Liquefaction flow behavior of Guamo sand

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1 470 From Fundamentals to Applications in Geotechnics D. Manzanal and A.O. Sfriso (Eds.) IOS Press, The authors and IOS Press. All rights reserved. doi: / Liquefaction flow behavior of Guamo sand OMAR JAVIER JIMENEZ JIMÉNEZ a,1 ARCESIO LIZCANO PELÁEZ b a Universidad Pedagógica y Tecnológica de Colombia, Tunja, Colombia b SRK Consulting, Vancouver, Canada Abstract. In soil mechanics there are many unknowns about the behavior of materials and even more great uncertainty generated sands, due to its gradation and shape of the grains is sometimes difficult to know their behavior and especially when trying to study by tests in laboratory. This research contains the results of analyzing the behavior of Guamo sand, representative of research in Colombia material by analyzing the effect of liquefaction flow by performing triaxial undrained (CU), for which he developed a series of tests taking into account the relative density of the material as well as the constructive effect of maintaining a constant gradation samples. Keywords. Liquefaction, Triaxial, phase-transformation. 1. Introduction Know the behavior of sands generates great uncertainty, this due to its gradation and particle shape, and its status conditions in the basement, where it is sometimes difficult to study their behavior and especially when aiming through laboratory tests, since is difficult to obtain undisturbed samples due to its structural condition, given this situation becomes important to implement techniques to know the status of these materials. In this particular case it was decided to perform a study of the behavior of sands, with reference Guamo sand, representative of research in Colombia material. Analyzing a phenomenon known as liquefaction, something that could risk the collapse of structures due to major changes in the status of stress material. However it is important to mention that the issue of the behavior of sands has been studied with great intensity worldwide, making very important studies that have marked this concept, as is the case Alarcon et al. [1], Yoshimine [2], Verdugo et al. [3], Lade et al. [4], among others. Thus the different phenomena encountered in the behavior of sands will be described, including the state of monotonic undrained failure which is simulated by triaxial tests. 2. Development triaxial test 2.1. Characterization of test material The material under study, corresponds to Guamo sand, sand taken as reference under multiple studies in Colombia, mainly by the Geotechnical Research Group at the 1 Corresponding Author: Tel ; Cel ; omarjjj@gmail.com (O. Jiménez)

2 O.J. Jimenez Jiménez and A. Lizcano Peláez / Liquefaction Flow Behavior of Guamo Sand 471 University of the Andes, such as Patiño (2006) [5], Arias (2006) [6], Gómez (2010) [7] and De la Rosa (2011) [8]. This material is from the town of Guamo, Tolima - Colombia, and is characterized as light gray, with a distribution of poorly graded sizes between 2 mm and mm particles and sub-angular shape, as shown in Figure 1. It has a mean particle diameter of D 50 = 0.51 mm G s= 2,624, void ratio maximum and minimum e max=0.916 and e min=0.546 respectively. In triaxial tests analysis is necessary to know the effect of generating rigidity rubber membranes, in order to make the appropriate corrections and stiffening effect required membrane penetration characterize the material used to make the module elastic, so tensile testing was carried out, finding a mean value of Em = 1350 kpa Sample preparation methods Figure 1. Grain size Guamo sand. Samples for conducting shaped triaxial tests were built with cylindrical diameter of approximately 50 mm height 100 mm, samples were mounted considering the thickness of rubber membrane that covers the entire sample. One of the main problems in the implementation of triaxial tests with sand, is the construction of the specimens; since from this first step the sample to be analyzed may change materially behavior fails, there are different techniques to construct these specimens, an important aspect in their construction is to seek to settle the specimen achieve adequate relative density, the methods used for mounting specimens was the method of dry deposition with funnel (DFD), this method was applied in research by Zhang et al. [9], Hyodo et al. [10] Saussus et al. [11] among others, as also the sedimentation method in water (WS) and moist tamping method (WT) technique that is widely used worldwide as recorded Gómez [7], Lade [4] used, Saussus et al. [11] Yoshimine et al. [12], among others Saturation In granular materials make the saturation process is somewhat complicated, because depending on the method used to manufacture the specimen, it can store a large content of air, thus making the saturation process may take too long and sometimes can reach the maximum pressure application equipment, exhausting pressure range to perform the consolidation of the material, as when assemblies for loose sands is made; due to this

3 472 O.J. Jimenez Jiménez and A. Lizcano Peláez / Liquefaction Flow Behavior of Guamo Sand problem achieving obtain a value of B = 1.0 according Sqempton parameter is a bit complicated, so it is possible to consider a saturated state when it reaches a value of B = Different authors recommend the use of CO 2 for saturation, however the method used in various tests was injection cycles, as described by Gómez [7], however it is important to note that the time waterflood in each cycle carried out depend on the stabilization of injection flow therefore should not handle a fixed time, but this time is variable which depends on the material density and degree of saturation which has the sample in each cycle waterflood Testing the effect correction rubber membrane In developing triaxial tests with granular materials, there is an effect generated by the rubber membrane element used in the assembly of the specimens; these membranes have two effects: one generated by the volumetric change at the time is applied confining pressure in chamber known as membrane penetration and the second generating the effect of lateral restraint with increasing axial deformation termed Restriction membrane. These effects are presented according Molenkamp et al [13], which has the form of correction for this effect on the performance of triaxial tests, this procedure was applied to all assay shown in Figure 2, an example of setting shown in a specimen tested. (a) q (b) q - p Figure 2. Correction for penetration and restriction membrane of Guamo sand, t = 0.82 mm. 3. Results of tests Triaxial tests conducted were developed under undrained conditions monotonic trial type (CU), to which 20 specimens consisting Guamo sand is failed, keeping the material uniform gradation but varying its relative density (Dr), in Table 1, the summary of tests performed indicating its main features to consider in the test performance is presented Sedimentation effect construction of samples As mentioned constructive method plays an important role in the behavior of the material, this test in Figure 3, the behavior of different samples that have been tested using different construction methods, while maintaining approximately the same void ratio is submitted, You can see the sand generates a change in the behavior of the trajectory of

