HALLETT Paul(1,3,4), DEXTER Anthony (1), BAUMGARTL Thomas (2), SEVILLE Jonathan (3), and HORN Rainer (2)

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1 Scientific registration no: 453 Symposium n o : 2 Presentation : oral - invit Changes to pore water pressure caused by indirect and direct tensile loading of unsaturated soil aggregates Variation de la pression de l eau dans des agrégats non saturés suite à l application directe et indirecte d une contrainte mécanique HALLETT Paul(1,3,4), DEXTER Anthony (1), BAUMGARTL Thomas (2), SEVILLE Jonathan (3), and HORN Rainer (2) (1) Soil Science Group, Silsoe Research Institute, Silsoe, Beds, MK45 4HS, UK (2) Institute for Plant Nutrition and Soil Science, Christian-Albrechts-University, D Kiel, Germany (3) School of Chemical Engineering, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Introduction The tensile strength of soil is an important physical property as it controls cracking caused by natural swelling and shrinkage cycles and fragmentation through tillage (Dexter, 1988). It is a complex property involving interactions between interparticle bonds, capillary stresses contributed by pore water, anisotropic stress distributions caused by the pore structure, and plastic deformation (Snyder & Miller, 1989). Most of the theoretical work conducted on soil tensile strength has been based on results obtained using simple laboratory test specimens formed using remoulded soil samples in which the pore structure is known (Harison et al., 1994, Hallett et al. 1995, Lima & Grismer, 1994). The more ambitious theoretical work (Snyder & Miller, 1989) and applied work on soil tensile strength used results of naturally structured soil with much more complex and difficult to quantify properties. Tensile strength measurements using undisturbed soil are important for assessing influences of soil management practices, and for predicting soil stability, fragmentation behaviour, and structural regeneration. A common natural test specimen for soil tensile strength measurements is the soil aggregate. There is contention concerning the suitability of using aggregates for these tests and the meaning of the tensile strength measurement. Many soil scientists argue that aggregates exist only in certain situations such as following cultivation. We refer to an aggregate as a structural entity with boundaries defined by the most extreme array of strength-diminishing pores of the next largest aggregate in size (Hallett et al., 1998a). Therefore, aggregates can also be mechanically released from the soil for investigation. This is a useful approach for examining scaling of structure since at each level of breakdown, the largest available pores are removed. (4) Present address: Soil Plant Dynamics Unit, Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA, UK p.hallett@scri.sari.ac.uk 1

2 The second aspect of contention is the testing procedure. The simplest and therefore most widely adopted technique for measuring the tensile strength of individual soil aggregates is the indirect tension test. In this test, the aggregate is loaded in compression at opposite poles which results in the development of a tensile stresses acting on planes passing through the longitudinal axis. A more complicated test is the direct tensile test in which the aggregate is adhered to loading grips and pulled apart (Farrell et al., 1967). This creates a tensile stress on planes perpendicular to the loading axis. With direct tensile tests, it is difficult to grip the samples and bending moments between the adhesive and aggregate may occur. The effect of the test procedure on the pore water pressure in unsaturated soil aggregates is still not known. We hypothesise that significant changes to pore water pressure will occur because of testing and the changes will be dependent upon the test procedure. This presents another significant problem to testing and the meaning of the tensile strength measurement since the pore water pressure at fracture has a large influence on tensile strength. Moreover, the complex pore structure of soil will complicate the prediction of these changes. In this paper, changes to the pore water pressure caused by indirect and direct tensile loading are examined both theoretically and through experiments where the pore water pressure is measured directly during testing using micro-tensiometers. These micro-tensiometers have a ceramic tip size of 1 mm, thus providing a local measurement of pore water pressure for aggregates as small as 10 mm diameter. Materials and Methods Soils Both artificial soil with a homogenous pore structure and highly heterogeneous soil aggregates were used for these experiments. The artificial soil was formed from a 667:1000 kaolinite (97.8% by mass < 1 ìm particle diameter) to fine sand (100% by mass < 180 ìm and 59.9% by mass < 63 ìm particle diameter) mass ratio. Soil aggregates mm in size were collected from a 2 m x 2 m quadrat in Bypass Field at the Silsoe Research Institute (Table 1). The pore water pressure of the samples was equilibrated using pressure plate apparatus. A bed of wet kaolinite was used to provide contact between the aggregates and the plate. Design and Use of Soil Microtensiometer The microtensiometer used to measure pore water pressure is similar to larger conventional tensiometers. It has a tip fabricated from thin-wall ceramic tubing with an outer diameter of 1 mm and air-entry pressure of 60 kpa. The ceramic tube is sealed at one end with a drop of melted glass and is inserted at the other end into a glass Soil Property UK Grid Map Ref. TL kg 100kg -1 % Clay 30.3±0.3 % Sand 42.2±0.6 % Organic Matter 4.3±0.0 Water Content 26.6±0.2 m 3 m -3 Degree of Saturation 0.71±0.00 kg m -3 Aggregate Dry Bulk 1579±17 Density Table 1: Properties of Bypass Field Soil tube which is melted around the ceramic to form a water-tight seal. The exposed ceramic tip is 2-3 mm long. A chamber, with a volume much greater than that of the glass tube, has a port which allows for total de-airing and houses a pressure transducer 2

