DEVELOPMENT OF A PRESSUREMETER TO OPERATE IN ALLUVIAL SOILS OF PUNJAB

Abstract

The use of different pressuremeters for in-situ soil testing is now well established. The pressuremeters, which provide high quality design parameters, are affordable on high cost projects only. Small cost projects permit the use of low cost techniques only (e.g. SPT), which often yield poor quality design parameters. Usually pressuremeters are employed for soil characterization using pre-bored or self-boring or full-displacement techniques. There was a need to develop a device in Pakistan that could be used as pre-bored as well as full-displacement pressuremeter for characterizing the alluvial soil deposits of Punjab province. Moreover, the new device should be simple, robust, cost effective and still produces parameters of high quality. This thesis describes the development of such a new in-situ testing device; namely the New Pressuremeter. Certain modifications have been made to the design of Newcastle Full-Displacement Pressuremeter (NFDPM), developed earlier by Akbar (2001) that has a radial displacement measurement system. The length of the probe of NFDPM is 420 mm (length to diameter ratio of 8.4), which creates handling problems in the field. To overcome this difficulty it was decided to reduce the length of the test section and to keep the length of the probe of the new device equal to the length of SPT split spoon sampler to get benefit from the available large SPT data. On the basis of this fact, the length of the probe of the new device was taken as 305 mm giving length to diameter ratio of 6.3. The analysis of a pressuremeter test is based on the assumption that the membrane expands as a right circular cylinder. With modifications, the new pressuremeter was developed using most of the local resources. The main body of the 48.2 mm diameter (D) probe is made of high strength stainless steel. Both ends of the main body are identical. The upper end of the probe is connected to drill rods used to push the probe into place. A pressure hose and electrical cable pass through the drill rods and are connected to the probe by a re-usable hydraulic fitting. A 45° stainless steel cone having a maximum diameter of 50.8 mm (surface area 28.5 cm 2 ) is screwed onto the base of the probe. The cone creates a cavity, which is 5% larger than the diameter of the probe. The oversize cavity helps in such a way that during installation, the friction between the membrane and the soil is almost eliminated thus ivpreventing the ends of the membrane from being pulled out of the clamping rings. This eliminates the need for a protective sheath (e.g. Chinese lantern) reducing the cost and making its assembly simpler. This device is robust and can be used in a greater variety of soil types. This device has displacement and applied pressure measurement systems and can produce stress-strain curves and unload-reload cycles. Using the new device following calibrations were carried out: • Calibration of pressure transducer • Calibration of the displacement transducer (Hall effect transducer) • Calibration for the system stiffness In situ testing was carried out at three locations of alluvial soil deposits of province of Punjab ranging from very soft to very stiff clays and loose to medium dense sands using the new device by full-displacement and pre-bored techniques. A total of 65 full scale pressuremeter tests [40 at Site-1(20 each using full-displacement and pre-bored techniques), 7 at Site-2 (all using pre-bored technique), 18 at Site-3 (9 each using full- displacement and pre-bored techniques)] were performed during this research. In situ testing with the SPT and CPT (CPT was not carried out at Site-1) was also carried out along with laboratory testing on disturbed and undisturbed samples to compare the results and validate the performance of the new pressuremeter. It is found that the soil parameters obtained using the new device as full-displacement and pre-bored pressuremeter compare well with those determined by laboratory and other in- situ testing devices. New correlations have also been proposed on the basis of test results obtained during this research work. The comparison of soil parameters determined from different sources proves the validity of the performance of the new device. However, more in-situ testing is recommended to build more confidence in the new device.