Ground Investigations - How Soil Tests Inform Foundation Design and Can save You Thousands of Pounds!

Ensuring a solid foundation is essential for any construction project, whether it’s your dream home, home extension or commercial project. This is never truer than in the fens when we can encounter a wide range of soil types from clay, silt, gravel, sand, peat etc.

This is where soil testing and ground investigations come into their own and are used to inform the foundations required for a project. In most instances, and certainly within Fenland, soil tests are required to demonstrate compliance with the Building Regulations, for every plot due to the varying soil conditions.

Our Approach

We pride ourselves on providing an in-depth science driven approach to soil testing, and unlike many others, ensure that our comprehensive approach includes moisture content and Atterberg limits as well as soil composition, ground water level and bearing capacities. Here is how approach stands out:

• 3m deep trial hole sampling – these holes are excavated by a JCB to 3m depth to assess the differing soil layers, providing insight to the ground composition and identify if peat is present and the level of any ground water.

• Hand Auger - If JCB access is not possible, or site constraints dictate otherwise, we can undertake 3m deep hand augers
  

• Shear Vane Readings – at relevant depths we measure soil resistance to shear forces using a shear vane meter to provide us with data on the safe load bearing capacity of the ground
  

• Plasticity – we take samples of the soil for laboratory testing. This determines the plasticity index and how it reacts to moisture fluctuations and the influence of trees and vegetation. Shrinkable soils, such as clay, can have significant volume changes as a result of moisture and nearby vegetation which can result in ground movement and affect the foundations of a building.
  

• Site Features – as part of the soil test we identify surrounding site features such as ditches, trees, hedges and consider their likely influence on the foundations based on the data gather from testing the soil 

 We collate this information in a detailed, but plain English report that defines the soil characteristics and recommends appropriate foundation solutions allowing detailed engineered solutions to be adopted if required.

Benefits of soil reports - £30k plus saving – an Example

We were approached by our client with regards to a single chalet bungalow dwelling in Chatteris. The previous structural engineers for the project had designed a pile a ground beam solution with no apparent soil testing occurring, rather assumptions on the ground conditions, bearing capacity and plasticity. The quotes for this work came in at over £50k and we were asked to review the designs and help the client.

Our first suggestion was to undertake a soil test so that we could better understand the soil conditions of the site. During our site visit we note the presence of surrounding hedgerow and trees as well as previously removed trees from the site.

The soil was identified as clayey silt changing to clayey sand as identified in the trial pit log. We took samples for testing and undertook a shear vane meter reading. The test results confirmed low shrinakbility and with this combined information and taking in to account the trees (present and removed) we were able to advise on a stepped strip foundation arrangement.

As part of our appointment we prepared a detailed foundation drawing and soil test report that was submitted and approved by Building Control.

The outcome of this was a traditional foundation arrangement that meets all Building Regulation standards and the client confirmed has saved them in excess of £30k in foundation costs – all as a result of undertaking a detailed soil test completed within weeks from client instruction.

Conclusion

Adequate soil testing at the earliest point of a project is crucial in determining the most appropriate foundation solution to ensure stability and longevity of your property. In addition, gathering the appropriate evidence at this stage will allow more detailed and informed decisions to be made and can result in significant savings of tens of thousands of pounds before you are out of the ground.

If you have a project where you may require a soil test, or would like any structural advice, please get in touch, we would be more than happy to help.

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