Determination of undrained shear strength parameters for buried pipeline stability in deltaic soft clays T.A. Newson and M.F. Bransby Division of Civil Engineering, The University of Dundee, Nethergate, Dundee, Scotland. P. Brunning and D.R. Morrow Stolt Offshore MS Ltd., Bucksburn House, Bucksburn, Aberdeen, Scotland. ABSTRACT Offshore pipelines are typically laid on the seabed and lowered into the seabed (trenched) over large sections to provide protection from shipping and fishing activities, or to stabilise the pipeline from hydrodynamic loads. Accurate estimates of the resistance to upward pipeline movement of the overlying clayey trench-backfill are important for design and analytical purposes. The undrained shear strength (c u ) of the upper layers of the seabed (particularly the initial 2 to 3 m below the mudline) is therefore a vital part of pipeline site investigations and is commonly found using the cone penetrometer (CPT). The limitations of standard CPTs may be overcome with the use of novel shaped penetrometers. The measured resistance can be easily used to calculate undrained shear strength using an unique bearing capacity or ‘bar’ factor. This paper describes the application of standard in situ cone penetrometer and T-bar tests from an ROV for the determination of undrained shear strength parameters for pipeline buried in deltaic soft clay. Comparisons with in situ plate bearing tests are also provided and the advantages of this novel form of in situ test are discussed. KEYWORDS: clay, in situ test, penetrometer, pipeline, backfill. INTRODUCTION Offshore pipelines are typically laid on the seabed and lowered into the seabed (trenched) over large sections to provide protection from shipping and fishing activities, to stabilise the pipeline from hydrodynamic loads or for thermal stability. Since pipeline rupture can potentially cause permanent or long-term environmental damage, the risk of exposure of the pipeline due to upheaval buckling, wave-induced liquefaction or erosion must be carefully assessed. Particularly at risk from upheaval buckling failures are pipelines in very soft clays, which are found towards the margins of estuaries and deltas. These may also contain fine sandy or silty laminations, and are commonly highly plastic. Accurate estimates of the resistance to upward pipeline movement of the overlying clayey trench-backfill are important for design and analytical purposes. The undrained shear strength (c u ) of the upper layers of the seabed (particularly the initial 2 to 3 m below the mudline) is therefore a vital part of pipeline site investigations and is commonly found using the cone penetrometer (CPT). This instrument provides a continuous measurement of undrained shear strength and also allows the stratigraphy of the profile to be identified. Unfortunately, the standard CPT (10 cm 2 ) is not very accurate in soft clay deposits due to the low tip resistances measured. To date, the empirical and theoretical solutions relating c u to tip resistances are difficult to apply objectively and accurate estimates of c u can therefore be difficult, and possibly misleading. These limitations may be overcome by varying the shape of the penetrometer, such that it causes symmetrical soil flow during penetration. This paper describes the application of standard in situ cone penetrometer and T-bar tests from a remotely operated vehicle (ROV) offshore for the determination of undrained shear strength parameters for pipeline buried in deltaic soft clay. Comparisons with in situ plate bearing tests are provided and the advantages of this novel form of in situ test for this type of application will be discussed. PENETROMETER TESTING Undrained shear strength profiling forms an important part of many site investigations involving soft clays. This is commonly achieved using a combination of laboratory shear strength tests on undisturbed samples and in situ testing methods. The most widely used of these in situ tests is the vane shear test (VST), which can be used to measure both the peak and residual undrained shear strength (c u ). However, despite its popularity, this test has a number of disadvantages: estimates of c u can only be taken at discrete, well spaced, depths in the profile (hence the test is quite slow and a continuous profile cannot be obtained) and thin layers of stiffer material can affect the results. In contrast, the cone penetrometer test (CPT) provides a continuous measurement of c u and, in addition, allows the stratigraphy of the profile to be identified. Unfortunately, the CPT is not very accurate in soft clay deposits due to the low tip resistances measured. Also, the deformation mechanism around the cone during penetration is asymmetric in the vertical plane, hence correction for overburden pressure and pore pressure is also required. To date, the empirical and theoretical solutions relating c u to tip resistance are difficult to apply objectively (they may require an estimation of an equivalent elastic stiffness, G) and estimates of c u can therefore be in error. Many of these limitations may be overcome by varying the shape of the penetrometer, such that it causes symmetrical soil flow during penetration. One such device is the T-bar penetrometer that was