Specialists in Soil and Water Sampling, Testing and Monitoring

Dynamic probing along proposed foundation lines checking to see if there are any weak zones, in this case swallow holes (Canterbury)


The Standard Penetration Test (SPT) is the most widely used intest in the UK. The test involvedriving a standard 50mm OD thick-walled sampler into soil at the bottom of a borehole, using repeated blows of a 63.5kg drop hammer. The SPT 'N' value is the number of blows required to achieve a penetration of 300mm after an initial seating drive of 150mm. The SPT is commonly used to obtain parameters for input into routine geotechnical design calculations. A wide range of parameters, for almost all soil and weak rock types can be obtained with ease and convenience and low cost. Geotechnical Services present SPT results on graphical depth profiles,

This method of exploration involves driving a rod with an oversize point or cone at its base into the ground by means of a sliding hammer. The hammer has a mass of 50/63.5kg and falls 500/750mm onto a steel anvil that drives the rods into the ground. The number of hammer blows required to drive the point 100mm into the ground is recorded (N100) and the results presented on a plot of Blow Count (N100) against Depth. At every metre increment, a new rod is attached to the "string" and a torque reading taken to measure the friction between the soil and the "string".
Dynamic probing is ideally suited to locating boundaries between materials with different density/resistance to driving. Correlations between Dynamic Probe results and other engineering parameters such as Standard Penetration Test (SPT) 'N' values and undrained shear strength have also been published.
Dynamic probing has an established track record in the investigation of sub-surface "soft spots", "hard spots" and voids. These include dissolution features in the chalk, old foundations and buried objects such as sewers, mine workings for example bell pits, and fissures associated with cambering or valley bulging, for example in the Hythe Beds in Kent.