Insitu testing and monitoring

AVZGeoEng provide a full suite of in-situ testing as a stand-alone service or as part of a wider investigation all backed by comprehensive technical and analytical support:

All the in-situ testing detailed below is carried out in-accordance with BS 1377: Part 9 and the Eurocode Part 2, Ground Investigation and Testing (published  2007).

SPT (Standard Penetration Test) is a dynamic, in situ penetration test used for providing information on geotechnical engineering properties of soil. The test takes place inside a borehole, using a thick-walled sample tube or cone. This tube has an outside diameter of 50mm with an inside diameter of 35mm. The sample tube is driven from the bottom of the borehole into the ground using a 63.5kg hammer, which is dropped freely through a distance of 760mm. The test results are deduced through the number of blows needed to drive the tube each 75mm into the ground, to a maximum depth of 450mm. The “standard penetration resistance” or “N-value” is calculated by the sum of the number of blows required for the last four 75mm increments of penetration, to a maximum of 50 blows.

Insitu California Bearing Ratio (CBR) Test is normally used for road, runways or car park pavement design to assess the strength of the sub-grade. The test involves driving a cylindrical plunger of a defined area into the soil at a uniform rate. This requires the use of a reaction load, (usually a four wheel drive vehicle), to provide the force.  Alternatively a profile of equivalent CBR can be measured using the Dynamic Cone Penetrometer (DCP) or Clegg Hammer Testing.

Plate Load Testing is particularly suited for the design of foundations or footings for buildings where it is considered that the mass characteristics of the soil would differ significantly from the results of laboratory tests, or where more precise values of settlement are required. These can also be used in verifying bearing capacity for crane and pile pads, deriving soil stiffness parameters and CBR values.

Mackintosh Probe is a lightweight and portable penetrometer. It is a considerably faster and cheaper tool than boring equipment especially when the depth of exploration is moderate and the soils under investigation are soft or loose. Correlations are developed between Mackintosh Probe results and those of the Standard Penetration Test (SPT), as well as, undrained shear strength (cu).

In-Situ Vane Shear Test is used to estimate the undrained shear strength of a soil, and is particularly appropriate for assessing very soft and sensitive clays, which can not be tested accurately in a laboratory as it is difficult to obtain an undisturbed sample. A rod with a four blade vane is pushed into the ground and rotated generally at a slow rate of 6o to 12o per minute to induce shear failure. After shearing, the slow rotation rate is resumed to determine the remoulded shear strength. The test can be done at the base of a borehole or trial pit, or at ground level. If the test is being conducted at the bottom of a drillhole it is important that the test area has not been disturbed by boring, Generally the test is conducted at a depth of five borehole diameters below the borehole base.

Groundwater and Gas Monitoring

Groundwater monitoring is carried out in order to obtain samples for testing or to monitor the water levels. The groundwater samples are recovered using either a dedicated disposable hand held bailer or a mechanical down-hole submersible pump.

Gas monitoring is undertaken to determine the levels of methane, carbon dioxide, oxygen and other gases which can be generated by made ground or backfilled materials. The gas flow within the borehole is dependent on the atmospheric pressure at the time the readings are taken.

Permeability Testing

Variable (falling and rising) head tests can be undertaken in boreholes or through standpipes / piezometers.

Packer permeability testing can be undertaken in boreholes using pneumatic packers.

Soakaway (infiltration) tests in Trial Pits can also be undertaken in accordance with BRE Digest 365. It should be noted that all permeability testing is time dependant and suitable allowance provided for testing in low permeability soils and rocks.