Loading...
Please wait, while we are loading the content...
Undrained monotonic and cyclic behaviour of a silty sand stabilized with colloidal silica
| Content Provider | Semantic Scholar |
|---|---|
| Author | Vranna, Antigoni Tika-Vasilikou, Theodora |
| Copyright Year | 2015 |
| Abstract | This paper presents a laboratory investigation into passive site stabilization of liquefiable silty sands by means of colloidal silica, CS. The improvement of the mechanical behaviour was examined, by conducting an extensive testing program comprising monotonic and cyclic triaxial tests performed on a quartz silty sand stabilized with CS. The stabilized specimen preparation method adopted in the tests is initially described and then results from the above tests conducted on treated and untreated specimens are presented. It is indicated that stabilization of a silty sand deposit with CS significantly improves both the undrained monotonic and cyclic resistance strength. Surprisingly, the monotonic undrained shear strength of samples that had been cyclically loaded to a double amplitude axial strain of at least 5%, was comparable to the monotonic undrained shear strength of samples that had not been cyclically loaded. Introduction Liquefaction of sandy soils is one of the major causes of damage in earth structures and foundations during earthquakes. Over the last decades widespread liquefaction-induced ground deformation and related damage to foundations has occurred as a result of urban expansion and building on liquefaction prone sites. Thus mitigation and prevention of damage due to liquefaction under existing developed sites is one of the main issues of seismic design. To this extent, the technique of passive site stabilization of liquefiable soil under existing structures has been proposed (Gallagher & Mitchell 2002). This method is based on the use of nanomaterials, such as colloidal silica, laponite and bentonite among others, and involves slow injection of the stabilization nanomaterial into the liquefiable soil by means of natural or augmented groundwater flow. In particular colloidal silica, CS, is an aqueous suspension of microscopic silica particles produced from saturated solutions of silicic acid, H4SiO4 (Iler 1979). In dilute solutions, CS has a density and viscosity similar to water and can be made to gel by adjusting the ionic strength or pH of a given solution. This property allows it to be injected or mixed with soil, so that after gelling colloidal silica blocks the void space in the soil and therefore alters its mechanical behaviour. The principal advantages of CS over other potential stabilizers are its excellent durability characteristics, its initial low viscosity and the ability to attain low permeability in grouted soils, long controllable and reproducible gel times, non-toxicity and its low cost. Previous studies on passive site stabilization by means of CS in laboratory, concerned mainly PhD student, Department of Civil Engineering, Aristotle University, Thessaloniki, Greece, avranna@civil.auth.gr Professor, Department of Civil Engineering, Aristotle University, Thessaloniki, Greece, tika@civil.auth.gr clean sands (Kabashima & Towhata 2000; Gallagher 2000; Gallagher & Mitchell 2002; Mollamahmutoglu & Yilmaz 2010; Vranna & Tika 2015) and very few (Díaz-Rodriguez et al. 2008) have been conducted on silty sands, mainly because of their lower permeability as compared to clean sands. With increasing application of passive site stabilization, however, there is need to better understand the behaviour of liquefiable sands that contain fines and are stabilized with CS under different loading conditions, as well as to assess the limits of the applicability of this improvement method to these soils. To this extent, a series of monotonic and cyclic tests was performed on a silty sand, stabilized with CS. The effectiveness of the CS stabilization was investigated by conducting also a series of monotonic and cyclic tests on untreated silty sand specimens. The results from the two series of tests are presented and discussed. Experimental Procedure Tested Materials The soil used in this study is a quartz silty sand with non-plastic fines content of fc = 10%. It has a specific gravity Gs =2.653, maximum and minimum void ratios of emax = 0.682 (ASTM D4254) and emin = 0.414 (ASTM D4253) respectively, a mean diameter D50 = 0.30mm and a uniformity coefficient of Cu = 4.13. Its gradation curve lies within the bound gradation curves, suggested