Field Vane Test (FVT)

Introduction

The field Vane Test (FVT) was developed for determining the undrained shear strength of soft clays and was used in Sweden as early as 1919 by Olsson (Kallstenius 1956). The vane was developed in part to overcome problems with obtaining undisturbed samples, especially in soft clay, for determining laboratory undrained shear strength. In the early 1950s, observations often showed that the shear strength from laboratory tests were low compared to back-calculated strengths from field cases. In soft clays, early use of the FVT often showed higher strengths as compared to traditional laboratory tests.

The results of undrained strength measurements obtained with the FVT in soft clays are often used as the reference value of in situ undrained strength for many engineers. Correlations developed to estimate undrained strength from other in situ tests, for example, the CPTU or DMT, often rely on reference values obtained from the FVT.

The most common application of the FVT is in soft to medium stiff saturated clays; however, there are a number of reported uses of the field vane in other materials, including organic soils and peat (e.g., Helenelund 1967; Northwood &c Sangrey 1971; Landva 1980; Faust et al. 1983; Muhuai et al. 1983). FVTs may be difficult to perform in very stiff overconsolidated clays or fissured clays. Test results in very stiff clays can be difficult to interpret as a result of questions related to drainage conditions, failure conditions, etc. FVTs conducted in these materials generally do not fit into the framework of normal FVT interpretation developed for soft clays.

Mechanics

The FVT is conducted by inserting a thin four-blade-vane into the ground and then rotating the vane to create a shear failure in the soil, as shown in Figure 5.1. The usual geometry of most vanes is rectangular with a height to diameter ratio (H/D) of 2 although special vanes with different H/D ratios may be used to evaluate anisotropy. Thin blades are normally used in an attempt to reduce the amount of disturbance to the soil structure and state of stress prior to shearing. The undrained shear strength of the soil is not measured directly in the test, but the maximum torque needed to cause rotation and failure of the soil is measured. To estimate the undrained shear strength from the torque, a number of simplifying assumptions need to be made regarding the failure mechanism.

Figure 5.1 Principle of Vane Shear Test.

Equipment

There are generally four ways that the vane may be inserted to perform the test: (1) using an unprotected vane advanced at the base of an open or cased borehole; (2) using an unprotected vane with a double-rod system without a borehole; (3) using a protective housing in which both the rods and vane are protected without using a borehole; or (4) using both unprotected rods and an unprotected vane without a borehole with a rod slip coupling located just above the vane. A schematic of these methods is shown in Figure 5.2. These methods are primarily related to vane tests performed on land.

The pushing thrust to advance the vane is usually provided by hydraulic pressure from a drill rig or a small reaction frame operated by either a hand crank mechanism or a portable hydraulic cylinder. The torque is normally measured using some form of a calibrated torque head operated by a simple hand crank located at the ground surface. The different techniques for deploying the vane have developed as a result of efforts to make the testing more efficient while at the same time retaining high quality testing. Each of the methods may be used in conjunction with a borehole, a condition that may arise out of necessity to advance through surface materials, such as fill, sand layers, or very stiff crusts. Geise et al. (1988) described a remote offshore vane that uses the housing as torque reaction down hole. Other successful uses of field vane offshore have been reported (e.g., Young et al. 1988)

Unprotected Vane Through Casing

When pushed at the bottom of a borehole as shown in Figure 5.2a, the vane is normally inserted through a casing or hollow-stem augers since an open borehole may collapse or squeeze, especially in soft clay. The use of rod centralizers inside the casing helps keep the vane rods vertical and centered at the base of the hole and also helps prevent bending of the rods during pushing. The casing or hollow stem augers then become the reaction base for the torque head.

This method of testing can be slow since the vane must be withdrawn after each test and the casing and borehole must be advanced to a new test depth before performing the next test.

