Engineering Properties of Surficial Marine Sediments
Authors: Wilson Jerry C. 
Published: October 2008

The engineering properties of 25 samples recovered from 5 geomorphic and sedimentary environments of the Southern California continental Borderland are reported. Samples were taken using a box corer yielding a relatively undisturbed sample suitable for engineering tests from the mainland shelf, basin slope, basin floor, and submarine canyon fan.

Testing included determination of specific gravity, grain size, water content, Atterberg limits, unit weight, void ratio, direct and vane shear strength, and consolidation characteristics.

Shallower water shelf sediments and deeper water basin and fan sediments form separate classes based on engineering properties. Slope sediments are transitional between these two classes, with overlap in both.

Shelf sediments are characterized by greater shear strengths, grain size, and unit weight, and by lower water content, specific gravity and void ratio. These sediments exhibit less compressibility (consolidation) than the basin and fan sediments.

Similar low specific gravities for the shelf and fan sediments indicate that the fan sediment source is similar to the shelf sediment source.

Water content and porosity have a direct relationship and decrease with depth in the sediment. Unit weight shows a slight, irregular increase with depth. Atterberg limits indicate that the deeper water clayey soils behave as organic and plastic silts, and as highly plastic clays.

Field vane shear tests indicate that undisturbed strength decreases from shelf, slope and basin to fan. The remolded strengths for all samples are approximately equal (50 to 100 psf) and uniform with depth. Sensitivities are generally between 1.5 and 2.5, but a maximum of approximately 4.0.

Laboratory direct shear tests indicate that the strength of the soils decreases in the order: shelf, slope, fan, basin. The increased ranking for the fan sediments is attributed to granular layers (turbidites) which cause development of some internal friction.

Average values of the compression index indicate that the submarine soils are progressively more compressible from shelf to slope to basin to fan. Laboratory determined compression indices are compared with values calculated using Skempton's equation empirically relating to liquid limit and specific gravity. Calculated values are higher than measured values for liquid limits up to approximately 100; lower for liquid limits greater than approximately 100.

Evaluation of submarine slope stabilities, using the infinite slope method, and these data indicate that all slopes sampled are stable. TWO of the seven slope stations show a potential for instability with increased deposition.

It is concluded that comparison of engineering properties from these sedimentary environments yields information on the geologic processes involved in their formation. In addition, these data aid in the design of permanent or temporary instrument installations on the sea floor.

INTRODUCTION

The increased use of the sea floor as a supporting surface for instrumentation, pipelines, habitats, platforms and vehicles has prompted serious study of the engineering properties of surficial marine sediments. Investigations by Keller, Richards, Moore, and others have developed values for these properties for continental shelf and deep ocean floor sediments. Other investigators have described problem areas in sampling and measuring techniques.