Why Ground Investigation Can Save Property Developers Millions

A Q&A with Mark Bradley of TFGI

When property developers assess a potential site, the visible parts of a project often receive the most attention: architectural design, planning potential, structural engineering, build costs and sale values. But one of the most important factors lies completely out of sight. The ground beneath a development can determine whether a project is straightforward, complex — or even viable at all.

Mark Bradley, founding director of Terra Firma Ground Investigation Ltd, explains the hidden ground risks that developers, landowners and homeowners need to understand before they build.

TFGI is a specialist ground investigation and geotechnical consultancy based in Buckinghamshire. The company works across ground investigation, geohazards, mine workings, sinkholes, water abstraction, dewatering, geothermal energy and geotechnical design advice. Mark Bradley personally oversees ground and site investigations, from initial project scoping through to reporting and recommendations.

In this interview, Mark explains why early ground investigation can save developers hundreds of thousands — and, in some cases, millions – of pounds.

Why should property developers think about ground conditions before they buy or build?

In our experience, we regularly see property developers looking to acquire, or already acquiring, parcels of land for development without doing enough research into the ground. They will invest in architects, interior designers, structural engineers and building works, but the ground itself can be treated as an afterthought. That is a serious risk, because the ground is what everything else is built on.

A developer once said to us: “We don’t see any return for the money we have to invest in the ground.” They saw ground investigation as a cost rather than an investment. But you quickly see the value if a large crack appears up the wall of a newly built house during or immediately after construction. We have been involved recently in a case where a newly built property had missed a significant ground risk. The new homeowner had been lying in bed when the house started creaking and cracking. And, as she lay there, she watched a huge crack propagate up her bedroom wall from floor to ceiling. At that point, she decided to get out quick!

That is an extreme example, but it shows why early investigation matters.

What kinds of ground features are you dealing with most often at the moment?

A large part of our current workload involves sites on chalk, particularly across the South East.

The key risks tend to be cavities within the ground. Those cavities may be natural or man-made. Natural cavities include those caused by chalk dissolution. Chalk is a form of limestone, and over geological time it can be dissolved by water moving down through the ground. Rainwater and groundwater can exploit fractures, joints and weaknesses in the Chalk, gradually enlarging them and creating pipes, voids or other karstic features. These features are not always obvious from the surface, but they can present a significant threat to both existing properties and proposed developments.

The second category is artificial, or man-made, cavities. These include old mine entries, adits, shafts and underground mine workings. In some cases, old voids can migrate upwards over time and eventually cause a surface collapse.

People often associate mining with coalfields in other parts of the country, but in the South East, chalk was mined for hundreds of years for agricultural and construction purposes. Chalk was used as a soil improver, in lime production, for building materials and in local industries. Many of those workings were small-scale, local and poorly recorded.

Can historic mines exist without anyone knowing about them?

Absolutely. We are dealing with an agricultural and industrial past where records may be incomplete, lost or never created in the first place. A lot of small-scale chalk mining was not always recorded on early Ordnance Survey maps. Some extraction may even pre-date the first detailed mapping of the area. Information gets lost over time. Land changes use. Shafts are filled. Surface evidence disappears. Houses, roads and gardens are built over the top.

So yes, residential or commercial properties can be built over historic workings with no obvious prior knowledge of their existence.

Do you have examples of this?

Hertfordshire has several well-known examples of ground collapses linked to historic chalk mining and dissolution features. The collapse at Fontmell Close in St Albans is one example. Investigations later indicated that the collapse was centred over historic mining voids at significant depth below the highway. The area had a long history of clay and chalk extraction, but that history was not necessarily obvious to the people living there.

Hemel Hempstead has also seen issues associated with historic chalk mining. At Highbarns, abandoned chalk mines were identified beneath houses, roads and public open space, leading to major investigation and stabilisation works.

These examples show how historic ground activity can remain hidden until a collapse occurs or until proper investigation is carried out.

Which areas of the South East are most at risk?

The risks tend to follow the geology. Chalk is present at or close to the surface across large parts of southern and eastern England. We often see natural and artificial cavities in a broad belt running from the Isle of Wight, through Hampshire, Berkshire, Surrey, Hertfordshire, Buckinghamshire and the Chilterns, and up towards East Anglia and Norfolk.

There are particular hotspots where local geology and historic land use combine. Areas we regularly think about include:

• The Chalfonts

• Hemel Hempstead

• Kings Langley

• St Albans

• Watford

• Rickmansworth

• Pinner

• Northwood

• Beaconsfield

• Reading

• Norwich and parts of Norfolk

• Parts of Hampshire, Dorset and the Isle of Wight

This does not mean every site in those areas is unsafe. It means developers should be alert to the possibility of chalk dissolution, historic extraction, shallow mine workings or other geohazards.

A quick phone call and some early desk research can often identify whether those risks are likely to be relevant to a particular site.

How do these ground risks usually show up?

