The sometimes unpredictable and highly variable nature of soil conditions in deepwater regions must be recognized in planning geotechnical site investigations. Examples of four different soil stratigraphies in deepwater are presented to demonstrate the variability and peculiar nature of some sediments and how important it is to plan for comprehensive programs both in the field and in the laboratory phases of a geotechnical site investigation. The authors will show how reliance on correlations based on index properties and failing to understand their limitations can lead to misinterpretation of soil properties.
The empirical procedures for offshore foundations design are directly influenced by the shear strength values selected for analyses.There is generally a large amount of scatter in measured undrained shear strength data from offshore borings because of various types of laboratory tests and sample disturbance. A comparison of strength data obtained on samples of cohesive soils from the Gulf of Mexico taken with a 2.25-in.-OD thin-wall tube percussion sampler and a 3.0-in.-OD thin-wall push sampler confirms that (1) the strength results for 3.0-in. push samples give the highest and most representative values with less scatter, (2) the unconsolidated-undrained triaxial compression test gives the best measure of undrained shear strength for cohesive soils, and (3) strengths interpreted from SHANSEP on push samples correlate well with conventional laboratory and in situ strength data.
In-situ vane shear testing to measure undrained shear strength in the offshore environment has experienced increased use and significant equipment improvements over the last decade. These changes arose from (1) the offshore petroleum industry's need to improve the quality of foundation site investigations for their structures, (2) technological advancements in mechanical and electrical systems, and (3) recognition by the geotechnical profession of the benefits of acquiring vane shear testing data for better interpretation of cohesive soil strength properties. This paper provides insight into the current international practice of in-situ vane shear testing by presenting results of a questionnaire to international experts. The paper also describes the historical developments that led to the current state of practice. A review of field vane deployment and important operational details is presented along with an appraisal of the applications of vane shear testing for foundation design purposes. The paper also presents various case studies demonstrating how vane shear testing data proved beneficial for offshore foundation design. Finally, the paper summarizes the author's opinions relative to the current state of the art of offshore vane shear testing and includes comments on possible future developments and applications.
The authors examined the results of 172 CKoU direct simple shear (DSS) tests. Their research has led to four important conclusions. Firstly, the test results confirm observations by other investigators that an acceptable correlation does not exist between the shear strength ratio, cu/?'v, and plasticity index, Ip. Secondly, consolidation pressure has a greater effect on cu/?'v than Ip and should be considered when evaluating DSS test results for use in a normalized soil parameter (NSP) procedure such as SHANSEP (stress history and normalized soil engineering properties). Thirdly, correlating cu/?'v results from DSS tests with only Ip could lead to either overestimating or underestimating in-situ undrained shear strength when employing an NSP procedure. Lastly, the authors demonstrate that correlations of DSS measured soil undrained shear strength, consolidation pressure and water content can provide a useful tool for evaluating in-situ undrained shear strength. Introduction Over the past thirty years, the DSS test has become widely used in geotechnical investigations, particularly in deepwater regions. The results of these tests are typically employed in some type of NSP procedure to evaluate in-situ undrained shear strength of clay deposits. The NSP procedure known as SHANSEP1 is a common method employed to perform this task. However, because of the expense and the considerable duration of testing associated with the SHANSEP procedure, laboratory testing is typically limited to a few tests within soil units defining the soil stratigraphy being evaluated. Subsequently, the results are often correlated with Ip To interpolate between SHANSEP test intervals to define the interpreted shear strength profile. A common correlation used to interpolate data between SHANSEP test intervals is that of the strength ratio, cu/?'v, and Ip. However, the authors' experience has been that a very poor correlation exists between cu/?'v and Ip when evaluated over a wide range of Ip common to offshore soils. This same opinion has been expressed by other investigators.2,3 To assess the reliability of the cu/?'v and Ip correlation, the authors examined the results of 172 DSS specimens tested in a normally consolidated state (OCR = 1). These tests were performed by five different geotechnical laboratories with extensive experience in performing SHANSEP-type testing. The soil specimens are from geotechnical site investigations conducted in six different offshore regions of the world. The plasticity characteristics of the 172 specimens examined in this study are presented in Fig. 1. This plasticity chart reveals that most of the specimens are highly plastic (CH) clays with liquid limits as high as 143 percent and Ip values as high as 101 percent. There are also several lower plasticity (CL) clays with Ip values ranging from 14 to 29 percent as well as some elastic silts (MH) and organic (OH) clays that fall below the A-line. The database does not include highly sensitive, cemented, or highly structured samples. Prevailing Strength Ratio - Ip Correlation In 1957, Skempton4 proposed the following correlation for normally consolidated clays based on field vane test results: (Mathematical equation available in full paper)
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.