3 which is used to measure the water pressure in the microtensiometer, corresponding to the pore water pressure of the soil in contact with the ceramic tip. A hole slightly smaller than the ceramic tip diameter was carefully bored into the test specimens to minimise the disturbance caused by the insertion of the microtensiometer. The microtensiometer tip was inserted into the sample and left to equilibrate, which took about 2 hours, before mechanical testing. Mechanical Testing Controlled mechanical strain was applied to the samples using a conventional loading frame. The outputs of a 5 kg load cell, an LVDT, and the microtensiometer were recorded using a data-logger. Figure 1 illustrates the loading configurations of the different mechanical tests. The indirect tensile test is simple, quick, and although elaborate equipment allows for more controlled testing, the indirect tensile test can be performed using very inexpensive, crude apparatus. The compressive force measured using the load cell, F, is converted to stress, ó, by using the approximation presented by Dexter & Kroesbergen (1985) in which, σ F = k 2 d where d is the geometric mean of the three principal axis lengths and k is a constant (1) σ Fastener & Grip Clamp Top Grip Soil Microtensiometer Removable Side Bottom Grip Nut Direct Tension-Remoulded Loading Platen Soil Aggregate σ Side View Inside Grip Free Moving Upper Fastener Upper Grip Soil Aggregate Lower Grip Grooves for Adhesive Indirect Tension-Aggregate Direct Tension-Aggregate Figure 1-Mechanical loading configurations for tensile tests. equal to Whether or not this value of k is appropriate is debatable and other authors have suggested different values (Rogowski & Kirkham, 1976; Hiramatsu & 3

4 Change of µ w to failure, kpa Oka, 1966). For the direct tensile test, σ=f/a, where A is the cross-sectional area of the incipient failure surface. All of the samples were loaded until tensile failure occurred. The loading rate was kept constant at 5 mm min -1 for the natural aggregates. A range of loading rates was applied to the artificial, homogeneous soil. Results and Discussion The influence of tensile loading on changes to the pore water pressure is most easily described for the ideal dogbone shaped test specimens which contain a simple pore structure and were saturated with pore water. In Figure 2, the applied tensile stress is seen to transmit almost directly to the change in pore water pressure, µ w. Such a change would be expected for quasi-saturated samples in which air-entry does not occur until the point of failure. The trend can be predicted easily by adapting Aitchison s extension to effective stress theory in which σ ' = σs + χ ( µ w), (2) Figure 2-Plot of changes to σ and µ w in where σ is the effective stress, σ s is the relation to strain, ε for a direct-tension strength contributed by soil bonds which bar sample. for these samples is negligible, and χ is the effective stress parameter. For these samples, χ equals the degree of saturation which is 1. 0 If different loading rates are 1:1 Line -2 applied so that viscosity also affects the change in pore water pressure (Hallett et -4 al., 1998b) (Fig. 3), pore water pressure changes are shown for these samples to -6 dominate the final failure stress. Therefore, it is inadequate to assume that -8 the final failure stress can be predicted from the pore water pressure at the onset -10 of mechanical loading. Existing models of soil tensile failure do not account for Change in σ to failure, kpa this effect (Snyder & Miller, 1989). Figure 3-Change in σ and µ w to failure for In Fig. 4 it can be seen that direct tensile the direct-tension bar samples. loading of natural soil aggregates also caused the pore water pressure to become more negative. Similar behaviour was found for 6 of the 8 replicates tested. A summary of the results is listed in Table 2. In the other two samples, the pore water pressure 4

