Assessment of deformation modulus of weak rock masses from pressuremeter tests and seismic surveys

The deformation modulus of intact rock can be determined through standardized laboratory tests for heavily jointed rock masses but this is very difficult, while in situ tests are time-consuming and expensive. In this study, the deformation modulus of selected heavily jointed, sheared and/or blocky, weathered, weak greywacke, andesite and claystone were assessed, based on pressuremeter tests, geo-engineering characterization and seismic surveys. Empirical equations based on GSI and RMR values are proposed to indirectly estimate the deformation modulus of the greywackes. For the andesites, the spacing of the discontinuities is greater than the length of the pressuremeter probe hence the intact rather than rock mass deformation modulus is obtained. The pressuremeter test results from the claystones could not be correlated with the field data; the relationship between the ratio of rock mass modulus to intact rock modulus and RQD appears to give a better estimation of the deformation modulus

Deformation modulus of heavily jointed-sheared and blocky greywackes by pressuremeter tests: Numerical, experimental and empirical assessments

Deformability of rock masses influencing their behavior is an important geomechanical property for the design of rock structures. Due to the difficulties in determining the deformability of jointed rock masses at the laboratory-scale, various in-situ test methods such as pressuremeter, dilatometer, plate loading tests etc. have been developed. Although these techniques are currently the best and direct methods, they are time-consuming and expensive, and present operational difficulties. In addition, the influence of the test volume on deformation modulus depending on the method employed is also important. For these reasons empirical equations to indirectly estimate the deformation modulus have also been recommended by several investigators as an alternative approach. In this study; the geomechanical quality of weak, heavily jointed, sheared and/or blocky greywacke rock masses, on which very concentrated civil works are continuing at the southern and southwestern parts of Ankara (Turkey), was assessed. The deformation modulus was determined by pressuremeter tests, the possible effects of variables on the derived deformation modulus from the pressuremeter test were evaluated by numerical methods, and the comparisons between the deformation modulus of the greywackes obtained from the pressuremeter tests and their geomechanical quality (GSI and RMR) were made. Numerical simulations revealed that the presence of a disturbed annulus around the borehole causes underestimation of the deformation modulus, while the effect of length to diameter ratio of the pressuremeter probe on the deformation modulus is minor. Based on the geoengineering characterization assessments, mainly two greywacke rock masses with different geomechanical qualities were identified. Geotechnical quality of one of these rock masses was verified by the back analysis of two slope failures. The empirical equations to indirectly estimate the deformation modulus of the greywackes using their GSI and RMR values yielded high coefficients of correlation

Comparison of undrained shear strength by pressuremeter and other tests, and numerical assessment of the effect of finite probe length in pressuremeter tests

There is a wide body of research reported in the literature which compare the undrained shear strength determined from various field and laboratory tests. The pressuremeter testing equipment and the related design techniques have been continuously refined. However, the undrained shear strength (S (u)) determined from the pressuremeter test (PMT) is generally higher than that obtained from other field or laboratory tests. In this study, numerical methods to consider the effects of length to diameter ratio (L/D) of the pressuremeter probe, and the procedural implications of soil disturbance and testing depth on the undrained shear strength determined from the PMT were investigated and possible correction factors depending on L/D ratio were established. In addition, in order to compare the variations in undrained shear strength of a clayey material with depth and testing method, a lightly overconsolidated and highly plastic clay, called "Eymir Lake clay" near Ankara (Turkey), was selected as the material of the study, and its values of undrained shear strength (S (u)), which were determined by PMT, field vane shear (FVT), conic penetration test (CPT), and laboratory tests, were compared. Based on the numerical analysis results, the correction factors depending on the L/D ratio of the conventional probes were suggested. The correction factors ranged from 0.83 to 0.96 for L/D ratios of 5.3 to 11, respectively, and they were determined to be independent of the rigidity values ranging from 25 to 200. It was also shown that overestimation of S (u) is independent of depth. In addition, due to very low permeability values of the Eymir Lake clay, it is concluded that for soils with coefficients of permeability lower than 10(-10) m/s, partial drainage around the pressuremeter probe is unlikely. Based on the comparison among the results from laboratory and different field tests, the values of S (u) determined from the theoretical Palmer's solution are higher than those from CPT, FVT, and CU tests. If the correction for L/D ratio is applied to the values of S (u) determined from the Palmer's solution, S (u) from pressuremeter approaches S (u) obtained from FVT; however, there is still a slight overestimation in S (u) obtained from PMT. This overestimation is probably due to the differences in the mode of failure and the presence of a disturbed zone around the pressuremeter probe. However, S (u) determined from the PMT by empirical methods is close to those determined from CPT, FVT, and CU tests