Preserving the Samples of SanFrancisco Bay Mud: A Case History

IGC 2009, Guntur, INDIA

PRESERVING THE SAMPLES OF SANFRANCISCO BAY MUD: A CASE HISTORY

S.K. Jain Assistant Professor, Department of Civil Engineering, Jaypee University of Information Technology, Waknaghat Distt. Solan H.P. –173215, India. E-mail: [email protected] Atul Nanda Assistant General Manager, Subsurface Projects Division, Engineers India Limited, 1 Bhikaji Cama Place, New Delhi– 110066, India. E-mail: [email protected]

ABSTRACT: Based on the guidelines given in standard geotechnical manuals, this paper presents an illustrated step-by-step procedure for preserving 5 inch (125 mm) Shelby tube samples of a soft clayey silt. Details included here are generally not found in standard manuals. It is shown that three layers of wax reinforced by two layers of gauge imparted enough stiffness to the 33 cm long cylindrical samples of soft clay so it could be turned upside-down and carried to a storage unit. The storage unit comprised of a steel closet incubated with a large dish of water. Laboratory tests, in particular the unconfined compression tests and UU triaxial test, are used to assert that the soil samples stored by the aforementioned procedure showed no signs of deterioration or loss in water content over a period of nine months.

1. INTRODUCTION Engineers, 1996; Nevada-DOT, 2005; Indian Standard, 1972). Five tubes, each approximately 15 inches (37.5 cm) This case history stems from a research project on the self- long, encased tightly in styrofoam boxes with styrofoam boring pressuremeter (SBPM) during the period 1975-1985 popcorn, arrived in Blacksburg on February 8, 1983. As at Stanford University and Virginia Tech. In the initial phase several months were to pass before the newly purchased of the investigation, field SBPM tests were carried out by triaxial testing equipment could be assembled, and a long Denby (1978) and Benoit (1983) at the Hamilton Air Force duration which is required in the testing of highly Base in Navoto, California (Denby & Clough, 1980; Benoit impermeable clays, it was thought necessary to extrude the & Clough, 1986). The soil underlying the base is locally samples from the tubes in order to prevent any deterioration known as the San Francisco Bay Mud. The soil is a highly of the soil (which occurs if the soil water reacts with the iron plastic silt of medium to dark gray color whose natural water present in copper tubes). content often exceeds its liquid limit, and thus, upon A steel extruder was designed and fabricated wherein the remoulding the soil turns into a thick “mud.” The soil is sampling tube could be held vertically and the sample extruded called “soft marine clay” by Duncan & Wright (2005). with the help of a hand-operated hydraulic jack. The San Francisco Bay Mud at the Hamilton Air Force Base has complete process is shown in Figure 1. In this essentially one- a saturated unit weight of 90 to 94 pcf (1.44 to 1.51 gm/cm³) man operation, a very small amount of force was required to and a plasticity index of 40, and is classified a MH soil in the push the entire sample (30 to 33 cm long) within the tube. Indian Standard Soil Classification system. The extruded sample was quickly coated with a layer of During the second phase of the investigation, undisturbed melted wax which was then covered with a single wrapping Shelby tube samples were taken from the SBPM test site and of gauze. A second layer of wax was now applied such that a comprehensive laboratory testing program was undertaken the entire gauze layer was completely soaked in the applied (Jain, 1985). The test data was needed for the finite element wax. The process was repeated with another application of modeling of the SBPM test (Johnston & Clough, 1983). gauze and wax coating. Layers of gauze acted as a reinforcement providing tensile strength to wax layers.

2. SAMPLING AND PRESERVING UNDISTURBED Thus, three layers of wax reinforced by two layers of gauze SOIL imparted enough stiffness to the cylindrical sample of soft clay so it could be turned upside-down in hands and carried Undisturbed piston tube samples of 5 inch (125 mm) to a steel closet, incubated with a large dish of water, for diameter were taken at the Hamilton Air Force Base on storage. The soil samples stored by this process showed no January 25, 1983, following standard sampling practices signs of deterioration or loss in water content over a period (ASTM D1587; ASTM D4220; U.S. Army corps of of nine months that were spent in the testing program.

