Clay shrinkage subsidence, exacerbated by tree root influence, had caused significant subsidence damage to two neighbouring blocks of flats in Oxford. It is considered the damage commenced during the severe summer drought of 2018, with further movement during the dry summer of 2019. The damage was considered progressive and seasonal.
Site Investigations were instigated, confirming both desiccated clay and the presence of tree roots to around 3.1m below ground level. Concrete strip foundations founded at 1.2m below ground were confirmed to be 0f 0.7m thickness.
The damage was localised, affecting corners of both blocks of flats, with typical subsidence cracking to internal and external masonry walls. Additionally, an internal section of the floor slab had dropped.
Oxford City Council fully considered the most viable and economic solutions available to arrest the movement, allowing superstructure repairs to commence. The solution required speed of design, a lack of disruption for the multiple tenants, short on-site duration and economic benefits.
Potential schemes utilising traditional underpinning or piling techniques would involve significant excavations and disruption, lengthy on-site duration, and added overall cost. With around 35 linear meters of foundations requiring stabilisation, outline costs for underpinning or piling were in the region of £65,000, excluding requirements for alternative accommodation.
Geobear presented a fully engineered scheme and quotation within 2 weeks of instruction. The design allowed for a total of 5 days on-site, with minimal enabling work, and no requirement to decant any tenants. Site Investigations, drain surveys and enabling work were all arranged by Geobear as part of the project.
The non-disruptive solution did not require any excavation or landscaping work on completion.
Across the two blocks, the design included stabilisation of 34 linear meters of strip foundations and stabilisation/lift of the damaged floor slab.
Enabling works were limited to some local exposing of services to enable safe drilling and temporary relocation of an ornamental planter.
Specialist clay geopolymers were injected into the ground at specified
horizontal and vertical spacings, to precise volumes. Laser levels were used to closely monitor all injection sequences, to ensure geopolymer filled all voids and fissures and met the underside of foundations. The geopolymer, specifically designed for clay soils increases the load-bearing strength of the soil and eliminates the harmful effects of shrink / swell seasonal movement.
During the drilling and injection works, drainage monitoring was carried out to ensure there was no damage to drain runs or material ingress.
During the works, unforeseen soil conditions were discovered, with localised pockets of solid lime interspersed within the clay. Geobear worked up design revisions on-site, liaising closely with the client throughout. Neither the programme of work nor the overall cost were affected.