Ground investigation and soil testing
Ground investigation (GI) and soil testing identify strata, groundwater, and geotechnical parameters so foundation and earthworks design can proceed on evidence rather than assumption. Trial pits, boreholes, and laboratory tests feed the interpretive report the engineer uses.
This site-preparation page explains how GI interfaces with preparation and dig packages — timing, what Mainline needs from the report, and why skipping GI to “get on site” is a false saving. Catalogue ownership of GI briefing sits on ground-investigation-before-build; commercial price impact of method choice lives in Insights on trial pits and boreholes.
Mainline typically delivers foundations and earthworks to the design that follows GI. We do not usually carry out the investigation ourselves, though we can recommend investigation contractors and work with their access requirements.
Preparation packages that ignore GI timing force foundation redesign into the dig window. The cheapest investigation day is almost always the one that happens before plant is hired for foundations.
review Piling Contractors in Tonbridge and Groundworks Contractors in Wandsworth.
When this topic usually comes up
When no recent GI exists for a new-build or extension; when warranty or lender conditions require investigation; when brownfield history raises contamination questions; or when foundation type is still open between strip, raft, and piles.
It also appears when preparation is ready to start but foundation drawings cannot freeze without boreholes or pits.
Book GI with enough laboratory turnaround before foundation freeze dates. Express testing helps; magic does not.
What it covers in practice
GI may include desk study, intrusive pits or boreholes, in-situ tests, sampling, and laboratory classification and strength testing. Contamination assessment may run in parallel on brownfield plots.
Preparation teams need to know where pits will be, how they are backfilled, and whether temporary access is required for rigs. Dig teams need founding recommendations, groundwater notes, and any sulphate or contamination flags that change concrete or spoil routes.
Do not invent allowable bearing pressures from this page — those come from the appointed designer reading the GI.
When pits are backfilled, note locations so later dig gangs are not surprised by soft pit backfill under formation. Poor pit reinstatement creates artificial soft spots.
Give investigation contractors the same access standard you expect for dig plant if deep boreholes or large pits are required.
Check whether GI exists, whether recommendations match the structural drawings, and whether contamination notes change spoil classification before clearance quotes are treated as final.
Read recommendations and limitations chapters, not only the summary table. Caveats about made ground thickness matter on site.
If this matches your project stage, send drawings and site details for a scoped quote — or call to talk through programme and access constraints.
Get an Accurate QuoteCommon triggers and situations
- No recent GI for the plot
- Design still choosing between shallow and piled foundations
- Brownfield or unknown fill history
- Warranty or lender requires investigation
- Groundwater or contamination risk flagged
How teams usually progress from here
Commission GI early enough to inform design, then deliver preparation and foundations to the issued drawings. Escalate if formation during dig contradicts the logged description.
When dig exposes ground that contradicts logs, pause pours and call the designer — that is competent practice, not delay tactics.
Practical checklist
- GI report and executive recommendations
- Contamination / sulphate notes
- Foundation drawings matching GI
- Access for investigation plant
- Escalation path if formation differs from logs
What affects programme and complexity
Late GI that forces redesign from strip to piles dominates programme cost. Access for investigation plant is usually cheap compared with redesign after mobilisation. Rate intent for construction sits on /groundworks-costs — GI fees are a separate professional service.
Standing time while waiting for lab results mid-dig is painful. Sequence GI so interpretive recommendations arrive before foundation drawings freeze.
Redesign fees and standing plant after late GI dwarf investigation invoices. Sequence truthfully.
Typical next steps
Step 1: Confirm GI scope and timing
What this step delivers: Investigation informs design before dig freeze.
Step 2: Complete intrusive works
What this step delivers: Logs and samples obtained.
Step 3: Interpret and design
What this step delivers: Foundation strategy locked to report.
Step 4: Construct to design
What this step delivers: Groundworks follow issued drawings.
Typical project contexts
- New-build plots before foundation freeze
- Extensions where ground is unknown
- Brownfield schemes needing contamination and geotechnical data
Frequently asked questions
Does Mainline carry out ground investigation?
Usually no. We construct to the design that follows GI and can recommend investigation specialists.
Can I start clearance before GI?
Often yes for soft strip, but do not freeze foundation dig or spoil classification assumptions until GI recommendations are understood.
Are trial pits enough?
Sometimes for simple sites; deeper or variable ground often needs boreholes. The geotechnical advisor decides.
What should I send Mainline from the GI?
The interpretive report summary, recommended foundation type, groundwater notes, and any contamination or sulphate flags that affect spoil or concrete.
How This Issue Is Normally Diagnosed and Repaired
Follow the typical path from problem identification through to resolution:
We provide these services across the UK, including
Get a Free Quote
Contact us for a no-obligation quote or to discuss your project. We'll advise on the best approach and provide clear pricing.
Get an Accurate QuoteHave a GI report? Send it with foundation drawings for a groundworks quote.
Related Services
Full-service groundworks for commercial and residential construction.
Piling ContractorsPiling for new builds, extensions and structural works on difficult or tight sites.
Foundation ContractorsCommercial and residential foundation contractors: strip, raft, pad and piled solutions to engineer's design.
Related Topics
Related Groundworks Guides
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Discuss Your ProjectGround investigation and soil testing FAQ
Does Mainline carry out ground investigation?
Usually no. We construct to the design that follows GI and can recommend investigation specialists.
Can I start clearance before GI?
Often yes for soft strip, but do not freeze foundation dig or spoil classification assumptions until GI recommendations are understood.
Are trial pits enough?
Sometimes for simple sites; deeper or variable ground often needs boreholes. The geotechnical advisor decides.
What should I send Mainline from the GI?
The interpretive report summary, recommended foundation type, groundwater notes, and any contamination or sulphate flags that affect spoil or concrete.
Need project guidance?
Share your site details and priorities. We will outline a practical scope, likely timeline, and next steps.