Measurement of stiffness of rock from laboratory and field
Naeem O. Abdulhadi & Amjad F. Barghouthi
Arab Center for Engineering Studies (ACES)
This paper compares the deformation modulus of rock measured from laboratory and field tests which were carried out as part of the site investigation works for a major project in Irbid, Jordan. Laboratory resonant column and torsional shear tests were performed at different confining pressures whereas ultrasonic velocity tests were conducted on unconfined rock specimens. In addition, empirical relationships were used for estimating the rock mass modulus employing the results of the uniaxial compression and point load strength tests. Field measurements comprised pressuremeter testing as well as seismic geophysical methods including down-hole and crosshole techniques. The static and dynamic in-situ stiffness measurements were found to be reasonably in good agreement with the laboratory values from the dynamic tests as well as empirical methods for estimating rock mass stiffness from uniaxial compressive strength results. INTRODUCTION
The modulus of deformation is undoubtedly the geomechanical parameter that best represents the mechanical behavior of rock mass. In particular, when it comes to underground excavations, this modulus becomes indispensable – whatever the type of design approach to be developed. Laboratory measurements have long been the reference standard for determining the mechanical properties of geomaterials. In addition, field tests to compliment the geotechnical investigation and laboratory testing has become an expedient and cost-effective way to determine the strength and stiffness parameters over an entire site.
The main purpose of this paper is to present and compare the stiffness obtained from static and dynamic tests determined in the laboratory and field from a comprehensive and integrated site investigation which was carried out by Arab Center for Engineering Studies (ACES) for a major project in Irbid, Jordan. The field tests comprised pressuremeter as well as seismic geophysical methods including down-hole and cross-hole techniques. On the other hand, the laboratory dynamic tests involved ultrasonic velocity, resonant column and torsional shear testing. In addition, uniaxial compression tests were carried out in which the stiffness was estimated from the compressive strength results employing well-established empirical relationships.
The deformation constants of a material are the most important parameters in any design and their determination involves the use of measuring techniques both for load and deformation. The amount of deformation that most of the rocks undergo is extremely small and its measurement requires special techniques. Deformation is defined as changes in shape (expansion, contraction, or other forms of distortion). It occurs usually in response to an applied load or stress, but it also may result from a change in temperature (thermal expansion or contraction) or water content (swelling or shrinkage). Deformability describes the ease with which rock can he deformed, and
its inverse, stiffness, the resistance to deformation. Deformability, like strength, depends mostly on the porosity and the degree of jointing of the rock under test. Pores and joints are the weakest and most deformable elements in the rock. Other factors influencing rock deformability are drying and vibration effects from blasting.
The rock mass deformations are calculated by means of modulus of elasticity values as obtained from laboratory tests on rock core specimens. In general, laboratory rock specimen test results do not represent the in-situ properties of the overall rock mass. This limitation has led to the development of several static and dynamic field methods. To define the quality of the rock masses based on rock mechanics parameters, both field and laboratory test results should be used in the design. Depending upon the...
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