Ground investigation in winter and unreliable results
Three of the results a planning application depends on are seasonal. How to get groundwater, soakaway and gas data an officer will accept.
Ground investigation winter work is not a problem in itself. Rigs run all year and laboratories do not care what month a sample was taken. The difficulty is that three of the results a planning application depends on are seasonal, and a single visit in the wrong conditions produces numbers that an officer or a consultee will not accept.
Groundwater level, infiltration rate and ground gas concentration all move with the season. Everything else in a site investigation, from soil classification to contamination testing, is stable.
Why the season changes the data
Groundwater responds to recharge. Rainfall infiltrates through the autumn and winter, levels rise to a seasonal maximum in late winter or early spring, and they fall through the summer as evapotranspiration takes over. The difference between the September low and the March high can be a metre or more in permeable ground.
That matters because design decisions are made against the seasonal high, not against whatever was in the borehole on the day. Soakaway invert levels, basement waterproofing, foundation design and gas protection measures are all set by the worst case, and the worst case is a winter number.
Ground gas behaves differently again. Methane and carbon dioxide migrate more readily when atmospheric pressure falls, because the pressure differential draws gas out of the ground. Monitoring in settled high pressure conditions can return readings that look reassuring and are meaningless.
Groundwater levels and the monitoring period
A single water strike recorded during drilling is not a groundwater level. It is a note of where water was encountered while a hole was being advanced, and it can be misleading in low permeability ground where levels take days to equilibrate.
What is needed is standpipes or piezometers installed in the boreholes and read over a period long enough to show the seasonal pattern. That usually means monthly readings across the winter recharge period at a minimum, and longer where the design depends on a fine margin.
The consequence for programme is direct. If the site investigation happens in June and the design needs winter levels, the monitoring runs until the following spring. A Geotechnical Assessment submitted with levels recorded only in summer will normally attract a condition requiring further monitoring, and on a basement scheme it may attract an objection.
Infiltration testing and the water table
Soakaway design follows an established test method involving repeat fills of a trial pit and measurement of the fall between fixed proportions of the effective depth. Three tests per pit is the conventional requirement.
Two seasonal traps sit in that. A test carried out in dry summer ground can overstate long-term performance, because the surrounding soil absorbs the first fills in a way it will not when already wet. And the design has to demonstrate clearance between the base of the soakaway and the seasonal high groundwater level, which cannot be shown without winter monitoring data.
Saturated winter ground creates the opposite difficulty. Tests in standing water are not valid, and a site that fails infiltration testing in February may simply be reporting the water table rather than the soil's permeability. The honest answer is to record the conditions carefully and, where the result drives the drainage strategy, to test in both conditions.
Where infiltration is not viable, the Drainage Strategy moves down the hierarchy to a watercourse or, last, to the sewer, and the water company's position becomes the critical path. Find that out early rather than after a failed test.
Ground gas monitoring and atmospheric pressure
Gas monitoring is a programme rather than a visit. Guidance for characterising ground gas risk expects a series of visits spread over time, including visits during low and falling atmospheric pressure, with the number of visits increasing with the assessed risk.
Winter helps here. Falling pressure events are more common through the autumn and winter, so a monitoring regime run in those months reaches the required conditions more quickly than one run through a settled summer.
Record the atmospheric pressure at each visit, along with the trend over the preceding hours. A Contaminated Land Assessment that reports flow rates and concentrations without pressure data is incomplete, and a reviewing consultant will say so. Gas risk assessment and the design of protective measures are work for suitably qualified and experienced persons, and the report should say who carried it out.
Access, plant and the cost of a wet site
The practical side is easy to underestimate. A cable percussion rig on a wet clay site needs tracking mats, and a wet site takes longer to work because plant moves slowly and reinstatement is harder.
Budget for it explicitly. Mats, a longer mobilisation, standing time when a pit floods, and the reinstatement of ruts across an agricultural field that a landowner will inspect afterwards. On a site with livestock or crops in the ground, access permissions may push the work into a specific window regardless of what the design programme wants.
There is an upside. Winter ground conditions reveal things a summer investigation misses, including where water stands, which drains are live, and where a made ground layer holds water.
The caveats an officer will notice
Reports contain standard limitation sections that nobody reads. The caveats that matter are the specific ones, and reviewers look for them.
- groundwater levels recorded outside the winter period, with no statement of the likely seasonal high
- infiltration tests carried out without recording antecedent rainfall
- gas monitoring visits without atmospheric pressure data, or with no visit during falling pressure
- a monitoring period shorter than the guidance recommends for the assessed risk
- conclusions drawn from a single round of readings
- a recommendation that further work be carried out at detailed design, where the planning decision depends on the answer
That last one is the most damaging. A report that defers the question the application turns on hands the authority a reason to refuse or to impose a condition that stops the development for a season.
Planning the investigation a season ahead
Work backwards from what the design needs:
- Decide in the first week of a project whether the scheme depends on infiltration drainage, a basement, or ground gas protection, because each of those needs winter data
- Commission the desk study immediately, since it costs little and sets the scope
- Install standpipes and gas wells during the intrusive works, even where the current design does not obviously need them
- Start monitoring before the autumn, so the winter readings are part of a continuous record
- Book the infiltration testing for a period you can characterise, and record antecedent rainfall
- Leave the drainage and foundation design until the monitoring is complete rather than designing on an assumption and revisiting it
Installing monitoring wells you may not need costs a fraction of returning to site. That is the single most useful habit on any scheme where the ground matters.
PlanScope drafts supporting documents such as drainage strategies and planning statements from your project details, flagging facts that need confirming. Ground investigation, gas risk assessment and geotechnical design require suitably qualified professionals. Browse all 66 report types to scope the ground-related evidence base.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Can ground investigation be done in winter?
Yes, and for some purposes winter is better. Drilling, sampling and contamination testing are not seasonal. Groundwater monitoring needs the winter recharge period to capture the seasonal high, and ground gas monitoring benefits from the falling atmospheric pressure events that are more frequent in autumn and winter.
How long should groundwater be monitored?
Long enough to show the seasonal pattern, which normally means standpipes or piezometers read at least monthly across the winter recharge period. A single water strike recorded while a borehole is being advanced is not a groundwater level, particularly in low permeability ground where levels take days to equilibrate.
Is a soakaway test valid in wet ground?
Tests carried out in standing water are not valid, and a site that fails in February may be reporting the water table rather than the soil's permeability. Equally, testing in dry summer ground can overstate long-term performance. Record antecedent rainfall and, where the drainage strategy depends on the result, test in both conditions.
Why does atmospheric pressure matter for gas monitoring?
Methane and carbon dioxide migrate more readily as atmospheric pressure falls, because the pressure differential draws gas from the ground. Monitoring only in settled high pressure conditions returns readings that look reassuring but say little. Record pressure and its recent trend at every visit, and include visits during falling pressure.
What caveat causes the most trouble in a report?
A recommendation that the critical question be resolved at detailed design, where the planning decision depends on the answer. That hands the authority a reason to refuse or to impose a condition that halts the development for a season. Resolve the question before submission where the scheme turns on it.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.