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Analysis of the flexural failure of an overhanging rock slab
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International Journal of Rock Mechanics & Mining Sciences 41 (2004) 475
SINOROCK2004 Paper 2B 24
Analysis of the flexural failure of an overhanging rock slab M. Cravero*, G. Iabichino CNR Istituto di Geologia Ambientale e GeoIngegneria, Torino-Italy, c/o Dip.to Georisorse e Territorio—Politecnico di Torino, C.so Duca degli Abruzzi n. 24, 10129 Torino, Italy
Abstract Flexural toppling is an instability process that occurs in natural or man made rock slopes with rock slabs or columns striking subparallel to the rock face and dipping antislope. The instability can be induced by tensile failure/propagation inside one or more columns. Previous contributions to the subject usually refer to the behaviour of a system of rock units, while this paper describes the failure of a portion of a single slab (LE10 m, HE9 m, hE0.8 m) of an overhanging face of a gneiss quarry. The failure developed both due to the breaking of the intact rock and to the linking of pre-existing joints. The mechanical parameters of the gneiss were laboratory tested, mostly for flexural strength, st; deformability and toughness KIc characterization, however, were used at slightly reduced values for the slab analysis. The slab, loaded by different load conditions, was firstly modelled as an inclined (a55 ) thin plate with two clamped edges. FEM models allow the deflection and the stress sf in the rock plate to be computed. Stress intensity factors KI were also evaluated, according to these results, by using a beam of equivalent length with a slit located at the beam clamped end. The LEFM approach was applied to the beam scheme through FEM models. If st is compared to the maximum sf or KIc to KI, an appraisal of the possibility of failure can be obtained for instance in terms of critical slit length lcr (Fig. 1). The results show that the ordinary load condition (self-weight) should satisfy slab stability, while small defects in the resisting section of the slab could be triggered by momentary, heavy load intensity (water U or seismic events S), and could eventually lead the slab to failure.
Keywords: Quarry slab failure; Mechanical parameters of gneiss; Thin plate; Beam; FEM model; LEFM analysis
0.2 h=0.8m
0.16 h=0.7m
lcr (m)
0.12
0.08
0.04
0 0
0.2
0.4
0.6
0.8
1
U (%)
Fig. 1. Slit length lcr inducing the critical stress intensity factor vs. water load U, for two different slab thicknesses. *Corresponding author. Tel.: +39-0115647615; fax: +39-0115647679. E-mail addresses: [email protected] (M. Cravero), [email protected] (G. Iabichino). For full length paper see CD-ROM attached. doi:10.1016/j.ijrmms.2003.12.146
International Journal of Rock Mechanics & Mining Sciences 41 (2004) 475
SINOROCK2004 Paper 2B 24
Analysis of the flexural failure of an overhanging rock slab M. Cravero*, G. Iabichino CNR Istituto di Geologia Ambientale e GeoIngegneria, Torino-Italy, c/o Dip.to Georisorse e Territorio—Politecnico di Torino, C.so Duca degli Abruzzi n. 24, 10129 Torino, Italy
Abstract Flexural toppling is an instability process that occurs in natural or man made rock slopes with rock slabs or columns striking subparallel to the rock face and dipping antislope. The instability can be induced by tensile failure/propagation inside one or more columns. Previous contributions to the subject usually refer to the behaviour of a system of rock units, while this paper describes the failure of a portion of a single slab (LE10 m, HE9 m, hE0.8 m) of an overhanging face of a gneiss quarry. The failure developed both due to the breaking of the intact rock and to the linking of pre-existing joints. The mechanical parameters of the gneiss were laboratory tested, mostly for flexural strength, st; deformability and toughness KIc characterization, however, were used at slightly reduced values for the slab analysis. The slab, loaded by different load conditions, was firstly modelled as an inclined (a55 ) thin plate with two clamped edges. FEM models allow the deflection and the stress sf in the rock plate to be computed. Stress intensity factors KI were also evaluated, according to these results, by using a beam of equivalent length with a slit located at the beam clamped end. The LEFM approach was applied to the beam scheme through FEM models. If st is compared to the maximum sf or KIc to KI, an appraisal of the possibility of failure can be obtained for instance in terms of critical slit length lcr (Fig. 1). The results show that the ordinary load condition (self-weight) should satisfy slab stability, while small defects in the resisting section of the slab could be triggered by momentary, heavy load intensity (water U or seismic events S), and could eventually lead the slab to failure.
Keywords: Quarry slab failure; Mechanical parameters of gneiss; Thin plate; Beam; FEM model; LEFM analysis
0.2 h=0.8m
0.16 h=0.7m
lcr (m)
0.12
0.08
0.04
0 0
0.2
0.4
0.6
0.8
1
U (%)
Fig. 1. Slit length lcr inducing the critical stress intensity factor vs. water load U, for two different slab thicknesses. *Corresponding author. Tel.: +39-0115647615; fax: +39-0115647679. E-mail addresses: [email protected] (M. Cravero), [email protected] (G. Iabichino). For full length paper see CD-ROM attached. doi:10.1016/j.ijrmms.2003.12.146