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SUMMARY: As a result of slope stability problems encountered around the perimeter of a peninsula extending into a deep excavation in rock, stabilisation measures became necessary. Stability analyses undertaken led to construction of an anchored retaining wall around the peninsula perimeter. The performance of test anchors and working anchors is examined.
RESUME: Par suite des problèmes de stabilite de la pente que nous avons rencontres autour de perimètre de la peninsule qui s'etend dans une excavation profonde dans le rocher, il devint necessaire de prendre des mesures de stabilisation. L'analyse de stabilite que nous avons faite, nous a amene à construire un mur de soutènement ancre autour du mur du perimètre de la peninsule. On a examine le bon fonctionnement des ancres à l'essai et de celles en service.
ZUSAMMENFASSUNG: Ausgangspunkt waren Probleme in der Abhangsstabilitat, die in Verbindung mit dem Perimeter einer Halbinsel mit tiefen felsexkavation auftraten, Stabilitatsanalysen fuehrten zur konstruktion einer verankerten Stuetzmauer um den Halbinsel - Perimeter. Die Leistung der Versuchsanker und dann der Arbeitsanker wird untersucht.
1. INTRODUCTION The site of the giant Sasol II oil-from coal complex is situated in the south-eastern Transvaal. The residual soils in this area are underlain by sedimentary rocks of the Ecca Group of Permian age, which have been subjected to extensive intrusion by igneous rocks primarily during Jurassic times. Rock slope stabilisation operations required in a deep excavation situated in the Ash Dewatering Area of the plant have provided information which is relevant to the implementation of effective stabilisation measures for rock slopes in general engineering practice.
2. DESIGN AND CONSTRUCTION OF ASH DEWATERING EXCAVATION The 9 m deep Ash Dewatering excavation extends over an area measuring some 200 m by 150 m as shown in Figure 1. However, from the northern side of the excavation there projects into the excavated area a peninsula of plan dimensions 100 m by 60 m, which supports two 32 m diameter reinforced concrete thickener tanks. An Emergency Bypass Pond is provided by a smaller excavation extending to a depth of 2,5 m below the base of the main excavation. The long term structural integrity of the thickener tanks is essential to the functioning of the entire plant. On the basis of the results of exploratory borings and with reference to experience with similar' subsurface profiles elsewhere in the Republic, the design of the Ash Dewatering excavation was based on a side slope angle of 2 vertical to 1 horizontal through the weathered rocks and a side slope angle of 1 vertical to 2 horizontal through the very highly weathered rock and residual soils situated within 1 m of natural ground level. However, where carbonaceous shale bedrock was to be exposed near the base of the main excavation, it was considered that a significant danger of retrogressive weathering in this material existed which could result in failure of the overlying material. Accordingly it was recommended that mortar, pneumatically applied over light reinforcement, be applied to the carbonaceous shale material immediately after excavation of the face so as to prevent deterioration upon exposure. Weep holes were to be left in the facing at regular intervals to allow dissipation of any water pressures which might otherwise develop behind the facing. However, it was considered advisable that the site be kept well drained and in particular that the ingress of surface water behind the crests of cut slopes be prevented. Excavation work in the Ash Dewatering Area commenced in July, 1976 and was completed in December, 1976. Little difficulty was experienced by the earthworks contractor in forming the deep excavation, though work was delayed on a number of occasions during the wet southern summer months when heavy rains resulted in the partially completed excavation becoming flooded due to ingress of storm water. However, shortly after the excavation works had been completed, evidence of distress in the rock slope faces comprising the southern boundaries of the Thickener Tank Peninsula was observed and in January, 1977 a limited rock slope failure took place at the south-eastern corner of the Thickener Tank Peninsula. As a result of this occurrence, and in view of the importance of the continuing stability of the Thickener Tank Peninsula to the future plant operation, a decision was taken to implement a detailed geotechnical investigation and analysis of the problem with a view to identifying the form and scope of the rock slope stabilisation measures required to ensure long term stability of rock slopes in the Ash Dewatering Excavation in general and around the Thickener Tank Peninsula in particular.
3. NATURE AND SCOPE OF ROCK SLOPE INSTABILITY The subsurface profile exposed in the vicinity of the Thickener Tank Peninsula of the Ash Dewatering Excavation comprises aIm thick layer of stiff silty clay which is underlain by 3,5 m of moderately weak, highly jointed dolerite.
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