FAQs – Ground & Water Solutions Centre

Choosing the right geotechnical and contaminated land Investigations partner is an important decision and before you make it you will undoubtedly have umpteen questions that need answers.
We have compiled a comprehensive list of FAQs we answer on a regular basis. And added in a few things we need to ask you.

Preparatory work

Why do you keep coming back with more questions?
You must be absolutely clear about the final outcomes of your project. It is vitally important that we work with you and your design team at the earliest stage to understand the current restriction and hurdles required to be overcome for your development. Whether this is costing the development to buy, overcoming a planning condition or objection or technical query from a consultee. Please send us as much information as you can, so we know how best to advise you to get you building.
We suspect utilities could limit suitable areas to dig. What do you need?
Please provide as much utility information as you have available. We can obviously obtain statutory plans, do a GPR, Cat, Genie, and hand pit.
Why have you raised an issue over site access?
The act of constructing a borehole is not an easy task. It generally requires heavy tracked machinery. Access must be checked. Generally, if you can get a 1.5t tracked digger to the location, then we can get our Terrier MK3 drilling rig there. Then it is just a matter of the density of the soils restricting depth. Although it may be that getting the equipment down is not so much of the issue (unless it is so hard it refuses), the deeper you go, and denser materials are more likely to stop the sample from being retrieved and the jacks only have so much power. So deeper boreholes might need a bigger rig, which require a larger working area and space for access.
How quick can I get my results?
Generally, standard Site Investigations (SI’s) can be completed within three weeks of site works. However, this can be a bit quicker for simpler reports and longer should we need to do more complex modelling, for example during basement impact assessments/damage category assessments, or ground-gas monitoring over a prolonged period.
Can I get an indication of the results earlier?

Of course. Post site works, we will phone you and discuss the results and their implications. You can get a written preliminary report within six working days of site works. At any point you can call the Engineer in charge and discuss your site.

Problems with boreholes / sampling

We do not understand why our previous SI was rejected?
Did the SI go deep enough for the design of piles? The NHBC requires the site to be investigated to at least, the depth of the deepest pile.
We have been told poor class samples do not allow for engineering review of ground conditions. Why?
If a continuous flight auger was used and mixed the soil column, disturbing the strata, that could be the problem. We recently provided a second opinion on a borehole that indicated 12m of Made Ground. In fact, it was natural soils being mixed with shallow loose Made Ground. A very costly mistake if missed.

Value / Judgement / Competence

I have had a cheaper quote for the boreholes than your price. Why are you more expensive?
Generally we find that when things are cheaper, it means less work is involved or less reliable information is being generated. In all the work we undertake, we must have the basic requirements of guidance in the back of our mind. This mainly relates to the class of sample retrieved from the ground (its level of disturbance), the tests that can be undertaken on such a sample and the size and type of sample needed for a specific test.

For example, in accordance with guidance, a sample derived from a Flight Auger/CFA rig is too disturbed even for general descriptive classification. Basic classification tests can only be undertaken on samples with a certain level of disturbance. This is the same for the strength tests used. Such as, Mackintosh Probes, which are only generally suitable for sensitive soils and are pointless where dense gravels are noted (e.g., in London, when undertaking a basement). Another example is SPT’s, which can be unreliable in chalk. Less reliable information may mean more conservatism in design, which may drive up construction costs unnecessarily.

Are we comparing like for like? Most of the time a price difference is due to a specification difference. Deeper/shallow boreholes, falling head instead of BRE soakage tests, no SPT’s, no UT100’s. It’s a bit of a minefield so please don’t be afraid to discuss it with us, each test has its pros and cons.

Who do you use for laboratory certification?
We use MCert and UKAS accredited labs, on a standard five working day turnaround (three days are available for a surcharge) and we ensure all samples sent do not deviate. You must be careful with the deviation. Wrong sample tubs or extended handling times can mean results are not as reliable as you think. This is in both the contaminated land and geotechnical world. For example, in the latter, a sample may appear to have a moisture deficit if left out to dry whilst awaiting collection.
Are you a competent person, in accordance with the guidance?
Yes, we are. We work very hard to maintain technical compliance. The desire to continually improve is imbedded in the company culture.

