Housing Development

Essington, Wolverhampton

This housing site contained a former opencast mine infilled in the 1980’s, with resulting made ground deposits in excess of 20mbgl. We had previously installed vibro stone columns on earlier phases of the site outside of the opencast area. For the opencast area, driven steel piling was considered as the foundation solution, with the proposed piles to be driven through the full depth of the opencast made ground soils, terminating in the bedrock beneath.

As the soils had been in-situ for 30-40 years, any self-weight consolidation of these materials would have ceased. Additionally, due to the significant depth of these soils, the lower materials were compacted to very high densities due to being effectively surcharged by the soils above, so much so that it was difficult to distinguish between these soils and the natural soils on site. As the top 5-8m of the ground was less compacted, we proposed to solely treat these soils with Vibro Stone Columns.
We proposed flat-bottomed raft foundations to compensate for partial depth treatment which ensured the house foundation would act as a single entity with resulting settlements being uniform beneath the entire plot footprint.

Accounting for the identified ground conditions, we undertook preliminary settlement design calculations to ensure settlements were within acceptable tolerances, whilst providing the required blanket bearing capacity of 50kN/m² beneath the entire footprint. In addition, our columns were spaced on a tight grid pattern to ensure uniform treatment beneath each plot.

The works were carried out over three visits between summer 2022 and early 2023, totalling just over 40 rig days on site installing more than 2200 columns to maximum depths of 8mbgl. All the columns were taken to either a ‘refusal’ depth in the denser made ground soils, or to our maximum treatment depth of 8m. Testing of our works was carried out in accordance with NHBC standards, using both plate load tests and dummy foundation tests, to assess both the quality of the Vibro Stone Columns and the settlement between the columns respectively.