Numerical Simulation of Direct Test for Rock
The shear behaviour of rock joint is a combination of complex phenomena such as normal dilation, asperity shearing or sliding, contact area undergoing shearing or sliding and stiffness of the surrounding rock mass. Hence, a constitutive model for joint behaviour has to consider a large number of assumptions and uncertainties. Considerable efforts have been made in the past by various researchers to explain the shear behaviour of rock joint. Based on their laboratory, analytical and numerical studies various shear strength models have been proposed by them. In the present study numerical modeling is done for simulating the experimental conditions. Numerical study is performed based on the Distinct Element Method (DEM) and using the Universal Distinct Element Code (UDEC) developed by ITASCA Consulting Group Inc. Direct shear test were carried out on numerically simulated rock joint using UDEC. The shear behavior of rock joint examined with asperity like 15-15 and 30-30 at different normal stress. The result of the laboratory shear test performed on the sample with asperity 15-15 and 30-30 at different normal stress is compared with the shear behaviour and failure process of numerically simulated rock joint. Indian Geotechnical Conference – 2010, GEOtrendz December 16–18, 2010 IGS Mumbai Chapter & IIT Bombay
DEVELOPMENT OF AN AUTOMATED LARGE SCALE DIRECT SHEAR TESTING MACHINE FOR ROCK
The shear behaviour of planar rock joints can be investigated in the laboratory by using a conventional direct shear apparatus where the normal load is kept constant (CNL) during the shearing process. However, shearing of non planar rock joint where dilation is restricted by the surrounding rock mass does not take place under Constant Normal Load (CNL) but rather under variable normal load where stiffness is constant called Constant Normal Stiffness (CNS) boundary condition. The shear strength of non planar discontinuities calculated under CNS boundary condition will be significantly higher than what is calculated under CNL boundary condition. In the past most of the researchers have carried out the test under Constant Normal Load (CNL) boundary condition. The equipment developed in the past for CNS boundary condition was either having difficulty in changing the boundary conditions or it was not useful for wide variety of rock joints. An automated servo controlled large scale direct shear testing machine on rock has been developed and fabricated to find the shear characteristics of rock under constant normal load and Constant normal stiffness boundary conditions at IIT Delhi. The Boundary condition can easily be changed by changing the input data in direct shear software. The Normal and shear load is applied through an electro hydraulic servo actuator unit which works on closed loop principle. The displacements are measured by LVDT’s mounted on the specimen. The data acquisition system has 16 channels, 2 channels for load cell, 6 channels for LVDTs and remaining 8 channels are free for additional input. The data acquisition system converts the mechanical and electrical signals in to the digital data. The output of signal is connected to CPU via cord. The load and deformation values are stored at desired intervals as note pad data. The direct shear software developed is having the facility to collect data and plot online graphs. Using developed equipment shear tests have been conducted under CNL and CNS boundary conditions, on the modeled rock joint having asperity angle 0°, 15–15°, 15–30°, 30–15°, 30–30° and 30–60° at different normal stress. The test results of only modeled rock joint with asperity angle 15–30°, at different normal stress under both CNL and CNS boundary conditions are discussed in the study due to space constraint.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.