MINING INSTABILITY

Area:  Existing Residential Property

Scope:  Ground Instability

Services:  Geotechnical Consultants

At the time of the ASL inspection Heras fencing had been placed around the hole in the patio to restrict access due to the unstable ground surrounding the hole.  The hole in the tarmac surfacing appeared to have increased and the hole was approximately 4.3m across at the base, resulting in the shallow soils being in a severely overhanging state.  There was evidence of damp/wet ground around the perimeter of the hole at the bottom.  Around the top of the hole the tarmac was unsupported and showing signs of distress and was sinking in towards the hole.  The material evident in the sides of the hole appeared to comprise sandy clay overlying mudstone and numerous small cracks were evident around the insides of the hole.

Due to the presence of significant loose soil debris in the hole it is recommended that these materials are removed to enable a detailed inspection to be undertaken to determine the cause of the collapse and allow a suitable reinstatement measures to be determined.

The site comprises a two storey brick built house with a parking area to the front and a tarmac patio and garden to the rear.

In May 2013 a sudden collapse of ground occurred in the rear patio area resulting in a hole approximately 2m across and 2.5m deep with soil debris and water in the bottom of the hole.  Further loss of ground continued over a period of days with the size of the hole increasing at surface and the water seeping away to leave a hole filled with soil debris.

ASL were requested to attend site and provide an assessment of the likely cause of the collapse, the on-going stability of the hole, the likely impact of the hole on the building and provide recommendations relating to the reinstatement of the hole and patio area.

The soil debris was removed from the hole using a tracked excavator and supervised by ASL.  A concrete slab was encountered at the base of the hole which was lifted to enable inspection of the ground beneath to check for loose ground or voids.  The ground beneath the slab was initially soft and wet although further excavation revealed competent natural clay materials at a depth of approximately 3m.  All loose and soft materials were trimmed from the sides of the excavation and removed from the hole.

It is considered that the purpose of the slab may have been the base for a tank or an underground shelter which had been removed but not backfilled.

As there was no evidence of any Made Ground, soft or loose materials or voiding in either the base or sides of the excavation the excavation was backfilled with concrete.  On completion of backfilling the hole has been suitably reinstated and did not present any further risks to the site.

ASL, acting as the Geotechnical Consultant, supervised the backfilling and provided a report confirming that the hole had had been suitability backfilled and present no further risks to the users of the care home.