POINTS TO REMEMBER FOR CIVIL ESTIMATORS

“EXCAVATION AND BACKFILLING WORKS”

Following are few general points to remember for Civil Estimators to prepare the tender/bid proposal in correct manner.

This document provides minimum installation, inspection and testing guidelines for the excavation and backfill of all required underground installations.

After reading this document, we will be familiar with the following:

• Excavation procedure • Subgrade preparation for foundation & concrete installation • Subgrade preparation for pipe bedding & electrical installation • Backfilling • What is meant by suitable fill materials • What is meant by unsuitable fill materials • What are the test results and documents required • Surveying • Information about the existing facilities • Protection of existing property • Clean up • What are the Inspection & tests required

Upon familiar with the above information, then only Estimator can price in proper way.

For shake of clarity, let us assume one of our building projects is having 100 numbers of isolated foundations. As per standards Contractor has to do Field Compaction Tests for 50% foundations only. Not 100% of foundations.

If estimator does not familiar about the requirement of these tests, he will estimate the price for 100% foundations. So bid/tender cost will be increased.

Note that overestimation and underestimation both are harmful to contractor. If it is underestimate contractor may lose the money from that project, in the other case contractor proposal will be out of range. Means contractor will never win the bid/tender.

EXCAVATION

Shall extend all lines and grades as required for the work from benchmarks and base lines.

Shall co-operate in protecting all control bench mark monuments and stakes.

All excavations for foundation construction shall conform to the locations and elevations indicated on the Approved for construction drawings.

Before starting work, the CONTRACTOR shall determine the nature and location of all existing above and underground services and facilities on site and in adjacent areas.

Shall take all the precautions including providing temporary supports, bridging, shoring, and safety barriers to protect the existing services and facilities from damage or interference during construction.

Shall notify the CLIENT in writing if any previously unrecorded services or other objects are discovered before or during construction. The CONTRACTOR shall record and give to the CLIENT accurate description, dimensions, location, and elevations related to reference datum and other relevant details of such services.

If CONTRACTOR needs to block drainage paths during construction, he shall provide temporary or permanent alternate routing during construction and restore the drainage.

If the CONTRACTOR damages existing structures or facilities, he shall repair or replace

them at no cost.

Required temporary roads shall be laid to provide access for construction activities. The temporary road should preferably coincide with permanent routes if possible.

Excavation shall extend a sufficient distance from edge of concrete walls or footings to allow for the working allowance i.e placement and removal of concrete forms, installation of underground services, and for inspection.(for these extend area shall follow as per standard method of measurements).

Excavation for underground pipes shall conform to the specified locations and elevations as indicated on the construction drawings.

• The recommended trench base width shall not be less than 450 mm for pipes up to and including 250 mm nominal pipe diameter, and shall not be less than the external diameter plus 200 mm on each side of the pipe for pipes with nominal diameter from 300 mm to 900 mm, and the external diameter plus 300 mm on each side of pipe for larger diameter.

• The trench width shall be sufficient to allow inspection of the pipe and pipe joints.

• For concrete pipes, excavation for bells and hubs shall be enlarged so that the full load is carried by the pipe barrel and the minimum bedding depth of 150 mm beneath the pipe is maintained.

Whenever the sides of excavations are in excess of 1.2 meters, the sides shall be sloped, shored or stepped to prevent hazardous conditions.

• Temporary slopes shall be cut at a minimum ratio of one and one-half horizontal to one vertical.

• Shoring, when required, shall be of adequate strength to safely sustain the loads imposed by the soil, any adjacent structures and any additional construction loads.

Every precaution shall be taken to prevent slips and falls of excavated or other material into the excavations.

Excavations adjacent to existing foundations (where the bottom of excavation is lower than the bottom of foundation) shall be limited to a minimum distance by an imaginary line drawn 30 degrees horizontally from the bottom corner of the existing foundation to the bottom corner of the intended excavation.

If excavated material is approved as backfill material, it shall be deposited at a location approved by the CLIENT but not closer than 3 m (or 1.5 times the depth of excavation, whichever is greater) to the excavation.

The height of the stockpiled excavation material shall be controlled so as to prevent danger to workmen, instability or damage to other structures or services.

If the excavated materials deemed unsuitable as fill shall be loaded and removed offsite to an approved dumping yard.

Shall notify the CLIENT of any obstructions encountered and shall provide support, protection, or removal of such obstructions as directed by the CLIENT.

All excavations shall be kept free from whatever water source so that the work shall be

constructed in the dry.

Excavations shall be performed in a manner to ensure proper drainage during the course of work.

All water encountered in the earthwork operation shall be disposed of by the CONTRACTOR so as not to damage public or private property or create a nuisance or health menace. The CONTRACTOR shall provide sumps, drain pumps and all other equipment of sufficient capacity as well as any other means needed to keep the construction area free from water until construction is completed.

All road base surface course materials, installed under previous work, shall be carefully excavated. These materials shall be replaced in the backfill process.

