Firstly, let me commend geoforum on this website. Exactly what I have been looking for!
Anyway, my question(s):
I am looking at the following two aspects of (offshore) pipeline-soil interaction:
1. SOIL RESPONSE DURING PIPELINE EXPANSION – a slow failure process involving up to a couple of meters of longitudinal displacement at either end of the pipeline, occurring over a period of days. The soil conditions in the areas of interest consist primarily of soft clays. 2. SOIL RESPONSE FOLLOWING PIPE-LAY ON A SLOPE – a fast failure process whereby the pipeline tries to slide down the slope immediately following ‘touch-down’ or ‘pipe-lay’. The soil conditions in the areas of interest consist primarily of stiff clays.
I am currently trying to come up with a means of mimicking these failure mechanisms in a laboratory, but have a few areas of concern. I am considering either a DSS (Direct Simple Shear) or a DS (Direct Shear) test. The main reason I have opted for these tests in place of any others is that they will allow me to fail the soil against the actual FBE (Fusion Bonded Epoxy) coated steel, or a sample of.
My first question is, which test lends itself best to each of the above failure mechanisms (1 and 2)?
My thoughts: 1.The failure process is slow, suggesting a drained test. The pipeline will have been in place on the seabed for many weeks, if not months, so it is safe to assume that a bond will have been achieved between the soil and pipeline. Perhaps a consolidated-drained test? 2.The failure process is fast, suggesting an undrained test. The pipeline will not have the chance to bond to the seabed soils - rather it will try to ‘fail’ immediately upon contact. Probably more like an unconsolidated-undrained test?
Anyone out there have any thoughts on any of the above?
Good questions. Probably not clear which path Mother Nature will take if failure/slippage was to occur (drained vs. undrained, or something in-between).
Difference between DSS and DS. This kills me. They are somewhat the same test. Consolidated one-dimensionally, then sheared horizontally to failure. The true simple shear (SS) would have complementary shear stresses on top & side and bottom & side. But commercial DSS only has on top and bottom. The sides move to create the shear strain. In the more common old direct shear box, a horizontal plane is cut. You used to be able to run DS undrained (ASTM, circa 1951), but now only slow drained DS to get effective stress strength parameters (c', phi') is okay. In contrast, the DSS is only used to get undrained shear strength (cu or su) and not for effective c' and phi' (see Ladd, 1970, ASTM STP 740). The DSS is really a drained test at "constant volume". The work by Lacasse et al. shows good comparison of true undrained with the constant volume (Dyvik et al. Geotechnique March 1987).
Actually, the undrained shear strength is just one stress path (at constant volume) to the effective stress paths. I would just run the DS (or DSS) tests in consolidated steps to get Cr, Cc, and preconsolidation (sp'). Then shear to failure to get Phi' (assume c' = 0). The undrained shear strength can be obtained from critical state soil mechanics:
su = 0.5 svo' (sin phi') OCR ^ L where capital L = 1-Cs/Cc
or the more well-known empirical approach based on experimental data (Jamiolkowski, et al. 1985, ICSMFE; Ladd, 1991 ASCE JGGE Terzaghi Lecture):
su = 0.22 svo' OCR^0.8 where svo' = effective vertical overburden stress.
You can assume an interface friction (tan delta) between the pile and soil.