Earth Retention Shoring System for Excavations – Part 1 of 2

The work specified herein includes the design, documentation, furnishing, and installation of earth retention systems for excavations.  The earth retention system shall be either singular or multiple stages comprised of soldier piles and laggings, sheet pile wall and bracing, raker or timber sheet pile wall bracing or secant piling bracing and tie-back systems or cantilever secant pile systems as indicated on drawings  and to the approval of the Engineer.

Unless otherwise specified or detailed on drawings the Contractor has the option of selecting a shoring method or combination of several methods to design, and to construct the shoring system as approved by the Engineer. However any approval accorded by the Engineer shall not relieve the Contractor of any of his obligations and responsibilities to provide a safe and sound shoring System.

The Contractor shall determine the size of excavation in order to accommodate the proposed method(s) of support.  However, the design shall evaluate the effect of excavation size and the proposed support system on the adjacent structures.  Special care shall be taken in positioning and shotcreting of secant pile walls to avoid any movement/gaps as this is vital for the permanent walls and its tanking system.

Adjacent structures are those that are within a distance equal to twice the total depth of the excavation away from the closest edge of the excavation.

Material Requirements

Structural steel for soldier piles, wales, rakers, struts, wedges, plates, waterstop and accessories, steel shapes shall be provided conforming to ASTM A36. Raker ties shall conform to ASTM A615 Grade 60. Timber sheeting and lagging where used shall conform to the following requirements.

• Moisture content shall not exceed 19 percent.
• Well seasoned timber approved by the Engineer.
• Wood treatment in accordance with FS TT-W-571.
• Allowable working stress of not less than 120 psi.
• Nominal thickness of not less than 3 inches.

Concrete and reinforcement steel shall be in accordance with relevant specifications when used for support of excavation.

General Workmanship Requirements

The earth retention system programme shall be in such a manner as to prevent undermining or disturbing foundations of existing structures or of work ongoing or previously completed work in accordance with the local authority requirements. Any excavation involving earth retention shall not be started until the earth retention submittal has been approved by the Engineer. Water table shall be lowered to reduce the hydrostatic pressures. Adoption of the indicated design or approval by the Engineer of the Contractor’s design will in no way relieve the Contractor of his responsibility for the successful performance of construction or any method of protection for adjoining property.

Contractor shall bear the entire cost and responsibility of correcting any failure, damages, subsidence, upheaval or cave-ins as a result of improper installation, maintenance or design. Contractor shall pay for all claims, costs and damages that arise as a result of the work performed at no additional cost to the      Client. Protect all existing utilities affected by construction from damage.

Services shall be exposed by hand digging if they exit within work area. The Engineer shall be notified if the existing utilities interfere with the earth retention system. The existing services utility /services shall be removed or diverted as approved by the Engineer. Elements of the support system shall not be spliced. The toe of the temporary earth retention system shall be extended by at least 1.5 meters below the bottom of the excavation or to bedrock to provide additional penetration for bottom stability and/or vertical capacity. The toe of the temporary earth retention system shall be extended by at least 1.5 meters below the bottom of the excavation or to bedrock to provide additional penetration for bottom stability and/or vertical capacity.

Soldier Piles and Lagging

Soldier piles shall be constructed by pre-drilling, or other approved pre-excavating methods.  Pre-drilled holes sized to accommodate the accepted pile cross section shall be provided.  Pre-   drilled holes shall be installed vertically and in indicated alignment.  Piles shall not exceed 0.5 percent out of plumb, provided that it does not reduce the basement wall thickness and overall structural layout and size.  The secant piles shall be cast using guide beams of appropriate sizes as approved by the Engineer. Misaligned or non-vertical holes shall be corrected to the satisfaction of the Engineer.

Pile tops shall be installed to embedment elevations shown on accepted shop drawings. Backfill the pre-formed hole with concrete by tremie methods to base of the excavation.  The appropriate grade concrete as specified shall be placed by tremie methods in accordance with ACI 304 in the remainder of the hole to the surface of existing ground and vibrating the concrete through the solider piles.  The secant pile wall will be provided with a capping beam of approved size and design other trimmings the guide beam and tops of piles.

Install lagging to retain the material type encountered in the excavation while allowing free draining of water. As the installation progresses, backpack the voids between the excavation face and the lagging with sand pack to establish a tight contact. Pack openings between lagging with hay or other material to allow free draining for water without loss of retained soil or sand pack. Coordinate the excavation with lagging placement.  Reduce the maximum height of unlagged face if water is flowing from the face of the excavation, or if soil to be retained moves toward the excavation.

