Topography rock mass strength and pore water pressure

Topography, rock mass strength and pore water pressure Jia-Jyun Dong, Yi-Ju Su and Chyi-Tyi Lee Graduate Institute of Applied Geology, National Central University, Jhongli, Taiwan Methodology - Slope performance curve Abstract Relief is a fundamental landscape reflecting the influence of uplift and erosion. Contrary to the traditional concept that the relief is dominated by incision, several researches indicate that the landscape-scale material strength play an important role on the landform process. However, it is difficult to obtain a representative strength parameters based on laboratory rock tests. Slope height and slope angle were frequently used to infer the strength of rock mass. In this research, a series of slope response curves will be proposed to constrain the rock mass strength. Non-linear Hoek-Brown failure criterion will be incorporated into the proposed model where the linear Mohr-Coulomb failure envelop seems oversimplified. Meanwhile, the influence of pore water pressure on the slope stability is considered. Consequently, the strength of rock mass could be inferred from the topography. Cases including stable and failed rock slopes with reported slope height and slope angle are used to validate the proposed model. The result shows that the strength parameter of rock mass could be reasonably inferred from the topography if the pore pressure can be evaluated. Rock mass classification RMR Hoek and Brown Failure Criterion GSI, mi=9, (σci)=35 MPa (Taheri and Tani, 2010) Back calculated GSI=30~40 Mohr-Coulomb Failure Criterion Real GSI FS=1 α=? Real GSI=28~33 Slope performance curve u GSI-based RMR-basedslope Measured slope performance anglesperformance and curves slope curves –heights dry slope wet u Importance of rock mass strength Bye, A. R. , Bell, F. G. , 2001. Stability assessment and slope design at Sandsloot open pit, South Africa, International Journal of Rock Mechanics & Mining Sciences, 38, 449 -466. Slope stability analysis u Verification of the slope performance curves Result(IV) (II) (I) Motivation Evert Hoek, 2000. Practical rock engineering Abstract number: EGU 2011 -1821; Session NH 3. 10/GM 6. 2 Evert Hoek, 2000. Practical rock engineering mi=5 Back calculated GSI=40 Real GSI=46~50 σci =250 MPa σci =35 MPa Result (III) u How to get representative rock mass strength? – Laboratory tests Scale effect – In situ tests Scale effect – Back analysis Feasible !! σci=3 MPa “Scale effect” of rock mass strength Back calculated Ru=0. 3~0. 6. mi=33 mi=9 Topography data from MOLA + rock mass classification system RMR l Wallrock: 50<RMR<65 mi=9, σci = 35 MPa l Interior deposits: 30<RMR<55 Schultz, 2002 Slope height vs. Slope angle of wallrock and interior deposits in Valles Marineris Conclusions Back calculated RMRs are significant lower than the field evaluated RMRs. Effect of pore pressure should be considered. Ru= u/σv=0 Ru=0. 3 l The strength of rock mass can be well constrained by the topography. l Pore pressure distributed in the rock slope is essential for back calculating the strength. l The influence of earthquake on the topography needs to be studied.