The competent safety inspector shall be capable of identifying existing and predictable hazards in the surroundings or working conditions that are hazardous to employees.

SUBGRADE PREPARATION FOR FOUNDATION AND CONCRETE INSTALLATION AFTER EXCAVATION

Upon completion of the foundation excavation as per AFC drawings levels, still contractor shall found that the surface is in loose condition, and then remove all the loose materials until hard surface will come and refill the excavation with either compacted Structural fill material or blinding concrete.

If a foundation is excavated deeper than the level shown on the drawings, then the over excavated depth shall be filled with Structural fill material and re-compacted or fill with blinding concrete in accordance with specification.

The bottom of the excavation shall be leveled and trimmed to receive the permanent works.

Upon completion of the excavation and preparation of the bottom of the excavation to receive the permanent works, the blinding concrete, membrane or such other protection shall be placed as soon as practical.

SUBGRADE PREPARATION AND BEDDING FOR PIPE AND ELECTRICAL INSTALLATION AFTER EXCAVATION

Bedding is the material on which a pipe or conduit is laid; backfill is the material placed

above the bedding.

Prior to placing bedding, the ground surface to receive bedding shall be inspected and approved by the CLIENT.

The upper 150 mm of existing soils shall be scarified and recompacted to the density specified in specification.

If existing soils cannot be compacted as specified in Specification, the CONTRACTOR shall place a 200 mm blanket layer of gravel or crushed rock before placing the bedding material.

Clean Sand shall be used as bedding and backfill material for coated and wrapped pipes, glass-fibre reinforced epoxy pipes, PVC pipes, underground electrical and instrument cables.

Sand shall pass a 2 mm screen and not more than 12 percent shall pass a 0.075 mm sieve.

Sand shall be uniformly compacted to a depth of 150 mm beneath the pipe or conduits with particular care given to the bedding at the haunches of the pipe.

Bedding material shall be placed in 150 mm loose layers and mechanically compacted according to the requirements of Section Backfill.

Sand shall be placed to a height of at least 300 mm above the top of the pipe, or electrical cable installation.

The upper 150 mm of bedding under concrete pipes shall consist of approved non-expansive granular materials.

Granular material shall conform to the following sieve analysis (ASTM D 422). Granular material shall be placed to a height of at least 300 mm above the top of concrete pipes

All material shall be approved suitable material consisting of granular soil with particles size as indicated in the specification. It shall have a California Bearing Ratio (CBR) of 30 or higher and less than 12 percent shall pass a 0.075 mm sieve unless indicated otherwise.

BACKFILL

Place, spread, water and compact backfill in strict accordance with the contract drawings.

Shall employ an independent geotechnical engineer approved by the CLIENT to verify that

the placement of the fill complies with the contract specifications and drawings.

The material shall be free from unsuitable material (as defined above) or particles larger than 75 mm in largest dimension.

All material used as backfill shall have a maximum Chloride content of 1% and a Maximum sulfate content of 0.5%.

If the excavated material is unsuitable as backfill, suitable material shall be obtained from borrow pits.

Care shall be taken during backfill operations to prevent damage to the permanent works. In particular, damage to membranes shall be repaired before backfill is continued.

Backfilling of foundations shall not be carried out until the foundations or structures within the excavation have been cured and protected in accordance with the Specification for Concrete Construction.

All forms shall be removed and the excavation cleared of loose soil, water and debris after approval of underground installations and just prior to backfilling.

Planking, strutting, sheet piles and other supports in excavations shall be removed as backfilling proceeds to maintain the stability of the side until backfill is completed. Temporary supports shall not be left in the ground.

Bedding and backfill shall be compacted by mechanical tamping wherever possible.

Hand-compacted backfill shall be permitted only when mechanical tamping is impractical.

Hand-compacted bedding shall be placed in loose lifts not exceeding 50 mm. Hand-compacted backfill shall be placed in loose lifts not exceeding 100 mm.

Machine-compacted bedding shall be placed in loose lifts not exceeding 150 mm; machine-compacted backfill shall be placed in loose lifts not exceeding 200 mm.

Backfill shall be placed symmetrically to prevent eccentric loading upon or against structures.

All bedding and backfill within structural areas (areas supporting foundations, concrete slabs, pavements, etc.) and 2m beyond shall be compacted to at least 95 percent of the maximum dry density as determined by ASTM D1557.

Backfill in non-structural areas (that are free of any underground and aboveground installations/pavement) shall be compacted to at least 90 percent of the maximum dry density as determined by ASTM D1557.

The fill material shall be conditioned with water such that at the time of compaction it is within +/- 2% of the optimum moisture content. Due allowance shall be made for evaporation in the elapsed time between measurement of moisture content and placement and compaction of the fill

Compaction of backfill by water jetting shall not be permitted.

The CONTRACTOR may use cement stabilized sand backfill in pipe trenches and adjacent

to foundations in exceptional circumstances, with the approval of the CLIENT.

