FAQ - frequently asked questions
What is a Ground Investigation?
The purpose of a ground investigation is to inspect, describe and sample topsoil, subsoils, and sometimes deeper bedrock. Usually the investigation results in a report that provides the interested party (typically a developer\builder) with advice about the type of foundation that will be required for new buildings, and whether or not the land is affected by: contamination, gas,or shallow mineworkings etc.
In advance of a ground investigation a desk study is completed. This typically comprises: an assessment of the site's history and environmental setting (geology, proximity of any landfill, watercoursesetc); a site visit (walkover survey); and the preparation of a conceptual site model. The conceptual model allows design of an appropriate ground investigation, with exploratory holes excavated by appropriate techniques, to appropriate depths, and at appropriate spacings.
There are a range of techniques available, and choice is dictated by a number of factors including:
- access – open land, or within a building etc;
- the anticipated ground; for example deep soft clays or weathered bedrock;
- the type of sample(s) required for subsequent laboratory testing;
- whether or not monitoring pipes are required.
The most commonly used techniques are:
|Trial pits - essentially a hole dug by a JCB mechanical excavator. Each hole is usually about 2.5m long, 1m wide and up to 4m deep.Trial pits allow a thorough examination of shallow soils.||Window sample boreholes – a 50mm to 100mm diameter hole can be advanced through soils to depths of about 5m, using a diesel-powered drilling rig.||Cable percussion boreholes – a 6" or 8" (150mm to 200mm) diameter hole can be advanced through soils to depths in excess of 20m, using a diesel-powered drilling rig.||Rotary boreholes – typically a 6" (150mm) diameter hole is advanced through rock to depths in excess of 50m, using a diesel-powered drilling rig.|
What is Trial Pitting?
Trial pitting is a simple and relatively inexpensive means of obtaining information about shallow ground conditions. A trial pit is essentially a hole dug by a mechanical excavator. Each hole is usually about 2.5m long, 1m wide and up to 4m deep.The spacing between trial pits is influenced by the anticipated geology, but the distance between pits is generally in the range 30m to 75m. Excavation is supervised by a Lithos Engineer who describes the materials encountered and recovers representative soil samples.
Typically a wheeled 4x4 JCB (or similar) backhoe excavator is used. However, a larger tracked machine, delivered to site on a low loader, is sometimes necessary. On more sensitive sites, (e.g. playing fields), a smaller tracked min-digger can be used to minimise damage to the ground surface.
When excavating through areas of hardstand a concrete breaker attachment is used prior to excavation. A tractor towed bowser is used if trial pits need to be filled with water for soakaway testing.
Trial pits are usually backfilled on completion, before excavation of the next pit. Soil arisings are put back in the hole in reverse order to excavation (e.g. topsoil last, so it forms the surface layer). In order to minimise future settlements, the backfill is compacted with the excavator bucket, and arisings are mounded over each pit to produce ‘graves’ of spoil.
Where fields are under crop, pits can often be placed along existing tramlines to minimise crop damage. Some rutting should be expected; especially at wetter times of the year.
On sensitive sites such as playing fields and landscaped areas, precautions can be taken to minimise damage to the ground surface. A lighter weight rubber tracked mini-digger can be used, and soil arisings can be placed on Visqueen or sheets of plywood. However, such measures are time consuming and will reduce the number of pits excavated during a single shift. Furthermore, a mini-digger is only able to reach depths of about 2m.
What is Window Sampling?
Window (or windowless) sampling involves driving steel tubes into the ground using a high frequency percussive hammer, driven by a hydraulic power pack associated with a diesel-powered mini drilling rig.The drilling rig is normally track mounted and some rigs are small enough to be driven through a conventional sized doorway.
The sample tubes are 1m or 2m long and those with a 'window' have a broad slot cut down one side. The soil material passes into the sample tube as it is driven into the ground. Drill rods are used to drive the sample tubes to greater depths. On reaching the required depth, the sample tube and drill rods are withdrawn using a mechanical jack. After removal from the borehole, the soil is inspected, described and sampled by the supervising Lithos Engineer.