There are three main stages at which problems can appear:

• The first is before development, during a proper site walkover or desk study. A trained eye may identify depressions, subtle hollows, unusual ground levels, possible old workings, historic extraction areas, or features shown on old maps and geological records.

• The second is during construction. When you start increasing the load on the ground, excavating, tracking heavy machinery across the site, or changing drainage patterns, weaknesses can become exposed. A feature that was stable under previous land use may become unstable once new loads or water pathways are introduced.

• The third is after construction. Subsidence, depressions or collapses can appear months or years later. Triggers can include leaking services, poorly designed drainage, soakaways, changes in groundwater flow, or prolonged periods of heavy rainfall.

  
  

That is why it is far better to understand the ground at the beginning rather than respond to failure at the end.

Sinkholes have been in the news a lot recently. Are all sinkholes the same?

No. The word “sinkhole” gets applied very broadly in the media to almost any kind of sudden ground collapse, but not all collapses have the same cause. Some are caused by natural geological processes. Others are linked to historic human activity, such as mining, quarrying, poorly compacted made ground, old wells, shafts or culverts.

In urban areas, a lot of collapses reported as sinkholes may actually relate to shallow infrastructure problems, such as poor utility installation, leaking pipes, damaged drains or water mains washing out the ground. The important point is to understand the cause. A small utility-related collapse may be localised. A collapse associated with a natural cavity or historic mine working may point to a much larger geohazard.

Are changing weather patterns making this problem worse?

We are certainly seeing more concern around surface collapses following periods of heavier rainfall. Water is a major trigger. It can wash material into existing voids, enlarge natural dissolution features, destabilise loose ground or enter old mine workings. It can also find new pathways through the ground after development, particularly if drainage or soakaways are poorly designed.

That does not mean every heavy rainfall event will cause a collapse, but where vulnerable ground already exists, water can be the factor that turns a hidden weakness into a visible failure.

This is especially important for developers because construction often changes how water moves across and into a site. A site that has been stable for decades under one use may behave differently once buildings, roads, services, drainage systems and soakaways are introduced.

What should developers do before acquiring a site?

Pick up the phone early!

The key is to get expert advice before you commit too much money. Early engagement can identify whether there are obvious risks beneath a site, what additional searches or investigations may be required, and whether the project is likely to remain financially viable.

That initial advice can save thousands, tens of thousands, hundreds of thousands – and, depending on the scale of the development – potentially millions of pounds.

Ground risk does not always mean a site cannot be developed. Often, it simply means the risk needs to be understood, priced and designed for. But if you only discover the problem once construction is underway, your options are usually much more limited and much more expensive.

What happens after that first phone call?

During an initial conversation, if we have the site location, we can usually take a quick look at published geological records straight away. That allows us to understand what is mapped beneath the site, whether the area is known for chalk, made ground, historical extraction, mining, dissolution features or other geohazards, and what the likely next steps might be.

The next stage would usually include an initial desk study, historical map searches, review of geological and mining records, and a site walkover. From there, we can advise whether intrusive investigation is needed, such as boreholes, trial pits, dynamic probing, geophysical surveys or targeted investigation of suspected voids.

The aim is always proportionate investigation. We are not trying to overcomplicate a project. We are trying to make sure the developer understands the risks early enough to manage them properly.

What are the warning signs developers or homeowners should look out for?

Warning signs can include new cracks in walls, doors or windows suddenly sticking, depressions appearing in gardens or driveways, cracking in roads or pavements, sudden changes in ground level, unexplained holes, or repeated drainage problems. However, some of the most serious features may show very little at the surface until they fail. That is why relying only on visible signs is not enough for a development site.

For developers, the right approach is to investigate before you build. For homeowners, if you notice sudden cracking, ground movement, a depression or a collapse, you should take it seriously and seek professional advice.

Is this only a problem for property developers?

Developers are often the focus because they are changing the load on the ground and investing large sums of money into a site. But this is also relevant to landowners, homeowners, insurers, local authorities and anyone buying a property in an area with known ground risks. If you have recently bought a new house and you start seeing signs of movement, you should not ignore them. It may be something relatively simple, but it could also indicate a deeper issue.

For anyone buying land or property in a known chalk area, particularly where there is a history of mining or ground instability, a proper understanding of the ground can be just as important as a structural survey.

What is your main message to developers?

Make the ground a priority, not an afterthought!

The ground is not a box-ticking exercise. It is fundamental to whether your project can be built safely, affordably and successfully. Early ground investigation gives developers options. Late discovery gives developers problems. A relatively modest investment at acquisition stage can protect a project from major cost, delay, redesign, insurance problems and, in the worst cases, structural failure.

If you are considering a site, especially in a chalk area or anywhere with possible historic mining, speak to a specialist early. It may be the most valuable call you make on the whole project.

Concerned about a site?

If you are acquiring land, planning a development or concerned about ground movement at an existing property, TFGI can help assess the risks beneath your site. An early conversation can identify potential geohazards, guide the right level of investigation and help you make informed decisions before costs escalate.

Email: enquiries@tfgi.co.uk

Phone: 01494791110