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In unweathered rocks, RQD indicates the joint intensity f whereasin weathered rock it gives a measure of the amount of compressible materialbut no indication of the infill compressibility.Ttfhere only tight clean joints are presents the correlation betweenRQD and allowable bearing pressure proposed by Peck et al (1974) 9 given inTable 6, may be used.Table 6. Allowable Bearing Pressure on Jointed Rock(Peck, H'anson £ Thornburn, 1974)RQD(%)Allowable Pressure(kPa)100 3000090 2000075 1200050 650025 30000 1000Note :(1) Use allowable pressure orunconfined compressive strength ofintact rock, whichever is less.(2) RQD is for rock in the zoneof influence of the foundation.For infilled joints deformation will be larger, end estimates ofthe joint infill compressibility may be required* The effect of joint infillingon allowable bearing pressure for a limited range of joint spacing and thicknessis given in the Canadian Foundation Manual (Canadian Geotechnical Society 9 1978),6.8 SLOPE FAILURE IN SURR0UWPING SOILThe overall stability of the ground surrounding the retaining wallshould be investigated, and calculations should be carried out on the fullrange of potential failure surfaces to ensure that an adequate factor of safetyagainst overall slope failure is maintained. The calculations should includethe influence of the surcharge from the wall on the slope. The minimum factorof safety required at a site is dependent on its hazard potential.Reference should be made to Chapter 5 of the Geotechnical Manual forSlopes, where detailed guidance is given on the Risk Category of a slope andthe minimum factor of safety required. The factor of safety should bedetermined for groundwater conditions associated with a 10 year return periodrainfall. Chapter 5 of the Geotechnical Manual also gives guidance on methodsthat may be used for carrying out the analysis.