Geotechnical Site Investigation

Site Investigation

Introduction

The main reason for a budget over runs on construction sites is due to inadequate understanding of the ground conditions. There are many unforeseen hazards in the ground that can only be identified by an appropriate ground investigation. A relatively small amount of investment at the outset of a project can provide large savings in the long term.

As part of the Phase II site investigation we also provide information on the ground conditions on the site, which will be required for foundation design.

We utilise a range of intrusive and non-intrusive techniques to undertake ground investigations. These include trial pitting, cable percussion boreholes, dynamic sampling, dynamic probing, plate load tests, cone penetration testing and geophysics.

We design our site investigations in line with the requirements of the “Guidelines for the Investigation by Drilling of Landfills and Contaminated Land”, published by the ICE Site Investigation Steering Group.

All staff undertaking ground investigations are engineers with appropriate qualifications and experience in geological/civil engineering disciplines, and hold current CSCS cards. All works are generally carried out in accordance with Eurocode 7 and the appropriate British standards.

All drilling rigs are operated by qualified drillers with NVQ’s in Land Drilling, CSCS certification and are first aid trained.

Drilling and Boring

We offer a full range of drilling options, from dynamic sampling and cable percussion boring in drift soils to rotary (open hole and cored) in bedrock (solid geology). Where required monitoring installations can be placed in the boreholes (gas/water monitoring wells to piezometers).

Cable Percussion Boring

Cable percussive drilling is the oldest and simplest form of drilling where advancement of the borehole is by hammering (percussion) a tool into the formation by cutting a hole or by bailing out the material. It is the most common form of drilling in the UK.

The type of rig is usually an “A” frame type, which arrives on site pulled by a four-wheel drive vehicle. Some cable percussion rigs are fitted to lorry bases and these may offer rotary drilling as well. This system of drilling is very common for geotechnical investigations but is widely used on environmental projects as well. This technique is used mainly for boring into and sampling superficial deposits rather than bedrock. The boring equipment is simple in design and comprises a cable passing from a winch, over a pulley at the top of the rig’s mast to the boring tools. The tools are lifted and dropped successively and, as they advance and the borehole is deepened, more cable is fed from the winch drum.

The boring tools are robust steel tubes with a solid weight fitted above them, around 1500 mm long and of diameters ranging from 140 mm to 300 mm. They are designed to sample the ground in that, as the open end hits the formation on the down stroke; material is forced inside and retrieved when the tool is withdrawn at the surface. Water is sometimes added to the borehole to assist in this. Different boring tools are available to suit different types of ground conditions. A shell (a round cylinder with a flap valve) is used in non-cohesive materials while a claycutter (with windows in its sides) is used in cohesive materials. When the ground contains layers of cobbles or boulders that inhibit the boring tools, a chisel can be used instead to break through and then boring resumes below.

As this technique is used mainly in soft ground conditions, the borehole may need to be lined with “casing” that is advanced at the same rate as the boring tools to maintain stability and minimise the entry of unwanted groundwater into the deepening borehole. In some circumstances, it may be necessary to use several different strings of casing and the more formations that are requiring sealing-off, the larger the diameter of casing that will be needed to start the borehole. Casings of 250 mm, 200 mm and 150 mm are conventional and it is the 150 mm diameter casing that is most commonly used. The technique may be used when there is a need to drill through an aquiclude into an aquifer. In these cases, the borehole is drilled in stages and at each change in casing diameter, a robust bentonite seal is placed at the base and allowed to rehydrate prior to further advancement of the borehole at the narrower diameter.

Soil samples can be taken for environmental and geotechnical purposes. “Bulk” disturbed samples can be taken from the drill cuttings and undisturbed samples can be taken using U38 or U100 sample tubes (denoting 38 mm and 100 mm diameters respectively with the latter also called U4 denoting 4 inch). In situ geotechnical tests such as SPT (Standard Penetration Tests) can also be carried out during drilling if required.

Cable percussion drilling is generally used for depths less than 40 m and typical drilling depths for ground contamination investigations are 5 m to 20 m. Significantly greater depths can be achieved from the rarer, very heavy duty, rigs.

“A” frame cable percussion rigs are fairly maneuverable as they are light but the length of the frame (6 m) prevents access to tight locations. Many contractors have “cut-down” mast rigs for access and operation in limited headroom locations if required. The method can generate considerable mess, particularly in soft clays and when shelling out sands and silts.

Rotary Boring

Rotary drilling techniques are employed where boreholes are required in very dense gravel or bedrock. Samples of bedrock are recovered in seamless plastic tubes for subsequent logging by a suitably qualified engineer and for laboratory testing.

