1. INTRODUCTIONUrban settings pose unique challenge to the construction Industry. Special features of urban areas are restricted movements, inadequate space for equipment, soil heterogeneity (including fill and remains of old foundations or other unexpected obstructions), effects of changes in the water table, foundation interaction (the detrimental effects of construction of new structures on the surrounding buildings).As the number of deep excavations in city is seen to increase exponentially so are the problems associated with their construction. Structures in the immediate vicinity of excavations, dense traffic scenario, presence of underground obstructions and utilities have made excavations a formidable task to execute. Clearly, deep excavations are posing mounting problems that demand a site specific and tailor made retaining solution. Even in complicated urban settings, deep retaining systems have been deployed successfully by overcoming construction challenges. This article describes some of the key retaining structures that are used.1.1 DEEP EXCAVATION SUPPORTING SYSTEMSSeveral in-situ support systems have been deployed for containing deep excavations. The criteria for the selection of these systems are excavation depth, ground conditions, ground water level, allowable vertical and horizontal displacements of adjacent ground, availability of construction know-how, cost factors, subsequent construction methodology, working space limitations etc. One of the key governing factors is the requirement of water tightness of the retaining structure. Following types of deep support systems are commonly used in metropolitan cities.(i) Diaphragm walls(ii) Pile walls (Contiguous, Tangent or Secant)(iii) Soldier pile with wooden lagging walls(iv) Sheet pile walls(v) Composite supporting systems that is, any of the retaining systems (i) to (iv) above strengthened by Anchors, internal strutting etc.Diaphragm walls, Contiguous Piles and Soldier piles with wooden lagging walls are addressed in the following section2. CONTIGUOUS PILE WALLSThere are different types of pile walls. Diameter and spacing of the piles is decided based on soil type, ground water level and magnitude of design pressures. Large spacing is avoided as it can result in caving of soil through gaps. In Contiguous bored pile construction, centre to centre spacing of piles is kept slightly greater than the pile diameter. Secant bored piles are formed by keeping this spacing of piles less than the diameter. Tangent piles are used when secant piling or diaphragm walling equipment is not available.General construction sequence for piling operation:(i) Centering of rotary rig on the proposed pile point(ii) Carrying out the boring operation (iii) Driving the casing to an approximate depth (iv) Maintaining the stability of the borehole simultaneously with bentonite slurry (v) Continuing boring operation in soil and rock using soil bucket and Soil/Rock auger (vi) After completion of boring, cleaning of borehole by bentonite flushing

Fig 1: Contiguous Pile Retaining System (vii) Lowering of rebar cage into the borehole(viii) Repeat bentonite flushing operation and subsequently(ix) Pouring concrete through tremie 2.1 Merits and DemeritsContiguous piles serving as retaining walls are popular since traditional piling equipments can be resorted for their construction. They are considered more economical than diaphragm wall in small to medium scale excavations due to reduction in cost of site operations. Common pile diameters adopted are 0.6, 0.8 and 1.0m. These piles are connected with a Capping beams at the top, which assists equitable pressure distributions in piles. These retaining piles are suitable in areas where water table is deep or where soil permeability is low. However, some acceptable amount of water can be collected at the base and pumped out. Contiguous piles are suitable in crowded urban areas, where traditional retaining methods would otherwise encroach the adjoining properties. Provision of Contiguous piles restricts ground movements on the backfill side, and thus protects the neighbouring structures, foundations and boundary walls from the detrimental effects of the excavation. Contiguous piles facilitate deployment of several independent sets of equipment and gangs along its alignment which can speed up its execution. They can be constructed using even the conventional piling equipments, and can be constructed in hard and rocky sub-soil conditions where diaphragm wall construction is difficult. Such retaining systems has advantage of employing varying diameter of piles in lieu of change in sub-surface conditions, or on encountering competent stratum at a depth which is different than that anticipated during design. Further, unlike the diaphragm wall which relies on the orthogonal geometry of the excavated area contiguous pile retaining system can constructed to form any shape in the excavated area. They are however, not considered suitable for construction in areas of high water table, as retention and containing water is not possible in contiguous piles. Perfect alignment of piles is often difficult to achieve at site, and this in turn is found to affect the dimension and alignment of the Capping beams.

