You’ve thought long and hard about how you’re going to develop or extend your property; you’ve appointed your Architect and Structural Engineer to turn you concept into a reality; and now you’re raring to go! You have all the information that you need to go ahead don’t you? Have you done a Ground Investigation?
Strong foundations are widely understood as being pivotal to the structural integrity of any building. What is often overlooked, by many outside the construction industry is what lies beneath: What do we have under the foundations? Soil? That’s pretty simple isn’t it? Well, no. Not really. The ground below the surface can come in many forms and vary significantly between different properties which may be within just a few metres of each other. We don’t know what’s below the surface until we excavate directly.
Understanding what type of ground is present below a property plays a fundamental role in designing suitable foundations. Constructing poorly designed foundations due to lack of knowledge of the ground can often have costly consequences.
How did they fix the leaning Tower of Pisa?
For many years professionals tried to stiffen the soft soil to no avail.
Professor Burland, a great English Geotechnical Engineer, solved the century old problem of the lean. Instead of stiffening the weak soil, he proposed weaken the stiffer soil. Slow, using small bore holes, soil was removed weaken the ground. the tower tilted back to a better lean.
Famously Professor Burland asked how straight the major of Pisa wanted it. the lean could have been completely removed and the tower straightened. But who would have wanted to visit it then!
In the UK we now use the “Burland Scale” to classify cracking to buildings.
Broadly speaking, the main types of natural ground in and around Greater London Basin are either gravel or clay. In some areas it may be a mixture of both. The Chilterns to the north and the Downs to the South are formed from chalk rising up.
In built up areas, over most natural layers of soil, ‘Made Ground’ has been laid down over time (the rubbish of millennia). This consists of man-made material such as rubble or soil that has been placed there artificially during previous developments and generations. The degree of Made Ground can vary significantly from site to site.
Types of survey
There is a wealth of data held about Britain’s green and pleasant land. Old bore hole records, geological maps, old rivers, flood data and historic land use maps can all be used to gain a picture of the site.
A Phase 1 Desk Study is an ideal way to determine what is below the site and the surrounding area. This can be carried out at RIBA Stages 2-3.
Ground and Site Investigations
There are numerous names used for investigations:
SI – Site Investigation
GSI – Ground site Investigations
FE – Foundation Exposures where the footings are exposed
Trial pits – a hole is dug to 2-3m to view the ground
This is done by a rig that penetrates the ground several metres below the surface. Soil samples at different depths are collected for analysis.
Often, gravity induced ‘striking’ of the soil is carried out to subsequently calculate the strength of the ground.
There are several types of Boreholes that can be drilled.
Shell and Auger
A tripod is erected and cores taken. These types of rigs provide some of the best results. They are ideal for open sites to fit the tripod and tractor unit on.
A drill is screwed into the ground and the soil taken. Unlike the shell and auger, the soil is disturbed and samples are not as good.
The rigs are smaller and can get into tighter spaces. Demountable rigs can be carried through a front door of a typical domestic property.
For shallower depths to around 6m, a small probe is forced into the ground. The resistance against the soil provides a guide to the soil type.
SPT – Soil Penetration Tests. A Cone Penetrometer is driven into the ground and the results recorded.
CBR – Californian Bearing Ratio this is test developed for highways to determine the ground stiffness.
Hydrogeology & Hydrogeological Testing
This is with regards to the flow of groundwater below a site. It is concerned with water tables and how water moves across a site. This type of investigation can monitor perched water above aquifers, water aquifers, tidal flows, seasonal variations, ground water conveyance. This data can also be used to advise on risks associated with flooding and assist with hydrogeologists with their conceptual ground models.
The water level of the ground is measured during an initial site investigation. For more thorough investigations, a monitoring well is left in place and repeat readings are taken over several weeks.
A standpipe is inserted into the ground. A plastic drain pipe is placed into the bore hole to prevent it collapsing and to allow the water table to be measured (known as “dipping”). Below is cap plate to a monitoring well.
To obtain more detailed information and data over time a piezometer is placed in the bottom of the hole and records water pressure.
Three monitoring points are needed to capture flow in both directions.
Time is also necessary to get a better understanding. The longer the time, the better the flow of water is understood.
Trial Pits (TP) & Foundation Exposures (FE)
Investigation into existing foundations is often needed. This involves digging with direct visual inspection and measurements taken on site.
Foundation exposures need not be too intrusive. A 300mm hole (1 foot square) can be used and a core taken.
Soil samples taken by the soil investigation team are passed onto accredited labs for further analysis. For example, tests to find the moisture content of soils with a significant clay content may be required. Atterberg limits can be found.
Often surveys involve a combination of the above
Many developments that Croft are commissioned on are on ‘Brownfield’ sites. These are areas that have already been developed or where groundwork has already been carried out. These are good for the environment as it reduces the demand to develop on ‘Greenfield’ sites. However, with Brownfield sites there are more uncertainties within the ground which can have implications on future development. These include: lack of consistency in the type of ground; excessive depths of made ground; obstructions within the ground (eg tree roots, buried services or foundations from previously demolished buildings) and contaminants within the soil.
Scope of investigations
It is possible for developments to proceed without the ground being investigated thoroughly. In this case the engineer will often make conservative assumptions on the strength of the ground. The risks of this approach may involve an over-design of the foundations as well as not being able to account for hidden obstructions or hazards in the ground.
A client’s decision to proceed with a ground investigation involves having to make financial allowances for intrusive works relatively early in the project. The benefits of having such works done at an early stage often outweigh the costs. Often clients may ask themselves ‘Can we afford to do this?’ when perhaps the question should be ‘Can we afford not to?’
The scope of the ground investigations will vary according to the scale of the development, the requirements from the local authority and the type of ground likely to be encountered. Croft have long-term working relationships with a number of ground specialists and are able to advise clients on the most suitable way forward.