Rotary drilling techniques are utilised where drilling into bedrock is required to check for shallow mine workings or to determine rock quality.  All our drilling rigs can undertake rotary coring to recover rock core samples at various diameters. The cores are then logged by experienced geoenvironmental engineers in accordance with the relevant British and European standards.

Alternatively the borehole can be drilled using rotary open hole methods in which samples are not recovered; rock cuttings are returned to the surface during the drilling and this allows a rapid examination of the strata. This method is often appropriate to locate shallow coal seams or mine workings and allows a number of boreholes to be completed in a short space of time.

Continuous monitoring of gases such as carbon monoxide, methane and carbon dioxide can be carried out while drilling through old mine workings.

Dynamic (Window) Sampling / Dynamic Probing

Window sampling is a percussive method of creating small diameter boreholes. Mostly the technique is achieved using hand-held equipment but some small drilling rigs have the capacity for window-sampling and the power of the rig enables window samplers to be driven to, and extracted from, greater depths compared to the manual technique.

A window sampler is a high tensile steel tube with a hardened cutting shoe to penetrate hard materials. Each sampler is usually 1 m or 2 m long with a series of “windows” or slots cut in the wall of the tube through which to view or extract soil samples. Samplers are driven down into the ground using a percussive hammer. A full set of window samplers usually consists of around four samplers ranging in diameter from 80 mm down to 35 mm. These are used systematically, starting with the widest and subsequently at reducing diameter to the required depth or limit of the technique. The depth limit tends to be around 8 m to 10 m although the technique is usually used to a maximum depth of around 5 m. In practice, the limiting factor tends not to be how deep the samplers can be driven in but rather whether the ground conditions are such that they may then be pulled back out. Coarse dense gravels, in particular, can grip the samplers and make extraction extremely difficult. The driller will assess the conditions at each location individually.

The start diameter is dictated by the ground conditions i.e. the softer the conditions, the wider the sampler set that can be driven in. Typically, however, a 60 mm to 80 mm diameter start sampler will be used, reducing to around 35 mm by around 4 m to 5 m depth. The full samplers are either jacked out manually or pulled from the hole using a hydraulic jacking system.

A full sampler will reveal a complete or partial ground profile although guidance from the driller will be needed on any compression that may have occurred and resulted in, for example, a 1.5 m sample occupying a 1 m space in the sampler. With some window sampling systems, samples can be recovered in a thin walled transparent liner placed inside the steel tubes. The liner is removed for visual inspection or capped at each end for transporting to the laboratory. Whatever system is used, the quantity of soil within the sampler is small and, particularly where the analytical suite is extensive, a sample may have to be taken from a 0.5 m or so section to ensure sufficient quantity. The advantage, however, is that once soil is taken for samples and backfilling purposes, there is rarely any excess requiring disposal.

Narrow diameter groundwater/gas monitoring installations can be constructed in the borehole once all samplers are withdrawn. Whilst the diameter of the standpipe is ultimately dictated by the diameter of the smallest (and lowest positioned) sampler, in practice it is usually possible to install a 19 mm or 35 mm standpipe with a fine gravel pack surround. With some systems, it is possible to drive a water well in through unsampled ground using the hammer unit.

This method is not suitable where there is a need to drill through an aquiclude into an aquifer as there is no way of protecting the aquifer from downwards migration of any contaminants from the ground above the aquiclude during drilling.

The equipment is generally brought to site in a van or four-wheel drive vehicle and from then on, it can be carried by hand around the site. In many circumstances the fact that this is largely a manual method is its advantage over even the smallest drilling rigs. Window sampling causes minimal disturbance to the ground surface (e.g. useful for investigations inside buildings, in a landscaped car park or between the sleepers of rail lines. The method can be used where access is very restricted (e.g. small operational sites and in gardens). It can also achieve rapid coverage to shallow depth (e.g. useful for establishing the thickness of a landfill cap over a large area).

Electric Dynamic (Window) Sampling

We also offer an electrical powered drilling unit which is capable of Windowless sampling, SPT’s etc., the same as a diesel/petrol powered rig. We replace the diesel power source with a unit that is electrically driven.

The power pack simply plugs into a 13amp socket/generator which in turn runs the hydraulics on the rig but, (being electric) it does not produce any fumes.

This is ideal for working in buildings, restricted area’s and petrol chemical plants etc.

We can also offer an electric powered hand held sampling unit for height/restricted areas.

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  • Head Office

    Hanover House
    Hanover Street
    Liverpool L3 3DZ
    0151 521 2539
    0151 909 3661
    enquiries@demeter-environmental.co.uk
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    Gemini House
    136-140 Old Shoreham Road Brighton
    East Sussex BN3 7BD
    01273 741 727
    brighton@demeter-environmental.co.uk