geoenvironmental Services P. Ltd., Hyderabad.

- Venkatappa Rao, G. and Sasidhar, R.S. (2009), Solid Waste Management and Engineered Landfills, Saimaster Geoenvironmental Services P. Ltd., Hyderabad.


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Soil nailing is a technique in which soil slopes, excavations or retaining walls are passively reinforced by the insertion of relatively slender

eflements - normally steel reinforcing bars. Such structural element which provides load transfer to the ground in excavation reinforcement application is called nail (Refer Figure 1). Soil nails are usually installed at an inclination of 10 to 20 degrees with horizontal and are primarily subjected to tensile stress. Tensile stress is applied passively to the nails in response to the deformation of the retained materials during subsequent excavation process. Soil nailing is typically used to stabilize existing slopes or excavations where top-to-bottom construction is advantageous compared to the other retaining wall systems. As construction proceeds from the top to bottom, shotcrete or concrete is also applied on the excavation face to provide continuity. In short Soil Nailing increases the shearing resistance of soil by acting in tension.

In the present era, soil nailing is being carried out at large in railway construction work for the stabilization of side lopes in existing track-road or laying of new tracks adjoining to an existing one (Refer Figure 4).

Development Of The Soil Nailing Technique

The soil nailing technique was developed in the early 1960s, partly from the techniques for rock bolting and multi-anchorage systems, and partly from reinforced fill technique (Clouterre, 1991; FHWA, 1998). The New Austrian Tunnelling Method introduced in the early 1960s was the premier prototype to use steel bars and shotcrete to reinforce the ground. With the increasing use of the technique, semi-empirical designs for soil nailing began to evolve in the early 1970s. The first systematic research on soil nailing, involving both model tests and full-scale field tests, was carried out in Germany in the mid-1970s. Subsequent development work was initiated in France and the United States in the early 1990s. The result of this research and development work formed the basis for the formulation of the design and construction approach for the soil nailing technique in the subsequent decades.


Fundamentals of Soil Nailing Technique

Figure 1. Soil nail with centralizers

Figure 2 depicts cross section of a grouted nailed wall along with some field photographs of the same in Figure 3.

Figure 2. Cross-section of a grouted soil nailed wall (a) Highway

Ground Engineering Soil Nailing


Various types of soil nailing

Various types of soil nailing methods that are employed in the field is listed below:

Grouted Nail: After excavation, first holes are drilled in the wall/slope face and then the nails are placed in the pre-drilled holes. Finally, the drill hole is then filled with cement grout.

Driven Nail: In this type, nails are mechanically driven to the wall during excavation. Installation of this type of soil nailing is very fast; however, it does not provide a good corrosion protection. This is generally used as temporary nailing.

Self-Drilling Soil Nail: Hollow bars are driven and grout is injected through the hollow bar simultaneously during the drilling. This method is faster than the grouted nailing and it exhibits more corrosion protection than driven nail.

Jet-Grouted Soil Nail: Jet grouting is used to erode the ground and for creating the hole to install the steel bars. The grout provides corrosion protection for the nail.

Launched Soil Nail: Bars are “launched” into the soil with very high speed using firing mechanism involving compressed air. This method of installation is very fast; however, it is difficult to control the length of the bar penetrating the ground.

Basic Elements of a Soil-nailed System

Figure 5 shows the cross-section of a typical soil-nailed cut slope. A soil-nailed system formed by the drill-and-grout method comprises the following basic elements:

Soil-Nail Reinforcement: A soil-nail reinforcement is the main element of a soil-nailed system. Its primary function is to provide tensile resistance. The reinforcement is typically a solid high yield deformed steel bar. Other types of materials, such as fibre reinforced polymer, can also be used as a soil-nail reinforcement.

Reinforcement Connector (Coupler): Couplers are used for joining sections of soil-nail reinforcing bars.

Cement Grout Sleeve: Cement grout, made of Portland cement and water, is placed in a pre-drilled hole after the insertion of a soil-nail reinforcement. The cement grout sleeve serves the primary function of transferring stresses between the ground and the soil-nail reinforcement. It also provides a nominal level of corrosion protection to the reinforcement.

Corrosion Protection Measures: Different types of corrosion protection measures are required depending on the design life and soil aggressivity. Common types of corrosion protection measures are hot-dip galvanising and corrugated plastic sheathing. Heat-shrinkable sleeves made of polyethylene and anti-corrosion mastic sealant material are commonly used to protect couplers.

(b) Railway

Figure 3. Application of soil nailed wall

Figure 4. Soil nailing in railway construction for laying of new tracks adjoining to an existing one

Soil-Nail Head: A soil-nail head typically comprises a reinforced concrete pad, a steel bearing plate and nuts. Its primary function is to provide a reaction for individual soil nails to mobilise tensile force. It also promotes local stability of the ground near the slope surface and between soil nails.

