TWK/Specific/TBT-TWK-004

Temporary Earth Retention

Temporary Works › Specific › Temporary Earth Retention

Presenter		Project
Location		Date

Temporary earth retention supports excavation sides during construction to prevent collapse onto workers and adjacent structures.
Common methods include sheet piling, soldier piles, king post walls, contiguous piles, secant piles, and diaphragm walls.
The retention system must be designed by a competent engineer, independently checked, and recorded on the temporary works register.
The design must account for soil type, groundwater, surcharge loading from plant and materials, and adjacent structure foundations.
Installation of retention systems must follow the designed sequence — excavating ahead of the support risks wall failure.
Monitoring for wall movement, deflection, and settlement of adjacent ground must begin before excavation and continue throughout.
Propping and waling systems transfer the earth pressure from the wall to internal supports — their installation is critical.
Dewatering behind or within the retention system affects ground pressures and stability of the wall and surrounding structures.
Removal of temporary retention must be planned, sequenced, and authorised — premature removal causes ground collapse.
All retention systems must be inspected at the frequency specified in the temporary works procedures for the project.

Prevent wall collapse	An earth retention wall failing inward buries workers under tonnes of soil — design, checking, and monitoring prevent this.
Protect neighbours	Retention failures cause settlement and damage to adjacent roads, buildings, and services — monitoring catches movement early.
Designed sequence	Excavating below the propping level before supports are installed removes the resistance keeping the wall stable — sequence is critical.

Don't

Ensure the retention design is completed, checked, and on the temporary works register.
Follow the excavation and propping sequence exactly as shown in the design.
Install monitoring instruments and record baseline readings before excavation starts.
Take monitoring readings at the frequency specified and compare against trigger levels.
Install waling and propping systems at the design levels before excavating further.
Assess the impact of dewatering on wall stability and surrounding ground conditions.
Inspect the retention system at the intervals required by the temporary works procedures.
Obtain formal authorisation before removing any temporary retention element.
Report any signs of wall movement, cracking, water seepage, or ground settlement.
Brief excavation teams on the designed sequence and the prohibition on digging ahead.

DON'T excavate adjacent to retention systems without a completed, checked design.
DON'T dig below the propping level before the waling and props are installed.
DON'T begin excavation without baseline monitoring readings recorded beforehand.
DON'T ignore trigger level exceedances in monitoring — investigate and act immediately.
DON'T delay propping installation — the wall relies on internal support at each stage.
DON'T lower groundwater without assessing the effect on wall pressures and settlement.
DON'T skip retention system inspections — undetected movement leads to sudden failure.
DON'T remove temporary retention without the TW coordinator's formal authorisation.
DON'T dismiss cracking, tilting, or seepage from retention walls — they indicate failure.
DON'T allow workers to excavate ahead of the designed retention sequence.

See also: Temporary Works Awareness | Excavation Near Structures

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