Window sampling is often used where constraints associated with existing buildings, operations and underground service runs, preclude trial pitting. This technique results in minimal disturbance of the surface, which can be beneficial where investigation of the ground beneath areas of hardstand is necessary - a 150mm diameter tarmac/concrete core can be lifted and put to one side.
However, window sampling only allows a more limited inspection of the ground than is possible via trial pits; especially made ground with a significant proportion of coarse material.
Boreholes are typically advanced to depths of around 4m; but greater depths are possible in the right ground. The diameter of each sample tube used decreases with depth; usually from an initial 100mm to around 40mm. The boreholes are either backfilled with arisings, or a gas\groundwater monitoring wells installed.
What is Cable Percussion Boring?
Cable percussion (also known as shell & auger) boring uses a mobile rig with a winch of 1 tonne to 2 tonne capacity, which is driven by a diesel engine, and has a frame about 6m high. The legs of the frame fold down to form a simple trailer that can be towed by a light vehicle.
Drill tools are worked on a wire rope using the clutch of the winch for the percussive action. Tools include a clay cutter for dry cohesive soils, a shell or baler, for granular soils, and a chisel for breaking up rock and other hard layers. The clay cutter and shell bring up disturbed soil, which is inspected, described and sampled by the supervising Lithos Engineer.
On completion each borehole is either backfilled, or a well is installed to monitor gas and\or groundwater level. Excess soil arisings are usually left at the borehole site, although they can be cleared away where necessary. Monitoring wells (typically 50mm diameter, black HDPE pipe) are protected at the surface with either a raised cover or a flush stopcock type cover at ground level.
What is Rotary Drilling?
Rotary drilling methods, in which the drill bit is rotated on the bottom of the borehole, are used to penetrate and sample bedrock. A fluid (usually air or water) is passed from the surface through hollow drill rods to cool and lubricates the bit, and transport drill cuttings to the surface.
Rotary drilling rigs can be mobilised to site on the back of a low loader, in the case of tracked machines, or driven to site in the case of lorry mounted wheeled rigs. Wheeled rigs are more suited to sites with significant areas of tarmac surfacing as damage can be caused by the metal tracks.
There are two basic types of rotary drilling:
- Open hole (or probe) drilling, where the drill bit cuts all the material within the diameter of the borehole, generating gravel-sized chippings – significant quantities of dust are emitted from the probehole when air is used as the flushing fluid, and gravelly mud is generated when water is used. Open hole drilling is most commonly used when investigating shallow mineworkings, but can also be used to install gas\groundwater monitoring wells in bedrock.
- Core drilling. A rock core is cut by a bit, passes up into the inner barrel and, at the end of the coring run, the core barrel assembly is lifted to the surface by raising and removing each drill rod individually. Core drilling is relatively expensive, but essential if quality data is required to assess issues associated with deep excavation, rock slope stability etc.
What is a Greenfield site?
Greenfield is a term used to describe an area of previously undeveloped land, in a city or rural location. Greenfield land includes that currently used for agriculture, landscaping, or just left to nature.
Whilst greenfield sites typically pose fewer problems to the Developer, it is not uncommon for there to be issues associated with: poor ground; steep slopes; abandoned mineworkings, migrating gas etc. Consequently, where new development is proposed, site investigation is required.
What is a Brownfield Site?
Brownfield land is "previously developed land", the UK definition of which talks of it being vacant, derelict or underused. Redevelopment of such land may be complicated by real or perceived contamination and geotechnical issues.
The government has a target that 60% of new housing development must be on brownfield land and the overall aim in the country is to recycle such land in preference to building on greenfield sites.
Contaminated land is dealt with through the development control (planning) system, to ensure it is made suitable for its new use. Where no redevelopment is currently proposed, contaminated land is dealt with by Part IIA of the Environmental Protection Act 1990, which looks at land in the context of its current use.
Both regimes are concerned with the risks posed by contamination to human health and\or the environment; and ensuring that risks are identified properly and managed.