From a more practical perspective all reports are overseen or undertaken by a Senior Engineer. Accreditations within the Company are with all major bodies that influence the industry including the Society for Brownfield Risk Assessment (SoBRA), Association of Geo-environmental and Geotechnical Specialists (AGS) and The Geological Society of London, with some members of the latter having reached Chartered Status. Influence within these bodies is maintained through working group participation.

A previous SI was not supervised. Will it be ok?
Are you getting value from your current provider? The best results follow having the best engineering judgement available on site. This could be the difference to founding at 2.0m instead of 2.5m, or 1.5m instead of 2.0m. Doing a 300mm capping instead of 400mm.

We will ensure we have experienced personnel, who know what they are looking for from a SI, on site as part of the drilling and technical team. We ensure quality information is built into our service provision from the start.

Will the reports be accepted by the Local Authority?
One of our tag lines is on time, on budget and approved. We will aim to get you the information you need to design your build and get the condition approved in the most cost-effective manner possible. Sometimes there is a bit of negotiation with the Local Authority, but we honour an agreement to provide a letter of return to the council should they ask. After all it is our product, if they need something explained or justified then it is our duty to do so.

However, sometimes, there are internal changes in a council policy, that are not communicated to all parties. This may be a change in policy on vapour risk assessments, groundwater risk assessments, groundwater monitoring, etc. In these circumstances a little extra bit of SI may be required to push it over the line.

For a Phase 1 Desk Study, can I get away with a Site Check Report or similar?
Based on Land Contamination: Risk Management (the guidance that underpins contaminated land risk assessment), the simple answer is no. The site check forms an important part, but it must be supplemented by a whole heap of information. This information is then assessed as a whole. If you just have a Site Check Report, you could miss important contamination sources or even be lulled into thinking the site is contaminated when it is not.
Can I get away with just a couple of soil tests?
The answer to this question depends on the circumstances of the SI/Report that is required, the regulatory framework for the report and your own evaluation of risk.

Less sampling and testing mean more uncertainty. If you can adequately manage that uncertainty, then great; but uncertainty can lead to conservatism and an over engineered design. So, we must always balance risk and reward. We request that you are open with us in this regard, so we can have an honest conversation about what you need.

Why are you now asking for more samples and data?
We cannot exactly predict the results of the chemical laboratory analysis that we commission. If we could, we would be billionaires!

Sometimes the results are uncertain, i.e., we get a couple of positives and the rest negative. More samples mean more certainty in the data, which lead to better evidence-based decisions being made and provides reassurance when a council officer reviews the report. We mainly want to avoid one scenario, the false positive. False positives mean you remediate a site when it is not required. Sometimes more info aids this.

Why do council requirements differ so much?
Some councils have very specific requirements to fulfil. Whether this is Camden Council and its Basement Impact Assessment/Damage Category Assessments or the concerns the Royal Borough of Kensington and Chelsea (RBKC) and Hammersmith and Fulham have with volatile vapour inhalation. Hackney Council has specific contamination risk assessment procedures for small developments, and West Sussex/West Berks and Wokingham have their concerns in relation to groundwater flooding.

The area’s specific geology and history will mean that councils will have different concerns. Therefore, a one size fits all policy may not be appropriate, so some adaptation of SI requirements may be needed in certain areas. The earlier we talk about this the better.


How come you missed the contamination?
You will be aware that we cannot excavate every single cm3 of Made Ground from the site, pass it all through a mass spectrometer and let you know the chemical make-up. Site Investigation and risk assessment is a balance between data and uncertainty. It is up to us to manage that balance with you.

Occasionally, unexpected contamination is found, beneath buildings or slabs not previously accessed or in the ground between trial pits where no source was identified for its presence.

