Ground investigation contractors and drilling standards

Ground investigation contractor competence is judged on three things: whether the drilling crew works to a recognised sampling standard, whether the person writing the logs is qualified to describe soils and rock, and whether the testing is done in an accredited laboratory. A contractor can hold every certificate going and still hand you data that a reviewing engineer will not rely on, because the samples were taken in the wrong class or the logs were written by a driller rather than a geologist.

The distinction that matters on a planning project is between the contractor who executes the investigation and the professional who interprets it. Both need competence, and they are different competences.

What separates a specification from a wish list

A usable specification states the purpose of each exploratory hole, the depth or the termination criterion, the sample class required at each level, the in situ tests and their spacing, the groundwater monitoring arrangement, and the laboratory schedule. A wish list says "four boreholes to 15 metres with soil samples".

The industry reference is the Specification for Ground Investigation published through the Institution of Civil Engineers with the Association of Geotechnical and Geoenvironmental Specialists. Used properly it turns an investigation into a priced, measurable contract rather than a day-rate arrangement where scope drifts. It also gives you a basis for rejecting work that does not meet the standard, which a loose scope does not.

Write the specification after the desk study, never before. The desk study tells you where made ground is likely, whether mine workings, landfill or alluvium are in play, and which contaminants are plausible. Without it you are guessing at depths and paying for the guess.

Drilling and sampling standards the contractor works to

BS 5930 is the code of practice for ground investigations in the United Kingdom, and it sits alongside BS EN ISO 22475-1 for sampling methods and groundwater measurements. The part people skip is sample quality. Sampling categories and sample classes define how much of the soil structure survives the process, and a Class 1 undisturbed sample for triaxial testing cannot be obtained with the technique used for a disturbed bulk sample.

If the geotechnical design needs effective stress parameters, the specification has to call for the sample class that supports that test. Discovering the gap at laboratory stage means going back to site.

Method choice follows the ground. Cable percussion suits clays and granular soils to moderate depths, rotary coring is needed in rock, window sampling is quick and shallow, and cone penetration testing gives continuous profiling without samples. Trial pits remain the most informative technique for shallow made ground and for foundation inspection, and they are the quickest way to find an obstruction. Where the site lies in a coalfield referral area, drilling into or near mine workings needs permission from the Coal Authority, and that consent has its own lead time.

Logging competence and who signs the logs

Field logs are engineering descriptions, and they are written to BS EN ISO 14688 for soils and BS EN ISO 14689 for rock. Descriptions follow a fixed order covering consistency or strength, colour, structure, the principal soil type and secondary constituents, and getting that order wrong makes the log hard to use and easy to dispute.

Ask who will be on site writing them. The answer should be a geotechnical or geoenvironmental engineer or an engineering geologist with a relevant degree and logged field experience, working under the supervision of a chartered professional who reviews and signs the final logs. It should not be the driller. Ask to see the name and experience before you appoint, because substitution after award is common and the difference in data quality is visible in the report.

Data should be delivered in the AGS digital format as well as in printed logs. That lets the designer, the contaminated land assessor and the drainage engineer all work from the same dataset without retyping it.

In situ testing and calibration records

In situ tests are only as good as their calibration paperwork. Standard penetration test results depend on the energy delivered by the hammer, so the rig's energy ratio calibration certificate should be current and supplied with the data. Cone penetration testing needs calibration records for the cone and its sensors. Permeability and infiltration testing has to follow a stated method, and for soakaway design the procedure in the BRE guidance on soakaways is the one drainage engineers expect to see.

Request the certificates as a deliverable rather than asking for them later. A reviewer who cannot see a calibration record will discount the test.

Laboratory scheduling and turnaround

Geotechnical laboratory testing should be carried out by a laboratory accredited for the tests in question, and chemical testing of soils for contamination assessment is expected to be done under the Environment Agency's monitoring certification scheme where the scheme covers the determinand. Accreditation is test-specific rather than laboratory-wide, so ask for the schedule of accredited tests and check that the ones you need appear on it. The choice of accredited testing laboratory affects whether the results can be relied on at all.

Turnaround is a programme risk people underestimate. Chemical suites come back in days, consolidation and triaxial testing take weeks, and a laboratory schedule agreed late pushes the report past the planning submission date. Schedule the long-duration tests first.

Supervision by the designer on site

The designer should visit while the investigation is live, and on anything beyond a simple site the specification should require it. Decisions get made in the field that nobody can make from an office: extending a hole because the founding stratum has not been proved, adding a monitoring installation where perched water appears, moving a position away from a service, changing a sample class because the ground is more granular than expected.

Eurocode 7 places ground investigation and interpretation within the design process rather than beside it, which is why geotechnical competence under Eurocode 7 is a separate question from contractor competence. The contractor delivers factual data. The designer produces the interpretative report, selects characteristic values and carries the design responsibility.

Reusing one investigation across several reports

A single well-specified investigation can feed four or five planning documents, which is the strongest argument for spending properly on it once.

1. Geotechnical parameters and foundation recommendations for a Geotechnical Assessment.
2. Soil and groundwater chemistry, ground gas monitoring and a conceptual site model for the land contamination work.
3. Infiltration test results for the drainage strategy and any SuDS design.
4. Groundwater levels and permeability for a Basement Impact Assessment where one is required.
5. Foundation and retaining wall information for tree protection design near root protection areas.

To make that work, the specification has to cover all of those purposes from the outset. Monitoring wells installed for gas cannot be added retrospectively, and infiltration tests at drainage depth rarely get done if only the structural engineer wrote the brief.

Land contamination Phase 1 and Phase 2 reports must be prepared in accordance with BS 10175 by suitably qualified and experienced people, and that is a legal and policy expectation rather than a preference. A drafted report gives that professional a structure and a policy framework to work within, and it is not a substitute for their judgement or their signature.

Illustrative example: a five-hole investigation on a former haulage yard, specified once, produced foundation parameters, a ground gas dataset over three monitoring visits, two infiltration tests at 1.5 metres, and groundwater levels for the basement design. Specified in three separate phases by three different consultants, the same information would have required two return visits.

You can browse the professional directory to find a reviewer for the interpretative work. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.