Slope Facing. A slope facing generally serves to provide the slope with surface protection, and to minimise erosion and other adverse effects of surface water on the slope. It may be soft, flexible, hard, or a combination of the three (CIRIA, 2005). A soft slope facing is non-structural, whereas a flexible or hard slope facing can be either structural or non-structural. A structural slope facing can enhance the stability of a soil-nailed system by the transfer of loads from the free surface in between the soil-nail heads to the soil nails and redistribution of forces between soil nails. The



most common type of soft facing is vegetation cover, often in association with an erosion control mat and a steel wire mesh.

Various issues affecting soil nailed slope

There are several factors that affect the feasibility and stability of soil nailing in slopes or excavations. As mentioned earlier, construction of soil nailing is subjected to favorable ground conditions. There are also various internal and global stability factors for soil nailed slopes.

Favorable Ground Condition: Soil nailing is well suited for Stiff to hard fine-grained soils which includes stiff to hard clays, clayey silts, silty clays, sandy clays, sandy silts, and combinations of theses. It is also applicable for dense to very dense granular soils with some apparent cohesion (some fine contents with percentage of fines not more than 10-15%). Nailing is not suitable for dry, poorly graded cohesionless soils, soils with cobbles and boulder (difficult to drill and increases construction cost), highly corrosive soil (involves expensive corrosion protection), soft to very

soft finegrained soils, and organic soil (very low bond stress or soil nail interaction force leading to excess nail length). Soil nailing is also not recommended for soils with high ground water table.

External Stability: The external or global stability of nailed slope includes stability of nailed slope, overturning and sliding of soil-nail system, bearing capacity failure against basal heave due to excavation. Sometimes long-term stability problem also come into picture, e.g., seasonal raining. In such cases, though ground water table may be low, the seeping water may affect the stability of nailed slope without facing or proper drainage system.

Internal Stability: It comprises of various failure modes of nailed structure e.g. nail soil pull-out failure, nail tensile failure, and facing flexural or punching shear failure.Such issues may be overcome by:-

Conducting adequate ground investigation and geotechnical testing for identification of soil parameters and ground characterization.

Performing in-situ test for soil nail interaction and nail strength.

Figure 5. Schematic Diagram of a Soil-nailed Cut Slope

(a) (b)

(c) (d)

(e)

Figure 6: Construction of soil nailing (a) Excavation (b) Mobile drilling rig, (c) Steel bar Installation, (d) Grouting Process (e) Stage construction


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Effective design of nailed slope system.

Construction procedure of nailed structure

Soil nailed structures are generally constructed in stages and it involves following steps:-

- Excavation till the depth where nails will be installed at a particular level

- Drilling nail holes- Nail installation and grouting- Construction of temporary shotcrete facing

Subsequent levels are then constructed and finally permanent facing is placed over the wall. Some of the field photographs of soil nail construction procedure are presented in Figure 6.

Design Requirement

Installation of nailing along the slope face increases the resisting force against the driving force of the soil mass in the failure zone. Hence, it can be regarded as a slope stabilization method. The fundamental principle of soil nailing is the development of tensile force in the soil mass and renders the soil mass stable. Although only tensile force is considered in the analysis and design, soil nail also resist bending and shear force in the slope. Through finite element analysis by Cheng (1998), has demonstrated that the bending and shear contribution to the factor of safety is relatively insignificant and the current practice in soil nail design (of considering only tensile force) should be good enough for the most cases. Nails are usually constructed at an angle of inclination from 10˚ to 20˚. Depending upon the climate of a particular region some sort of thickness of corrosion zone is assumed for an ordinary steel bar soil nail. As in Hong-Kong practice, a thickness of 2 mm is assumed as the corrosion zone so that the design bar diameter is totally 4mm less than the actual diameter of the bar. The nail is usually protected by galvanization, paint, epoxy and cement grout. Alternatively, fiber reinforced polymer (FRP) and carbon fiber reinforced polymer (CFRP) may be used for soil nails which are currently under consideration. There are several practices in the design of soil nails. The effective nail load is usually taken as the minimum of

- the bond strength between cement grout and soil,- the tensile strength of the soil nail and - the bond stress between grout and the nail.

Design Considerations

A soil-nailed system is required to fulfil fundamental requirements of stability, serviceability and durability during construction and throughout its design life. Other issues such as cost and environmental impact are also important design considerations.

- Stability: The stability of a soil-nailed system throughout its design life should be assessed. The design of a soil-nailed system should ensure that there is an adequate safety margin against all the perceived potential modes of failure.

- Serviceability: The performance of a soil-nailed system should not exceed a state at which the movement of the system affects its appearance or the efficient use of nearby structures, facilities or services, which rely upon it.

- Durability: The environmental conditions should be investigated at the design stage to assess their significance in relation to the durability of soil nails. The durability of a steel soil-nailed system is governed primarily by the resistance to corrosion under different soil aggressivity.

- Economic Considerations: The construction cost of a soil-nailed system depends on the material cost, construction method, temporary works requirements, buildability, corrosion protection requirements, soil-nail layout, type of facing, etc.

- Environmental Considerations: The construction of a soil-nailed system may disturb the ground ecosystem, induce nuisance and pollution during construction, and cause visual impact to the existing environment. Appropriate pollution control measures, such as providing water sprays and dust traps at the mouths of drillholes when drilling rocks, screening the working platform and installing noise barriers in areas with sensitive receivers, should be provided.