5 Figure 4-Response of σ and µ w to direct tensile strain, ε applied to soil aggregates. Figure 5-Response of σ and µ w to indirect tensile loading of soil aggregates. fluctuated during loading and the change in pore water pressure at fracture was negligible. Indirect tensile loading of soil aggregates sometimes caused the pore water pressure to become less negative throughout the test (Fig. 5). Fluctuations in the pore water pressure change also occurred for some of the indirect tension tests with the pore water pressure becoming less negative at the onset of loading and then more negative at a point where the slope of ó/å increased (Fig. 6). Mechanical testing of natural soil is always subject to extreme variability. The results suggest this variability may be caused partly by the unpredictable changes to the pore water pressure caused by mechanical deformation. The changes in µ w are also far too unpredictable to use for obtaining as defined in Eq. 2. Moreover, the indirect tensile tests required a much greater calculated stress to cause fracture. This could be a result of bending moments produced with the direct tensile testing approach, failure in indirect tensile testing by combinations of shear, compression and tensile stresses, or an inadequacy of Eq. 1. Anisotropic distribution of stresses within the aggregate caused by the pore structure probably caused fluctuations in pore water pressure under mechanical loading. With increased mechanical strain, deformation may change the volumetric stress from compression to tension or vice versa. Change in σ to failure Change in µ w to failure Direct Tension [8] 2.24 ± ± 1.06 Indirect Tension (Total) [18] ± ± Indirect Tension (Positive change in µ w from start to failure) [6] ± ± 0.78 Indirect Tension (Negative change in µ w from start to failure) [12] ± ± Table 2-Summary of mechanical response of soil aggregates to direct and indirect tensile loading. The numbers in [] are the number of samples. Standard errors are listed. 5

6 More ideal soil samples containing known crack distributions need to be tested to assess this effect properly. Moreover, based on the concept of the aggregate hierarchy (Dexter, 1988), the degree of saturation and hence χ increases with decreasing aggregate size. Localised air-entry into the largest intraaggregate pores could cause pore water redistribution and fluctuations in its pressure. Effect of Loading Configuration Changes in stress caused by external loading by the indirect tensile test would be expected to have an opposite effect on pore water pressure than the direct tensile test because of the volumetric strain imparted to the aggregate. Volumetric stress change, p, is evaluated from the principal stresses, σ 1, σ 2, σ 3 as Figure 6-Fluctuations in response of µ w and σ to indirect tensile strain, ε. σ 1 + σ 2 + σ 3 p =. (3) 3 Positive stress values denote compression. Using photoelastic stress analysis, Frocht & Guernsey (1952) determined that for a sphere under indirect tension, σ 1 =σ 2 =-0.45F/A and σ 3 =+2.59F/A, where F is the applied load and A the area of the meridional plane of the sphere. For indirect tensile loading, p evaluated at the centre of a sphere using Eq. 3 is F/A. A positive value indicates that the sample is under volumetric compression. The results of the direct tensile test are much different. Given that the stress conditions for this test are σ 1 =σ 2 =0 and σ 3 =-F/A, Eq. 3 evaluates p as F/A. The negative value indicates that the sample is under volumetric tension. In summary, the indirect tensile test would be expected to cause the pore water pressure to become less negative with increasing load whereas an opposite trend would be expected with direct tensile testing. The volumetric stress probably does not account for all of the differences found between the direct and indirect tensile tests. In particular, it does not explain why in some of the indirect tensile tests the pore water pressure fluctuated with increasing loads. This effect could be due to the effect of the structural pores on the stress distribution which was not included in Frocht & Guernsey s (1952) analysis. Flattening of the sample in indirect tensile testing may also affect the change in pore water pressure. Flattening causes a greater transmission of the compressive load resulting in a diminished tensile stress transmission along the central longitudinal axis (Frydman, 1964). With greater flattening or ductility, the plastic shear deformation becomes dominant. The aggregate shape also affects the contact with the loading platen and therefore the stress transmission. This is one source of variability in strength testing using the indirect tension test (Dexter & Kroesbergen, 1985). 6

