EAP provide Special Foundation Design after Subsidence Investigation.
This case involves a detached house that was constructed in 1971, founded on clay. The condition of the site and whether trees were removed prior to construction was not known. In 1976 crack damage and distortion was found to affect the building. Investigation carried out at the time concluded that the building was founded on 250mm diameter piles which were believed to be 5m long. Ground heave was active and the piles were inadequate to resist the ground heave.
Remedial work was not carried out to address foundation movement; however work was carried out to remedy crack damage apparent at the time, together with some brick reinforcement work.
In 2016, a survey carried out in connection with the house purchase concluded that the building was extremely affected by crack damage which was believed to be due to differential vertical movement that was currently active. As a result EAP were commissioned to investigate the cause of the damage.
Initially level and crack width monitoring was installed; this confirmed that vertical movement was active. A 119mm differential in level exists between front and rear right corners. To determine the cause of the movement ground investigation was commissioned.
A single borehole drilled in March 2017, provided U100 samples for laboratory testing which determined elevated soil suction at 3.3m in conjunction with increased cohesion between 3.5 and 5m and a deficit in moisture content. This it may be concluded that the ground retained a degree of desiccation at depths between 3.5 and 5m.
It would appear that further swelling of the ground was possible as the suctions continued to dissipate to the equilibrium conditions. Consequently, anticipating further heave, the remedial scheme design involved reinforced tension piles with sleeve to the upper section and anti heave protection below a reinforced concrete slab.