If very fine sand and/or silt are encountered, take measure to retain the material in place and prevent loss of ground and/or movements which may cause damage to adjacent buildings, structures or utilities (Construction contingency plan).

Hard/Soft Secant Pile Walls

The design and construction of guide walls shall be the responsibility of the Contractor and shall take into account the actual site and ground conditions and the equipment to be used on site to ensure stability and avoid undercutting as appropriate.

Guide walls shall be constructed in reinforced concrete or other suitable materials. The minimum depth of guide wall shall be 500 mm and the minimum shoulder width shall be 300 mm for walls in reinforced concrete.

Self –Hardening Concrete Mixes

The Contractor shall submit details for the self-hardening concrete mix (soft pile) proportions to be used prior to commencing the Works. Cement materials, aggregates (if used), additives and water shall be in accordance with applicable specification. Trial mixes shall be prepared for each mix unless there are existing data showing that the mix proportions and method of manufacture will produce hardened material of the strength, permeability, shrinkage properties and durability required, having adequate workability for compaction by the method to be used in placing.

All testing shall be carried out in an approved specialist laboratory. No variations outside the limits set out in the proportions shall be made nor in the original source of materials without demonstrating compliance with the specification. Self-hardening mixes shall be checked for compliance with the mix proportions. Cylinder samples shall be made at the rate of four samples for each 50 m³ of self hardening concrete or part thereof.

The Contractor shall keep a detailed record of the results of all tests on self-hardening mixes and their ingredients. Each test shall be clearly identified with the pile to which it relates and the date it was carried out.

Support Fluid: Where support fluid is used to maintain the stability of the pile bore, it shall be in accordance with the relevant clauses of the Bored Concrete Piles Specification”. (Bentonite).

Boring Requirements

Boring Near Recently Cast Piles: Piles shall not be bored so close to recently cast piles, which contain workable or partially set concrete piles or self-hardening mixes that a flow of concrete or instability could be induced or damage caused to any installed piles.The Contractor’s sequence of construction shall  be  submitted  prior to work commencing.

Stability of Bore: Temporary casing shall be used in unstable ground with or without support fluid to maintain the stability of the bore. The process of advancing the bore and the temporary casing shall be such that soil is not drawn into the bore from outside the area of the pile and cavities are not created outside the temporary casing.

Cleanliness of Pile Base: Prior to placing steel or concrete, the Contractor shall remove from the base of the pile as much loose disturbed and re-moulded materials as practical and in accordance with the method of construction.

Cleaning of Support Fluid: Prior to placing steel or concrete, any support fluid including water shall be wholly or partly removed and replaced while maintaining the fluid head if it does not comply with the Contractor’s limits for support fluid prior to concreting.

Steel Reinforcement: Shall be placed in accordance with the relevant clauses of specification. The quantity of steel reinforcement shall be as designed by the Contractor and approved by the Engineer.

Placing and Compacting Concrete: Placing of concrete shall be in accordance with relevant clauses of specification. The concrete work ability shall be determined using the slump or flow table. The slump or flow shall be measured at the time of discharge into the pile boring and shall be in accordance with the limits as specified under performance requirements.

Tolerances Requirements

Guide Wall: The finished face of the guide wall closest to any subsequent main excavation shall be vertical to a tolerance of 1 in 200 and shall represent the reference line. There shall be no ridges on the face and the centerline of the guide wall shall not deviate from its specified position by more than ±5 mm. in 3 m. The minimum clear distance between the guide walls shall be the pile diameter plus 25 mm. and the maximum distance shall be the pile diameter plus 50 mm.

Secant Piles: At cut-off level the maximum permitted deviation of the pile center from the center point shown on the setting out drawings shall be 25 mm in any direction except that an additional tolerance of 10 mm shall be permitted for each additional 1.0 m that the cut-off level is below the top of the guide wall.

The exposed face of the pile shall be vertical within a tolerance of 1:100 in any direction. An additional tolerance of 100 mm will be allowed for concrete protrusions resulting from cavities formed by over break in the ground. Where very soft clay layers or peat layers are anticipated or obstructions are to be removed prior to or during boring, an additional over break tolerance will be agreed with the Engineer.

The Contractor shall carryout necessary preparation of the secant pile wall surface in terms of chipping and cleaning of piles to be readied for gunting to the approval of the Engineer. prior to its handover to the basement contractor. Gunnting will be carried out by the Main Contractor.

