8 Execution

8.1 General

The phases of execution differ with the type of wall and the type of supporting fluid used. In the general case where a supporting fluid is used, the basic sequences are:

— for cast in situ concrete diaphragm walls:

  • 1) excavation, generally with a bentonite suspension;
  • 2) cleaning the excavation;
  • 3) placing the reinforcement;
  • 4) concreting;
  • 5) trimming;

— for precast concrete diaphragm walls:

  • 1) excavation, generally with a self-hardening slurry, sometimes with a bentonite suspension;
  • 2) cleaning the excavation. When a bentonite suspension is used, it is replaced by a self-hardening slurry. If required by the design, a stronger material such as mortar or concrete may be placed at the bottom of the excavation, to support the precast panel and applied loads;
  • 3) placing the precast panel.

— for cut-off slurry walls:

  • 1) excavation with a self-hardening slurry. In some cases (e.g. excavations of long duration) , a different supporting fluid may be used, which has then to be replaced by the self-hardening slurry;
  • 2) when required, placing elements such as membranes, reinforcement or sheet piles;
  • 3) trimming and protective capping.

— for plastic concrete walls:

  • 1) excavation, generally with a bentonite suspension;
  • 2) cleaning the excavation;
  • 3) concreting;
  • 4) trimming.

8.2 Tolerances

8.2.1 Panel

The width and depth of the panel excavation shall not be less than the design values.

For retaining walls, horizontal deviations of the exposed face of a panel, at the top of the guide-walls, shall be less than 20 mm towards the main excavation and 50 mm away from the main excavation for cast in situ panels, and less than 10 mm in either direction for precast panels.

For cut-off walls, the deviation of the location of the wall measured with respect to its axis can be larger than for retaining walls and will depend on the project.

For retaining walls, verticality of the panels (including their ends) shall be within 1 % in both transverse and longitudinal directions.

Where boulders or other obstructions are present in the ground, this tolerance may have to be increased.

When the joint between panels is formed by cutting into the hardened material of the previously cast adjacent panel, it shall be checked that the hardened material is cut over an adequate horizontal distance. The minimum horizontal distance depends on the ground type, the depth, the type of material and the cutting tool.

Protrusions at the exposed face of cast in situ panels should not exceed 100  mm beyond the plane of tolerance. Larger values may have to be allowed in cases where the maximum particle size in the ground exceeds 100 mm or where the ground is soft or loose.

The offset between two adjacent panels at their joint location shall not exceed limits which would have an adverse effect on the performance of the wall.

8.2.2 Reinforcement cage

The tolerance on the total width of the reinforcement cage shall be ±10 mm.

The tolerance on the elevation of inserts (couplers, starter bars, inserts for anchors,...) after concreting shall be ±70  mm.

The tolerance on the elevation of the top of the cage after concreting shall be ±50  mm.

The tolerance on the horizontal position of the cage along the axis of the wall after concreting shall be ±70 mm.

8.3 Preliminary works

8.3.1 Working platform

The working platform shall be stable, above the water table, horizontal and be suitable for traffic of heavy equipment and lorries. The area along the line of the wall shall be clear of underground obstructions.

The working platform should usually be 1,5 m above the highest water-table anticipated during excavation, taking into account possible fluctuations.

In the case of a construction site on a slope, horizontal platforms shall be prepared.

Material used for the construction of the working platform or for backfilling excavations shall be of suitable quality and well compacted or stabilized.

Leakage from nearby drains or pipes, which can affect the construction of the diaphragm wall, shall be stopped or diverted.

8.3.2 Guide-walls

The purpose of guide-walls is to ensure alignment of the diaphragm wall, to serve as a guide for the excavating tools, to secure the sides of the trench against collapse in the vicinity of the fluctuating level of the supporting fluid, and to serve as a support for the reinforcement cages or prefabricated panels or other elements inserted in the excavation until the concrete or self-hardening slurry has hardened. They should also be suitable to support the reaction forces of stop end extractors when necessary.

In the case of cut-off walls excavated continuously with a backhoe, guide-walls may not be necessary, should ground conditions permit.

Guide-walls are usually made of reinforced concrete and constructed in situ, preferably cast against the ground, with a depth normally between 0,7 m and 1,5 m depending on ground conditions.

Guide-walls shall be designed and constructed to resist the loads to which they will be subjected, including the traffic of equipment and adjacent structures, without undue deformation or displacement, and to allow the positional tolerances concerning the panel to be met.

Guide-walls should be propped apart until the excavation of the panel takes place.

The distance between the guide-walls should normally be between 20 mm and 50 mm greater than the design thickness of the diaphragm wall. In the case of polygonal or irregular shaped walls, it may be necessary to increase the distance between the guide-walls.

The top of the guide-walls should normally be horizontal and have the same elevation on both sides of the trench. Usually, the inside face of one of the guide-walls is used as the reference face to establish the position of the diaphragm wall.

EN 1538:2000 Execution of special geotechnical works — Diaphragm walls