7 Design considerations
The terminology used to define the dimensions and details of panels is shown on Figures 1 and 2. Design shall take into account tolerances given in 8.2.
The panel dimensions should take into account the dimensions of available excavating equipment, the method and sequence of excavating, panel stability during excavation and concrete supply, as well as the appropriate information in clause 4.
The design wall thickness is equal to the width of the excavating tool. A larger wall thickness may be taken into account provided it is justified by site measurements.
The panels shall be designed as vertical elements, normally with the same horizontal cross-section throughout their depth. In some cases, the horizontal cross-section may be reduced below a certain depth.
The design of the wall shall take into account the discontinuity of the reinforcement at the joints between the panels and between adjacent cages in the same panel. Sufficient space shall be allowed between reinforcement cages of adjacent panels to accommodate the type of joints to be made and to take account of the construction tolerances.
A reinforced concrete capping beam is usually constructed along the top of reinforced concrete diaphragm walls, where it is necessary to distribute loads or minimize differential displacements. In rare cases where it is necessary to provide structural continuity across the joints, special techniques are available.
7.2 Panel stability during excavation
The length of the panels shall be such as to ensure the stability of the trench during excavation. The trench stability during excavation includes two aspects:
- the stability of the soil grains at the walls of the trench;
- the overall stability of the excavation.
The trench remains stable as a result of the stabilizing forces of the supporting fluid acting against the walls of the trench. In the case of bentonitic suspensions, the supporting effect in fine-grained soils is due to the formation of a filter cake. In coarser soils, this effect is caused by stagnation of the bentonitic suspension after a limited penetration into the pores of the soil. In the case of polymer solutions, the supporting effect is caused by the seepage pressure of the liquid flowing into the soil. The penetration depth, which increases with time, is significant in the case of silty or sandy soils, but remains small in the case of clayey soils.
The main factors affecting the stability which can be controlled during the execution are:
- the properties of the supporting fluid;
- the level of the supporting fluid;
- the length of the panels;
- the time during which the trench is left open, relative to the soil and groundwater conditions (possible loss of shear strength of the soil with time).
The excavation tools or procedures, especially where chiselling or blasting are used, may have an influence on the stability of the trench.
The stability of the trench shall be determined on the basis of comparable experience, stability calculations, or trial excavation(s) on site. When the comparable experience is considered to be insufficient, the second or third option shall be adopted.
Comparable experience is defined as experience which relates to similar works in similar conditions. In particular, the following items shall be considered in the comparison:
- soil and rock properties;
- groundwater pressures;
- adjacent structures;
- construction methods.
This experience shall be well documented or otherwise clearly established. Experience gained locally is considered to be particularly relevant.
The stability calculations shall take account of the following factors:
- stabilizing forces due to the supporting fluid;
- groundwater pressures;
- earth pressures, including the three-dimensional geometry of the problem;
- shear strength parameters of the soils;
- effects of adjacent loads.
In the case of trial excavation(s) , an adequate safety margin shall be allowed in the design of the diaphragm wall trench.
The level of the supporting fluid shall be adjusted with respect to the highest piezometric level anticipated during excavation, and shall always remain at least 1 m above the highest piezometric level.
In the case of very soft soils, it may be necessary to raise the level of the supporting fluid and/or to increase its density during excavation, and to minimize the time during which the trench is left open.
In the case of highly permeable, coarse soils or where there are voids in the ground, loss of supporting fluid may occur and, as a consequence, special measures should be adopted, for example:
- increasing the shear strength of the fluid by increasing the bentonite content in the suspension;
- adding a filler material to the bentonite suspension, either at the mixing plant or directly in the trench;
- in the case of voids, filling the trench to an appropriate depth with lean mix concrete or other suitable material, and re-excavating;
- grouting the layers concerned before excavating the trench.
7.3 Socketting into rock
Where diaphragm walls are required to be socketted into bedrock, the following shall be taken into account in the design:
- the function of the wall;
- the properties of the rock such as strength, structure (fissuring, bedding, etc.) , degree of weathering, and possibly permeability;
- the slope of the rock surface both in transverse and longitudinal directions of the diaphragm wall;
- the ability to penetrate the rock with the tools to be used. The design may need to include special solutions such as:
- variable depths along the base of individual panels or between panels;
- doweling into the rock at the base of panels with steel bars, tubes, beams, etc.;
- base grouting.