Technológie

Pit retaining and sealing

Retaining wall

Definitions

A retaining wall is defined, for the purpose of this chapter, as a slim vertical structure used for ground retention during excavation. Unlike gravity retaining walls, the weight of a slim retaining wall has little effect on its capacity to balance the pressures. A slim retaining wall acts like a series of juxtaposed vertical beams holding back the pressures exerted by the soil, water and existing structures. It is supported by struts or ground anchors and is embedded into the ground below excavation level, in order to:

• mobilise the passive pressure at the toe,
• allow the wall to support vertical loads if necessary,
• provide hydraulic stability – for which a continuous wall is required.

There are two main types of retaining walls:

• continuous walls: diaphragm or precast walls, sheet piling, slurry trenches, secant piles, etc.
• discountinuous walls: Berling and Parisian-type wall, Lutetian and Moscow-type walls, contiguous piles, shotcrete,etc.

Diaphragm wall

A diaphragm wall is a reinforced concrete wall that is cast in sections or panels excavated in the ground. The trench held open during excavation, and installation of reinforcement and concrete by the use of a supporting slurry. The slurry forms an impervious deposit (cake) on the walls of the trench, isolating the hydraulic pressure of the slurry from the surrounding soil and ground water, such that this pressure exerts sufficient outward force to keep the trench open.

The slurry mix can be based on the use of bentonite, or polymers or a mixture of the two. The temporary guide walls are constructed in advance and consist of two reinforced-concrete sections each about 30cm thick and 1m deep. The guide-walls have several functions:

• to provide physical confirmation of the location of the wall,
• to guide the excavation tool,
• to provide a reservoir for drilling mud,
• to provide a fixed support for suspension of the reinforcement cages.

Individual panel lengths are determined by a number of factors including trench stability and the sensitivity of the surroundings to movement. Typically they do not exceed 7m. The wall can be constructed very close to existing structures though a minimum clearance is required for the thicknes of the guide wall.

When excavation of a panel is complete the slurry is treated to reduce the quantity of solids in suspension to a predetermined acceptable level.

Thereafter, the reinforcement cage is installed and concrete poured using a tremie pipe.

The joint between adjacent panels can be achieved in one of two ways:

• by use of a temporary steel stop end allowing the placement of a waterstop across the joint and providing at the same time a guide for the excavating tools,
• by cutting back into the concrete of the previously constructed panel when excavating with a Hydrofraise®.

The standard thickness of diaphragm wall are: 0.50m, 0.60m, 0.80m, 1.00m, 1.20m, 1.50m and 1.80m.

Excavating tools fall into two main categories:

• Cable operated and hydraulic grabs
• Hydrofraise®

Diaphragm walls are ideal for use in water – bearing ground. The wall can be used as either temporary or permanent structure, and in the latter case it is incorporated into the permanent works.

Reinforced slurry wall

This technique lies between the Berlin and diaphragm walls insofar as it provides a temporary but watertight wall. Trenches are excavated under a bentonite cement slurry, using a tool such as a grab, in the same way as for a diaphragm wall. Vertical steel sections or sheet piles are lowered into the fresh, liquid slurry.