Project Description

PROJECT OVERVIEW

Bachy Soletanche were awarded a contract to design and construct a temporary hard/firm secant pile retaining wall to facilitate the remodelling of an existing dock entrance. The 1300mm diameter secant wall piles were expected to terminate in the top of the Dolerite. The excavation within the wall was to extend down to the top of rock at an approximate depth of 26.5m. The solution, developed by Bachy Soletanche and BAM Nuttall, involved the installation of steel dowels through the bases of the hard piles to provide restraint at the toe of the wall. Ground anchors were also installed through the existing mass concrete gravity retaining wall to ensure its stability. The anchor safe working load was 3000kN.

DESIGN

The secant wall, dowels and ground anchors were designed by Bachy Soletanche to provide a single composite solution. The dowels were modelled in different ways in a series of WALLAP analyses for the design of the secant pile wall to determine the effective ‘prop’ load to be provided by the dowels. A number of potential failure mechanisms were considered in the design of the dowel, with the dowels checked in shear and in tension. Each ground anchor comprised a 16 strand tendon installed in a 240mm diameter bore within the existing mass concrete of the wall. The full anchor length of up to 23.5m was located entirely within the existing wall as required by the Engineer.

A key factor to consider in the design was the interaction amongst anchors in the same group in terms of the cone pull-out resistance. As the ground anchors were temporary, corrosion protection was not required. design cut-off level, saving a considerable amount of time and money.

The project was a good example of Bachy Soletanche and Costain’s ability to coordinate the various ongoing activities of a major project via efficient planning, good teamwork and site management. High standards of safety and quality were also maintained throughout the project for the delivery of a high quality building foundation.

GROUND CONDITIONS

The secant pile wall was installed through 26m of made ground into the top of the Dolerite rock. The dowels then extended 2m into the strong Dolerite. The ground anchors were installed entirely within the body of the existing mass concrete wall.

STEEL DOWELS

The 273mm diameter steel reservation tubes were fixed into the reinforcement cages installed in the male/secondary wall piles. A Casagrande M6A rig fitted with a down the hole hammer (DTHH) was used to extend a 240mm diameter bore out of the bottom of the pile and 2m into the strong Dolerite. The bore was filled with grout, introduced via a tremmie from the base of the hole. The steel dowel was then lowered into position with 2.0m in the rock and 2.0m as an anchorage length inside the pile. The grout was topped up as necessary and a packer fitted into the top of the reservation tube to allow a pressure of 2.0bar to be applied to the grout.

GROUND ANCHORS

The 240mm diameter anchor bores were drilled using the down hole hammer (DTHH) through the concrete wall at the specified angle of 10º to the vertical. Once the required bore length had been reached the DTHH was withdrawn and the anchor was lowered into the hole using an attendant crane. The anchor bore was then filled with grout, introduced via a tremmie from the base of the hole.

All of the anchors were tested to 125% of the working load. Three No. anchors were subjected to suitability testing and the remainder to acceptance testing. The anchors were then locked off at 100% of the working load.

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