Contiguous piled walls are used where the groundwater table is below the excavation level and can be installed using rotary bored or CFA techniques. They’re normally the fastest and most efficient piled wall option.
The wall consists of bored piles typically installed at centres 150mm greater than their diameter. Retained soil is exposed during excavation, and permanent works generally require an additional reinforced concrete lining to secure exposed soil, and resist long term groundwater pressures.
Secant pile wall - hard/soft or hard/firm
This method offers the fastest and most efficient solution when short-term water retention is required, due to excavation below groundwater level. Primary piles are constructed with a pre-calculated gap between them, using a 'soft' cement-bentonite mix or 'firm' concrete.
Secondary piles, formed of structural reinforced concrete, are then installed between the primary piles, cutting into them on either side, with a typical interlock of 150mm. These walls may still need a reinforced concrete lining for permanent works applications, depending on the particular requirements of the project. They will generally keep a basement excavation reasonably dry for follow on construction works.
Secant wall – hard/hard
Hard/hard wall construction is similar to a hard/firm wall but, in this case, the primary piles are constructed in higher strength concrete and may even be reinforced.
King post walls are used for retaining ground where small settlements and ground movements are not critical and are often temporary.
A line of bored piles are installed with long steel H-Sections projecting from them. The gaps between the H-Sections are then filled with timber inserts, such as sleepers or pre-cast concrete planks. These are slotted in horizontally during the excavation.
A bored pile retaining wall can stand unsupported and acting in cantilever up to a limit. For deep excavations, piled retaining wall designs may require temporary propping, bracing or anchoring. It is important the construction sequence is discussed between the pile designer, the main contractor and the temporary works engineer.
These can be, for example:
- braces or props across the excavation from wall to wall. These are heavy and obstruct headroom in the excavation
- raking shores, founded in the excavation and propping diagonally upwards against the walls
- ground anchors to tie the wall back, leaving the excavation unobstructed; but there are issues for adjacent landowners
- berms, or banks of earth left in place, unexcavated, acting as mass-stabilisation for the wall