Standard Penetration Test (SPT)

Standard Penetration Tests (SPT’s) are performed in boreholes at regular intervals during drilling operations in granular (sands/gravels) soils, stiff or stoney cohesive (clay/silt) soils, and weak rocks. As a standard practice the test is normally conducted at 1m depth increments to 5m depth and at 1.5m increments thereafter as the borehole is advanced.

 

Test results record the number of driving blows that are required to drive a standard split sampling barrel (or solid cone) 300mm below the base of the borehole; following an initial 150mm of seating blows. Blows for each 75mm driving increments are added together to derive a standard ‘N’ Value, which is related to the in-situ density and shear strength of the soils. The result can also be empirically related to a range of engineering design parameters and is commonly used for pile design purposes.

 

Borehole Vane Test

Vane tests are undertaken at the base of borehole in soft silt or clay soils, where it provides an indication of the peak and residual shear strength of the in-situ soils, without the effects of sampling disturbance.  The test is normally undertaken at specified intervals between undisturbed samples, when required to meet the Engineers scheduled design requirements.

 

The test is also used to calibrate the results of in-situ CPTU piezocone (Dutch cone) tests in unconsolidated deposits. This combined approach can provide a rapid and cost effective option for the investigation for large sites.