Earth Retention Shoring System for Excavations – Part 2 of 2

Diaphragm & Guide Walls Requirements

The design and construction of the diaphragm wall including all its associated works such as guide walls, anchors, etc., shall be the responsibility of the Contractor and shall take into account the actual site, ground conditions, the equipment to be used and the local authorities requirements/regulations.

The design and construction of the guide walls take into account the actual site and ground conditions and the equipment to be used on site to ensure stability and avoid undercutting of the guide wall. Guide walls shall be constructed in reinforced concrete or other suitable materials. The minimum depth of guide wall shall be 1000 mm.

Concrete and Steel Reinforcement: All materials used such as Cement and cementitious materials, aggregates, additives, water and steel reinforcement shall be in accordance with relevant specification.

Support Fluid: Unless otherwise specified support fluid shall be Bentonite. Bentonite, additives, mixing and testing and clean water shall be in accordance with applicable specification.

Panels: The thickness of the panels shall not be less than the specified thickness on the approved shop drawings. The length of panel may be varied to suit the Contractor’s equipment subject to any upper limits approved by the Engineer. Within these constraints the Contractor shall be responsible for selecting panel dimensions which ensure stability and that movements remain within the criteria set herein. If in the Contractor’s opinion the specified panel dimensions are not adequate to ensure stability, he shall inform the Engineer at the time of tender.

Excavation: Excavation for panels shall not be so close to other panels which have recently been cast and which contain workable or unset concrete that a flow of concrete or instability could be induced or could cause damage to any panel. The Contractor’s planned sequence of construction shall be submitted prior to commencing work.

A suitable guide wall shall be used in conjunction with the casting method to ensure stability of the strata near ground level until concrete has been placed. During construction the level of support fluid in the excavation shall be maintained within the guide wall or stable ground so that it is not less than 1.5 m above the level of external standing groundwater at all times.

In the event of a loss of support fluid from an excavation, the Contractor shall notify the Engineer of his intended action before continuing the work. Prior to placing steel or concrete the Contractor shall clean the base of the excavation of as much loose, disturbed and remolded materials as practical and in accordance with the method of construction and shall wholly or partly remove and replace support fluid while maintaining the fluid head if it does not comply with the Contractor’s stated limits for support fluid prior to concreting.

Steel Reinforcement: The number of joints in longitudinal steel bars shall be kept to a minimum. Joints in steel reinforcement shall be such that the full strength of each bar is effective across the joint and shall be made so that there is no detrimental displacement of the reinforcement during the construction of the panel, following the guidance of ASTM Standard.

Reinforcement shall be maintained in its correct position during concreting of the panel. Where it is made up into cages, they shall be sufficiently rigid to enable them to be handled, placed and concreted without damage. If the cage is to be welded together, welding shall be carried out to the requirements of related ASTM Standards.  Details of the procedures should be submitted prior to the commencement of the Works.

Spacers shall be designed and manufactured using durable materials, which shall not lead to corrosion of the reinforcement or spalling of the concrete cover. Details of the means by which the Contractor plans to ensure the correct cover to and position of the reinforcement shall be submitted prior to commencing the Works. The minimum projecting bond lengths required by the Particu­lar Specification shall be observed. The Contractor shall prepare reinforcement detail construction drawings for each panel and these shall be submitted prior to commencing the Works.

Concrete Placing Requirements

The work-ability and method of placing of the concrete shall be such that a continuous monolithic concrete panel of the full cross-section is formed, and that the concrete in its final position is dense and homogeneous. Concrete shall be transported from the mixer to the position of the panel in such a manner that segregation of the mix does not occur.

Before commencement of concreting of a panel, the Contractor shall satisfy himself that the supplier will have available sufficient quantity of concrete to construct the panel in one continuous operation. The concrete shall be placed without such interruption as would allow the previously placed batch to have achieved a stiffness which prevents proper amalgamation of the two concrete batches. No spoil, liquid or other foreign matter shall be allowed to contaminate the concrete.

The concrete work-ability shall be determined using the slump or flow table in accordance with ACI: 318. The slump range or target flow for concrete placed through support fluid using a tremie pipe shall be 150 mm ± 50. Internal vibrators shall not be used to compact concrete within a cast in place panel. The concrete shall be placed through a tremie pipe in one continuous operation. Where two or more pipes are used in the same panel simultaneously, care shall be taken to ensure that the concrete level at each pipe position is maintained nearly equal.