Merits And Limitations

The soil nailing technique offers an alternative design solution to the conventional techniques of cutting back and retaining wall construction.

Merits

(a) It is suitable for cramped sites with difficult access because the construction plant required for soil nail installation is small and mobile.

(b) It can easily cope with site constraints and variations in ground conditions encountered during construction, e.g., by adjusting the location and length of the soil nails to suit the site conditions.

(c) During construction, it causes less environmental impact than cutting back and retaining wall construction as no major earthworks and tree felling are needed.

(d) There could be time and cost savings compared to conventional techniques of cutting back and retaining wall construction which usually involve substantial earthworks and temporary works.


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(e) It is less sensitive to undetected adverse geological features, and thus more robust and reliable than unsupported cuts. In addition, it renders higher system redundancy than unsupported cuts or anchored slopes due to the presence of a large number of soil nails.

(f) The failure mode of a soil-nailed system is likely to be ductile, thus providing warning signs before failure.

Demerits

(a) The presence of utilities, underground structures or other buried obstructions poses restrictions to the length and layout of soil nails.

(b) The zone occupied by soil nails is sterilised and the site poses constraints to future development.

(c) Permission has to be obtained from the owners of the adjacent land for the installation of soil nails beyond the lot boundary. This places restrictions on the layout of soil nails.

(d) The presence of high groundwater levels may lead to construction difficulties in hole drilling and grouting, and instability problems of slope surface in the case of soil-nailed excavations.

(e) The effectiveness of soil nails may be compromised at sites with past large landslides involving deep-seated failure due to disturbance of the ground.

(f) The presence of permeable ground, such as ground with many cobbles, boulders, highly fractured rocks, open joints, or voids, presents construction difficulties due to potential grout leakage problems.

(g) The presence of ground with a high content of fines may lead to problems of creeping between the ground and soil nails.

(h) Long soil nails are difficult to install, and thus the soil nailing technique may not be appropriate for deep-seated landslides and large slopes.

(i) Because soil nails are not prestressed, mobilisation of soil-nail forces will be accompanied by ground deformation. The effects on nearby structures, facilities or services may have to be considered, particularly in the case of soil-nailed excavations.

(j) Soil nails are not effective in stabilising localised steep slope profiles, back scarps, overhangs or in areas of high erosion potential. Suitable measures, e.g., local trimming, should be considered prior to soil nail installation.

Conclusion

Soil nailing is embrassed by practicing engineers as a

highly competitive well proven technique. Soil nailing has certain similarities to both reinforced earth and anchoring, although its particular operating principles and construction methods give it a firm and distinct identity. Similar considerations distinguish it from allied insitu soil reinforcing techniques such as reticulated root piles and soil dowelling. Most applications of soil nailing to date have been associated with new construction projects such as foundation excavations and slope stabilization, for both temporary and permanent works. The system has equal facility in a wide range of remedial projects, and indeed it is most likely that nailing will find its wide applications in the India in this field, bearing in mind the prevailing economic trends. It is to be hoped that the growth of the technique in India can be fostered by practical research collaborations between industry, the universities and government, in the manner of developed countries like France, Germany, United States of America and United Kingdom, who are the current leaders in this field.

Reference

1. FHWA (2003), Soil NAIL walls, Geotechnical Engineering circular No 7 , Report No FHWAO-IF-03-017, Federal Highway Administration.

2. Juran I., Gerge.B.Khalid F. and Elias V. (1990a): “Kinematical Limit Analysis for Design of Soil Nailed Structures”, J. of Geotechnical Engineering, ASCE, vol 116, No 1, pp. 54-71.

3. Juran I., Gerge.B.Khalid F. And Elias V. (1990b): ‘Design of Soil Nailed Retaining Structures, Design and Performance of Earth Retaining structures’, J. of Geotechnical Engineering, ASCE, Vol 116 pp. 54-71.

4. Guide to Soil Nail Design And Construction, Geotechnical Engineering office Civil Engineering and Development Department The Government of the Hong Kong Special Administrative Region.

5. Soil Nailing For Stabilization Of Steep Slopes Near Railway Tracks, Prepared by Dr. Amit Prashant, Ms. Mousumi Mukherjee, Department of Civil Engineering Indian Institute of Technology Kanpur, Submitted to Research Designs and Standards Organization (RDSO), Lucknow

6. www.williamsform.com/Ground_Anchors/Soil_Nails_Soil_Nailing/soil_nail_soil_nailing.html

7. http://www.classes.ce.ttu.edu/CE5331_013/

8. http://www.geofabrics.com/docs/Tamworth.pdf

9. http://www.wmplanthire.com/slope_stabilisation.htm

10. http://www.keller-ge.co.uk/engineering/case-studies

11. New Directions in LRFD for Soil Nailing Design and Specifications, C. A. Lazarte (GeoSyntec Consultants, Columbia, Maryland, USA), G. B. Baecher (University of Maryland, College Park, Maryland, USA 0, J. L. Withiam (D’Appolonia, Monroeville, Pennsylvania, USA)


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In construction, underpinning is the process of strengthening and stabilizing the foundation of an existing building or other structure. Underpinning is a

repair process that strengthens foundations which have been weakened by a variety of factors. In the process of underpinning, the area underneath the load of the foundation is repaired, made strong or reinforced.

