conducting tests in cycles of loading and unloading. Hence, the behaviour of loading and unloading curves in last 2 cycles is almost similar.
Surface measurement
Plate loading test (PLT), and
Flat jack test (FJT), Large scale testing
Plate jacking test (PJT), and
Radial jack test (RJT), Drill hole testing
Goodman jack test (GJT), and
Dilatometer test (DT). Indirect methods
Rock Mass Quality (Q) by Barton et al. [17, 18]
Rock Mass Rating (RMR) by Beiniawski [14]
Geological strength index (GSI) by Hoek and Brown [19]
Rock mass index (RMi) by by Palmstrom [20]
Based on the site conditions and availability of resources at project for test site preparation, the different methods are utilised to evaluate the modulus of deformation of rock mass. It is always suggested to utilise at least two methods in the field in addition to indirect methods. There are following reasons to conduct the deformability tests in five cycles:
Construction activities in rock mechanics for dam and underground structures are done in stages/cycles. Dam loading is applied on rock mass in stages and it takes some time due to construction to apply full loading on rock mass. Similarly, large underground structures are also excavated in stages.
Rock mass in drift is disturbed due to blasting.
Joints get closed by doing testing in cycles (opening of joint due to stress release during excavation of drift by drill and blast method).
Equipment gets adjusted during loading and unloading in first cycle. It is, therefore, suggested to repeat the first cycle second time to calculate modulus value at low stress or first cycle may be avoided for calculating modulus of deformation. In-situ rock mass conditions are created by
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Number 4 December 2015
Final value of modulus of deformation is obtained for in-situ rock mass from last loading/ unloading cycle. Hence, it is very important to decide the final loading in all deformability tests.
In view of above justifications, the deformability tests are conducted by using 5 cycles of loading and unloading to determine the deformability characteristics of rock mass. The final stresses expected to be transferred by structures to the rock mass must be made available much before the start of in-situ testing. The structures may be dam to be founded on rock mass and stresses to be transferred to the rock mass foundation are due to self weight and filling of reservoir including dynamic loading during earthquake. It should also include the effects of loading and unloading during operation of reservoir. The stresses may be due to unloading during excavation of tunnel and its loading during filling of hydraulic tunnels. The maximum stress may be applied equal to 1.5 to 2 times the stress due to structure. It may further be divided into five cycles of loading and unloading with application of maximum stress in fifth cycle as per ISRM [16]. The behaviour of rock mass is almost similar during fourth and fifth cycle of loading and unloading (Fig. 1). The modulus of deformation is determined based on loading in fifth cycle. The variation in modulus of deformation with applied stress is given in Table 1. In the case of Trap rock mass, the modulus of deformation increases from 2.49 GPa to 6.75 GPa as the applied stress increases from 10 MPa to 60 MPa. It is confirmed from Table 1 that the modulus value increases with the increase in applied stress during testing. Hence, it is very important to select applied stress properly during testing. Table 1: Applied stress and modulus of deformation Rock type
Applied stress MPa
Trap
10 20 40 60
Modulus of deformation GPa 2.49 3.69 5.47 6.75
The Bridge and Structural Engineer