What is a Deep Excavation Support

Deep excavation support is the temporary structural system used to keep the sides of an excavation stable while drainage, sewer or other underground works are carried out below ground level. In drainage engineering, it becomes particularly important when workers, pipes, chambers or repair equipment have to be positioned in an excavation whose sides cannot safely remain unsupported.

The support system has to resist ground pressure and, where relevant, additional effects from groundwater, nearby buildings, roads, stored materials and construction plant. Its purpose is not simply to stop loose soil falling into the excavation. It has to maintain a sufficiently stable excavation geometry throughout the temporary works, including excavation, installation or repair, and subsequent backfilling.

There is no single depth at which every excavation automatically requires the same support system. Ground conditions, excavation geometry, groundwater and nearby loads can make a relatively shallow excavation difficult to support, while different conditions may permit another excavation to be formed differently. For deep drainage works, excavation support therefore has to be selected from the actual temporary works conditions rather than depth alone.

Ground Pressure Changes as the Excavation Gets Deeper

Before excavation, soil is confined by the surrounding ground. Removing soil from one side of a vertical plane changes that stress condition and allows the exposed ground to move towards the excavation.

A support system resists this movement. The forces involved generally increase with depth because the vertical stress in the soil increases with the weight of material above it.

For a simplified soil profile, vertical total stress from the soil’s own weight can be expressed as:

σv = γz

where σv is vertical stress, γ is the unit weight of the soil and z is depth.

If a soil had a unit weight of 18 kN/m³, for example, the vertical stress attributable to that soil would be approximately 18 kPa at 1 m depth and 90 kPa at 5 m depth. This is only a simplified illustration. It does not directly give the horizontal load that must be applied to a real excavation support design.

Horizontal earth pressure depends on soil properties, groundwater, wall movement and the support arrangement. Cohesive and granular soils also behave differently, and layered ground can produce a pressure distribution that cannot be represented accurately by one uniform soil property.

The resulting support loads can be affected by:

• excavation depth and width;
• soil type, strength and density;
• groundwater level;
• layering and changes in ground conditions;
• movement permitted by the support system;
• nearby foundations;
• traffic and construction plant;
• excavated material or other surcharge loads near the edge.

Surcharge is particularly relevant during drainage works in streets and developed areas. A vehicle, excavator or stockpile close to the excavation applies additional load to the ground. Part of that load can be transferred laterally towards the retaining system.

This is one reason why spoil placement matters. Material removed from an excavation has substantial weight, and placing it immediately beside the edge can increase loading at the same time that the ground has already lost lateral support.

Ground movement outside the excavation is also important. A support system can remain standing while allowing enough deformation to affect a nearby road, utility or shallow building foundation. Temporary works design may therefore be governed by acceptable movement as well as ultimate stability.

Support Systems Resist the Ground in Different Ways

The appropriate support arrangement depends on the excavation and available working space. Drainage works can use proprietary trench systems for some excavations, while deeper or more constrained works may require sheet piles, soldier pile arrangements, contiguous or secant pile walls, or other engineered temporary retaining systems.

These systems do not all behave in the same way.

A trench box, for example, is primarily intended to provide a protected working zone within an excavation. It should not automatically be treated as equivalent to a designed earth-retaining wall that prevents ground movement outside the excavation.

Sheet piles form a relatively continuous wall by driving or pressing interlocking sections into the ground. Depending on the design, the wall can extend below formation level so that the embedded portion contributes resistance.

Bracing can then be installed across the excavation to reduce wall movement. Walers distribute loads along the retaining wall, while struts transfer forces from one side of the excavation to the other.

Typical characteristics can be compared as follows:

Internal bracing creates a particular challenge for sewer construction because the excavation is not empty space. Large pipes, manhole sections, lifting equipment and personnel all need to move between the support members.

The position of struts therefore has to satisfy structural requirements while preserving the access needed to complete the drainage work. Removing or relocating a strut simply because it obstructs a pipe installation can change the load path of the entire temporary support system.

