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DISCUSSION on A Device For Extracting Large Intact Soil Samples
Developed and used in a remote region of Bolivia by Türker Mirata |
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DISCUSSION on A Device For Extracting Large Intact Soil Samples
Developed and used in a remote region of Bolivia by Türker Mirata |
This discussion has two parts: in the first, suggestions are made regarding the methods described by the authors; in the second, a reminder is given of an in situ test, developed as a simpler substitute for the in situ large shear box test, which the authors aimed at avoiding by the methods reported.
The steel stakes (nails) hammered around the plinth of soil result in an inevitable densification of the unsaturated soil. Much less disturbance would have resulted, had the split box, described by the authors, been used directly to confine the sample, as the periphery of the proposed plinth of soil was excavated in stages, following the procedure described in the Earth Manual (USBR, 1998), gently pressing the box down, to confine the exposed portion of the sample after each stage of excavation. The rest of the operations described by the authors could have then been applied.
Immersing an unsaturated sample of soil in water, in the way described, can never cause full saturation of the soil. This operation should more appropriately be called 'soaking' and what it does basically is to increase the moisture content and lower the existing suctions in the soil. The soil remains unsaturated, and this is probably the main reason for the high value of angle of internal friction measured. A strain rate of 0.4 mm/minute appears rather fast for the test to be qualified as 'drained' for such a size of clayey sample. The shear strength parameters reported by the authors are therefore assumed to be undrained parameters for a soaked, unsaturated clay, for the estimates in the rest of this discussion.
The authors rightly agree that it would have been preferable to carry out in situ tests, if this were feasible. The in situ wedge shear test ( iswest) (Mirata, 1974, 1991) was developed just for such circumstances where the application of a conventional large shear box test in situ would be impractical. Outlined by Mirata et al. (1998), this test entails the protection of a wedge of soil, having a shear plane area of 250 mm x 360 mm, by a 10 mm thick steel mould, while the wedge is sheared from the parent soil by means of a single hydraulic jack acting through two grooved plates with thirty 12 mm diameter steel balls rolling in between. The soil itself is used to provide the necessary reaction, and as both the normal and the shear load is applied through the single jack, the need for dead loading is eliminated. The total mass of equipment required to perform such a test is about 85 kg, and Cascini (1992), who has had the necessary equipment made on the basis of the information given in the original paper (Mirata, 1974) reports testing times as low as 2.5 hours for one test. More detailed information about the equipment, testing procedures, and the use of appended computer programs for the detailed evaluation of all three versions of this test is available in a recently updated manual (Mirata, 2000). Assistance needed during the application of the test may be reduced by the use of the semi-automatic displacement recorder (sadir) described in the accompanying technical note (Mirata, 2001).
The inclination of the slope and the relatively shallow depth of the potential failure plane render almost impossible the development of sufficient passive resistance to enable such a test to be performed directly at the orientation of the potential failure plane reported by the authors. What could have been done was to perform two tests, the first using a test mould with the angle between the shear plane and the direction of loading equal to 30o and the second (shown by dashed lines in Fig. 1) using a mould with the corresponding angle 45o. The same loading pit could be used for the second test, with wooden blocks placed at the rear of the pit. In a second pair of similar tests, if the direction of loading is reversed as shown in Fig. 2, the average inclination of the shear plane in each pair of tests using the same mould angle would match the inclination of the slope. Using the shear strength parameters measured by the authors, the weights of the various equipment used by the Author, and equation (22) given by Mirata (1974), which neglects the change in area during shear, the expected load at failure (P1, P2, P3 , and P4in Figs 1 and 2) would be around 1.36 kN, 3.91 kN, 0.96 kN, and 3.47 kN respectively. Using the simple equations for the evaluation of the test (Mirata, 1974, 1991), or feeding these values into the computer program IWPW77, making use of the sample data files (Mirata, 2000) yields normal stress - shear strength pairs (kPa) of (10.2, 8.9), (32.3, 26.0), (9.8, 9.3), and (32.1, 26.0) respectively for the four tests. Four tests are generally adequate to define the undrained shear strength parameters of the soil with sufficient accuracy, and as seen, the range of normal stresses are close to those on the potential failure plane. For somewhat lower normal stresses, test moulds with angles 25o and 40ocould be used in which case, by a similar analysis, it can be shown that the range of normal stresses would be reduced to between about 7.6 kPa and 20.0 kPa.
REFERENCES
1. Cascini, L. (1992). Discussion. Géotechnique 42, No. 4, 645-648.
2. Mirata, T. (1974). The in situ wedge shear test -- a new technique in soil testing. Géotechnique 24,
No. 3, 311-332. Corrigenda: Géotechnique 24, No. 4, 698; 25, No. 1, 157-158; 36, No. 1, 144; 37, No. 3, 420; 38, No. 1, 163.3. Mirata, T. (1991). Developments in wedge shear testing of unsaturated clays and gravels.
Géotechnique 41, No. 1, 79-100. Corrigenda: Géotechnique 41, No. 2, 296; 41, No. 4, 639.4. Mirata, T. (2000). Manual for Wedge Shear Testing of Soils. 170 pp. Obtainable from ULAKBIM (Turkish Scientific & Technical Documentation Centre); contact: Mrs. C. Yazicioglu at: cevza@ulakbim.gov.tr or ulakbim@gov.tr
5. Mirata, T. (2001). A semi-automatic displacement recorder. EJGE Paper 0107 in Vol. 6, 2001
6. Mirata, T., Gökalp, A., and Sakar, M. (1998). Achieving higher normal stress levels in the
prismatic wedge shear test. EJGE Paper 9804 in Vol. 3, 1998.7. USBR (1998). Earth Manual. Part 1, third edition. US Department of the Interior, Bureau of Reclamation, Denver.