Underpinning comes in many types and suits different kinds of repair projects. In order to make the right choice, it is necessary to understand the structure of the whole foundation, the supports used and the factors that cause the foundation to crack or get damaged.

Reason for Underpinning

Underpinning may be necessary for a variety of reasons:

- The original foundation is simply not strong or stable enough, e.g. due to decay of wooden piles under the foundation.

- The usage of the structure has changed. - The properties of the soil supporting the foundation

may have changed or was mischaracterized during planning.

- The construction of nearby structures necessitates the excavation of soil supporting existing foundations.


Basics of Underpinning System its Application & Benefits

Ground Engineering Underpinning System


About the Process

Underpinning is accomplished by digging underneath shallow footings and extending the foundation by pouring concrete to extend the reach of the foundations in depth or in breadth so it either rests on a stronger soil stratum or distributes its load across a greater area. Use of micropiles and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of a grout.

Subsidence usually occurs in lowrise and mediumrise buildings which usually have shallow foundations or cellars up to about 3m deep. Subsidence and settlement are taken as interchangeable terms, meaning the sinking of ground on which a structure is founded. Ground can also rise, in

which case the movement is known as “heave”. Depending on the cause of subsidence or heave, horizontal stretching or squeezing of the ground can accompany the vertical movement.

When movement is, or is likely to become excessive, so that the use or safety of the building is compromised, this is when underpinning is generally the best solution.

Common Methods of Underpinning Process

There are several types of underpinning methods to choose from, and each will be best for some situations only.

Method 1: The traditional mass concrete underpinning method is a good example. The process is best suited for shallow underpinning and involves excavating the weak soil underneath the foundation and replacing it with mass concrete which provides more strength. Since the concrete base is now stronger than the soil that used to hold the foundation, the weight of the structure is carried in a much more efficient manner.

Method 2: Another method involves the use of a beam and base. This method makes use of the traditional way of constructing mass concrete bases but also incorporates beams to serve as a support for the existing foundation. Its load is transferred to a concrete beam that is constructed underneath it. The beam then transfers the load to a mass concrete base which spreads it evenly for support. The construction of the beams depends on the architecture of the structure built above the foundation.

Method 3: Another type of underpinning process, called mini-piled underpinning, involves transferring the load of the structure and the foundation to stable soils found deep under the ground. Usually, the depth of the soil exceeds 5 meters. The constructed piles are cased in steel and have a diameter ranging from 15 cm to 30 cm. The piles are driven into drilled holes, making them rest on a stable soil below ground level. Some piles may be constructed as to reach 15 meters below ground.

Process of Basement Underpinning

Basement underpinning pertains to the process of repairing or re-strengthening an existing foundation of a building. Basement underpinning is usually referred to as a repair method but it is also advised by landscapers for prolonging the durability of a residential building. Homeowners planning to undertake basement underpinning should have a basic understanding of what it entails since it quite expensive.

Basement Underpinning Basics

The most established form of basement underpinning uses

Dry rot is a type of fungus that decays wooden piles under foundations

Cavity created by the underpinning process to be filled with concrete to extend reach and depth of the foundations



a combination of concrete and piers. Some people might refer to this as ‘traditional underpinning’ but this is actually a slight modification since the earliest form of underpinning didn’t use piers. While the concrete adds to the basement’s overall durability, the presence of piers increases its load-bearing strength.

Basement underpinning requires the creation of rounded or squared holes depending upon the kind of excavation tools used and the shape of the piers. Ideally, the holes should be at least 24 inches apart. Excavation tools include the use of backhoes and soil loaders that required for scooping-out and piling the dug-up soil. Some people prefer using shovels but this can make the entire process rather tiring.

- Pier Piling: Once the excavation has created enough space, the extra support in the form of piers is introduced. This could be in the form of using:

- Resistance Piers—they are installed using advanced machinery wherein they are driven through the soil without needing any screws or brackets. These are ideally suited for bigger homes.

- Helical Piers—these are installed using the conventional system of screwing the piers upon a frame. These piers are difficult-to-manipulate once installed, since the framework secured with screws cannot be undone.

- Pouring Concrete : Some landscapers prefer to pour concrete upon the pier frame too to make it more durable while some just like to pack some garden soil. The outer edge of the piers is secured along the perimeter of the basement by pouring concrete. The temporary support structure is removed. Concrete pouring is done in the traditional manner using tools like shovels, trowels and concrete rakes. The outer edge of the concrete-covered edge is smoothed with a concrete cutter.

Safety Issues to be Given Utmost Importance

Most types of underpinning involve digging holes under buildings in confined spaces. The existing structure is expected to defy gravity and temporarily arch over the excavation. Collapses can occur. The risks must be identified and managed well before and during the underpinning process. Following are list of safety activities that needs to be considered for underpinning process:

- Investigate services before digging - Check that underpinning pits cannot flood or be gassed - Strengthen superstructure before digging - Check that walls above are strong enough to support

themselves over pits - Support sides of excavations - Ensure that workers can escape from pits easily - Use threaded couplers instead of dowel bars to connect

reinforcement rods between sections of shallow mass concrete underpinning

- Ensure safe access and ventilation to pits - Use a Banksman to oversee safety.