This can be dealt with fairly easily. Discuss the occurrence with your engineer and we can visit, assess the issue, and provide advice on dealing with it. This can be done without it becoming a big problem causing delays.


Do I need ground-gas monitoring if I have deep Made Ground, Alluvium, or a backfilled gravel pit nearby?
It is not an automatic yes. Most of these sources, unless in exceptional circumstances, will not be sufficient to produce ground-gas in any significant quantities. There will be site specific details which will have to be reviewed, but such sources do not automatically mean expensive and time-consuming monitoring.
If I have CO2 over 5% or CH4 over 1% in my monitoring data, does that mean membranes and gas protection?
Gas protection is always a consideration, but definitely not a must. Ground-gas monitoring results need referencing back to the Conceptual Site Model (CSM) and detailed evaluation of measurements, flow, sources, and the proposed development taken into account first.
Is it easier just to install a membrane?
No No No! Gas protection measures are actually quite difficult to get right and verified appropriately. Any drop in standards during installation and verification, may mean time consuming and costly risk assessment, to determine if they were needed in the first place and possible retrofitting. It is much easier to get the risk assessment right in the first place and definitively determine requirements.

Delays caused by site issues

Concrete and hardstanding breaking out is delaying our SI. What should we do?
Sometimes you come across a site, with concrete hardstanding, where the concrete is just absolutely rock hard. It can barely be scratch by a modern handheld jack hammer and blunts drill bits. This is not your average lean mix or similar, but proper old school concrete. In these circumstances, if suspected, it may be better to use diamond drillers to cut out the locations.
Why is my report taking so long?
We are invested in you and your project, and our primary goal is to get you building as quickly as possible. That said, with a Contaminated Land or Geotechnical report, being almost the equivalent of a master’s dissertation; it takes a bit of time to collect accurate high-quality data, formulate an opinion, discuss uncertainties, and produce the report.

And getting it right is in all our best interests.

Why do you have to repeat the soakage tests?
We found the ground was unstable when we started to flood the trial pits. The last thing we need is for a trial pit to go from 2m long and 0.45m wide, to 3.0m long and 1.5m wide, due to collapse when the water is introduced. In these circumstances it is sometimes better to consider backfilling the pit with flint rejects (or similar approved certified clean gap graded material) and install a well to keep things safe. More expensive initially but less in the long run as this way we can get usable data.
Why is the regulator questioning the borehole locations sunk during the previous Site Investigation?
It is probably due to poor GPS coordinates. Knowing exactly where boreholes and trial pits are is vitally important, especially if they reveal a hotspot of contamination, which you then need to remediate at a later date.
Dense ground has resulted in our application being refused. Why?
Generally, there is a requirement to prove the ground conditions to a depth comparable to the height of the building. This is sometimes impractical, but it gives you an idea of the balance that must be struck. The deeper you go the more expensive it will be. Therefore, there is a trade-off between depth, equipment, uncertainty, risk and cost.


Do I need someone to independently test my imported Topsoil?
Sadly, yes you do. There have been too many instances when Topsoil has been imported to site, with certificates which are a few months or even up to a year old; and although the certs suggest the material is clean, fibrous asbestos, lead or alternative contaminants have been found. Unfortunately trusting that the certificates represent what you have imported, is a no go.
What about muckaway and WRAS?
People sometimes forget that the appropriate classification of materials for removal from site (Non-Haz, INERT or Haz) is an important consideration in construction. The cost of waste being removed from a site can have vast implication on build finances. Ensure that your SI covers this aspect, so you do not get caught out later down the line with unexpected costs.

Determining appropriate water supply connection pipe materials is a small, but still important aspect of SI. The possibility that hydrocarbons can leach through plastic pipes and impact water supply, is an important consideration in brownfield development, which is sometimes overlooked.

The WRAS requirements look at levels of hydrocarbons, such as PAH’s and TPH’s (plus VOC’s and other compounds should the Desk Study suggest their presence) at the proposed depth of pipe laying and deepening on the results suggest the correct pipe materials for the site. We assume that you do not want to be laying expensive barrier pipe when you do not need too.


