West Hill, South Croydon

The proposed development was to comprise the construction of a block of flats with the superstructure comprising 4 (central and north-eastern portion) and 3 (south-western portion) storeys.

Client: Deco Design & Build Ltd

Site Investigation Value: £5,300

Project Overview:

The proposed development was to comprise the construction of a block of flats with the superstructure comprising 4 (central and north-eastern portion) and 3 (south-western portion) storeys. The building included a full basement beneath its entire footprint with excavations ranging between ~2.2m and ~4.7m below original site levels at its western and eastern edges respectively.

The proposed development is to include areas of soft landscaping comprising communal open space.

Site Investigation Overview

The works comprised an intrusive ground investigation to assess the feasibility of a raft foundation scheme for the new building, as proposed by the Client and the design team. The original brief submitted to Soils Limited, comprised cable percussive drilling. However, considering the anticipated ground conditions (chalk) and following CIRIA574 guidance that emphasises the limited reliability of SPT “N” blow count data when deriving chalk grade for foundation design, Soils Limited further recommended trial pitting at the proposed basement slab level.

What Soils Limited Brought To The Table

The scope of the investigation comprised:

– 1No. cable percussive borehole to 20m;

– 3No. machine excavated trial pits;

– Geotechnical laboratory testing;

– Chemical laboratory testing.

The trial pitting investigation allowed chalk description and classification, including hardness, dry density, discontinuity spacing/aperture and engineering behaviour in full alignment with CIRIA574. This assisted Soils Limited to confidently validate the bearing capacity and stiffness values that were to be adopted by the design team for the proposed raft foundation.

Furthermore, the trial pitting revealed the presence of a potential in-filled solution feature, commonly found in the chalk, which could potentially lead to unacceptable settlements. Following discussion with the Client, it was estimated that site elevations were to be reduced further any potential in-filled solution features would be exposed and removed as part of the basement excavation. Soils Limited advised the Client accordingly, in order to ensure that visual inspections were undertaken at the basement formation-level by a qualified Geologist/Geotechnical Engineer to validate the aforementioned hypothesis.