Insitu Testing

Insitu testing is any test that is conducted on or in the soil of a site, most commonly standard penetration tests (SPT) and cone penetration tests (CPT) but extends to cover testing from onsite hand shear vane testing to plate load testing.

Standard Penetration Tests

Standard penetration tests (SPTs) are conducted as a down borehole test making use of either a split-spoon sampler or a cone rod.  SPT testing can be done by making use of window sampling, cable percussion or rotary / sonic borehole rigs.

A trip hammer of a standardised weight (63.5 kg) is dropped from a standardised height (76 cm / 30 inches) to drive either the split-spoon or cone into the ground, from which the number of blows is recorded to drive the sampler or rod into the ground for 45 cm.  The first 15 cm of this drive is called the seating drive whilst the following 30 cm is used to calculate a ‘N-value’, if the N-value exceeds 50, the test is considered to have “refused”.

Interpreted results are used to determine the engineering properties of the soil.

Cone Penetration Tests

Cone penetration tests (CPT) require a cone penetrometer to be pushed into the ground using a cone penetration test rig at a standard rate whilst data is recorded at regular intervals during the penetration.

CPT rigs have the capability to change the measuring device within the testing instrumentation to piezocones for pore water pressures, magnetometer cone for UXO / ferrous objects, seismic cone for seismic testing, membrane interface probe for continuous gaseous sampling and monitoring with more cone varieties on offer for a range of testing requirements.

Dynamic probe

Dynamic probing (DP) is a method of in situ testing that allows for quick data gathering across a site without the need for sample collection. Dynamic probes work on the same principle as SPT’s and can be conducted from a standard window sample borehole rig, super heavy dynamic probes (DPSH) are the most popular type of dynamic probing test rigs.

The penetrative cone is attached to a 1.0 m section of rod with 100 mm intervals marked upon it. Depending on the chosen method, the drop height can be adjusted.  The amount of blows it takes for the rod to pass through each 100 mm increment is recorded and interpreted into an ‘N-Value’.

Handheld Dynamic Cone Penetrometer

A handheld version of dynamic probing can be conducted, known as a TRL DCP.  TRL (Transport Research Laboratory) DCP (Dynamic Cone Penetrometer), these tests involve dropping an 8 kg weight from a height of 575 mm, unlike SPTs and DPs, the mm/blow is recorded and subsequently converted into a Californian Bearing Ratio (% CBR). CBR’s are typically used for designing roads and pavements.

Plate Load Tests & Californian Bearing Ratio

Plate Load Testing (or Plate Bearing Testing) is an in-situ field test used to determine the ultimate bearing capacity of predominantly soils, but also rock and man-made materials such as concrete and macadam.

Plate Load Testing requires an imposed load to be applied through a plate of known diameter, typically 300 mm, 450 mm and 600 mm.  Load applied in kiloNewtons per metre squared (KN/m²) making use of a hydraulic jack is measured through UKAS calibrated dials measuring the deflection to 0.01 mm. the deflection of the plate measured in millimetres can identify the settlement within a soil with a known forced load whilst for more ridged materials a shear strength can be identified on the basis that the capable load applied is enough to shear the material.

In situ or laboratory Californian Bearing Ratio test can be conducted and is defined as the ratio of force per unit area that is required to penetrate a soil to a depth of approximately 1.25 mm/min.

Soil strength presented as %C BR worked from Plate Load Tests or Californian Bearing Ratio Tests identifies the ultimate bearing capacity of predominantly soils, but also rock and man-made materials such as concrete and macadam.

Hand Shear Vane

Hand shear vanes are designed to measure the undrained shear strength of cohesive soils using interchangeable vanes, chosen depending upon the expected strength of the soil.  Operation of hand shear vanes require the vanes to be pushed into soil approximately 5 cm and twisted.  Once the soil has sheared the torsion device within the hand shear vane will turn a dial which will present the maximum deflection of force as a value or into KPa depending on the vane type used.

Soil is assigned a strength in KPa can be used to determine engineering properties of the soil.