Basement underpinning

Basement underpinning is recommended for many reasons, like:

- The existing basement is showing signs of extensive cracking, moisture seepage or is sinking into the underlying soil bed.

- The existing basement is leaking out the internal heat even after repeated, basic repairs with mortar and insulation treatment.

- The existing basement needs to be strengthened for bearing additional weight as more floors are being added to the house.

- Construction of large buildings in the vicinity along with moisture-friendly soil has caused large-scale soil compaction below the basement, causing overall instability of the house.

Steps followed in Basement Underpinning

- Installation of Temporary Support: At the time starting the project, a temporary support-like structure is created. This is done to ensure that the basement doesn’t collapse during underpinning. Further, it provides a base upon which heavy materials like piers can be moved.

- Excavation : Excavation or removal of soil from under and around the basement is vital. This provides access to the underside of the basement, creating space that is used for installing the stabilizing additions (piers). Before excavating the soil, one should have a detailed plan that explains the angles and spots wherein the piers would be inserted. Unnecessary excavation can lengthen the project and weaken the basement.


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Common Problems Encountered in Underpinning

Though Underpinning is very good for stabilizing an old sinking building of structure, yet all is not well with this technique. There are lot of cases where one will end up with serious problems with Underpinning. Underpinning of an existing foundation is typically required whenever a new excavation compromises the stability of the soils supporting that foundation. underpinning involves extending a building’s foundation downward, usually by adding concrete under the existing foundation wall. It is a specialty operation, which even under the very best conditions, has considerable risk associated with it. There are some conditions that make it even more difficult to perform without causing damage. These are as follows:

the perimeter footings was mostly successful. However, the church building had interior foundations for walls and columns that were not underpinned. The drawdown of the water table caused consolidation of the silty soils, which in turn caused settlement of these interior elements. The interior walls developed severe cracks and the slab-on-grade experienced extensive settlement. As a result, the building was vacated.

(C) Sandy Soils

Sandy soils pose dual problems

- They settle when vibrated. Pile driving is one potential source of vibrations. Even if the permanent building does not use piles, the contractor may decide to use piles as part of the temporary soil retention systems. For example, soldier pile walls with wood lagging are commonly used. The settlement caused by vibration of cohesionless soils can affect not only the foundations immediately bordering the excavation, but can also cause settlement of interior foundations and slabs-on-grade.

- They spill out. Cohesionless soils have no ability to stand vertically. So if sheeting or lagging is not installed as the excavation progresses, sandy soils will spill into the excavation, causing the building to lose foundation support.

Precautions to be Taken in Underpinning

Underpinning is a messy, noisy and traumatic operation for buildings and their occupants alike. Unless sophisticated and expensive jacking systems are incorporated, the underpinning will almost inevitably promote some additional subsidence as the works settle in. If a structure is partially underpinned, for example one house in a terrace, then future damage may recur as the rest of the non-underpinned structure continues settling. For these reasons, underpinning should be avoided if at all possible.

Underpinning is not necessary from a purely engineering viewpoint in the following situations:

- Where the cause of the ground movement has ceased and is unlikely to recur, repairing the damage should be sufficient

- Where the rate and total magnitude of anticipated ground movement is unlikely to significantly threaten the structural strength, stability or integrity of a building during its required lifespan, periodic repairs and redecoration should suffice. Doors and windows may have to be eased from time to time or changed for other types which are more tolerant of frame distortion.

(A) Rubble Foundations

Older buildings may use foundations composed of large stones, which may or may not be mortared together. These rubble foundations, while perfectly adequate for distributing gravity loads to the soil, are not well suited to bridging over underpinning pits. They lack the continuity that is inherent in reinforced concrete footings, or even unreinforced concrete footings. In some cases, it may not be feasible to underpin these foundations. Instead, it may be necessary to install a retaining wall next to the wall that is designed to withstand the lateral load due to the surcharge.

(B) High Water Table with Silts and Clays

In one case that the author investigated, a high-rise building was constructed next to an abutting church. The basements of the highrise building extended several levels below the church, which required underpinning the perimeter foundations of the church. In addition, the presence of a high water table meant the site would need to be dewatered. The underpinning operation for

Rubble foundations



Some Underpinning Cases

Photographs of some underpinning jobs are shown below:

Figure 1 shows some general pictures of underpinning in foundations.

be installed with minimal site disturbance and under low headroom conditions.

(a) (b)Figure 1. (a) Approach pit and pit below foundation (b) Dry packing the underside of the footing

Figure 2. Combining Foundations by Underpinning

Figure 2 shows combining isolated foundations through underpinning.