for liquefiable soils. Its permeability was estimated within the range from 0.91∙10 to 1.33∙10 m/s. Ludox SM-30 was selected as the stabilizing agent of specimens, supplied as a 30% by weight silica solution with a viscosity of 5.5cP, a pH of 10 and an average particle size of 7nm. Distilled water was added to the initial solution in order to obtain concentrations of 6% and 10% CS. Gel times of the studied solutions were investigated by conducting viscosity measurement tests by means of a rotating Brookfield viscometer. Figure 1 presents typical test results for CS = 10% solutions with the same pH value and different salinity (Fig. 1a) and vice versa (Fig. 1b). It is noted that gel time was defined as the elapsed time for which the tested solution viscosity is equal to η = 3.5cP. Beyond that value, viscosity increases rapidly and eventually the solution transforms into a rigid gel. It was decided to employ a CS gel time equal to 10 and 11 hours for the CS = 10% and 6% solutions, respectively, which was determined by adjusting the pH value to pH = 6.0, as well as the appropriate NaCl concentration of the solutions. Testing Programme Cylindrical specimens (height/diameter ≈ 100mm / 50mm) were prepared at various densities, using the undercompaction method, as proposed by Ladd (1978), both for the untreated and treated silty sand. Saturation was achieved by percolating throughout the specimen, first carbon dioxide gas (CO2) and then de-aired water. Following, the CS solution was likewise injected into the specimens until it filled the soil voids. The procedure was assumed complete when a solution volume equal to four times the soil specimen volume was extracted from the top of the specimen. The viscosity of the CS solution remained low (η < 3.5cP) throughout the specimen percolation process. Figure 1. Variation of viscosity, η, with time, t, for CS = 10% solutions with (a) pH = 6.00 and different NaCl concentrations and (b) 0.15N NaCl concentration and different pH values. After the setting of CS, specimens were placed in a constant temperature and humidity chamber for a curing time of five times the CS gel time (η > 1000cP). Saturation of treated samples prior to testing was not performed, due to the infilling of pore spaces with CS and the possibility of damaging the formed CS bonds. It was assumed, therefore, that total p = (σα + 2σr) / 3, and effective p ́ = (σ ́α + 2σ ́r) / 3, confining mean stresses, coincide. The monotonic testing programme consisted of undrained isotropically consolidated tests on untreated and unconfined compression, as well as undrained consolidated tests on treated silty sand specimens. All types of tests were performed using a closed-loop automatic cyclic triaxial apparatus (M.T.S. Systems Corporation) (Vranna 2015). In the monotonic tests, specimens after isotropic consolidation under p ́0, were subjected to undrained compression at a constant strain rate of 0.1%/min. In the cyclic triaxial tests, a sinusoidally varying axial stress (±σd) was applied at a frequency of f = 0.1Hz, under undrained conditions. In this work, the occurrence of double amplitude axial strain, εDA = 5% is used as a reference point to define cyclic softening of both treated and untreated specimens. For this reason, a series of cyclic triaxial tests with different cyclic stress ratios, CSR = σd / 2p ́0, was carried out in order to determine the number of load cycles, N, required for the development of εDA = 5% both for the treated and untreated specimens. In view of the typical number of load cycles of actual earthquakes (10 to 20 for an earthquake of M7.5 magnitude), in this work the onset of liquefaction and thus the cyclic resistance ratio, CRR15, is considered as the cyclic stress ratio, CSR = σd / 2p ́0, required to produce εDA = 5% in 15 load cycles. Confining stresses of either 100kPa or 300kPa were used in the tests. Test Results Monotonic Response Figure 2 presents the Mohr-Coulomb peak shear strength envelopes of the untreated specimens as well as the specimens treated with CS = 6% and 10%, at a relative density, Dr = 31-37%. It is shown that the increase in strength of the treated specimens over that of the untreated is due to the increase in both friction angle, but most mainly in cohesion. This indicates that introduction 0.01 0.1 1 10 100 time, t (h) 1 10 10 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://secure.tcc.co.nz/ei/images/ICEGE15%20Papers/Antigoni_83.00.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
