Most soft clays in their natural state exhibit a small degree of overconsolidation resulting from changes in ground water level, delayed consolidation, or other causes. The overconsolidation ratio is commonly in the range of 1.0 to 2.5. Under surface loading pore pressures in such a deposit will develop as for an elastic material until the effective stresses reach a yield condition or failure condition. In the latter case the soil can continue to carry additional total stresses within confined zones and pore pressures in these zones will continue to increase. In the former case the soil will continue to deform plastically until it reaches failure, showing, in general, different pore pressure responses in these two phases. Thus pore pressure response at any point in a soft clay deposit under increasing surface loading may show two or three distinct phases, although in some cases the plastic and failure responses may be almost indistinguishable. In this report evidence is presented for the yield condition from a study of published laboratory test results. Expressions are then developed for the pore pressures to be expected in the field for each of the three phases -- elastic, plastic, and contained failure -- as a function of the vertical total stress increment at any point in the soil mass. (Author).

Although problems associated with total stress tension arise in soil structures such as earth embankments and multilayer pavements, no standard tension test for soils exists. An unconfined direct tension test, either strain controlled or load controlled, is described, which can be used to determine the tensile (total) stress-strain characteristics of compacted clays. The overall tensile strains in the specimen are determined by observing the displacement of two surface markers spiked into the specimen with a displacement measuring optical device (DMOD) capable of measuring to one-micrometer displacement. An alternative radiographic technique of monitoring strains has shown that the distribution of average tensile strains in the specimens is substantially uniform. Moreover, the failure of the tension specimen occurs in midsection, indicating a uniaxial transmission of tensile force to the specimen. Comparison of standard triaxial unconfined compression tests with unconfined tension tests shows that elastic moduli are not equal in compression and tension for compacted clays at a given moisture content.

A comparison has been made between consolidated undrained tests on separate soil specimens and multistage testing of a single specimen. Three series of samples were prepared, two of the series consisting of remolded kaolin in each of which the specimens were lightly overconsolidated under K0 conditions, and one series consisting of undisturbed specimens of soft silty clay from a site at Fulford near York in England. It is concluded from these tests that although some differences arise from the two methods, the results from multistage tests are adequate for practical purposes both in the prediction of failure conditions and in the prediction of deformations and pore pressure response under working loads. In view of the number of advantages accruing from multistage testing, it is recommended that this method should be used in the routine triaxial testing of soft lightly overconsolidated clays of low sensitivity.

Small vane and penetrometer devices have been developed to measure soil properties in models during centrifuge operation. To aid the interpretation of vane data from centrifuge models a series of tests was conducted in cakes of kaolin clay in a normal gravity field. In the experimental program the effects of disturbance during insertion, delay between insertion and rotation, rate of rotation and geometry of vane blades were investigated. The results were used to devise a scientific method of operation of the vane apparatus. Subsequent tests were all performed with a vane size of 18 mm diameter by 14 mm high. Penetrometer tests were conducted in cakes of kaolin and Gault clays. Vane shear strengths were used to calibrate the apparatus. The ratio of cone resistance to vane strength was found to increase with increasing over-consolidation ratio. The values also differed considerably between the two clays. Thus, while the penetrometer has the advantage of providing a continuous profile of resistance with depth, some care is required in converting this measurement into undrained shear strength.

Three triaxial tests were conducted on clay shale specimens from Laneport Dam. The specimens were 3 in. long by 1.4 in. in diameter. The purpose of the test program was to evaluate the influence of temperature variation on pore pressure in clay shale, and stress-strain and pore pressure behavior due to incremental isotropic and axial and lateral stress changes. The results of this experimental program are given in this report.