The hopper and pipe of the tremie shall be clean and watertight throughout. The pipe shall extend to the base of the panel and a sliding plug or barrier shall be placed in the pipe to prevent direct contact between the first charge of concrete in the tremie and the support fluid. The pipe shall at all times penetrate the concrete, which has previously been placed with a minimum embedment of 3 m and shall not be withdrawn from the concrete until completion of concreting.

At all times a sufficient quantity of concrete shall be maintained within the pipe to ensure that the pressure from it exceeds that from the support fluid and workable concrete above the tremie base. The internal diameter of the pipe of the tremie shall be of sufficient size to ensure the easy flow of concrete. It shall be so designed that external projections are minimized, allowing the tremie to pass within reinforcing cages without causing damage. The internal face of the pipe of the tremie shall be free from projections.

The depth of the surface of the concrete shall be measured and the embedded length of the tremie pipe recorded at regular intervals corresponding to each batch of concrete. The depths measured and volumes placed shall be plotted immediately on a graph during the concreting process and compared with the theoretical relationship of depth against volume.

Tolerances Requirements:

Guide Wall: The finished internal face of the guide wall closest to any subsequent main excavation shall be vertical to within a tolerance of 1 in 200 and the top edge of the wall shall represent the reference line. There shall be no ridges or abrupt changes on the face and its variation from its specified position shall not exceed ± 15 mm in 3 m. The minimum clear distance between the guide walls shall be the specified diaphragm wall thickness plus 25 mm and the maximum distance shall be the specified diaphragm wall thickness plus 50 mm.

Diaphragm Wall: At cut-off level the maximum deviation of the centre-line of each panel from the specified position shall be 15 mm and an additional tolerance of 8 mm for each 1.0 m that the cut-off level is below the top of the guide wall shall be permitted unless otherwise stated. The exposed wall face and the ends of panels shall be vertical within a tolerance of 1:120 or as specified in the particular specification. An additional tolerance of 100 mm will be allowed for concrete protrusions resulting from cavities formed by over break in the ground.

Steel Reinforcement: The longitudinal tolerance of the cage head at the top of the guide wall measured along the excavation shall be ±75 mm. The vertical tolerance of the cage head measured relative to the guide wall shall be +150/-50 mm. The reinforcement shall be maintained in position during concreting of a panel.

Stop-ends: Temporary stop-ends shall be of the length, thickness and quality of material adequate for the purpose of preventing water and soil from entering the panel excavations. Each temporary stop-end shall be straight and true throughout. The external surface shall be clean and smooth, i.e. free from distortions that may affect panel integrity during removal of the temporary stop-end. Stop-ends shall be rigid and adequately restrained to prevent horizontal movement during concreting. The Contractor shall notify the Engineer prior to the removal of each stop-end.

Concrete Level: If the cut-off level for the panel is less than 1 m below the top level of the guide walls, uncontaminated concrete shall be brought to the top of the guide walls. If the cut-off level is greater than 1 m below the top level of the guide walls, concrete shall be brought to 1 m above the cut-off level specified, with a tolerance of ±150 mm. An additional tolerance of +150 mm over the above tolerances shall be permitted for each 1000 mm of depth by which the cut-off level is below the top of the guide wall.

Where more than one tremie pipe is used the concrete shall be brought up to 1 m above the cut-off level specified with a tolerance of ± 250 mm. After each panel has been cast, any empty excavation remaining shall be protected and shall be carefully back-filled as soon as possible, with material in accordance with the Particular Specification. Prior to back-filling, panels shall be clearly marked and fenced off so as not to cause a safety hazard.

The Contractor shall be responsible for the repair of any joint, defect or panel where on exposure of the wall visible running water leaks are found which would result in leakage per individual square meter in excess of that stated in the Particular Specification. Any leak, which results in a flow emanating from the surface of the retaining wall, shall be sealed.

