The ground investigation is the most fundamental part of the site investigation as a whole. Following the results of a Phase I preliminary site investigation, when a Phase II site investigation is required which en-compass’s the ground investigation. The ground investigation involves the fieldworks part of any project and includes intrusive work such as trial pits and boreholes. Trial pits are excavated either by hand adjacent to existing foundations or mechanically to provide an insight into the shallow ground conditions at a site. Trial pits are more cost effective than boreholes and can enable a greater profile of the soils but are generally restricted to around 3.50m depth with a standard 3CX type machine. The advantages of boreholes is that greater depths can be achieved and more detailed soil and rock strengths can be recorded. Also a borehole will disturb a far smaller area than a trial pit – too much ground disturbance is not ideal as it can affect the construction.
Borehole types and methods include:
Window sample and windowless sample boreholes; using a tracked rig and can reach depths of around 6.00 to 8.00mbgl depending on ground conditions. Hand held equipment is used on sites with difficult access or where borehole positions are on existing slopes. Window sample boreholes the cheapest form or borehole and can usually drill approximately six locations in a day. They are ideal for chemical sampling, installing ground gas and ground water monitoring wells for subsequent environmental monitoring, and for in-situ geotechnical testing of the shallow underlying soils. However they are restricted to shallow depths.
Cable percussion boreholes; also referred to as “cable tool” or “shell & auger”, tripod rigs that can reach significant depths but are limited to drilling through soils or drift geology. Cable percussion boreholes are used when information is required at greater depths than around 8.00mbgl. Depending upon the ground conditions and final depths required, in may be necessary to commence drilling in larger diameter casing and reducing as depth to facilitate the drilling process. GeoCon generally use 6″ and 8″ casing on the majority of our sites, however we do have the capability to use 10″ and 12″.
GeoCon also use difficult access cable percussion rig called “cut down rigs” which are used on site with difficult access or low head room such as inside existing buildings.
Rotary boreholes; Rotary techniques are used to drill through rock / solid geology. There are two types or rotary borehole, rotary open hole and rotary cored. Rotary open hole boreholes are drilled on projects such as mining investigations when the structural details of the rock is not necessarily important but we are more interested in the presence of voids that could indicate shallow mine workings. Rotary cored boreholes are drilled when structural details of the underlying rock are required. Generally used in applications such as when a proposed development has a heavy loadings, when the site is underlain by shallow bedrock, sites with slope stability issues or when geological faulting is present at a site and could affect any proposed developments.
Probe holes; probe holes are utilised on projects where they may be a former mine shaft on the site. Probe holes are carried out by a rotary drilling rig but rather than drilling, the drill rods are “pushed” into the ground until they are unable to continue due to being restricted by rock head. Holes are probed in a spiral pattern at approximately 1.00m spacing. Over an area of usually 15 -20m squared, we can collect information regarding the general depth to rock head beneath the site, then if a mine shaft is present the drill rods will continue to a greater depth than the adjacent probe holes – this indicates the location of the mine shaft.
GeoCon use a variety of ground investigation techniques to ensure the correct approach is utilised on each project. Every project is bespoke and every site is different, that’s why GeoCon use our specialist knowledge to design every ground investigation on an individual basis.
