Received 10 September 2009; accepted 11 December 2009 *Corresponding author. Tel: 98 9124598214 E-mail address: kaveh.ahangari@gmail.com doi: 10.1016/S1674-5264(09)60212-X Use of casing and its effect on pressure cells AHANGARI K 1,* , NOORZAD A 2 1 Department of Mining Engineering, Islamic Azad University, Science and Research Branch, Tehran, Iran 2 Power and Water University of Technology, Tehran, Iran Abstract: Although pressure cells have been produced and installed successfully for decades, the accuracy of measured pressure is often inadequate. Due to large differences between the stiffness of pressure cells and the surrounding media, there is a considerable difference between applied pressure and that measured from pressure cells. It is often difficult and expensive to make a pressure cell with stiffness (modulus of elasticity) similar to the surrounding material in which it will be embedded. In order to improve this situation, a casing material with proportional dimensions is recommended as a means to obtain reliable results. In our study, the effect of using casing in the installation of pressure cells is investigated, providing the characteristics of casing. Some practical recommendations are presented to improve the accuracy of the results using casing. Keywords: instrumentation; stress measurement; pressure cell; casing 1 Introduction The most commonly used instrument for measur- ing soil stress is the hydraulic type pressure cell that provides relative measurements. The in-situ pressure can however be over- or un- der-registered by pressure cells depending on the stiffness ratio of the soil to the cell, the various boundary conditions and other factors which are spe- cific to the installation field [1-5] . It is difficult to measure the actual pressure in the field. Dunnicliff and Green described in details many factors that affect the measurement of stress in soils [1] . Among those factors, they discussed the aspect ratio, the soil to cell stiffness ratio, the size of the cell, the loading type such as non-uniform and point loads, the placement and effect of temperature variation. Because of the differences between the stiffness of pressure cells and surrounding media in which they will be installed, the measured pressure differs con- siderably from existing pressure. The problem will be discussed for two scenarios: the first case is when an installed pressure cell has higher stiffness than the surrounding media, so that the pressure concentrates on it (hard-inclusion); the second case concerns a condition where the cell is softer than the surrounding media, to which a high pressure should be transferred (soft-inclusion). In the first case, a very large pressure and in the second case a very small pressure will be measured (Fig. 1) [6-8] . $ $ % % σ m σ σ m σ (a) Pressure cell stiffer than (b) Pressure cell softer than surrounding media surrounding media Fig. 1 Stress distribution in area of installed pressure cell Ideally, pressure cells should have the ability to fit different types of installation in order to match the surrounding soil stiffness, but this concept would be difficult to implement, considering that soil stiffness is not constant. It would also be too expensive for manufacturers prefer the use of different type of models instead of fitting one model for a whole range of needs [1] . During installation, fine grained soils are placed as casing around the cells to eliminate direct contact between pressure cells and coarse material and crushed stone, which may lead to a point load on the cell pad as depicted in Fig. 2 [9-11] . There have been only a few reports in the literature on the use of casings for installation of pressure cells. Mining Science and Technology 20 (2010) 0384–0390 MINING SCIENCE AND TECHNOLOGY www.elsevier.com/locate/jcumt