This paper presents recommendation for conformity assessment for vane shear tests (VST) at rotation rates faster than standardized by ASTM and equivalent societies/institutes. The proposed rationale is based on compensating the standard requirements by recording downhole torque and rotation versus time. Such supplementaty measurements are commonly available when using advanced onshore and offshore VST systems. These systems allow a good check on use of "time to failure" for correcting a faster rotation rate to the standard rate. An obvious benefit is a reduced overall field testing time.
The vane shear test (VST), or field vane test, is one of several in situ test methods commonly used to determine undrained shear strength and strength sensitivity in water-saturated clay and silts soils. It has a long history of use1,2 Figure 1 present basic test principle.
A perceived advantage of the ST is its theoretical model for data interpretation, ie. Lower bound limit analysis for well-defined failure planes. Basic output consists of undrained shear strength, suvfor undisturbed conditions. Undrained shear strength for remoulded (or "residual"), sur conditions can also be obtained and, hence, the strength sensitivity is defined by St = suv/sur Empiricism is still required to link this output with the parameter values required for common geotechnical calculation models. These models generally include simplifications so that factors such as strength anisotropy and time-dependency are accounted for only in an approximately manner.
In geotechnical practice, the VST method competes primarily with continuous in situ penetration test methods. The cone penetration test (CPT) is the most widely used method such T-bar penetration and ball penetration tests is increasing3.
The VST is slow test in comparison with the CPT, T-bar or ball penetration tests. This paper presents recommendations for a conformity assessment4 for VST at rotation rates faster than the standards of ASTM International and equivalent societies/institutes5,6,7. The rationale is based on compensating the standard requirements by recording downhole torque and rotation versus time. Such supplementary measurements are commonly available when using advanced on shore and offshore VST systems.
All current VST standards fall in the group "method specifications" i.e if the method complies, then some undefined measurement accuracy may be presumed. In comparison, the more recent CPT standards provide for "performance specifications", i.e. requirements for accuracy of measurement.
Advanced VST systems emerged in the time of rapid growth of offshore oil and gas development8. They are now in common use, mainly for offshore site characterization (www.fugro.com) but also for advanced onshore application (www.envi.se; www.geotech.se). As for conventional VST systems, advanced systems allow tests to be conducted beneath the terminus of an open borehole during rotary drilling operations, or alternatively, by using direct-push technology where the vane is deployed at the front end of pushrods with or without a housing chamber.
Figure 1: Procedures for field vane testing during boring advancement
The advanced VST systems surpass the measuring requirements set by ASTM International5, British Standards Institute (BSI)6 and the International Organization for Standardization (ISO)7.