ABSTRACT

A remotely controlled vane probe has been developed and tested for measuring in-situ shear strengths of sub-bottom clays in marine environments. This device, an adaptation of the long used vane shear test, is employed in conjunction with wire-line soil sampling operations from either a fixed or a floating base.

The development included extensive onshore testing at Venice, Louisiana, accompanied by conventional onshore and offshore sampling and testing methods. Results indicate that shear strengths measured by the Remote Vane are comparable to those obtained with laboratory tests on high quality samples and with conventional field vane tests. Subsequent to its onshore trials, the device has been operated successfully from a floating base at several offshore locations.

The primary function of this new marine geotechnical procedure is to permit more accurate determination of the in-situ soil strength profile for use in designing pile foundations and bottom support systems and in investigations of marine slope stability. Remote Vane test results obtained at the '; Venice site are compared with a load test on a 12-inch diameter open-end pipe pile, driven to 147-foot penetration. In addition, Remote Vane results obtained from a floating vessel at an undisclosed offshore location are presented and discussed.

Introduction

The shear strength of ocean bottom clays must be known to predict the performance of bottom supported structures. In the offshore area, deep penetration studies are usually required for

  • predetermining the axial and lateral load carrying capacity of individual piles and pile groups used to support fixed platforms,

  • predicting the bearing capacity of the foundation elements used to support the legs of jack-up drilling rigs, and

  • calculating the stability of slopes.

The techniques and equipment needed to investigate the strength of ocean bottom clays to these deep penetrations are necessarily influenced by the environment in which they are employed. In the marine environment, speed and simplicity are required for any economically acceptable soil exploration technique. Experience has shown that a wireline soil sampling operation which can be performed from a floating base meets these requirements. Operations have been routinely conducted in water up to 500 feet deep and to soil penetrations as great as 500 feet using this system.

Inherent in wire-line sampling. however. is some sample disturbance which affects soil shear strength measurements. To overcome this problem while retaining the advantages of a floating operation. a remotely controlled, wire-line operated, vane shear probe (hereinafter called the Remote Vane) has been developed and tested to measure clay shear strength in situ. This device. an adaptation of the long used vane shear test. is employed in, conjunction with wire-line soil sampling operations.

This paper describes the new device and its operation and discusses its application to stability problems involving ocean soils.

Review of In-Situ Techniques and Investigations

In the offshore oil industry. there is generally the need for both shallow and deep penetration tools to measure in-situ soil shear strength.

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