595 Preserving the Samples of SanFrancisco Bay Mud: A Case History

Fig. 1: Photographs Demonstrating the Process of Extrusion and Preservation of the Undisturbed Samples of Bay Mud

596 Preserving the Samples of SanFrancisco Bay Mud: A Case History

3. LABORATORY TESTING OF BAY MUD Table 2: Results of Unconfined Compression Tests on Undisturbed Samples of San Francisco Bay Mud The laboratory effort in this investigation was directed Test Depth Water content ' S S towards the consolidation behavior of the Bay Mud. In σ vo u u m % 2 2 kN/m ' addition, a set of triaxial tests were carried out as the data kN/m σ vo was needed in constitutive modeling. A summary of the VUC-1 6.1 92 54.2 18.3 .34 testing program including a brief description of why each test VUC-2 6.1 90 54.2 17.8 .33 was undertaken, is presented in Table 1. Of particular interest ' here are the unconfined compression tests and the σ vo = effective overburden pressure unconsolidated – undrained triaxial tests. These tests were Su = undrained shear strength carried out to ascertain the quality of the samples. 3.2 UU Triaxial Test Table 1: Plan of Laboratory Testing One unconsolidated test was performed which gave a S /σ ´ Type of test No. of Purpose of test u vo tests ratio of 0.33, nearly equal to the average of unconfined Unconfined Compression i) Determine anisotropic compression test. A failure strain of 6–7% was noted. Tests 4 strength properties ii) Ascertain quality of 3.3 CU Triaxial Test samples Oedometer Tests Undrained tests were conducted on isotropically consolidated (a) Stress-controlled 9 Investigate the anisotropic samples with a back pressure of 90 psi. The stress state in all (b) Strain-controlled 3 Consolidation properties samples was brought initially to the virgin consolidation line Triaxial Tests through a consolidation pressure of at least 1.5 times the in- (a) UU 1 Ascertain quality of situ overburden stress, and subsequently unloaded to induce samples OCR effect if desired. Table 3 summarizes the useful (b) Consolidated- 6 i) Determine information extracted from the triaxial tests. Failure strains Undrained consolidation appear to have reduced to 3 to 4% as against 6 to 10% properties under reported previously (Bonaparte & Mitchell, 1979). Such isotropic pressure ii) Determine strength reductions in failure strains are attributed to a use of back properties under pressure. varying OCR The other laboratory tests including the consolidation and (c) Consolidated- 4 i) Determine Drained consolidation drained triaxial tests are well recorded in Jain (1985). The properties under test data was recently used by Jain & Nanda (2008a, b). isotropic pressure Prior to this investigation, the San Francisco Bay Mud at the ii) Data needed for Hamilton Air Force Base had been a subject of studies at the constitutive modeling University of California, Berkeley and at Stanford University for over two decades. Bonaparte & Mitchell (1979) have 3.1 Unconfined Compression Tests summarized the laboratory and field tests until 1978. The Two unconfined compression tests were conducted on results of our laboratory tests matched very well to those cylindrical samples of 1.4 inch (35 mm) diameter and 2.8 included of Bonaparte & Mitchell (1979). inch (70 mm) in height. Table 3: Results of Triaxial Tests To open a tube of stored Bay Mud, first a circular cut was Test Test type made by piercing the gauze-wax shell by a sharp knife. The

No. 2

´ u tube was then sliced through with the help of a wire saw. c f σ ´ % / S A / f φ i u