Figure 5.2 Methods of inserting field vane, (a) unprotected vane through a cased borehole, (b) unprotected vane with protected rods, (c) protected vane and rods, and (d) unprotected vane and rods with slip coupling.

Open hole drilling with just a surface casing may also be used. The measured torque will include a small unknown component of soil friction on the rods just above the vane, which is difficult to account for in the data reduction. Additionally, the drilling may produce unknown disturbance and stress relief effects at the bottom of the hole, both of which may lead to errors in interpretation. Because of these difficulties, this method of testing is considered to be the least desirable approach to FVT.

Protected Rods and Unprotected Vane

The use of a double-rod system to advance the rods and vane as shown in Figure 5.2b increases testing efficiency and eliminates the need for a borehole, provided that the system can be advanced by pushing from the surface. Some difficulties may be encountered pushing through fill or a very stiff crust near the surface, and usually an open or cased hole is needed. The outer protective rods act to prevent the inner vane rods from bending during pushing and help eliminate friction on the inner rods. Advance of the system is stopped above the test depth, and then the inner rod is pushed further to force the vane out ahead of the outer protective rods while the outer rods are held fixed.

Protected Rods and Protected Vane

The use of a protective vane housing was presented by Cadling & Odenstad (1950) as a means of eliminating rod friction and protecting the vane from bending but still allowing the vane to be advanced efficiently. The system is shown in Figure 5.2c and allows the vane and rods to advance while being fully protected. The advantage of using a protective vane housing is that the vane itself may be machined with very thin blades in an attempt to reduce soil disturbance (e.g., Geonor SGI Vane Borer H-10). To perform the test, the system is pushed to a depth just above the test depth, and then the vane is advanced just ahead of the protective housing while the inner rods remain free from contact with the soil.

Unprotected Rods and Unprotected Vane with Slip Coupling

In this case, both the vane rods and vane are unprotected as shown in Figure 5.2d and are pushed from near the surface without a borehole (e.g., Nikon Vane Borer M-1000). A rotary slip coupling is located between the rods and vane. The slip coupling allows a rotation of the rods of about 15° before the vane is engaged. The torque measurement during this initial rotation represents the friction acting on the rods alone without any torque being applied to the vane. Once the vane is engaged, the measured torque represents both rod friction and soil resistance from rotation of the vane. The portion of the measured torque attributed to just the vane is obtained by subtraction. This method of testing is efficient, especially in soft and very soft clays where the rods and vane may be inserted with little pushing effort provided the rods do not bend. Figure 5.3 shows a photo of the torque head of the Nikon Vane Borer. A sample recording of the measured torque showing the rod friction, peak strength, post-peak strength, and remolded strength scribed onto the paper disc is shown in Figure 5.4.

Vanes

Different manufacturers supply different size vanes to suit different soil conditions. Common sizes include vanes with nominal diameters of 50mm (2 in.), 65 mm (2.5 in.), and 80 mm (3.1 in.). Some vanes have a tapered blade cross section and are tapered at the base and have rounded corners at the top as shown in Figure 5.5. Rectangular vanes with blades

Figure 5.3 Vane shear test in progress using Nikon M-1000 Vane Borer Torque Head attached to casing.

Figure 5.4 Sample wax chart recording of torque from Nikon M-1000 Vane Borer.

Table 5.1 ASTM D2573 recommended dimensions of field vanes

of constant cross section are preferred to reduce uncertainties with interpretation of the results. Table 5.1 gives different sizes of vanes suggested by ASTM D2573. A vane height to diameter ratio of 2 is specified in ASTM D2573.

It is noted by ASTM D2573 that “the selection of the vane size is directly related to the consistency of the soil being tested, that is, the softer the soil, the larger the vane diameter”; however, this may cause some problems with interpretation as will be discussed. Also, because of the dimensions given in Table 5.1, the level of disturbance created by all of the vanes is not consistent, since the perimeter ratio (defined in Section 5.6.1) for each vane is not constant.