No abstract

After many decades of research, the issue of sample disturbance is still important as regards to determining reliable and representative soil parameters for foundation design in soft clays. Parallel laboratory tests have been carried out on high-quality block samples and ordinary piston tube samples from 12 deposits of soft Norwegian marine clays. Undrained triaxial and direct simple shear (DSS) tests on samples reconsolidated to the in situ effective stresses show that sample disturbance has a significant effect on the measured stress–strain–strength behaviour: the more disturbed the sample, the lower the shear stress at small strains and the higher the shear stress at large strains. Breakdown of the clay structure, including cementation bonds, is the assumed cause of lower shear resistance at small strains, whereas at large strains the shear resistance is governed mainly by the water content, which for soft clay samples, reconsolidated to the in situ effective stresses, will be lower, and the strength thereby higher, the more disturbed the sample. The work described herein also includes the effects of the consolidation procedure; in addition to the reconsolidation technique, both stress history and normalized soil engineering properties (SHANSEP) and delayed consolidation tests have been carried out.Key words: soft clays, sample disturbance, consolidation procedures, stress–strain–strength behaviour, stress–strain–time behaviour.

Data have been reviewed from sites in Europe and North and South America as well as published data from South Africa. The review has concentrated on dissipation data from piezocone tests (CPTU) to compare predicted coefficient of consolidation and permeability values using published interpretation techniques with available reference values. The results of this review have shown that the theoretical solutions provide reasonable estimates of the in situ coefficient of consolidation. Results were evaluated for pore-pressure data from different locations on the piezocone, and the least scatter in results was obtained with the pore-pressure element location immediately above the cone tip. A new correlation has been proposed to estimate in situ horizontal coefficient of permeability (kh) from piezocone dissipation data. Key words : in situ, coefficient consolidation, cone penetration test, permeability.

Difficulties in obtaining high-quality soil samples from deepwater sites have necessitated increasing reliance on piezocone, T-bar and ball penetration tests to determine soil properties for design purposes. This paper reports the results of an international collaborative project in which a worldwide high-quality database of lightly overconsolidated clays was assembled and used to evaluate resistance factors for the estimation of intact and remoulded undrained shear strength from the penetration resistance of each device. The derived factors were then compared with existing theoretical solutions to evaluate the influence of particular soil characteristics. The overall statistics showed similar levels of variability of the resistance factors, with low coefficients of variation, for all three types of penetrometer. However, correlations of the resistance factors with specific soil characteristics indicated that the resistance factors for the piezocone were more influenced by soil stiffness, or rigidity index, than for the T-bar and ball, while the effect of strength anisotropy was only apparent in respect of resistance factors for the T-bar and ball relative to shear strengths measured in triaxial compression. In the correlation between the remoulded penetration resistance and remoulded strength, the resistance factors for remoulded strength were found to be higher than those for intact strength and with a slight tendency to increase with increasing strength sensitivity but insensitive to soil index properties. Based on an assessment of the influence of various soil characteristics, resistance factors are recommended for the estimation of intact and remoulded undrained shear strength from the penetration resistances of each device for soil with strength sensitivity less than six.

The use of the cone penetration test (CPT) for offshore soil investigations is particularly important in sands where it proves problematic and often impossible to take undisturbed samples. CPT data can provide reliable indications of in situ soil properties and/or in situ stress conditions. The geotechnical literature, however, abounds with different procedures for interpreting CPT data in sands.A comprehensive data base for sands was established mainly from large scale calibration chamber tests performed at several institutions. The data base also included results from field tests described in the literature and from NGI's experience. The paper presents an evaluation and updated recommendation of Schmertmann's method for estimating relative densities from recorded cone resistance. It also evaluates various procedures for interpreting the drained friction angle from cone resistance and presents correlations between the constrained deformation modulus and cone resistance. Finally, the paper recommends other in situ tests to complement and enhance the interpretation of CPT data in sand, and points out the necessity for simultaneous sampling and laboratory testing. ---------~--------:RENCES Parkin, A.K. and T. Lunne (1982) Boundary effects in the laboratory calibration of a cone penetrometer for sand. An updated correlation between relative density, Dr and Fugro-type electric cone bearing qc. Waterways Experiment Station, Vicksburg, Miss. Contract report, DACW 39-76-M 6646. 145P. Schmertmann, J.H. (1978) Guidelines for cone penetration test: performance and design. United States. Department of Transportation. Federal Highway Administration Offices of Researc and Development, Washington, D.C. Report, TS-78-209. 145p. Bellotti, R., V. Ghionna, M. Jamiolkowski, M.E. Manassero and E. Pasqualini (1983) Evaluation of sand strength from CPT. To be presented at the International Symposium on Soil and Rock Investigations by In-Situ Testing. Paris, 18th-20th May, 1983. Baldi, G., R. Bellotti, V. Ghionna, M. Jamiolkowski and E. Pasqualini (1982) Design parameters for sands from CPT. A method for determining the friction angle in sands from the Marchetti dilatometer test (DMT).

Publication informationMarine Geology, 226 (1-2): 145-165Publisher Elsevier A cost efficient way of investigating shallow sediments and for collecting soils data for 2 many offshore geotechnical problems, e.g. anchoring of floating structures with suction 3 piles or evaluation of submarine slope stability, is to carry out seabed sampling to say 4 25 m below seabed and in addition perform seabed CPTUs to say 40 m. Based on a 5 review of data collated from the literature and from in house project experience at the 6 Norwegian Geotechnical Institute it was found that Kullenberg type piston core samples 7give poor recovery and samples, which are increasingly disturbed as penetration 8increase. This means that laboratory tests on these samples will give results that are not 9 representative of in situ conditions and will give incorrect parameters for foundation 10 design. Many different factors control the quality of the samples recovered but the need 11 for an effectively stationary piston a core retainer and improvements to the physical 12 sampler parameters, e.g. diameter / wall thickness ratio, cutting edge angle, inside and 13 outside friction are perhaps the most critical. The importance of each of these factors is 14 discussed in detail in addition to the need for instrumentation and the way to penetrate 15 the sampler. Recommendations are given in terms of criteria for the design of a new 16 sampler, which the authors feel will give better results than most samplers used in 17 practice today.

Results of 43 load tests on driven and jacked piles performed at eight calyey sites have been back-analysed using piezocone test data. A method to evaluate the axial capacity of piles has been proposed in which the pile unit skin friction and the unit end bearing resistance are computed from the net corrected cone resistance, qnet. Correlation factors between piezocone and pile load tests were back-calculated using this approach. As most of the pile tests performed were tension tests, special emphasis was given to the evaluation of the pile unit skin friction. Key words: clay, pile, piezocone, cone penetration test, axial pile capacity, skin friction.

Stress history is usually determined by interpreting the results of oedometer tests to obtain the preconsolidation stress using traditional methods such as those proposed by Casagrande and Janbu. Both these methods are based on the assumption that the soil experiences a change in stiffness, from a stiff response to a soft response, close to the preconsolidation stress. Disturbed soils, however, especially overconsolidated clays of low plasticity, exhibit a soft response at low stresses, thereby making the interpretation of oedometer tests using traditional methods extremely difficult. Using data mainly from the Haltenbanken area, offshore Norway, a study was undertaken to investigate the various methods and testing procedures available to determining stress history. Oedometer verification tests were also performed on reconstituted specimens to compare interpreted values with the actual applied preconsolidation stress. This paper describes the results of the study by illustrating the variation in the interpretation methods and testing procedures used to determine preconsolidation stress.Key words: preconsolidation stress, yield stress, overconsolidated, low plastic clays, oedometer test, stress history.