They are installed before the ground is dug out, not placed into an existing trench.

Boxes and hydraulic shores assume the excavation stays open long enough to place support.

Sheets remove that assumption because the retention exists before the dig does.

They are driven from the surface into undisturbed ground.

Excavation then proceeds inside a wall that is already there.

That is why they are the answer for ground that will not stand up at all.

Running sand, saturated silt, made ground and anything below the water table.

These materials do not stand vertically even briefly, so placed support cannot be installed.

Made ground and previously backfilled trenches have no reliable strength, and urban service work is full of both.

Groundwater turns otherwise workable ground into flowing material.

Deep excavations in mixed ground frequently need sheeting for the lower sections.

Ground investigation before the work is what identifies these conditions rather than discovering them at the dig.

It restricts flow but is not watertight - dewatering is usually still needed.

Interlocks reduce but do not eliminate seepage between sheets.

Water can also flow beneath the toe of the sheets if they do not penetrate an impermeable layer.

Sealed interlock systems exist for applications needing better water control.

Dewatering changes the ground conditions and can cause settlement outside the excavation.

Water management and ground support are related design problems rather than separate ones.

Driven by excavator-mounted hammer, vibratory driver or press - and the method is chosen by what the surroundings tolerate.

Impact hammers are effective and noisy, and generate significant vibration.

Vibratory drivers are faster in granular ground and transmit vibration into the surroundings.

Pressing is much quieter and near-vibrationless, which is what allows work close to buildings.

Vibration is the usual constraint near existing structures, services and sensitive equipment.

Obstructions in made ground can stop a sheet short, which then leaves a gap in the retention.

Waling beams and struts, installed progressively as the excavation deepens.

The sheets retain the ground, and the framing resists the load the sheets carry.

The sequence - dig a stage, install a frame, dig the next - is part of the design rather than the method.

Digging past a frame level before installing it exposes the sheets to loads they were not designed to carry alone.

Frame positions and sizes come from the design, which accounts for depth, ground and surcharge.

Removal follows the reverse sequence as backfilling progresses.

Pulling leaves voids that must be backfilled, and near structures extraction can cause more movement than driving.

The volume the sheet occupied becomes a void as it is withdrawn.

Ground can flow into that void, causing settlement at the surface some distance away.

Grouting during extraction is used where settlement cannot be tolerated.

Sheets are sometimes left in place permanently for exactly this reason, and that is a design decision with a cost.

Extraction near services and structures needs its own assessment rather than being treated as the end of the job.

Commonly, and the driving plant usually comes with them or with a specialist subcontractor.

Sheet piling is a specialist operation, and most general contractors subcontract it.

Hired sheets are reused many times and vary in condition, with damaged interlocks the main issue.

A damaged interlock can cause a sheet to declutch during driving, leaving a gap in the wall.

Design, driving and extraction are usually a single package for that reason.

Left-in-place sheets are lost from the hire fleet and charged accordingly, which affects the decision to extract.