The red sandstone that underpins much of Exeter's higher ground can deceive you. It looks competent at outcrop, but weathering often turns the upper metre into a loose, gravelly sand that crumbles under load. We have opened exploratory test pits on sites from Heavitree down to the Exe Valley, and the contrast between the dry sandstone of the ridge and the soft alluvial clay near the river is stark. A test pit logged to BS 5930 lets us see that transition directly — something no borehole log can match in terms of lateral detail. For projects near the historic quay, where made ground overlies the old river channel, we routinely combine trial pits with CPT soundings to map the depth to natural gravel before designing a foundation strategy.
A well-placed trial pit in Exeter's variable ground can save more in redesign costs than the entire investigation budget.
Scope of work in Exeter
Critical ground factors in Exeter
The mistake we see too often around Exeter is a desk study that stops at the geological map and assumes the sandstone is uniform. A developer we worked with on a site near Pinhoe had done just that — until a trial pit uncovered a 1.8 m thick lens of soft, organic silt trapped in a paleo-channel. That lens would have caused differential settlement under the proposed strip footings, and the structural engineer had to switch to a reinforced raft. Without the trial pit, the first sign of trouble would have been cracks in the blockwork six months after handover. Our approach is to treat every pit as a chance to challenge the desk study, not just confirm it, so that buried services, old backfilled cellars and unexpected soft spots get caught before they become a claim.
Our services
Every test pit we dig in Exeter feeds into the wider ground model, whether it is a standalone investigation for a rear extension or part of a multi-method campaign for a new housing estate.
Machine-excavated trial pits
Using a 3-tonne excavator with a flat bucket to open clean faces, we log strata, take samples and measure groundwater ingress on sites with vehicle access across Exeter.
Hand-dug inspection pits
For rear gardens, listed buildings and areas with buried services, we excavate by hand to 1.2 m or deeper with temporary support, minimising vibration and risk.
In-situ density and strength testing
Sand replacement density tests and hand vane shear readings taken directly on the pit floor to give the structural engineer immediate, defensible design parameters.
Combined pit and borehole campaigns
Where the pit hits refusal on rock or the water table is high, we pair the trial pit with dynamic sampling or CPT to extend the depth of investigation without losing the visual record.
Common questions
What does an exploratory test pit cost for a domestic project in Exeter?
For a single machine-dug pit to 2.5–3.0 m depth with a factual report, expect a range from £360 to £620 depending on access, muck-away and the number of samples sent to the laboratory. Hand-dug pits fall at the lower end unless shoring is required.
How deep can you go with a trial pit in Exeter's ground?
Most machine-excavated pits reach 3.5–4.0 m before stability or groundwater becomes a problem. In the alluvial clays near the Exe, we often stop at 2.5 m because the trench sides start to soften quickly once the water table is exposed.
Do I need a trial pit if I already have a borehole log?
Often yes. A borehole gives you a vertical profile at a point; a trial pit exposes the lateral continuity of strata, reveals fissures, root penetration and old foundation remnants that a borehole can miss. On Exeter's sloping sites, we use pits to track how the weathered zone thickness changes across the footprint.
How quickly can you mobilise a trial pit team in Exeter?
We can usually have a machine and logging engineer on site within five working days of instruction, provided we have received the utility drawings and the client has cleared the excavation area.
What happens to the pit after you finish logging?
We backfill the same day, compacting in layers with the excavated spoil. If the hole has penetrated a clay layer that could act as a drainage path, we place a bentonite plug to restore the natural seal. The surface is left level and safe for site traffic.