Steel Reinforcement: A vertical tolerance of +150/-50 mm. shall generally be permitted on the level of steel projecting from a pile cut-off.

Capping Beam Requirements: After the construction of the secant pile wall the top portion of the secant piles shall be chipped to have a sound surface for casting the capping beam.  A capping beam shall be constructed to the sizes dictated but the design requirements and cured prior to the stressing of tie back anchors to transfer the anchor load evenly to the shoring system and to function as a single unit. The design and construction of the capping beam shall be the responsibility of the Contractor and shall take into account the actual site and ground conditions.

Back filling of Empty Bore: Where concrete is not brought to the top of the guide wall, the empty pile bore shall be back filled as soon as possible with approved material. Prior to back filling, bores shall be clearly marked and fenced off so as not to cause a safety hazard.

Water Retention: The Contractor shall be responsible for the repair of any joint, defect or pile where on exposure of the wall visible running water leaks are found. Any leak, which results in water flow emanating from the surface of the retaining wall, shall be sealed.

Performance Requirements

a) Concrete for soft piles –    20 N/mm²

b) Concrete for hard pile –    40 N/mm²

c) Slump –    175 ± 25 mm

Contiguous Pile Walls

The design and construction of guide walls, shall be the responsibility of the Contractor and shall take into account the actual site and ground conditions and the equipment to be used on site to ensure stability and avoid undercutting as appropriate. Guide walls shall be constructed in reinforced concrete or other suitable materials. The minimum depth of guide wall shall be 500 mm and the minimum shoulder width shall be 300 mm for walls in reinforced concrete.

Boring Near Recently Cast Piles: Piles shall not be bored so close to recently cast piles which contain workable or partially set concrete piles to avoid instability or cause damage to any installed piles.

Stability of Bore: Temporary casing shall be used in unstable ground with or without support fluid to maintain the stability of the bore. The process of advancing the bore and the temporary casing shall be such that soil is not drawn into the bore from outside the area of the pile and cavities are not created outside the temporary casing.

Cleaning Of Support Fluid: Prior to placing steel or concrete, any support fluid including water shall be wholly or partly removed and replaced while maintaining the fluid head if it does not comply with the Contractor’s limits for support fluid prior to concreting.

Steel Reinforcement: Shall be placed in accordance with applicable specifications.  The quantity of steel reinforcement shall be as designed by the Contractor and approved by the Engineer.

Placing and Compacting Concrete: Placing of concrete shall be in accordance with applicable specifications.  The concrete workability shall be determined using the slump or flow table. The slump or flow shall be measured at the time of discharge into the pile boring and shall be in accordance with the limits as specified under performance requirements.

Tolerances Requirements

Guide Wall: The finished face of the guide wall closest to any subsequent main excavation shall be vertical to a tolerance of 1 in 200 and shall represent the reference line. There shall be no ridges on the face and the centre-line of the guide wall shall not deviate from its specified position by more than ±5 mm. in 3 m. The minimum clear distance between the guide walls shall be the pile diameter plus 25 mm. and the maximum distance shall be the pile diameter plus 50 mm.

Contiguous Piles: At cut-off level the maximum permitted deviation of the pile centre from the centre point shown on the setting out drawings shall be 25 mm in any direction except that an additional tolerance of 10 mm shall be permitted for each additional 1.0 m that the cut-off level is below the top of the guide wall. The exposed face of the pile shall be vertical within a tolerance of 1:100 in any direction.

An additional tolerance of 100 mm will be allowed for concrete protrusions resulting from cavities formed by overbreak in the ground. Where very soft clay layers or peat layers are anticipated or obstructions are to be removed prior to or during boring, an additional overbreak tolerance will be agreed with the Engineer.

Steel Reinforcement: A vertical tolerance of +50/-50 mm. shall generally be permitted on the level of steel projecting from a pile cut-off.

Backfilling of Empty Bore: Where concrete is not brought to the top of the guide wall, the empty pile bore shall be backfilled as soon as possible with approved material. Prior to backfilling, bores shall be clearly marked and fenced off so as not to cause a safety hazard.

Water Retention: Any leak which results in water flow emanating from the surface of the retaining wall shall be controlled by dewatering.

Performance Requirements:

a) Concrete for hard pile –  35 N/mm²  Cylinder compressive strength

b) Slump –  175 ± 25 mm.