Do I need BRE365 compliant tests to aid my drainage design?

Technically, and in most circumstances, yes! Some people will do what is called a BRE365 equivalent test, which is basically the same as a pV test for a drainage field (treated foul). The trouble with this is the holes that pV tests are undertaken in are too small to comply with BRE365 (1m long and 0.30m wide). So, such tests can only provide an indication, alongside tests like falling head tests in boreholes and it may well be, the council drainage officers will ask for full compliance on checking.

Can you complete a BRE365 test in a day?
This depends on the geology. BRE365 requires three consecutive fillings with at least 75% of the water draining on each one. We have witnessed this completed in chalk, in under 10 minutes (1×10 -3/-4m/sec). However, only if the rate is ~1-2x 10 -5m/sec (perhaps typical for gravels) can this be undertaken reasonably in a day. Especially if the base level of the soakaway is 1.5 – 2.0m below existing ground level (bgl). Mid to late 1 x 10 -6m/sec can take overnight, so the test could last three days. If the infiltration rate is slower, it could take all week.
Is there much point in carrying on past a day?
This very much depends on your site, the available drainage area, and alternatives available to you. With surface water flooding being more prevalent in design today, getting compliant infiltration rates, and being able to properly size soakaways early in design, should be a must. However, if you have a suitable alternative, you only need to prove they do or don’t work, then no, there is not.
Do I need to confirm overwinter groundwater levels to supplement SUDS design?
In some councils yes. West Sussex requires a site has twice monthly groundwater readings over winter, and then BRE365 compliant tests undertaken during winter months. West Berks and Wokingham, require an analysis of how high groundwater levels may affect soakaways. Please review this early in the design process and let us know if there is a requirement to review groundwater levels as part of the design. You do not want to be getting to the spring wanting to build, with an overwinter groundwater monitoring condition.

General problems

What other issues could cause problems and add cost?

Sometimes it is just the simple things: Like digging a trial hole to a depth of 2.5/3m, directly in line with a proposed foundation trench. It creates disturbed soils to depth on a foundation line, which have to be dug out at a later date.

Is hidden shallow groundwater an issue?
Groundwater strikes in boreholes can sometimes be hidden by the soils being smeared within the hole created or the casing being used. In reality sometimes a machine excavated trial pit is the easiest and best way to determine if traditional footings can be utilised. It gives a practical perspective on digging a trench and potentially filling it with concrete.
You have highlighted chalk dissolution as a risk, really?

Yes, chalk dissolution can cause significant issues with respect to getting out of the ground and drainage design, so should not be taken lightly. But it’s a matter of risk assessment and mitigation. It can be assumed to be present and mitigated against, or fully investigated depending on the risk, rather than just referencing the BGS ground dissolution maps. We will discuss this with you every step of the way and make sure you are building and out of the ground in the most cost-effective way possible.

Do we need a CBR test?
There are about six different types of CBR test and which one, depends on the anticipated soil type and the proposed end-use. For example, adoptable roads generally want worst case conditions revealed for design; so the council’s highways department may want a Soaked Laboratory CBR test. They may also want Equivalent CBR tests undertaken by Plate Load. Other methods include non-soaked laboratory CBR’s, In-situ CBR’s (4×4) and equivalent CBR’s from DCP’s. All have their place. Please just talk to the team about which one best suits your situation.


Can I talk to you about the report so I can fully understand its conclusions?
Of course you can. We have a post project phone call, to make sure the conclusions of the report are understood and agreed. Any uncertainties are talked through, and understood, and firm conclusions drawn. It is your development and what we do impacts it, especially as getting out of the ground is the most difficult part on most sites. Therefore understanding our report is key. Making sure that conclusions are carried through to both Structural and Civil Design stages is absolutely critical. We offer a post report issue service, so we can attend design team meetings or just chat everything through on the phone.



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