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.