Prior to placing cement stabilized sand backfill, the CONTRACTOR shall have completed and received approval from the CLIENT for all pipe testing.

The CONTRACTOR shall not use cement stabilized sand in cable trenches.

The CONTRACTOR shall not use cement-stabilized sand in the top 600 mm below finished grade.

Cement stabilized sand shall contain at least 100 kg cement per cubic meter of sand and the water-cement ratio shall not exceed 0.8. Material for cement stabilized sand shall consist of:

a. Sand as specified above.

b. Cement: cement shall conform to Specification for Concrete Supply.

c. Water: water shall conform to Specification for Concrete Supply.

d. Admixture/ may be used.

Shall place backfill in the trench from the chute of the Mixer Truck onto the centre of the trench; he shall keep lateral pressure on the trench equal and shall not fill above 600 mm below final grade.

After inspection and approval, the CONTRACTOR shall fill the balance of the trench with approved backfill material.

Settlement of the backfill material shall be made good with suitable fill material and compacted in accordance with Section backfill.

Road base surface course materials removed in the excavation work shall be replaced in the backfill process.

Suitable Materials:

Shall mean materials that are in conformance with the specifications and are approved by the CLIENT.

Shall be free from refuse, stones or rocks larger than 75mm.

The stability / bearing capacity of the compacted fill or embankments shall be sufficient for the superimposed loads.

Material shall be classified / described in accordance with ASTM D2487 or D2488.

Unsuitable Materials:

Organic material, logs, stumps and perishable materials.

Materials with undefined properties.

Materials having moisture content greater than the maximum required.

Building rubble and domestic and industrial wastes.

Clay, silt and other loose or soft soils not in accordance with compaction requirements.

Dredging material.

Material containing gypsum or other soluble salts

INSPECTION AND TESTING

Inspect the excavation bottoms and will observe the filling and compacting operations to verify that the backfill is being constructed in accordance with the contract specifications and approved drawings.

All backfill and bedding materials, whether imported from outside the site or excavated from the cut areas, shall be inspected and tested.

Imported material shall be in accordance with above clauses. On-site proctor testing in accordance with ASTM D1557, will be made by the On-Site Testing Laboratory Services.

A minimum of one proctor test will be made for every 2000 cubic meter of material and if the source of material changes or the material itself changes.

Testing to verify the degree of compaction will be done by the On-Site Testing Laboratory.

All testing shall be performed under the supervision of the CLIENT.

The On-Site Testing Laboratory will provide a written report documenting the test results and verifying that the work is in conformance with the specification and the contract drawings.

When test results indicate that compaction is not as specified, the material shall be removed and recompacted to meet specification requirements.

For On-Site Testing Laboratory Services will make field compaction tests in accordance with ASTM Test Method D1556 (Sand Cone Method) or ASTM Test Method D6938 for each layer of backfill.

The minimum requirements of compaction tests shall be as follows:

• A minimum of one compaction test will be made under each equipment/structural foundations.

• For building foundations, compaction tests shall be carried out under 50 % of

the total foundations.

• For backfilling above/around the foundations, one compaction test shall be made for each 10-20 cubic meters of fill (depending on the size of area being backfilled).

• A minimum of three compaction test will be made for every 100 meter length of

trench back fill placed.

Tests will be made for each lift of fill material placed.

If the surface is disturbed, the compaction tests will be made in the compacted materials below the disturbed zone.

When these tests indicate that the density or moisture content of any layer of fill or portion thereof does not meet the specified density or moisture content, the particular layer or portions shall be reworked until the specified density and moisture content have been obtained.

The CONTRACTOR is responsible for providing the appropriate laboratory test results performed on the import materials including bedding materials to substantiate their properties and their conformance with the specification requirements.

All testing shall be performed by an independent laboratory and certified by a professional engineer.

The CLIENT reserves the right to also independently test any of the import materials. Import materials shall be approved in writing by the CLIENT prior to placement on the project.

PROTECTION OF PROPERTY AND PERSONNEL

Shall protect all active utility lines including temporary lines, existing lines, systems and services already installed in the course of the project and any other lines encountered during excavation.

If any of these lines are damaged, repaired or replaced at the CONTRACTOR’S cost.

Shall barricade open holes and depressions occurring as part of this work, and post warning lights on property adjacent to or with public access.

Shall protect structures, utilities, pavements and other facilities from damage caused by his operations.

CLEAN UP

Upon completion, leave project site clear of debris and surplus material. All debris and

surplus materials shall be disposed of to the approved dumping yard.

RECORDS

The following test results or documents are submitted to the CLIENT within 3 days of completion of the subject work.

Survey records field notes, computations and other records for the installation, including backfill.

Method statement shall be produced and submitted to CLIENT for review/approval.

Submit a Water Control/Dewatering Plan for review and approval.

CLIENT reserves the right to inspect materials and workmanship standards at all stages of the Project and to witness any or all tests.

Shall provide the Work & Inspection plan to client for review 30 days after award

.