Site Investigation

Window sampling

The percussive window or windowless sampling method involves driving cylindrical steel tubes into the ground using a hydraulic hammer. The drilling rigs are mounted on rubber tracks and disturbance is minimal making the rigs suitable for use on environmentally sensitive sites. The rigs are ideal for work on sites with restricted access, close-up against walls or inside buildings, where it would be otherwise difficult to obtain samples. The rigs can be tracked up slopes and can be driven through a standard domestic doorway. In areas where there is low-headroom or access limitations hand-held equipment can be used.

Sampling is achieved by a percussive action. When using the rubber tracked rig the drive mechanism consists of a chain-driven drop weight that repeatedly strikes an anvil, driving the rods and sample tubes into the ground. When using the hand-held rig the samples are driven using a pneumatic hammer. Both systems can drive sample tubes into all soil types, but they are not designed for sampling or coring in hard rocks.

The sample tubes are usually 1m long and have a broad slot, or window, cut down one side. The soil is pushed into the sample tube as it is driven into the ground. Drill rods are used to drive the sample tubes to greater depths. The weight and the drop height are adjustable, enabling standard penetration testing and dynamic probing to be carried out as well as soil sampling.

On reaching the required depth, the sample tube and drill rods are extracted using a hydraulic jack. Where the ground is unstable, steel casing can be installed as the sample tubes are driven to prevent the sides of the borehole from collapsing when the tubes are extracted. After the sample tubes are withdrawn from the ground, the soil core is logged and sampled from the window.

Windowless sampling can also be carried out using a conventional sample tube combined with an inner plastic liner to enable easy removal of the core from the sampler. This system is used to rapidly penetrate to the depth at which the sample is to be taken, or to provide a continuous core section. The sample is retained within the plastic liner which can then be split on site for examination or taken to a laboratory for analysis. This method of sampling reduces the potential for cross-contamination of the soil during the drilling process.

Window and windowless sample tubes are selected in a sequence of reducing diameters. Various diameters are available (35 to 80mm) and the sample tubes are selected according to the ground conditions encountered. The total depth which can be achieved will depend upon the soil type and the presence of obstructions. Depths of up to 10m are possible in certain soil types where the borehole remains open without support, but typically sampling depths of between 2 and 5m are obtained.

Percussive window sampling can also be carried out using a portable jack-hammer system. The sample tubes are driven into the ground using a hydraulic hammer and are then retrieved with the use of a compact hydraulic jack which takes its power from the same hydraulic pack used to power the hammer.

This system is ideal for use in sites with restricted access where conventional window sample rigs are unable to be used. Extra-long hydraulic hoses can be provided to allow the power pack to be located away from the working area, this could be used to alleviate the problem of exhaust fumes when working in confined spaces.