Monitoring of surface levels and borehole inclinometers at the Geldenhuis Interchange, an area undermined at a depth of approximately 450m, has indicated that differential movements are occurring in the rock mass across a dyke contact. A zone of complex block adjustment appears to exist within a 5m to 10m wide zone across the contact. Surface monitoring at the Cleveland site has revealed that subsidence at that location has been continuing for a period of at least 10 years. These two case studies illustrate that mining at considerable depths below surface can cause surface subsidence.
The Geldenhuis Interchange, located approximately 10 km to the east of Johannesburg at the Intersection of the N3 and M2 motorways (Fig. 1),is underlain by the workings of the Geldenhuis Deep Mine at a depth of about 450 metres.
As a result of damage to one of the bridge structures (Bridge B77) at the interchange, observed by Messrs Scott and De Waal Inc. during a programme of widening and rehabilitation of the N3 Section 12(1) and considered possibly to be related to undermining, Investigations were carried out to assist with the design of a new bridge. These investigations, carried out by Messrs Steffen, Robertson and Kirsten Inc, included the monitoring of Inclinometers installed in 6 cored boreholes, and the monitoring of surface levels at and in the Immediate vicinity of the structure.
Approximately 1 km west of the Geldenhuis Interchange, sudden subsidence occurred some 10 years ago. This area of subsidence, referred to as the Cleveland site, is also underlain by workings of the Geldenhuis Deep Mine, but at a shallower depth of 200 m to 250 m. The surface area affected was estimated to be 30 000 m2. The subsidence event has been described previously by Stacey and Rauch(2), and the present paper contains an update on the vertical displacements Which have taken place over the past 10 years.
The area in the vicinity of the Geldenhuis Interchange is underlain by strata of the Johannesburg Subgroup of the Witwatersrand Supergroup. The strata dip southwards and are frequently intersected by vertical or near vertical diabase intrusions with variable strike orientations. The site is underlain by gold bearing reef horizons of the Main Reef Group which have been partially mined out. Past mining activities concentrated on three separate reefs; the South Reef, the Main Reef Leader, and the Main Reef. These reef horizons dip to the south at angles of 25° to 30° The extents of stoping on the reef horizons and the underground positions of the dykes are shown on Figs 2, 3 and 4.
Several dykes traverse the area in the vicinity of the interchange. The most significant of these are:
A north-east, south-west striking dyke which underlies the interchange itself, and dips approximately 85° to the south-east.
An east-west striking dyke located approximately 100 metres to the south of the interchange. The dip of this dyke is unknown but is probably sub-vertical.