Even small projects get the 100% G&W attention to detail

by | Feb 7th 2024

Even small projects get the 100% G&W attention to detail

7th February 2024

Geotechnical

A client in Oxfordshire planning to build a new house on greenfield land saves time and money thanks to the re-classification of underlying chalk. This was made possible by allowing Ground & Water to follow up on questionable findings of the initial site investigation.

Our Client

Ground & Water was working with a local firm of architects and the house builder.

Preliminary-Assessment

The Initial Investigation involved drilling five windowless sampler boreholes to depths of between 3.95 – 5.45m bgl with Standard Penetration Tests at 1.00m intervals. Boreholes were terminated early due to the density of strata. Three Super Heavy Dynamic Probes were undertaken through the base of boreholes to a final depth of 5.00m bgl. A Clay-with-Flints Formation was noted to a maximum depth of 1.00m bgl with the Seaford Chalk Formation noted to full depth in the boreholes. Minimum foundation depths were initially advised to be at 1.10m bgl, within the Seaford Chalk Formation. This was to bypass the Topsoil/Clay-with-Flints, and then found on competent moisture stable soils.

 

Our Challenge

Having been tasked with investigating the ground conditions at a site in Oxfordshire to determine foundation requirements, depths and bearing capacity for the new residential dwelling; the initial investigation raised concerns about the bearing capacity of the bedrock underlying the site thanks to the disturbed nature of the samples recovered. Following a worst-case scenario would have jeopardised the project, so Ground & Water’s challenge was to investigate further to confirm the true bearing capacity of the chalk.

The Ground & Water Approach

Following the initial investigation and preliminary assessment it was decided however, that foundations constructed at the depths initially advised would have a bearing capacity of 100kN/m2. This was lower than the original design for the house and the whole project would have needed to be re-modelled and changed to accommodate for this. The reasoning for the low bearing capacity was because the only chalk identified was through windowless sampler boreholes.

The process of this drilling can cause chalk to be recovered as disturbed samples, not showing their natural form, meaning that the chalk must be assumed as a worse case scenario “Dm” chalk. “Dm” chalk is structureless, with the main soil type being a SILT. If this was the main soil type on-site, then the original worse case scenario 100kN/m2 would have been kept in place. However, it was advised that the bearing capacity could be safely raised to 200 – 225kN/m2 if the chalk could be inspected via trial holes – in situ. A second intrusive investigation was required and approved.

The return Investigation

 A return visit to the site was scheduled once the pit for the foundations had been dug. This was inspected, logged and sampled by a Ground & Water engineer. Thanks to the logging, and once the samples had come back from the lab to clarify, the chalk was classed as “Dc”. “Dc” chalk is also structureless, however the main soil type is a GRAVEL, rather than a silt. This type of chalk is therefore much stronger and has a better bearing capacity.

 

The Outcome

Once the site had been visited and the new description of the chalk confirmed, the bearing capacity could be safely raised to 225kN/m2. This meant that even a quick site visit to confirm the in-situ soils, saved the client time and money. This time and cost saving resulted from not needing to re-design the whole structure as originally planned, or even to design different types of foundations or widths to accommodate the original (low) bearing capacity log. A great outcome for our client which was down to investing in a second site visit and even more thorough investigation.

Ground & Water works on all sizes of project. Call us today to see how we can help with yours. Back to case studies

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