3. DIAPHRAGM WALLS4

6

Diaphragm walling is a technique of constructing a continuous underground wall from the ground level. Diaphragm walls provide structural support and water tightness. These reinforced concrete diaphragm walls are also called Slurry trench walls due to the reference

Fig 2: Diaphragm Wall

given to the construction technique where excavation is made possible by filling and keeping the wall cavity full with bentonite-water mixture during excavation to prevent collapse of Vertical excavated surfaces. These retaining structures find following applications: earth retention walls for deep excavations; basements, and tunnels; High capacity vertical foundation elements; Retaining wall foundations; water control. These are also used as a permanent basement walls for facilitating Top-down construction method.Typical wall thickness varies between 0.6 to 1.1m. The wall is constructed panel by panel in full depth. Panel width varies from 2.5m to about 6m. Short widths of 2.5m are selected in less stable soils, under very high surcharge or for very deep walls. Different panel shapes other than the conventional straight section like T, L are possible to form and used for special purposes. Steps involved in the construction of diaphragm wall can be broadly listed as follows: Guide Wall Construction along alignment Trenching by crane operated Grab or hydraulic grab Bentonite flushing Lowering reinforcement cage Concrete using Tremie It must be remembered that Diaphragm wall are constructed as a series of alternating primary and secondary panels. Alternate primary panels are constructed which are restrained on either side by stop end pipes. Before the intermediate secondary panel excavation is taken up, the pipes are removed and the panel is cast against two primary panels on either side to maintain continuity.

Fig 3: Construction of Diaphragm Wall

3.1 Merits and DemeritsDiaphragm wall construction is relatively quiet and minimum noise and vibration levels make it suitable for construction in urban areas. The water tight walls formed can be used as permanent structural wall and are most economical when use in this manner. The finished structural wall formed prior to excavation allows subsequent construction of the basement in a water tight and clean environment. Once the diaphragm walls are constructed, work can be planned to proceed simultaneously above and below the ground level. There is a minimum of space wasted.Diaphragm walls however, require the use of heavy construction equipment that requires reasonable headroom conditions, smaller cranes can be used though this could compromise efficiency. They are not considered efficient means in hard and rocky grounds, where the conventional grabs are undeployable. 4. SOLDIER PILE WITH WOODEN LAGGING SYSTEMSoldier pile and lagging walls are some of the oldest forms of retaining systems used in deep excavations. Soldier piles are driven/ bored at regular interval and allowed to gain strength. Excavation proceeds step by step after placement of Soldier piles at the periphery of the excavation. Depending on the ground conditions, wooden laggings are placed spanning from one soldier pile to another. At some predetermined levels, horizontal Waling beams and supporting elements (struts, anchors or nails) are erected. Ground anchors are increasingly used in such supports due to easy access to equipments.This type of retaining system involves the following broad based activities:(i) Constructing soldier piles at regular intervals (1 to 3m on centre typically)(ii) Excavating in small stages and installing wooden lagging.(iii) Backfilling and compacting the void space behind the lagging.4.1 Merits and DemeritsSoldier pile and lagging walls are the most inexpensive systems compared to other retaining walls. They are also very easy and fast to construct. These are found to be suitable for soils with some cohesion and without water table. They are commonly preferred in narrow excavations for pipe laying or similar works, but are also used for deep and large excavations in conjunction with struts.The major disadvantages of soldier pile and lagging systems are that they are primarily limited to temporary construction. They cannot be used in high water table conditions without extensive dewatering. Poor backfilling and associated ground losses can result in significant surface settlements. They are not as rigid as other retaining systems. Because only the flange of a soldier pile is embedded beneath subgrade, it is very difficult to control basal soil movements.5. CASE STUDY5.1 Diaphram wall construction at CC-01 Stretch of Delhi Metro CorridorTable 1: Diaphragm wall construction at CC-01 Stretch of Delhi Metro Sr. No.ProjectWall Thickness(mm)Wall length(m)Wall Depth(m)

