Geotechnical and
Environmental Services

Specialists in the investigation, assessment and reclamation
of development land.

Phase 2 Investigation

Geotechnical and environmental ground investigation

Ground investigation involves the inspection of soils and rock layers by the excavation or trial pits and/or the drilling of boreholes.  Different techniques are applicable to different geology and site conditions.  The soils and rocks encountered are inspected by a supervising Geo-Environmental Consultant to provide a detailed engineering description of the ground.  Samples are obtained from the exploratory holes for laboratory analysis.

The intrusive investigation techniques we employ are detailed below:

Trial Pits

Often the cheapest form of investigation, excavation of trial pits by hand or machine allow detailed inspection of the ground due to the volume of soil exposed.  Hand dug inspection pits can be used when only relatively shallow soils need to be inspected (usually less than 1.5m depth) or to expose existing foundations.  A wheeled backhoe loader is the usual machine for digging trial pits which are typically 3 to 4.5m deep, 0.5m wide and 3m long.

Window Sampling

Hand held percussive window sampling is a useful method of drilling narrow diameter boreholes particularly where access is difficult.  As all the elements are able to be moved by hand, as long as there is room to walk we are able to drill this type of borehole.  The name stems from the fact that the borehole is formed by driving a steel tube into the ground.  This tube is withdrawn and a slot, or window, down the side of the tube allows inspection and sampling of the soils.

Window sampling can be forwarded to depths of 3m to 6m depending upon ground conditions.  Shallow boreholes can also be formed by hand augers, but unless the ground is soft these are usually limited to less than 2m depth.

As with all boreholes, after drilling is completed it is common to install a monitoring well to allow gas and/or groundwater samples to be obtained on subsequent visits to the site.  The well consists of a plastic pipe, typically 35 or 50mm in diameter, which is slotted along part of its length.  This slotted section is known as the response zone.  The response zone is surrounded by gravel, but is isolated from other parts of the borehole by use of a seal formed with a special type of clay known as bentonite.

Windowless Sampling Boreholes

This type of borehole is formed by a small tracked drilling rig with samples retrieved in thin plastic liners.  With side carriers removed, a normal type of this rig can usually pass through a gap of just under a metre.  For sites with more difficult access, special break down rigs can be employed that can be hand carried to the borehole location and re-assembled.  Borehole depths of up to 6m is typically, but in exceptional circumstances up to 15m depth can be achieved.  This type of drilling rig has the advantage of being able to undertake Standard Penetration Tests (SPT) and allows the use of casing which is an advantage in certain ground conditions.

Cable Percussive Boreholes

This simple and ancient form of drilling involves repetitive dropping of a tube into the soil under its own weight.  The sample is obtained from the clay cutter head in fine soils or a bailer for wet granular soils.  As the borehole progresses SPTs can be undertaken and relatively undisturbed samples can be obtained.  The rig consists of a tripod together with a diesel powered winch.  The rig folds down to form a trailer which is towed into place by a four wheel drive vehicle.  Typically these boreholes are 15 to 25m deep, but depths of double that can be achieved in soils, but only thin weak rock layers can be penetrated.

Rotary Boreholes

The most expensive form of drilling is rotary, but is the only choice if shallow rock is encountered.  These can vary in size from small tracked units to large rigs mounted on four wheel drive trucks.  Rotary open hole techniques break the rock into small fragments and so recovery of any samples is limited.  In contrast, rotary coring retrieves excellent samples.  There are no practical limits to the depths that this drilling method can achieve.