What is a standard penetration test or SPT?

A standard penetration test, abbreviated to SPT, is used when carrying out ground site geotechnical investigation works and the results are used for treating subsidence. It uses a device called a split-spoon sampler attached to the end of a metal rod.

This is driven into the ground by dropping a hammer weighing 63.5kg from a height of 76cm on to an anvil attached to the end of the drive rod. When the sampler has penetrated 150mm into the ground, the number of blows is recorded, and this is called the ‘seating drive’. The process is repeated until another 300mm of penetration has been achieved. The blow-count required to penetrate the last 300mm is known as the standard penetration resistance or N-value. It is important that the same weight is used for pre and post treatment investigation testing and when comparing tests carried out by different parties on the same site, as different weights will give different blow counts and a false impression of ground strength. The sample obtained during the SPT is used for lab analysis to determine the geological context and behavioural type of the material.

This method of testing is similar an SPT; however, it does not sample the soil and instead uses a cone on the end of the metal rod for the in-situ determination of the resistance of soils and soft rocks. For this reason, it is commonly called Dynamic Cone Penetrometer (DCP) test.

As with SPT, a hammer of a given mass and falling height is used to drive the cone. The penetration resistance is defined as the number of blows required to drive the penetrometer over a defined distance. A continuous record is provided with respect to depth, but no samples are recovered.

The DCP method differs from a standard penetration test; however, as the N-count is taken at 100mm increments, the N-count from an SPT and a DCP are not directly comparable. Methods are available to correlate between the results however, as the penetration of a cone will be different to that of the spoon-sampler of the standard penetration test, such correlations will always be subject to variability and should be used with caution.

Before subsidence treatment work begins, a test is conducted on the soil under a subsidence affected building/structure and the initial bearing capacity is determined. The test is repeated post-treatment works to verify the specified bearing capacity value has been achieved.

Mainmark have a long-standing expertise in ground investigation works and use SPT and DCP test results to design value engineered solutions giving the required depth of treatment, number of injection points with depth and the required quantity of Teretek® geopolymer resin. Such solutions have been utilised in a wide variety of residential and commercial buildings, and infrastructure, such as roads, railways and bridges. For a DCP test we use a 30kg weight for both pre and post testing.

When a listed building, comprising of a basement plus first and second floor, situated close to a busy city street in Chester, began suffering from subsidence, the client was looking for a non-intrusive and controlled way to improve the load bearing capacity of the ground beneath this iconic local building.

Mainmark carried out pre-treatment DCP tests and used the results to work closely with the client to come up with a detailed project plan. This mapped out the precise injection points and depths of Teretek® geopolymer resin required to support the existing floors and any foundations beneath the perimeter and separating walls of the building. The tests were repeated after treatment and confirmed, as you can see by comparing the blue pre-treatment result with the red post-treatment result on the graph below, that ground strengthening had been achieved to arrest the subsidence and, although it was not a requirement of the project, lift was achieved in some areas.

You can read more about the use of resin injection in this project here. If you require assistance with subsidence related issues affecting your project or require a survey, please contact our team of experts.