This paper presents the results of rock slope surface stability analyses using the application of Hoek and Bray (2004) and Block Theory of Goodman and Shi (1995) from 834 fracture orientation measurements of the ten rock slope surfaces along the road, cut through the volcanic rocks of Hon Ngang formation, around the Hon Ngang island, Kien Hai district, Kien Giang province, Vietnam. The analytical results have been determined, the average percentage of plane failure can occur on the total rock slope surfaces is the largest, namely the average percentage of plane failure is 18.41%, the wedge failure is 9.65% and the toppling failure is 9.98%. Because the road is excavated around the island, the direction of the rock slope surfaces is also varied accordingly. The analytical results of the rock slope surface stability in different directions of the road are determined: the average value of fractures in the direction N - S, dip to W for plane failure is 17.48%, wedge failure is 10.85%, and toppling failure is 5.65%; the average value of fractures in the direction N - S, dip to E for plane failure is 22.69%, wedge failure is 13.01% and toppling failure is 10.78%; the average value of fractures in the direction NWW - SEE, dip to NEN for plane failure is 21.41%, wedge failure is 10.17% and toppling failure is 11.72%; the average value of fractures in the direction E - W, dip to S for plane failure is 12.76%, wedge failure is 5.13% and toppling failure is 10.91%. Besides, the analytical results have also identified five rock slope surfaces which appear key blocks: HN-01 (key block 129o/24o); HN-04 (key block 070o/33o); HN-11 (key block 035o/70o); HN-13 (key block 079o/36o); HN-14 (key block 122o/35o). These results have also shown that the existence of key blocks on the rock slope surface in the N - S direction, dip to E at the survey locations HN-1, 13 and 14; and the rock slope surface in the NWW - SEE direction, dip to NEN at the survey locations HN-4 and 11. These results have important significance to support for protecting slope surface safety.
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