Tom Lunne

Interpretation of Cone Penetrometer Data for Offshore Sands

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 NGIs experience. The paper presents an evaluation and updated recommendation of Schmertmanns 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.

Procedures used to obtain soil parameters for foundation engineering in the North Sea

It is difficult and expensive to perform high‐quality soil investigations for platforms and pipelines offshore in deep waters and restless seas. This is especially true in the North Sea where the “weather window”; is small, and careful planning and timing of the soil investigation is essential. Field equipment, drilling, sampling and in situ soil testing procedures are described herein. The need for some laboratory testing on‐board, run simultaneously with the sampling operation, is stressed, and the sample handling and laboratory testing procedures used on‐shore are described and evaluated. The state of the art is discussed, and some future trends and desirable developments outlined.

The Fourth James K. Mitchell Lecture: The CPT in offshore soil investigations - a historic perspective

For the last 40 years, the CPT has played a key role in offshore soil investigations, mainly in connection with oil and gas development, but also for other purposes. The offshore application of CPT has been an important factor with regards to development of equipment, data processing and interpretation. Each of the following aspects are discussed in terms of historic development, present status and future challenges: - Deployment method and equipment. - Penetrometer design and construction. - Data acquisition, processing and quality control. - Standards and guidelines. - Interpretation of results and application in foundation design. It is shown that these developments have greatly enhanced the reliability and applicability of the use of the CPT in soil investigations and design. However, there are still important challenges to be faced in the future

Observed Settlements Of Five North Sea Gravity Platforms

The observed settlements of 5 North Sea gravity platforms installed in the period 1975 to 1978 are presented. They are all founded on overconsolidated clays and dense sand. For each platform the systems for measuring settlements are described and some available methods for computing immediate and long term settlements are reviewed. The foundation base width varies between 89 and 140 m, and the net foundation pressures imposed on the seafloor vary between 200 and 300 kn/sq m. The major part of the long term settlements occurs in the upper part of the foundation soils, within a depth of 30 to 40% of the foundation width. Observed consolidation settlements range between 50 and 230 mm and are completed within 9 to 40 months. The observed data have been used to determine empirical relationships between strength and compressibility properties for the soil. 12 references.

Design Parameters for Offshore Sands: Use of In Situ Tests

Traditionally, North Sea sands have been tested by laboratory tests on reconstituted samples and by in situ cone penetration tests. A number of other in situ test devices are already available for offshore use or will most likely be available in the near future. These include the piezocone, the pressure-meter, the dilatometer, nuclear density and electrical resistivity probes, the seismic cone and the screw plate.

On the influence of overconsolidation effect on the compressibility assessment of subsoil by means of CPTU and DMT

Abstract The paper contains the analysis of the influence of overconsolidation effect on the values of constrained moduli, assumed by means of two most popular in situ advanced tests. The tested soils included two geological formations: normally consolidated tills of the Pomeranian phase and overconsolidated tills of the Posnanian phase. The overconsolidation ratio (OCR) was derived from CPTU, DMT and oedometric tests. The tests revealed that for the assessment of changes in constrained modulus in the subsoil with CPTU and DMT, the formulas determining the relationship between cone resistance, DMT results and constrained moduli requires empirical coefficient different for soils of varied genesis.

Effect of sample disturbance on cyclic shear strength of normally to lightly OC clays

Abstract Tests on Onsøy clay (I p = 24–27%) were performed to study the effect of sample disturbance on undrained static and cyclic shear strength. Samples with different degree of disturbance from sampling were used and parallel sets of monotonic and cyclic undrained laboratory tests were performed on block and 54 mm composite piston samples. A silty Drammen clay (I p = 19%) which was studied earlier was included in the study. Test results indicated that there are significant effects of sample disturbance on both static and cyclic undrained shear strengths for both Onsøy and silty Drammen clays.

Sample Disturbance of Onsøy Clay Due to Gas Exsolution

High quality block clay samples at Onsoy were obtained by using the Sherbrooke sampler. Pore water with various degrees of gas dissolved were introduced to soil specimen under a back pressure simulating 1500 m water depth. Then total stresses were released, resulting in gas coming out of solution before trimming samples for advanced tests. Results from both Triaxial compression tests (CAUC) and Direct simple shear tests (DSS) showed that sample disturbance is increasing due to gas exsolution up to η = 20% and there is less increase in effects on undrained shear strength for η = 20 to 67%. Δe/ei and anisotropy ratio indicate that sample disturbance due to gas exsolution has more influence on DSS and CRSC tests than on CAUC tests. CRSC tests showed that preconsolidation stress is decreasing with increasing degree of gas saturation. Tube sampling disturbance of onsoy clay from previous study is discussed and compared with disturbance due to gas coming out of solution.

Effect of Time After Sampling on Laboratory Test Results

Until very recently, standard procedure in North Sea oil investigations has been to perform mostly simple index tests on soil samples in the offshore field laboratory. More sophisticated laboratory testing on samples is carried out in the onshore laboratory with the purpose of determining soil design strength and deformation parameters for foundation design. These onshore laboratory tests are normally carried out one to several weeks, sometimes months, after sampling (see Andresen et al., 1979).