Modulus of elasticity/deformation of rock mass is the most important parameter in rock engineering. Flexible dilatometer is the convenience approaches to determine in situ modulus of elasticity and deformability in Siah Bishe Pumped storage power plant in Iran. Data interpretation and some limitation of application of dilatometer are presented in this paper. Engineering judgment has important role in the test data interpretation. Also some additional hardware and software assessors are used in order to increase accuracy.


Several methods have been used to measure modulus of elasticity/deformation in rock mass. Parameter such as duration, accuracy, technical accessory and device, expert technical and price are the main parameters have effect the selection test methods. Flexible dilatometer test has several important specification which cause applied it's for more underground rock structures. Siah Bishe pumped-storage power plant at the north of Iran has experienced Flexible dilatometer tests. In rock mechanics the first successful application of high-capacity flexible dilatometers were reported in 1964 by Panek et al. at the United States Bureau of Mine, and in 1966 by Rocha et. al. at the National Civil Engineering Laboratory (LNEC) in Portugal [4].

Dilatometer Test

Dilatometer test has been applied for measuring deformation parameter of rock mass in a borehole. Some of important advantages of this test are: low cost, repeatability of test at several depths of borehole with nearest situation to intact condition and probability of anisotropy assessment. Important limitation of this method is limited volume of rock mass affected in test (About 1/3 cubic meter). Usually stress state in this method is lower than other method which is depending to test phenomena. Actually this test has been done for location that is far from the structural loading situation. Thus damping stress by distinct from loading point cause not necessary to applied high stress in the rock mass at the test borehole wall. The modulus rang which measured in this method are between 0.005 to 25Gpa. Test method is presented in several sources such as ISRM [1]. Therefore introduce device, accessory, borehole situation, test result, interpretation of data are presented in the next part.


Elastometer-2 model 4018 was used in Siah Bishe pumped- storage project which made by OYO Co. from Japan [7]. This dilatometer can be applicable to the wide rang of ground from soft to hard rock. Flexible probe has 72mm diameter (hard or soft) and length of loading area is 520 mm. devices that measure borehole wall deformation with deferential sensor. The maximum applicable pressure is 20Mpa and displacement sensors are two arms with 19.99mm with 1 micron accuracy, Figure 1. Water was used for expanding probe. This device can measure dilation only in one direction [6]. Continue pressure with additional accessory 512 was applied (2 bar/sec) in order to eliminate pulse pressure effect, constant rate of pressure and eliminate the time depending probe specification.

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