The correct depth of a ground investigation depends on what you need to establish. Whether designing foundations, installing piles, assessing contamination or investigating drainage, the objective is to obtain reliable information at the depths that matter to the proposed development. Investigating early can also help establish key design parameters before layouts and drainage strategies are finalised.

Foundations and piling

Trial pits are often suitable for investigating shallow foundations, allowing direct inspection of soil strata, made ground and existing foundations. Boreholes and in-situ testing may be required to assess deeper strata, groundwater conditions and the suitability of the ground for foundation support.

For piled foundations, investigations must establish the ground profile, identify suitable founding strata and assess pile capacity and settlement. Under established Eurocode 7 guidance, investigation depth below the anticipated pile toe should generally extend by the greatest of 5 m, three times the largest pile base diameter, or the smaller plan dimension of the pile group. The geotechnical designer should confirm the appropriate depth against the applicable standards, geology and proposed pile design.

Identifying the potential need for piling at an early stage helps avoid boreholes terminating too shallowly and the cost of a second mobilisation.

Contamination assessment

Contamination investigations should target potential sources and pathways identified by the desk study and conceptual site model. Trial pits can assess shallow made ground, while boreholes allow investigation of deeper strata and groundwater. Sampling depths should reflect changes in ground conditions and potential contaminant distribution rather than relying solely on fixed intervals.

Infiltration testing and drainage design

Early infiltration testing can establish whether sustainable drainage systems (SuDS) are feasible and inform the design of soakaways, infiltration features and soft-landscaped areas. Preliminary infiltration tests may sometimes be undertaken in suitable boreholes during the initial ground investigation, providing an early indication of infiltration potential at relevant depths.

Where preliminary results are promising, full testing in accordance with BRE Digest 365 may be required to establish design infiltration rates for a proposed soakaway. BRE 365 testing typically involves excavating a trial pit representative of the proposed soakaway's depth and geometry, filling it with water and monitoring the rate of water-level reduction.

However, a full BRE 365 test is not necessarily worthwhile if existing borehole logs and test results demonstrate that clay soils extend through the proposed infiltration depth and are unlikely to accept sufficient water. The evidence can be presented to the Local Authority or drainage approving body to explain why infiltration is unlikely to be viable and propose an alternative drainage strategy. Clay should not automatically be assumed impermeable, and the approving authority may still require formal testing.

For drainage fields serving septic tanks or sewage treatment systems, percolation tests follow a separate procedure under Approved Document H in England. These tests assess soil suitability for wastewater infiltration and help determine the drainage field area required. They are not interchangeable with BRE 365 tests.

Can one mobilisation cover everything?

Often, yes. A coordinated investigation can combine foundation assessments, contamination sampling, preliminary infiltration testing and other ground assessments during one site visit. A single excavator may be sufficient for several shallow investigations, although deeper boreholes or specialist testing may require additional equipment.

The key is to investigate early and design the investigation around the development proposals. Establishing ground conditions, foundation parameters and infiltration performance before designs are finalised can avoid unsuitable solutions, unnecessary excavation and repeat plant mobilisation. A well-planned investigation can therefore reduce costs while providing the information needed for informed design decisions.

Technical references

• Eurocode 7: Geotechnical design — Ground investigation and testing. • BRE Digest 365 — Soakaway design — methodology for soakaway testing. • Approved Document H: Drainage and waste disposal — includes guidance on drainage fields and percolation testing in England. • Environment Agency: Land Contamination Risk Management — guidance on risk-based contamination assessment.