Dewatering And Groundwater Control In Excavations

An excavation below the water table is not a hole with water in it. It is a hole that water is actively trying to flow into, from every direction, continuously.

Managing that flow is what decides whether the excavation stays stable, and whether the buildings around it move.

Why it matters structurally

Three mechanisms, and all three cause real damage.

Base instability. Water flowing upward into the base of an excavation reduces the effective stress in the soil. Push that far enough and a sandy base becomes a liquid, a condition where the soil has no strength at all. In clay, upward water pressure under a thin remaining layer can lift the whole base as a plug.

Wall instability. Water pressure on the retained side adds to the soil pressure, and the combination can exceed what the shoring was designed for. This is the same arithmetic as [drainage behind retaining walls](/blog/drainage-behind-retaining-walls), applied to a temporary wall with far less margin.

Settlement outside the site. This is the one that produces claims. Lowering the water table outside the excavation increases the effective stress in the ground there, which compresses it, which settles whatever is sitting on it. The settlement can extend well beyond the site boundary, and it affects buildings nobody was working near.

The two approaches

Exclusion, meaning keep the water out. A cut-off wall taken down into an impermeable layer, so the excavation sits inside a box and very little flows in. Diaphragm walls, secant piles, sheet piles and grout curtains all do this.

More expensive up front, and it removes the settlement risk outside the site almost entirely, because the water table beyond the wall is not lowered. On a constrained urban site next to existing buildings, this is usually the right answer regardless of the cost comparison.

Dewatering, meaning pump the water out and let it flow. Sumps, wellpoints, deep wells or ejectors, depending on the soil and the depth.

Cheaper and simpler, and it lowers the water table in a cone extending outward from the site, which is where the settlement comes from.

The choice is a risk decision about the neighbours more than a cost decision about the hole.

Matching the method to the ground

Sump pumping, for shallow excavations and low flows. Water is allowed to enter and collected at the base. Carries a risk of drawing fines out with the water, which causes settlement and undermines the base.

Wellpoints, a ring of closely spaced shallow wells on a header pipe under vacuum. Suits sands and silty sands and a limited drawdown depth.

Deep wells, individual pumped wells, for greater drawdown and higher permeability ground.

Ejector systems, for fine-grained soils where the flow is low but the drawdown needs to be deep.

Recharge wells, which put water back into the ground outside the excavation to hold the external water table up and protect the neighbours. The specific remedy for the settlement problem, and the thing worth doing before damage rather than after.

Which method works depends on the permeability, and permeability is the single most variable soil property there is. It cannot be estimated from a borehole log with any confidence, which is why a pumping test is the standard on anything consequential, and why the investigation described in [understanding geotechnical investigation reports](/blog/understanding-geotechnical-investigation-reports) has to be read for what it does not establish as much as what it does.

Drawing fines is the quiet failure

Water flowing through soil at any speed carries the fine particles with it.

Which means a dewatering system that is running clear is working and one that is running dirty is removing the ground. Persistent fines in the discharge means voids are forming somewhere, usually behind the shoring or under an adjacent footing, and the settlement that follows appears suddenly rather than gradually.

Monitoring the discharge for fines is as important as monitoring the flow rate, and it is routinely not done.

What gets monitored

Groundwater level, in piezometers inside and outside the excavation, so the drawdown is measured rather than assumed.

Settlement of adjacent structures, on survey marks read to a baseline established before work started. Without that baseline, nothing can be demonstrated either way, which is the whole point of a [dilapidation report](/blog/importance-of-dilapidation-reports) and the [post-construction version](/blog/post-construction-dilapidation-reports-why-they-matter).

Flow rate, because a rising flow means the system is losing ground somewhere or an unexpected layer has been intersected.

The discharge, for fines.

Shoring movement, by inclinometer or survey, which is the direct indicator of whether the wall is behaving as designed. This runs alongside the general controls in [excavation near existing structures](/blog/excavation-near-existing-structures-best-practices).

Discharge, which is an approval not a detail

The water has to go somewhere, and that is a consent question before it is a plumbing one.

Volume, quality, sediment, pH and any contamination all govern whether it can go to stormwater, to sewer, or requires treatment. A system designed without the discharge resolved is a system that gets switched off, and an excavation that cannot be dewatered is an excavation that floods.

On a site where contamination is suspected, the discharge requirements can change the whole method and push the decision toward exclusion, because treating a continuous flow is expensive in a way a one-off is not.

Switching it off

The condition people forget.

Once the structure is built, dewatering stops and the water table returns. The completed basement then has to resist the full hydrostatic pressure, including uplift on the base slab, which is a different and larger load case than anything during construction.

A basement that was designed for the dewatered condition floats. That is the subject of [basement tanking and hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift), and the sequencing of when dewatering can be turned off is part of the structural design rather than the contractor's choice.

ACSES provides geotechnical engineering and excavation assessment across Sydney. Talk to us about a project.