Figure 3. Process of Helical Pile Underpinning

Figure 4. Underpinning with Pile and Reinforced Concrete Raft

Figure 5. Underpinning to a flank wall to enable the construction of a Reinforced Concrete Basement

Figure 3 shows process of helical pile underpinning. Helical piles consist of specially made augers or helical sections which are drilled and left in place to form the pile. Their capacity is proportional to the installation torque and typically ranges between 10 and 30 kips. They can

Underpinning of a building with piles and reinforced concrete raft is shown in Figure 4. The work is being carried out by Falcon structural UK.

Underpinning to a flank wall to enable the construction of a Reinforced Concrete Basement is shown in Figure 5.

Benefits of Underpinning System

- The process of underpinning the foundation makes buildings accessible for inspection, correction and improvement. As underpinning is usually undertaken in older homes, older household plumbing, electrical and insulation systems can be replaced with new ones. Also, one will have a new concrete floor. Overall, one will get long term savings in energy, safety and comfort of the entire house.

- Foundation underpinning dramatically increases the usability of the below ground space, up to fifty percent



of the available room in a house. In doing so basement may simply become an entertainment field, spa, training room, etc.

Conclusion

In the construction world, underpinning is a process in which you strengthen and also stabilise a foundation of a building which could be potentially dangerous if no action was taken. A few of the reasons and benefits one should consider are as follows....

- The existing foundations are not strong / stable enough - underpinning will correct these problems.

- The use of the building has changed - Possibly you have changed the purpose of a building, maybe from a house to a shop - underpinning will allow the building to take more pressure.

- The soil properties that the building lies upon have changes - there could have been some movement in the ground, perhaps through subsidence - underpinning will help strengthen the building in this case.

- New buildings or structures added to the surrounding area. If a new house is build next to an existing property, then underpinning maybe needed to strengthen the first property.

- Land cost has increased. If the price of land increases by a great deal, it maybe cheaper to use underpinning with a view to improving the existing property (perhaps adding a new floor) rather than purchasing more land.

Hence significant advantages can be gained by early coordination and understanding of the specific requirements that must be met by the underpinning and excavation support systems. Incorporating the design of the underpinning system into the overall project deliverables has the potential of resolving unforeseen problems during construction.

Reference

- http://www.buildingconservation.com

- http://www.doityourself.com

- http://www.aquamasterplumbing.com

- Getting to the Bottom of Underpinning, David B. Peraza, P.E., STRUCTURE magazine 17 December 2006.

- http://www.falconstructural.co.uk

- http://ezinearticles.com

Photo Courtesy:www.alphastructural.com www.great-southern-exterminating.com www.thebasementspecialists.ie www.knollmeyerbuildingcorp.com mohlermasonryblog.wordpress.com


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Concrete structures in aggressive environment (Marine / Industrial area) are subjected to chloride attack. In order to protect concrete structures from

chloride attack in aggressive environment, it is necessary that protective coatings are applied to concrete and steel both to prevent ingress of chlorides.

Protective surface coatings are primarily used to protect new or repaired concrete surfaces from future chemical attack (e.g. against sulphurous and nitrous pollutants) and the ingress of aggressive liquids and gases (e.g.water borne chlorides or atmospheric carbon dioxide). Some corrosion induced damages in concrete are shown in Figure 1. This concrete surface protection can also be specified to be: water-repelling through impregnation and pore blocking; elastic and crack-bridging (to different degrees of elasticityat different temperatures); resistant

to different chemicals (such as in containment zones); or abrasion and wear resistant (e.g. on horizontal concrete surfaces such as balconies and car park decks).In order to meet all of the different requirements for concrete facades and horizontal concrete surfaces on different structures and in different exposure conditions, there are a wide range of different protective concrete coating products and systems required.

On reinforced concrete facades single pack, acrylic resin based anti carbonation coatings are normally ideal as the protective coating solution. Also ideal are protective coatings based on, elastic film-forming, styrene acrylates or other copolymer resin based products, which should be used when additional crack-bridging properties are required. On horizontal decksfor combined chemical and wear resistance, the best protective deck coating


Protective Coating for Exposed Concrete Surfaces

Concrete Surface Coatings


products are usually 2 component epoxy and polyurethane resin based solutions that are specifically designed to accommodate this additional stress and exposure.

Figure 1. Corrosion induced damages in concrete

Codal Provision

The characteristics and performance requirements for protective surface coatings to be used on reinforced concrete are defined in the European Standard EN1504 Part 2, with the appropriate product selection then to be made in accordance with the specific requirements and exposure conditions of your project.

Various Types of Concrete Coatings

All conventional concrete coatings are multi-component systems, and the two-component epoxy/amine and urethane (isocyanate/polyol) are the most common. These epoxy and urethane coatings require more than one day before return to service. Faster curing systems include polyaspartic and methyl methacrylate coatings, which can be returned to service in hours instead of days. Following are the five basic types of concrete coatings that are available in the market-

- Penetrating Concrete Sealers-Penetrating concrete sealers are most often used to coat exterior concrete surface, especially those that are vulnerable to damage from freezing-thawing conditions and other harsh outdoor elements. These sealers dry to a natural matte finish, invisibly protecting the surface without changing its appearance. This type of coating works by penetrating into the concrete and creating a chemical reaction that shields the concrete from moisture and chemicals commonly used to deice surfaces.

