Ground conditions have a significant influence on the planning, design and construction of civil engineering projects. The strength, depth and nature of the rocks and soils have a major impact on the foundations for structures and the slope and width of the earthworks. The re-usability of excavated material is also an important factor in determining an Earthworks Strategy for the Proposed Scheme. All of these factors affect the required land take as well as the project economics and environmental impacts.
Types of Ground Investigation
The main exploration methods are using trial pits and boreholes.
Trial Pits
Trial pits are typically 3m x 1m on the ground and excavated up to a depth of approximately 4.5 metres. They are normally excavated by a hydraulic back-hoe (JCB or tracked digger). The use of trial pits allows a sound visual assessment of soils and taking of large samples for testing. Upon completion of the excavation and sampling, excavated material is returned to the trial pit, compacted in layers and the surface turf and grass replaced.
Boreholes
A cable percussion tripod rig is normally used to sink boreholes. A typical percussion rig consists of an engine-powered winch and tripod frame which is easily collapsed for transportation and can be towed by a 4x4 vehicle. Boreholes can be up to 250mm in diameter and up to 20 metres deep. Samples allow identification of soils and layering and high quality samples are taken for strength and suitability testing.
When hard rock is encountered, lorry or track-mounted core-drilling rigs must be used to extend boreholes. These are more substantial plant, accompanied by an air compressor. Cores are typically 100mm diameter and provide a lot of information about the strength and fracturing of rock, both in excavation and its potential for re-use.
Standpipes may be installed in some boreholes to allow the level of the water table to be monitored. The steel standpipe normally projects about 300mm above ground and fencing will be erected around standpipes to protect livestock and to prevent damage to farm machinery. We would like standpipes to remain in place until construction works start and staff may visit occasionally to record the groundwater levels.
Boreholes (except those fitted with standpipes) will be backfilled on completion
There are range of additional “probing” methods that might be used in special circumstances:
CPT Probing
CPT probing is a rapid technique that involves pushing a 50mm steel probe into the ground to obtain an indication of the character of the underlying soils. Measurement of the force required is a direct measurement of the strength and compressibility of the ground.
Based on a lorry chassis, this technique is most useful on soft ground and can reach a depth of 20-25m. The hole usually closes up automatically on withdrawal of the probe.
Hand-held probing
Where the ground is too soft or too steep for mechanical plant access, then hand-held tube samplers and probes will be used.
For sampling, a 75mm tube is hammered into the ground using a “kango”- type hammer. Typically up to 4m deep, this gives samples roughly equivalent in value to a small borehole, though boulders and stones can severely limit its depth.
To map the extent of peat, especially for widespread bogs, we anticipate a large number of rapid hand-held probes using a 50mm diameter rod to sound the depth of the peat along the line of the route.
For all probing techniques, the ground normally closes up immediately upon withdrawal of the probe, leaving no lasting impression.
Window Sampling
Window Sampling utilises a track mounted rig (see below) to drill small diameter holes to enable samples to be taken and install monitoring well is required. Window Sampling involves driving open-sided steel tubes approximately 40-80mm in diameter into the ground using a jack or percussion hammer. The tube is then retrieved, and a disturbed sample is obtained through the 'window' in the tube.
Standpipe installation
Standpipes may be installed in some boreholes to allow the level of the water table to be monitored. The steel standpipe normally projects about 300mm above ground and fencing will be erected around standpipes to protect livestock and to prevent damage to farm machinery. We would like standpipes to remain in place until construction works start and staff may visit occasionally to record the groundwater levels.
Reinstatement
Inevitably, excavating holes will leave some residual signs especially in the case of boreholes and trial pits. We will endeavour to return the topsoil and turf or other surfacing to the completed hole, though inevitably there will be a slight mound until the earth settles back to its original level. Some rutting of access routes will be unavoidable, but we will endeavour to minimise this by using the agreed access routes and re-levelling the worst affected areas.