Characterization of Soft Deepwater West Africa Clays: SHANSEP Testing is Not Recommended for Sensitive Structured Clays
Authors: Le Manh-Hai , Nauroy Jean-Francois , De Gennaro Vincenzo , Delage Pierre , Flavigny Etienne , Thanh Nguyen , Colliat Jean-Louis , Puech Alain , Meunier Jacques 
Published: October 2008

With the development of deepwater fields offshore West Africa, in water depths currently ranging between about 600 and 1,500 metres, geotechnical characterization of the sediments has become an issue since the properties of the soft Gulf of Guinea clays differ from those of the equivalent Gulf of Mexico or North Sea clays. In particular, the highly plastic (plasticity index Ip over 80 %) West Africa deepwater clays have a very low unit weight, are highly compressible, and have a relatively strong natural structure.

As part of a joint research effort carried out within the CLAROM framework in France, a high quality triaxial testing programme was carried out on clay specimens from an oilfield located in 1,300 metres of water. The paper describes the laboratory test procedure and the results obtained, comparing the results of SHANSEP testing with the results of other triaxial tests at low confining stresses representative of the in situ conditions.

With the SHANSEP testing procedure, the micro-structure of the soft West Africa clay is altered by the reconsolidation stresses in the over-consolidated range through large consolidation deformations. The paper shows that the SHANSEP procedure provides undrained shear strength values that are similar to those of a disturbed or reconstituted sample. High quality triaxial testing under confining stresses representative of the in situ stress level appears as the only way to provide geotechnical parameters that are representative of the in situ clay strength. Therefore, the SHANSEP triaxial testing procedure is not recommended for the soft sensitive structured clays from deepwater West Africa sites.

Introduction

As compared to other marine sediments, the clays of the Gulf of Guinea (hereinafter called " GoG??), recovered from several sites at 600 to 1,300 m of water depth, present some unusual and specific physical properties (Puech et al. [1]): high water contents up to 8 m depth (with w changing from 250 % down to 150 %), high plasticity indexes (typically IP = 150 and wL = 200 % at the seabed, IP = 80 and wL = 120 % at 8 m depth), low shear strength (Cu = 5 - 30 kPa to about 20 m depth), and sometimes an atypical peak of in situ CPT net cone resistance at the seabed (qnet = 120-200 kPa).

The compression behaviour of GoG sediments has been interpreted within the general framework proposed for the analysis of the mechanical behaviour of structured natural clays (De Gennaro et al. [2]). The post sedimentation history of GoG clays, giving rise to an increased in situ strength, is quantified by an effective vertical yield stress which is higher than the in situ stress (i.e., corresponding to an " apparent?? over-consolidation ratio).

The undrained shear strength of the clay can be measured in the laboratory by several methods (e.g., lab vane test, direct simple shear or triaxial tests, etc.). In the triaxial test, the shear strength depends on the reconsolidation procedure. In practice, two different approaches are possible. The first one, which was suggested by Bjerrum [3], involves reconsolidating the soil sample to effective stresses equivalent to the in situ stresses. The other method, originally proposed by Ladd and Foott [4], entails the application of vertical effective stresses much greater than those in situ, coupled with a normalization technique (so-called " SHANSEP?? for Stress History And Normalized Soil Engineering Properties).