- Acrylic Sealers-Acrylic sealers work by creating a thin layer of protective film over the surface of the concrete. They are available in both solvent and water-based formulas and you can select the sheen level you desire. This type concrete coating can be used either in interior or exterior spaces. Acrylic sealers are economical but are not as durable as epoxies and polyurethanes. When used on interior floors, acrylic sealers require regular maintenance and waxing to reduce wear and

the appearance of black heel marks. Because acrylic sealers dry quickly, they are a good choice when a short down time is important.

- Polyurethanes-Polyurethanes are also available in both water-based and solvent formulations. They can be used inside or out and are available in a selection of sheens. Polyurethanes are almost twice as thick as acrylic sealers and produce an extremely durable cover. They are a good option for high traffic areas because they provide strong resistance to heel marks, scuffs and staining. A polyurethane finish is non-yellowing and dries to a transparent sheen.

- Epoxy-Epoxies are a versatile choice that is available in a one or two component application. They can be clear or can have sand, colored quartz or vinyl chips incorporated in them so a variety of colors and patterns can be achieved. Because epoxy finishes are vulnerable to UV damage, these products are normally used on interior surfaces. Epoxies produce a hard, abrasion-resistant coating that is durable and long-lasting. Careful surface preparation is vital to the success of an epoxy coating.

- Concrete Overlays-Concrete overlays like Flex-cement contain polymer resins, cement and aggregates. The polymer resins make this coating strong and durable. Overlays have become very popular in recent years because they offer a cost-efficient way to restore old, damaged concrete surfaces without having to go to the time, trouble and expense of ripping out the existing flooring. Concrete overlays can be used to smooth and level uneven surfaces and when you choose a concrete overlay, you’ll experience minimal downtime, because they cure quickly. It may be only a few hours before freshly coated surfaces are able to support foot traffic.

Overlays are good choices for either interior or exterior surfaces. They are one of the most versatile options available when it comes to design possibilities. Skilled installers can create a myriad of designs and colors by using special techniques and tools. Overlays can be stamped, stenciled, textured or stained. The design possibilities are basically unlimited with this type treatment.

Protective Coating Over Existing Concrete Surface

Protective coating-Protective coatings are used to protect concrete from degradation by chemicals and subsequent loss of structural integrity, to prevent staining of concrete, or to protect liquids from being contaminated by the concrete.The proper use of protective coatings is the extremely effective means of preventing concrete deterioration and corrosion in water and waste water applications. The function of a coating isto act as a barrier that prevents either



chemical compounds or corrosion current from contacting a concrete substrate.The coating s effectiveness of fulfilling this function depends on its degree of integrity (being a completely continuous film or freedom from imperfection or defects), its ability to bond to the concrete substrate, and its ability to insulate against the passage of corrosion current (dielectric strength) or chemicalions. Corrosion protection by coatings for water and waste water pipelines is the implementation of a well-balanced cycle of the following four equally important elements:

- Specifying and using a proper coating system- Proper surface preparation for the coating system- Proper application of the coating system- Quality inspection of the coating system

A. Condition Assessment of Concrete for Protective Coating Application-

Because conventional sealers and paints are a comparatively thin and weak layer applied to the surface, they are wholly dependent upon the integrity of the concrete surface to maintain integrity oftheir protective film. In other words, paints and sealers are not structurally independent of the surface and, to be effective, must only be applied to a sound substrate. If the concrete has been in service without the benefit of protective coatings, the surface of the exposed concrete is likely deteriorated to some extent. A detailed inspection of the surface conditions has to be performed during the design stage or provisions must be made in the contract for varying conditions.The recommended practice and procedure for assessing the condition of the concrete by the American ConcreteInstitute (ACI) include visualexamination, nondestructive evaluationtest (NDT), and destructive tests (ACI364.1R). A typical NDT is shown in Figure 1.

Figure 2. A typical NDT in progress

B. Surface Preparation for Concrete Coating

Since the adhesion of the coating will be limited by the strength of the surface, the deteriorated concrete must be removed to reveal a sound surface prior to application of any coating. Depending on the extent of deterioration, rebuilding of the surface with a mortar prior to coating application may also be necessary. No general surface preparation standard exists for concrete however, most coating projects have unique conditions and special requirements that must be evaluated to determine which will best meet the objectives of the engineers and owners. Some of the important factors for selection of surface preparation methods are:

- Substrate condition- Owner requirements- Material requirements- Application conditions

Below Figure 3 shows surface preparation process.

Figure 3 Surface preparation

C. Concrete Coating

Protective coatings play a critical role in extending the service life of concrete surfaces in harsh environments. When subjected to the harsh conditions inherent in water and waste water treatment facilities, the role of the coating system in protection of concrete surfaces is particularly important for a number of reasons:

- The coating system must protect the concrete from chemical attack and deterioration by hydrogen sulfide, carbon dioxide, and chloride attacks.

- The coating system must protect the concrete from microbiological attack. Microorganisms, which cause the breakdown of organic matter also, can cause deterioration of concrete (through the formation of hydrogen sulfide),resulting in spalling and sloughing.

