Modelling rock slope behaviour and evolution with reference to Northern Spain and Southern Jordan

Abstract

The geomorphological behaviour of steep jointed rock slopes has been studied using distinct element computer models. In order to model steep slopes effectively, methodologies need to be combined from the studies of environmental modellers, geomorphologists and engineers. The distinct element method is ideal for the study of the development of jointed rock masses, where the failure is controlled by the nature of the discontinuities. Theoretical modelling identified that block size is a key control affecting the deformation of rock masses. Deformation of rock masses with smaller block assemblages is greater than for rock masses composed of larger block sizes. This is due to the increased magnitude of joint normal closure. Catastrophic failure is less likely in slopes with smaller block sizes because the shear strength is greater in a closely jointed rock mass. These slopes are more likely to undergo gradual deformations. Block-size effects are also responsible for influencing the failure mechanism of rock masses. As block size decreases, the magnitude of block rotation increases and the failure mechanism changes from sliding to toppling. The effect of slope scale on the deformation properties of the rock masses has also been investigated. Two field locations, the Picos de Europa mountains, northern Spain and Wadi Rum, southern Jordan, have been chosen to provide a link between the theoretical modelling and classic rock landforms which are controlled by the discontinuity geometry. Given the sporadic and infrequent occurrence of failure events at the field sites, a computer modelling approach has been adopted to analyse slope behaviour. In the Picos de Europa, slope deformations are deep-seated, with sliding and toppling being the dominant modes of failure. Much of the slope deformation in these mountains is a result of post-glacial rock-slope deformation. The sandstone inselbergs of Jordan show a range of morphologies from rounded hills to vertical cliffs. The morphology of the inselbergs is related to the intact rock strength; stronger Red lshrin sandstone forms vertical slopes, whereas the weaker Disi sandstone forms rounded domes. Jointing in the area is sub-vertical with horizontal bedding and computer simulations have shown that toppling is the dominant mode of failure in these inselbergs. Comparison of computer model output suggests that different failure mechanisms have distinct failure signatures. Catastrophic, deep-seated failures are characterised by a long period of acceleration as the failure propagates through the rock mass and infinite velocity is reached. Non-catastrophic slope movements, such as self-stabilising topples, are characterised by short periods of acceleration followed by small creep movements at a constant velocity. Computer modelling has indicated that scale effects do exist in the modelled rock masses from the Picos de Europa and particularly Wadi Rum. In areas where jointing is constant, the relative block size of the rock mass decreases as slope scale increases. The greater numbers of blocks along with greater in situ stresses influence the failure of the slope. Cosmogenic dating was used to temporally constrain UDEC model output and provide a better understanding of rock slope failure mechanisms in the Picos de Europa and Wadi Rum. Dating indicated delayed paraglacial adjustment was the triggering mechanism for slope failure in the Picos de Europa, whereas failures in Wadi Rum appeared to be closely linked with wetter climatic conditions.