One of the halves was immediately sealed the gauge wax ε E ´ kN/m S c Depth, m Depth, process, and stored. σ Induced OCR OCR Induced The shell of the other half was removed with the help of a UU UU 6.5 - - .33 - 6–7 - - butcher knife. In most cases, only two cuts were necessary – CU-2 IC-U 6.5 72.7 1 .34 .86 3–4 27.2º - one vertical and the other horizontal (on the bottom CU-3 IC-U; const. p 6.5 107.1 1 .37 1.0 3–4 36º 530 periphery) – and the entire shell could be separated as one CU-4 IC-U; const. p 6.5 92.4 1.2 .40 .79 3–4 33* 974 unit. Four samples were extracted from a tube of CU-5 IC-U; const. p 6.5 101.4 1.5 .44 .68 4–5 34.7* 1035 approximately 6 inch long. *Based on c = 0. σc´ = effective consolidation pressure The Table 2 presents the results of the two unconfined Af = pore pressure parameter at failure compression tests. In the table, Su denotes the untrained shear Ei = initial Young’s modulus strength and, σvo´ , the effective overburden pressure. εf = axial failure strain

597 Preserving the Samples of SanFrancisco Bay Mud: A Case History

4. CONCLUSIONS Denby G.M. (1978). “Self-Boring Pressuremeter Study of the San Francisco Bay Mud”, Ph.D. Thesis, Stanford The case history described here lends support to the method University, California. of preserving soil samples by wax and gauge. Three coatings Denby G.M. and Clough G.W. (1980). “Self-Boring of wax with two layers of gauge, along with a steel closet Pressuremeter Tests in Clay”, J. Geotech. Engrg. Div., with a bowl of water, were found enough to preserve the ASCE, 106(12), 1369–1387. samples of soft clay for nine months. Indian Standard (1972). IS: 2132-1972, Code of Practice for Thin Walled Tube Sampling for soils. ACKNOWLEDGEMENTS Jain, S.K. (1985). “Analysis of the Pressuremeter Test by Authors express gratitude to Dr. G.Wayne Clough for help FEM Formulation of the Elasto-plastic Consolidation”, and guidance in this work. Ph.D. Thesis, Virginia Tech, Blacksburg, Virginia. Jain S.K. and Nanda A. (2008a). “Constitutive Modeling of San Francisco Bay Mud”, In Geomechanics in the REFERENCES Emerging Social and Technological Age, Proc. 12th Int. ASTM D1587 Practice for Thin-Walled Tube Sampling of Conf. IACMAG, Goa, 3158–3165. Soils. Jain S.K. and Nanda A. (2008b). “Determining Horizontal ASTM D4220 Preserving and Transporting Soil Samples, Permeability of San Francisco Bay Mud”, Proc. Indian Revised 2000. Geotechnical Conference, I.I.Sc. Bangalore, Dec. 2008, Benoit J. (1983). “Analysis of Self-boring Pressuremeter 24–27. Tests in Soft Clay”, Ph.D. Thesis, Stanford University. Johnston P.R. and Clough G.W. (1983). “Development of a Design Technology for Ground Support for Tunnels in Benoit J. and Clough G.W. (1986). “Self-boring Pressuremeter Soil–Vol. I: Time Dependent Response Due to Consolidation Tests in Soft Clays”, J. Geotech. Engrg., ASCE, 112(1): in Clays”, Stanford University Report to the Dept. of 60–78. Transportation, Rep. No. DOT-TSC-UMT-82–54. Bonaparte R. and Mitchell J.K. (1979). “The Properties of Nevada-DOT (2005). NDOT Geotechnical Policies and San Francisco Bay Mud at Hamilton Air Force Base”, Procedures Manual, Chapter 6: Geotechnical California, Report, Department of Civil Engineering, Investigation Procedures. University of California, Berkeley. U.S. Army Corps of Engineers (1996). Soil Sampling, Duncan J.M. and Wright S.G. (2005). “Soil Strength and Engineering Manual 1110-1-1906, Department of the Slope Stability”, Wiley NJ. Army.

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