Groundworks are the foundation of every project — literally. When they go wrong, the consequences are expensive. Here are the most common groundworks problems we encounter in Cambridgeshire, and how we solve them.
1. Clay Soil Heave
The Problem
Clay soil expands when wet and shrinks when dry. This movement can crack foundations, distort walls, and stick doors. In Cambridge, much of the soil is clay — particularly in Cherry Hinton, Trumpington, and surrounding villages.
The Solution
- Deeper foundations: Foundations below the "zone of influence" — typically 1.5–2m in clay
- Trench fill: Solid concrete less susceptible to movement than strip foundations
- Heave precautions: Compressible boards inside the trench absorb upward pressure
Cost Impact
Deeper foundations in clay can add £3,000–£8,000 to a standard extension compared to good ground.
2. Trees and Root Damage
The Problem
Trees draw moisture from the soil. In clay, this causes shrinkage. When trees are removed, the soil rehydrates and expands. Both scenarios cause foundation movement.
The Solution
- Deeper foundations: Foundations must go below the root zone — often 2m+
- Root barriers: Prevent roots from reaching foundations on larger sites
- Wait period: If removing trees, wait 2–3 years before building to let the ground stabilise
Cost Impact
Foundations near trees can cost 50–100% more than standard depths.
3. High Water Table
The Problem
When the water table is close to the surface, foundations flood before concrete can be poured. This causes delays, contamination, and weak foundations.
The Solution
- Dewatering: Pumps remove water temporarily during excavation
- Sump pumps: Permanent drainage for ongoing water management
- Waterproof concrete: Specified for permanently wet conditions
Cost Impact
Dewatering adds £500–£2,000 to foundations, plus potential ongoing pump maintenance.
4. Poor Soil Bearing Capacity
The Problem
Some soils can't support the weight of a building without significant settlement. Made-up ground (previously filled), sand, and silt are common culprits.
The Solution
- Raft foundations: Spread the load over a larger area
- Piled foundations: Transfer load to deeper, more stable ground
- Ground improvement: Compaction, grouting, or replacement of poor soil
Cost Impact
| Foundation Type | When Used | Cost Premium |
|---|---|---|
| Standard strip | Good ground | Baseline |
| Raft | Poor bearing | +20–30% |
| Piled | Very poor ground | +50–100% |
5. Drainage Issues
The Problem
Existing drains in the wrong place, collapsed drains, or no drainage for new connections. This can delay projects by weeks.
The Solution
- CCTV survey: Inspect existing drains before starting
- Divert existing drains: Often cheaper than building around them
- Soakaway design: For surface water when sewer connection isn't possible
Cost Impact
Drainage diversions typically cost £1,000–£3,000. New connections to mains sewers: £1,500–£3,000.
6. Contaminated Ground
The Problem
Former industrial sites, petrol stations, or agricultural land can have contaminated soil that requires special handling.
The Solution
- Soil testing: Desk study and site investigation before purchase
- Remediation: Remove, treat, or cap contaminated soil
- Gas protection: Membranes for sites with ground gas
Cost Impact
Remediation can add £10,000+ to a project. Always investigate before buying land.
7. Access Restrictions
The Problem
Narrow side passages, no rear access, or restrictions on skip placement. This affects how materials are delivered and waste removed.
The Solution
- Mini excavators: Smaller machines for tight spaces
- Manual excavation: When machines can't reach (more expensive)
- Alternative disposal: Bagged waste or multiple smaller skips
Cost Impact
Poor access typically adds 20–30% to groundwork costs due to slower progress and manual labour.
How to Avoid Groundworks Problems
Before You Buy Land
- Get a soil investigation report
- Check for trees within 30m of the planned build
- Ask about previous site use (contamination risk)
Before You Start Building
- Trial holes to check ground conditions
- CCTV survey of existing drains
- Proximity check for trees, boundaries, services
During Groundworks
- Regular inspections by building control
- Keep detailed records (photos, depths, changes)
- Address problems immediately — don't wait
Need Help with Groundworks?
DNDH Construction has tackled every groundworks challenge in Cambridge — from clay heave in Cherry Hinton to high water tables in Histon. We survey properly, quote accurately, and don't hide problems.