Sequence is consequently part of structural behaviour. A deep excavation may be formed in stages, with support installed as excavation progresses. Waiting until the full depth has been excavated before installing support can expose the ground to movements that the designed sequence was intended to prevent.

Groundwater Can Control the Stability of the Excavation

Groundwater introduces a separate set of forces and failure mechanisms. A retaining system designed only for dry soil conditions may not behave as intended if water levels rise behind it.

Water pressure increases with depth below the relevant water surface. For static water, the pressure increase is approximately 9.8 kPa per metre of water depth. A 4 m difference in water level can therefore correspond to approximately 39 kPa of pressure at the lower level.

The effect is not limited to direct pressure against a retaining wall. Groundwater flowing towards an excavation can carry fine particles, reduce effective stress in the soil and create instability at the base or around gaps in the support system.

Potential groundwater-related mechanisms include:

1. water entering through joints or openings in the retaining system;
2. erosion and transport of fine soil into the excavation;
3. uplift pressure acting beneath the excavation base;
4. instability caused by upward seepage;
5. softening of susceptible ground;
6. increased lateral loading where water cannot drain away.

Dewatering may be required to control these conditions, but lowering groundwater is itself an engineering intervention. Pumping can change pore-water pressures outside the excavation and, in some soils, contribute to settlement of surrounding ground.

This is particularly significant in urban sewer repairs where buildings, buried services and roads may be close to the excavation. The objective is not simply to keep the bottom dry enough for workers. Water control and excavation support have to function as a combined temporary works system.

Leakage from existing drainage infrastructure can complicate the situation further. A damaged sewer, water main or surface water pipe can introduce water locally even where the natural groundwater table is lower.

The excavation base also needs attention. Stable-looking walls do not guarantee that the base is safe. Hydraulic uplift or weak underlying soil can produce a failure mechanism that originates below the working area rather than at the visible excavation face.

Drainage Construction Places Specific Demands on Temporary Support

Deep sewer work differs from many ordinary excavations because the structure being installed usually has to pass through the supported area. The temporary works must provide enough space for pipe sections, chambers, bedding materials and lifting operations while continuing to restrain the ground.

Pipe level is often fixed by hydraulic requirements. A gravity sewer cannot simply be raised substantially to make excavation easier if the required upstream and downstream invert levels dictate a deeper route.

Existing services add another constraint. Gas, electricity, telecommunications, water and other drainage pipes may cross the excavation or run close to it. These can restrict where piles, sheet piles, anchors or other support components can be installed.

A workable support layout therefore has to reconcile several requirements at the same time:

• maintain ground stability throughout the excavation sequence;
• control movement that could affect adjacent structures and utilities;
• provide sufficient working space at pipe and chamber level;
• allow pipe sections and equipment to be lowered into position;
• accommodate existing services;
• permit the drainage installation to be backfilled correctly;
• allow temporary support to be removed or otherwise dealt with in the intended sequence.

Backfilling changes the structural condition again. As suitable fill is placed and compacted around the new drainage installation, temporary support may be progressively withdrawn where the system and method permit. The sequence needs to avoid leaving unsupported voids or disturbing the bedding and side support required by the new pipe.

This interaction is especially important for flexible drainage pipes, whose installed structural behaviour depends partly on the surrounding soil. A sewer pipe can be correctly manufactured and still perform poorly if temporary works removal leaves inadequate or disturbed side support.

The support system itself also has to remain compatible with the final underground structure. Sheet piles or other elements cannot be withdrawn without considering whether removal will disturb surrounding ground, newly compacted backfill or adjacent services.

Deep excavation support is therefore governed by temporary load paths that change as construction progresses. Excavation removes confining ground, retaining elements and bracing introduce new load paths, groundwater control alters hydraulic conditions, and backfilling progressively restores support around the completed drainage installation. The stability required at one construction stage cannot be assumed to remain unchanged at the next.