7 Conclusions The effect of mechanical loading on soil pore water pressure has been shown to be affected significantly by the mode of loading and the pore structure. Effective stress theory is too simple in its present form to predict the contribution of pore water to soil tensile strength. Most importantly, effective stress theory does not account for spatial fluctuations in pore water pressure which may be caused by the pore structure. The results of the indirect tensile tests question severely its applicability to non-dry aggregates. Whereas direct tensile testing resulted in pore water pressure becoming more negative (strain hardening), the indirect tensile tests sometimes showed an opposite trend. This effect could be attributable to the volumetric stress generated within the sample. Bibliography Dexter, A.R Advances in characterization of soil structure. Soil Till. Res., 11, Dexter, A.R. & Kroesbergen, B Methodology for determination of tensile strength of soil aggregates. J. Agric. Engng. Res., 31, Farrell, D.A., Larson, W.E. & Greacen, E.L A model of the effect of soil variability on tensile strength and fracture. Prepared for presentation at: 1967 Meeting-Am. Soc. Agron., Washington, D.C., November 5-10, 19 pp. Frocht, N.M. & Guernsey, F. Jr A special investigation to develop a general method for three-dimensional photoelastic stress analysis. Nat. Advis. Comm. Aeronaut. Tech. Pub Washington, D.C. Frydman, S The applicability of the Brazilian (indirect tension) test to soils. Aust. J. Appl. Sci., 15, Hallett, P.D., Bird, N.R.A., Dexter, A.R. & Seville, J.P.K. 1998a. Application of fractals to the scaling of aggregate structure and strength. Eur. J. Soil Sci.(in press). Hallett, P.D., Seville, J.P.K., Dexter, A.R., Baumgartl, T. & Horn, R. 1998b. Strain-rate dependency of capillary stress changes in wet particle agglomerates measured directly using microtensiometers. Powder Tech. (in press). Hallett, P.D., Dexter, A.R. & Seville, J.P.K The application of fracture mechanics to crack propagation in dry soil. Eur. J. Soil Sci., 46, Harison, J.A., Hardin, B.O. & Mahboub, K Fracture toughness of compacted cohesive soils using the ring test. J. of Geotechnical Engng-ASCE, 120, Hiramatsu, Y & Oka, Y Determination of the tensile strength of rock by a compression test of an irregular piece. Int. J. Rock Mech. & Min. Sci., 3, Lima, M.A. & Grismer, M.E Application of fracture mechanics to cracking of saline soils. Soil Sci., 158, Rogowski, A.S. & Kirkham, D Strength of soil aggregates: influence of size, density, clay and organic matter content. Med. Fac. Landbouww, Rijksuniv., Gent 41/1-1976, Snyder, V.A. & Miller, R.D Soil deformation and fracture under tensile forces. [In] Mechanics and related processes in structured agricultural soils (ed. W.E. Larson). Kluwer Academic Publishers. pp Toll, D.G A framework for unsaturated soil behaviour. Géotechnique, 40, Keywords : aggregate, water potential, soil mechanics, microtensiometer Mots clés : agrégat, potential hydrique, mécanique des sols, microtensiomètre 7