4 O.J. Jimenez Jiménez and A. Lizcano Peláez / Liquefaction Flow Behavior of Guamo Sand 473 effort showing greater contractante effect when the method of WS, while its effect is reduced when working with DFD method, while the WT method demonstrates a intermediate behavior. Table 1. Summary triaxial tests Test eo Dro (%) ec Drc (%) 3 (kpa) M.P. t (mm) GU WT 0.40 GU WT 0.40 GU WT 0.40 GU WT 0.40 GU WT 0.40 GU WT 0.40 GU WT 0.40 GU WT 0.40 GU WT 0.40 GU WT 0.40 GU WT 0.40 GU WS 0.49 GU WS 0.45 GU WS 0.40 GU WS 0.40 GU DFD 0.40 GU WT 0.40 GU WT 0.48 GU WT 0.25 GU WT 0.82 Where: e o = Void ratio before consolidation e c = Void ratio after consolidation Dr o = Relative density before consolidation Dr c = Relative density after consolidation M.P.= Method of preparation This effect produces an incomplete knowledge of the material is taken, so different authors recommend working with the WS method, which allows contractante meet fully the behavior of the material, however, this method is quite complex for the preparation of samples since it is not easy to reproduce samples with low specific gravity, making it impossible to know the behavior in a loose state of the material, which is important in characterizing the material. Therefore, the method using the WT is convenient, although not fully reproduce the contractant effect, however the method may be reproduced low density specimens allowing knowledge on the behavior of the material loose state. This effect generated by the construction method shown in Figure 4 (a), where it is observed as the phase transformation line (PTL) changes depending on the construction method.

5 474 O.J. Jimenez Jiménez and A. Lizcano Peláez / Liquefaction Flow Behavior of Guamo Sand a) 3 = 100 kpa b) 3 = 400 kpa Figure 3. Behavior generated by construction method 3.2. Answer sand undrained monotonic loading One of the main important states in reaching the stage of cutting a material is determined by the critical state or stable state, to achieve this state is important to seek the necessary deformation so that the deviator stress reaches a steady state, the state critic can be achieved in equal magnitude when tests are performed by taking an equal void ratio after completion of the consolidation phase, where I applied a varying confinement always get to this state with equal magnitude, proof of this is presented in Figures 4, 5 and 6 wherein the behavior achieved in samples with approximately equal void ratios, the critical state is reached at a constant value equal deviator stress, in each case changing the path behavior of principal stresses. In each scheme can be seen as the path of effort reaches the critical state line (CSL), the slope having this line from the origin at an angle that correlates the angle of critical friction for this item corresponds to a value of 34.5 º Analysis of the behavior of liquefaction flow To analyze the behavior of liquefaction flow is important plot all results in the same scheme must therefore be represented in the plane e - p ', where a number of states that develop over a test indicated cut, this allows you to draw a series of lines been known as steady state line (SSL) line undrained instability (UIL) line of quasi-steady state (QSSL) and phase transformation line (PTL) of each initial condition containment; in Figure 7 the graphical analysis of the results obtained for 20 trials in undrained conditions occurs, however in relation to the state of phase transformation (PTS), the lines should be made depending on the construction method of sample, in this case only shown in Figure 7, the result according to the WT method, since the other two methods worked the number of trials are very minimal.

6 O.J. Jimenez Jiménez and A. Lizcano Peláez / Liquefaction Flow Behavior of Guamo Sand 475 (a) q - p (b) q - Figure 4. Undrained test, loose condition. (a) q - p Figure 5. Undrained test, average condition. (b) q - (a) q - p Figure 6. Undrained test, dense condition. (b) q -

7 476 O.J. Jimenez Jiménez and A. Lizcano Peláez / Liquefaction Flow Behavior of Guamo Sand Figure 7. Analysis of results e - p' (method WT), Guamo sand 4. Conclusions This research allowed to know the mechanical behavior of sand Guamo, using laboratory tests and was the main event of triaxial compression, finding different swing states which may have a granular material as presented different theories here evaluated, where there is similarity in it. According to the obtained results showed that the critical angle is not dependent friction specimen construction method, but depending on the construction method is the response characteristics of the material. The liquefaction flow can only occur when the material is fully saturated, so when triaxial tests are conducted is important to minimize the presence of air in the specimen, otherwise the behavior exhibited at the stage of failure of the material will affected, reducing the trajectory contractante which is the main cause of presenting the phenomenon of liquefaction. It is thus clear that when triaxial tests are performed with sand, it is important correction assay membrane effects in terms of penetration and restrictions, so it is advisable to work with thin membranes with thickness less than 0.40 mm, greater thicknesses produces a greater restriction on the specimen side at the stage of failure, which exhibits greater change in relation to thin membranes; despite the thin membranes that allow greater penetration effect is less noticeable than that produced by the restriction membrane. Guamo sand is vulnerable to the liquefaction of low-grade, determined by a small contractante behavior, being evidence of liquefaction at low flow confinement pressures and Dr <25%. References [1] Leonards G.A. Alarcon, G.A. and J.L. Chameau. Undrained monotonic and cyclic strength of sands. Geotechnical Engineering, 114(10): , 1988.

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