Diaphragm Wall Finishing Requirements

Upon completion of the Diaphragm Wall Construction and prior to take-over by others (Piling / Main Works Contractor) the following conditions shall be satisfied by Enabling Works Contractor to the approval of the Engineer:-

• Any Protrusions (Bulge) noticeable on the surface of the Diaphragm Wall shall not exceed 70 mm. The Enabling Works Contractor shall carefully scrub-up the Wall Surface to ensure that all un-acceptable Protrusions (Bulges) are removed.  The Enabling Works Contractor shall submit for Engineer approval, Method Statement dealing with the process of Protrusion (Bulge) removal.
• Removal of bulge in the diaphragm wall must not expose the reinforcement of the wall. A minimum concrete cover of 2 cms must be maintained throughout.
• Removal of the bulge must in no way affect the Structural integrity of the diaphragm wall.
• Depressions in the diaphragm wall will not be acceptable and they must be filled with high strength concrete / grout depending on the size after cleaning the surface and removing loose particles of concrete. A Method Statement shall be submitted for Engineer’s approval.
• Where excavation is Part of Enabling Works Contractor Scope of Works he should clean the Surface of the Diaphragm Wall to remove all Soil Deposits, Dirt, Dust and other deleterious matter and leave in Dry, Acceptable Condition.

Steel Sheet Piling Requirements

A comparatively small displacement of soil is caused during driving and suitable sections can be driven into almost any soil except strong rocks. Reference should be made to grade 5275 P for the properties of mild steel and grade 535 P for high yield steel to BS EN 10025:1990 4). Where steel piling is manufactured to other standards, care should be taken that the design stresses to be used are compatible with that particular quality of steel.

The structural design of the steel sheet piling should be in accordance with BS 449.2 the allowable stresses given therein may be increased by 12 % for temporary works of short duration

Driving Equipment’s Requirements

Pile Hammer: Air-, steam-, hydraulic-, or diesel-powered type capable of consistently delivering adequate peak-force duration and magnitude to develop the ultimate capacity required for type and size of pile driven and character of subsurface material anticipated.

Hammer Cushions and Driving Caps: Between hammer and top of pile, provide hammer cushion and steel driving cap as recommended by hammer manufacturer and as required to drive pile without damage.

Leads: Use fixed, semifixed, or hanging-type pile-driver leads that will hold full length of pile firmly in position and in axial alignment with hammer.

Driving Piles

Continuously drive piles to elevations or penetration resistance indicated or established by static load testing of piles. Establish and maintain axial alignment of leads and piles before and during driving. Retain first paragraph below if predrilling is permitted. Predrilling is generally prohibited for friction pilings but, if approved by engineer, predrilling can be an effective method of penetrating hardpan, cemented strata, hard clay, or dense compacted clay. Revise to suit Project or if prejetting or other methods to facilitate pile driving are permitted.

Predrilling: Provide pre-excavated holes where indicated, to depths indicated. Drill holes with a diameter less than the largest cross-section dimension of pile.

• Firmly seat pile in predrilled hole by driving with reduced energy before starting final driving.
• Location: 102 mm from location indicated after initial driving, and 152 mm after pile driving is completed.
• Plumb: Maintain 25 mm in 1.2 m from vertical, or a maximum of 102 mm, measured when pile is above ground in leads
• Batter Angle: Maximum 25 mm in 1.2 m from required angle, measured when pile is above ground in leads.

Cutting Off: Cut off tops of driven piles square with pile axis and at elevations indicated.

Pile-Driving Records

Maintain accurate driving records for each pile, compiled and attested to by a qualified professional engineer. Include the following data.

• Project name and number
• Name of Contractor
• Pile location in pile group and designation of pile group.
• Sequence of driving in pile group.
• Pile dimensions.
• Ground elevation
• Elevation of tips after driving.
• Final tip and cutoff elevations of piles after driving pile group. 9. Records of redriving.
• Elevation of splices
• Type, make, model, and rated energy of hammer
• Weight and stroke of hammer.
• Type of pile-driving cap used
• Cushion material and thickness
• Actual stroke and blow rate of hammer.
• Pile-driving start and finish times, and total driving time.
• Time, pile-tip elevation, and reason for interruptions.
• Number of blows for every 305 mm of penetration, and number of blows per 25 mm for the last 152 mm of driving
• Pile deviations from location and plumb
• Preboring, jetting, or special procedures used
• Unusual occurrences during pile driving

