DEVELOPMENTS IN THE DESIGN OF TRIAXIAL EQUIPMENT FOR TESTING COMPACTED SOILS

The economic design of earth structure demands accurate knowledge of the shear strengths and pore pressure parameters of partially compacted soils. Tests may take several weeks and the difficulties suggest improvement of equipment and possible automation. Recent research proves the necessity of measuring the pore air and pore water pressures if the degree of saturation is below 0.9. Failure to take into account residual suctions (sometimes of 40 lb/sq.in) can lead to underestimation of stability during construction of an embankment. In the longer tests the sources of error, of which eleven are listed, must be overcome in the standard equipment. Measurement of Pore Water Pressure in Partially Saturated Soil. Accurate pore water pressure measurements can only be taken if the disc has capillaries fine enough to remain fully saturated when the air pressure exceeds the water pressure. The Aerox Celleton Grade CC2 disc has been used satisfactorily and is illustrated and fully described. With this disc a pressure transducer is used which gives seven listed advantages. Measurement of Pore Air Pressures. A glass fibre disc is found suitable and is built into the 4in dia top cap together with a transducer, diaphragm and strain gauge rosette. A photograph of the various parts together with a section through the cap illustrate the device clearly and the author fully describes the methods employed. Semi-Conductor Strain Gauge Pressure Transducers. As an alternative to commercial transducers, which are expensive, a workshop made transducer is illustrated and explained. Measurement of Volume Changes. A perspex cell has been designed similar to that of Frederick (1962), with a brass bush through which the loading ram can pass. The cell is then centred before being clamped. Calibration tests are given. Measurement of Axial Stress. When measuring by proving ring, an error of up to 5 per cent may arise through friction in the bush. This is overcome by placing a load cell in the base of the apparatus. A description of the cell and its mounting follows. Deflecting only 1.25 x 10-3in per 1,000lb load, the load cell is stiff and stores little strain energy. Undrained Triaxial Tests. To overcome air diffusion through the rubber membrane two sheets of slotted aluminium foil, illustrated and dimensioned, are used. The method of use is described. The strips are free to slide over one another and are placed neither in tension nor compression. The use of transducers reduces manual operation. By connecting them to a time switch automatic readings can be taken. Transducers incorporating semi-conductor strain gauges seem superior to those with the conventional strain gauges. The apparatus developed is prototype and modifications are to be expected. /CEA/