Trial pits and boreholes: what lies below your walls

When cracks appear in plaster or brickwork, the first step is finding out why the walls are moving. A visual check inside and outside tells part of the story, but the real answers sit below ground level. That is where trial pits and boreholes come in.

These investigations form the core of a subsidence survey in Hastings. The specialist firm digs or drills close to the affected walls to examine the foundations and the soil they rest on. Without this step, any repair work would be guesswork. Knowing exactly what supports your home means the right method gets chosen first time, whether that involves mass concrete underpinning or another approach entirely.

The ground beneath Hastings varies from street to street. Sandstone, clay and made-up land all behave differently when wet or dry. Digging down removes the uncertainty.

A trial pit is a narrow trench dug by hand or small machine right next to the wall. It goes down past the bottom of the existing foundation so the specialist can see how deep the footings are and what they sit on.

In older homes around the area, footings are often shallow. Some Victorian terraces were built directly onto compacted earth with little more than a few courses of brick below ground. A trial pit shows this clearly. It also exposes the soil layers: sand, clay, chalk or rubble left over from earlier building work.

If water is pooling at the bottom of the pit, that points to a drainage problem washing out the ground. The specialist records the depth, takes photographs and notes the soil type. This information feeds directly into the report and shapes the repair plan. For houses on steep streets, trial pits also show whether the ground has slipped downhill over time.

Sometimes a trial pit cannot go deep enough. If the suspect layer sits several metres down, or if the house needs mini piled underpinning driven into stable strata, the specialist drills a borehole instead.

A borehole is a narrow shaft, typically 150 millimetres across, drilled through the soil using a portable rig. As the drill advances, it brings up cores of earth. The specialist lays these out in order to read the geology like a cross-section. They measure where soft clay gives way to firm sandstone, or where a void appears because old tunnels have collapsed.

In parts of Hastings built over former quarries or caves, boreholes are the only reliable way to find empty space below the foundations. The depth required depends entirely on what the drill finds. Once the target layer is confirmed, the hole is filled back in and the site left tidy.

The ground under Hastings is not uniform. Wadhurst Clay shrinks when it dries and swells when it rains, which causes seasonal cracking. Hastings Beds sandstone is firmer but can fracture along natural joints. Made ground near the coast often contains loose fill that compresses under weight.

A trial pit or borehole identifies which of these materials sits under your specific walls. That distinction matters because each soil type demands a different response. Shrinkable clay might need resin injection to stabilise the moisture content, while loose fill could require concrete poured beneath the existing footings.

The specialist compares the soil findings with the pattern of cracks recorded during structural monitoring. When the two match, the cause of movement is confirmed. When they do not, further pits may be needed until the picture makes sense.

Once the pits are dug and the boreholes drilled, the specialist compiles everything into a report. This document states the footing depths, soil types, moisture readings and the likely cause of subsidence. It then recommends a repair method suited to those exact conditions.

For a house where the footings are simply too shallow, mass concrete underpinning extends them down to a firmer layer. Where access is tight or the stable ground is deep, mini piling reaches further without needing large excavations. If the problem is limited to a single corner, crack repair combined with localised ground treatment may be enough.

The investigation stage prevents unnecessary work. Digging first means the specialist firm does not pour concrete where it is not needed, and does not miss a hidden drain leak that would undo the repair within a year. Your home gets the specific fix the ground demands.