- The coating system must protect the concrete from spalling and cracking caused by moisture penetration. Moisture enters the concrete and isdrawn inside by capillary action. In cold temperatures the moisture freezes in the pores, swells, and causes spalling. Prior


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to the late 1980s, relatively thin film coatings based on coal tar epoxyand amine cured epoxy formulations provided effective corrosion protection of concrete for up to 10-12 years.A typical coating might, for example, be only 12 mils (.012 ) thick; it would seem impractical to provide sufficient protection for the concrete structures. Incontrast, to a typical sealer and paint approach, a polymer lining has sufficient cohesive strength to provide substantial film integrity independent of the substrate. It will also provide a cost effective method to deal with anirregular concrete surface that has resulted from deterioration.

Figure 4 shows the coating process in an exposed concrete surface.

Figure 4. Coating Process in an exposed concrete surface

Basic checks for the coating processes

When preparing a protective coating strategy over the exposed concrete surface from start to finish it is essential to prepare it in a comprehensive manner so that all the aspects are covered. Some guidelines are given to cover all the aspects below:

Step 1

Supply of Information- Prior to the engineer giving approval of a particular paint type, the contractor shall supply information which will satisfy & comply with the requirements of the specification.

Step 2

Coding system-all primers, paints and solvents to be used in the works shall be identified by a unique coding system, relating to the batch of raw materials from which the product was manufactured and the date of manufacture.

Step 3

Storage life-storage life shall normally be a minimum of one

year. If the storage life is known to be shorter, the expiry date must be marked on the container prior to dispatch from the manufacturer’s factory.

Step 4

Method of using paint components-preparation and application techniques for all components of the paint system shall be stated.

Step 5

Surface preparation of concrete-recommendations for preparing the surfaces of concrete shall be given, including the following:

- The minimum age,- The maximum moisture content and measuring method,- The equipment to be used for preparing the concrete

surface, and- The materials suitable for filling defects in the concrete.

Step 6

Dry film thickness and coverage- The minimum and maximum DFT limits for each component of the paint system shall be given for a specific temperature. The corresponding coverage in L/m2 shall be quoted for prepared concrete surfaces typical of low strength (Grade 20) and high strength (Grade 40) concrete cured under site conditions, in order to achievere commended DFT values.

Step 7

Over coating-drying and over cutting times of the pretreatments and coats of the paint system shall be given for a particular temperature and relative humidity.

Step 8

Physical properties-all components of the paint system shall be capable of unique identification such that any substitution, dilution or adulteration of the paint can be identified. The Contractor shall provide test data and methods of test for the following properties of each applicable primer, paint and solvent used in the system:

- Specific gravity, - Volume of solids,- Viscosity, - Fineness of pigment grind,- Infra-red spectrograph,- Pyrolysis gas chromatography of the binder,- Ash content.

Step 9

Durability-

- The suitability of the coating for application on damp, alkaline, cement-based materials shall be stated.


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- The decorative life of the paint shall be stated, in terms of the colour-fastness of the finish coat and resistance to chalking, loss of gloss and atmospheric dirtying.

- The life of a paint system prior to the need for recoating shall beat least 10 years. Examples shall be cited of where the paint systemhas achieved this life.

- The paint system shall be capable of withstanding cleaning withhot water (in the range between 40C and 50C), detergent and scrubbing action without losing adhesion, softening or changing in colour or gloss.

Step 10

Health andSafety-

- The contractor shall supply health and safety data relating to the storage and application of all components of the paint system. As aminimum, the check list contained in Table 1 shall be completed.

- The effects of solvent and vapour build-up on the environment in the vicinity of the paint applicator shall be monitored, and the loss of volatiles per unit area of paint in terms of minimum air exchange rates in confined areas shall be determined.

- The in-service performance of the paint under conditions of fire shall be given, making particular reference to the surface spread of flame, and the toxicity and opacity of combustion products.

Conclusion

Concrete coatings have multiple capabilities of providing aesthetics and protecting structural concrete from weathering. They are viable for use on all concrete elements including barrierwalls, girders, abutments and wingwalls and all types of foundations. Severalof these coatings reduce the permeability of concrete (waterproofing) and limit the intrusion of harmful moisture and deicing salts. All of those coatings should provide enhanced aesthetics(compared to textured masonry coatings and stains) and better reparability. Beneficial coatings need to be identified by the engineers and tested to determine their effectiveness in protecting concrete. Due to the wide latitude of properties of these different coatings, guidelines need to be developed for determining where they are best suited (new and maintenance concrete application) to make their application more effective.

Reference

- Applying linings to concrete surfaces in water and wastewater environments J. Peter Ault, P.E., PCS, T. Kyle Greenfield, PCS, SasanHosein, M.S.Corrpro Companies, Inc

- www.texcote.com, www.remmers.co.uk- model specification for protective coatings for concrete, Civil

Engineering Department Hong Kong Government, 1994.- www.ConcreteRepairSite.co.uk- www.sika.com.au- www.matcoinc.com


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BC India

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16th National Congress on Corrosion & its Control

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