Reinforced and Prestressed Concrete Sheet Pile

The following recommendations apply to reinforced concrete sheet piles

• General, the design , manufacture and handling should be in accordance with 7.4.2 of BS 8004:1986
• Concrete .for the various conditions of driving and exposure the concrete mix and strengths should be in accordance with table 12 of BS 8004:1986
• The reinforcement should conform to BS 8110-1 or BS 5400-4, BS 5400-7 and BS BS 5400-8 for resisting the applied forces. the lateral reinforcement is important in resisting the driving stresses. the cover requirements in BS 8110-1 should be the minimum requirements
• The dimensions depend on design, the thickness should generally be in the range of 120 mm to 400 mm .the normal width of a concrete wedge shaped to assist close driving
• Sheet piles are provided with tongued and grooved joints trapezoidal, trian-gular or semicircular in shape. Driving and also minimize the seepage of soil through the wall after completion. Water tightness may be achieved through specially designed interlocks or through grouted joints
• Driving, the recommendations in 7.4.2.5 of BS 8004:1986 should be followed in driving concrete sheet piles

Prestressed Concrete Sheet Piles

The main advantages are:

• High strength in bending and ability to withstand tensile stresses during driving and to take hard driving
• Relatively crack – free in handling , pitching and driving.
• Economical design for given loads and moments
• Greater durability in different environments.

The requirements for materials, manufacture and driving of prestressed concrete piles should be as set out in BS 8110-1 or BS 5400-8 and 7.4.3 of BS 8004:1986. In the design tensions up to maximum of 50% of the modulus of rupture(in tension) may be permitted provided the ultimate strength requirements are satisfied .in this respect the guidance for class 2 structures in 4.3.4.3 of BS 8100-1:1986 is appropriate.

Tieback Excavations

Tiebacks and anchorages shall be installed in soil or rock in accordance with approved shop drawings and as indicated. Tiebacks shall be installed through sleeves or holes provided in the earth retention system. Methods of installation which prevent the loss of ground due to erosion or jetting during the installation of tieback casing, anchors and anchor head assemblies shall be used. Anchor head assemblies shall be used to prevent the flow of water or movement of soil through annular spaces openings below groundwater.

Tiebacks with pressure grouted anchor zones of the single stage or regroutable type shall be installed.   Each tieback must be capable of being retensioned. Tieback heads shall be protected from deliberate or accidental damage. Test each tieback installed for support of earth excavation to verify and establish the tieback capacity.

Design piles or other support system members incorporated in a system utilizing tiebacks to have the capacity of resisting the vertical components of tieback loads without settlement during any stage of the excavation and construction.

Maintenance of Soil Supports

Maintain steel members for bracing and replacement lagging on hand throughout lagging and bracing work and other earth retention operations to protect the work and for use in case of accident or emergency. Seal leaks that occur in the walls as excavation progress. When tiebacks are used release tension in tieback as the excavation is back-filled. Maintain and remove supports at the limits of construction.

Removal of Earth Retention System

Remove earth retention system except where indicated to remain in place.  No steel or wood sheet piling or soldier piles shall be withdrawn if the tip of the sheeting is driven below a reference line drawn to the slope of one horizontal to one vertical from the outside bottom edge of any foundation or from the spring line of any pipe.

Remove the earth retention system without endangering the construction, under this or other contracts, other structures, utilities, or property as approved by the Engineer. Immediately backfill all voids left or caused by withdrawal of earth retention systems with screened gravel or select borrow by tamping with tools specifically adapted for that purpose.

Remove bracing for earth retention. Removal shall follow the requirements specified above. Earth retention systems and bracings shall be cut-off at least 1.5 meters or to such depths below finished ground surface, all earth retention system left-in-place, as per the requirements of the concerned authorities.

Tests and Inspections: A minimum of 10% of the tie backs shall be tested to 125% of the approved working load  and its elongation shall be attended to for the approval of the Engineer.

Storing of Suitable Excavated Material: During excavation, materials suitable for backfill and fill shall be temporarily stockpiled on the Site at sufficient distance from the sides of the excavation to avoid over-loading and prevent any cave-in or mixing with the concrete during the construction of foundations.  Thereafter the suitable material shall be moved to a designated stockpile area out of the Site as directed by Engineer.

Disposal of Unsuitable and Surplus Excavated Material: Upon the order of the consulting Engineer, all unsuitable and surplus excavated materials shall be immediately removed, loaded and transported off Site area by the Contractor to dumps located by and approved by the concerned Authorities.