1. Kashmere Gate Station80055220-22

100060

2Mandi House Station80033624

1000140

3Lal KilaStation80033620-22

1000280

4Jama Masjid Station80030017-18

Among the various strectches of Delhi Metro rail corridor, diaphragm wall construction features at Kashmere Gate station have been discussed here.5.2 Diaphragm wall at Kashmere Gate StationIn Kashmere Gate area, diaphragm wall was used to support 17m excavation. This station area comprised rectangular access area at the centre with vertical launching shafts at the ends. The total length of diaphragm wall including the access and shaft portion was 612m (Table 1) and 125 panels were used with their width varying from 3m to 6m. A minimum width was governed by the minimum split width capability of the grab. M35 concrete was used for the diaphragm wall, with main rebar diameter of 36mm. Total weight of reinforcement cage varied from 16 to 24 MT depending on the panel width. Laps in rebar were avoided by employing threaded couplers. extra nuts were use which was threaded at special fabrication shed at the site. Few of the concrete panels of the diaphragm wall at the launching shaft were constructed using 40mm and 25 mm diameter fibre reinforcement to facilitate cutting operation through the end diaphragm walls using TBM machines. Trenching for diaphragm wall was carried out using rope operated grabs of 6.50T to 8 T capacities with the help of cranes (75T capacity). The entire trench was stabilized with bentonite slurry by restricting its specific gravity to a maximum value of 1.2. The vertical alignment of the wall and shape of cross section were checked using Koden ultrasonic drilling monitor, which gave the precise output in from of continuous log. Inclinometers were installed to monitor the movement of the diaphragm walls during various stage of excavation.

Fig 4: Trenching with Mechanical Grab Fig 5: Cage Lowering for D-Wall at Kashmere Gate site5.3 Soldier Piles and Wooden Lagging supported system at CC-01 stretch of Delhi Metro Project, near Jama MsjidExcavation was required at the Cut and Cover portion of TBM launching shaft at Jama Masjid site of CC-01 stretch. Soldier piles with timber lagging were provided to support the boundary of the excavation. The average depth of excavation varied from 3m to 16m.The supporting system comprised soldier piles spaced at 1.8m c/c and with a closer spacing of 1.6m c/c near the launching shaft. Wooden laggings of thickness 100mm to 120mm were supported between the soldier piles. Three levels of Struts were provided at depths 3.285, 7.285, and 10.831m below the established ground level (EGL-209.80m). Additional level of Waler beam with pre-stressed rock anchors were provided 2m above the excavation level. Rock anchors with capacity of 86.4T, spaced at 3.6m c/c, were embedded 6m into the quartzitic bedrock to meet the bond strength considerations. King Post consisting of H-shaped structural steel section NBP 600 x 220 x 154.4 was employed. Boring operations was carried out with conventional way using Rotary rig till 3 to 4 m depth, casing was then lowered into the drilled hole with necessary verticality. Further, boring & drilling was carried out with auger / soil bucket and boring operation was terminated after a penetration of 0.6 m into weathered Quartzite Rock. Throughout, the boring operation, borehole wall was stabilized with bentonite slurry having specific gravity of 1.04 at supply point to about 1.11 for the flushed out flow. On termination of borehole kingpost along with reinforcement cage provided for bottom 1.5 m length was lowered into the borehole. On termination of borehole kingpost along with reinforcement cage provided for bottom 1.5 m length was lowered into .Fig 6: Soldier Pile With wooden lagging Systemthe borehole.The kingpost was supported and thereafter the annular space in the borehole was backfilled with sand. Excavation of about 1.5 to 2.0 m was considered free standing for few hours and in the ensuing period wooden laggings were inserted into the web portion of adjacent H shaped soldier piles. Excavation operation was followed by insertion of lagging, and was continued till the required excavation depth was achieved.6. CONCLUSIONIn the above sections, the retaining systems suitable in the urban environment were outlined. Restrictions of space in the urban settings compel use of deep vertical excavations which require supports that are designed to consume minimum construction space. Retaining systems like diaphragm wall, contiguous pile wall sand soldier piles with wooden lagging and sheet pile wall has been described in this article. We also discussed the case study of Diaphragm wall and Soldier pile wall which has been extensively used in the Delhi metro rail construction projects.7. REFERENCES1. Puller, M., Deep Excavations: A practical Manual, Thomas Telford, London, UK, 19982. Venkata Ramasubbarao Godavarthi: Contiguous Pile Wall as Deep Excavation Supporting System3. Donald R.McMahon:A DSM wall for excavation support4. S.S. Gue & Y.C. Tan Design and Construction Considerations for Deep Excavations5. M. Ufuk Ergun, Deep Excavations6. Mehtod Statement of Soldier Pile Phase-III CC-01 Project7. Mehtod Statement of Diaphragm Wall Phase-III CC-01 Project