Several Dilatometer and Plate Load Tests were carried out as part of a site investigation program for the Roudbar-Lorestan Dam Project in the South-West of Iran in order to determine the deformation characteristics of the rock masses in that area. The results showed that the modulus of deformation measured by these two methods is variable for the same individual rock mass. This paper aims to compare and discuss the rest


The deformation modulus is an important parameter for characterizing the mechanical behavior of rock masses. This parameter is used to assess the stability of structures built in or founded on rock. The Plate Load Test (PLT) and Flexible Dilatometer Test (FDT) are two conventional methods among in-situ tests to determine the in-situ modulus of deformation. Both tests were considered as part of a large site investigation program at the suggested location for the Roudbar-Lorestan Dam, near the city of Aligudarz in the south-west of Iran. The investigations aimed to provide geotechnical parameters in order to analyze the suitability of this location and also to obtain required design parameters for the construction of the dam. The rock masses in this area are mainly grey to brownish grey, well bedded limestone and dolomitic limestone, with alternation of marly limestone, shale, and marl

Test Procedure

Both testing methods are based on the theory of elasticity which considers the rock mass as an elastic, isotropic and homogeneous medium. Due to these assumptions, the modulus derived from both tests should be similar. In real conditions however, this is not the case and the modulus determined from PLT is usually greater than values derived from the FDT which will be discussed in the following text.

Dilatometer Testing

Several dilatometer tests have been conducted in a number of locations at the Roudbar-Lorestan dam site following the standard procedure proposed by the International Society of Rock Mechanics [1]. The device used, a High Pressure Dilatometer HPD73, can apply a pressure of up to 20 MPa to the ground, and can expand from an initial diameter of 73 mm to nearly 100 mm [2]. The expansion of the instrument is measured by means of six circumferentially fixed strain gauged leaf springs that follow the movement of the inside of the membrane and the internal pressure is measured by a transducer within the body of the HPD. The instrument is intended for use in soft to medium strength rocks [3] but may also be used in harder rock masses with proper calibration. It is capable of resolving movements of less than 1 micron and pressure changes less than 1 kPa, however it requires corrections for membrane thinning since the strain arms are covered by the membrane.ults

Plate Load Testing

Compared to the dilatometer test, the PLT is more expensive and time consuming, however, the stress distribution and the larger volume of rock mass involved in the test, makes it more reliable.

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