Stiffness Module and Compression Strength of Polymer-Treated Sand

According to the increasing demand of suitable soils under different types of foundation in the geotechnical engineering projects, many researchers try to find the best type of additives to enhance the strength of soils and to improve its mechanical properties. It is a big challenge to define the stress–strain behavior of the soil because it is complex and nonlinear. Young’s modulus (E) and the shear modulus (G) of the soil are not constant; it may significantly change with the strain level.

Moreover, the small-strain stiffness is an important parameter for various geotechnical design applications, including small-strain dynamic analysis such as those used to predict the soil behavior or soil–structure interaction during earthquakes, explosions, machine vibrations or traffic vibrations. Small-strain stiffness may also be used as an indirect indication of other soil parameters, as it (in many cases) correlates well to other soil properties. For example, when studying the hardening process of polymer treated soil, an increase in stiffness and compressive strength can be expected with increasing inter-particle cementation. At small strains, the stiffness is relatively high, while at strains close to the failure the stiffness is low. However, the behavior was sufficiently constant and linear below an approximate strain level of 0.001% [1].

The objective of this research is to study the availability of Free–Free Resonant frequency method (FFR) in measuring the Young’s modulus and Poisson ratio for (ETS). Moreover, detecting the effect of additives on the strength of treated sand by applying compression test on both types of treated specimens: (CTS) and (ETS).

The small-strain stiffness is usually determined in the laboratories by direct methods such as the bender/extender elements, or by indirect methods, such as the resonant column test. The free–free resonant frequency method (FFR) is a simplified testing procedure (based on the resonant column-testing concept) that has recently been used for the characterization of cement-treated soils [2, 3, 4, 5, 6, 7]. FFR testing was applied by allowing a cylindrical specimen to vibrate at its fundamental frequency and then, the stiffness was evaluated from the measured fundamental frequency, density and length of the specimen through a straightforward formula based on theories of one-dimensional wave propagation in an elastic rod. However, the interpretation of stiffness from the FFR results might be affected by the uncertainties that related to the boundary conditions (uncertainties of which the laboratory is not perfectly free) or to the diameter-to-length ratio (aspect ratio) of the specimen [4, 6] and to the nature of material whether it is isotropic or not.

The components of the specimens in this research was sand, epoxy and cement. The specific gravity of sand was equal (G_S= 2.6399) and grain size distribution was analyzed as shown in Fig. 1 according to ISO/DIS 17892-3. One type of epoxy was used, but it consists of two components A&B and the table 1 summarize the available properties for that type of Epoxy. In addition, a blast-furnace slag cement was used consisting of approximately 70% ground granulated blast furnace slag, 26% portland clinker and 4% gypsum and its minimal normalized mortar strength at 28 days was 42.5 N/mm² [8].

Figure 1.

15 cylindrical specimens were prepared and divided according to the epoxy additives percentage from 1% up to 5% with 1% step into five groups as shown in Fig. 2. Each group contains 3 specimens. In addition, nine cylindrical specimens have been prepared and divided into 3 groups according to the cement additive percentage 10%, 20%, 30% (Fig. 2a). Each group contains three specimens. Unfortunately, upon extrusion, the cemented specimens has failed. Therefore, another group of three specimens was reprepared containing 50% of cement + 50% of sand (Fig. 2b). Lastly, two groups of prism specimens were prepared (in order to apply the tests as explained in ASTM standards for the both shapes of specimens) more than shape of material, each group contains 2 specimens, at two epoxy additives percentages 3% and 4%, as shown in Fig. 3, just for comparison.

Figure 2.

Figure 3.

The process of preparing the specimens was as follows: the sand and the additives were mixed dry in a dough mixer for about 5 minutes until reaching a homogeneous paste form. The consistency of the paste after mixing remained plastic. Following, the mixture was poured into steel cylindrical molds of different aspect ratios and into wood molds for prism specimens. Then, the cylindrical molds were vibrated lightly in order to remove any trapped air bubbles. Then, the cylindrical specimens were cured for maximum one week, in a conditioned room at about 20°C, and for two weeks for the prism specimens (during this period, no FFR testing could be done on the specimens). Following the period of curing, the specimens were strong enough to be extruded from the molds and table 2 displays the final dimensions of the specimens after extrusion. Next, the FFR testing has been applied on cylindrical specimens and prism specimens, however, the compression test was applied on the cylindrical specimens only.

The objective of this research was firstly, to validate the free–free resonant frequency method and its interpretation to determine the small-strain stiffness moduli of polymer-treated soil. Secondly, to check the effect of epoxy additives on the strength of sand and comparing with the effect of cement on the same type of sand. The reliability of the measured fundamental frequencies obtained from the FFR testing were evaluated through calculating Young modules and Poisson ratio. According to previous studies [14] [15] [16] Young modulus (E) for sand is ranging (10–30) MPa but, upon adding the epoxy to the sand, its (E) modulus increased. For instance, E for the smallest specimens (highest aspect ratio) increased from 45 MPa up to 104 MPa at 1%, 5% respectively. This is a positive indication about the effect of the epoxy on the sand. E modulus increase when aspect ratio decreases i.e. the length of specimen increases. For our specimens, we keep the same diameter for all specimens but change the length so, the value of stress is still the same whereas the strain value changes. According to Hook law, the smallest specimens (highest aspect ratio) show the lowest value of Young modulus. Furthermore, Poisson ratio decreased as much as additives percentages increased, for example, it dropped from 0.3 to 0.15 at 1%, 5% respectively for aspect ratio 0.6, and this is logic trend between Young (E) and Poisson. At compression test on both types of treated soils: epoxy treated specimens and cemented treated one, the maximum load boomed! The maximum load, that specimen could bear, increased from 490 N up to 13300 N at 1%, 5% respectively. The maximum load of ETS at 3% epoxy additives (6670N) is a twice of the strength of the CTS at 50% cement additives (3264N). In addition, the smaller the sample, the greater its strength was observed. Finally, the above results have encouraged me to complete in researching and looking for another type of polymer additives and trying new scenario on cylindrical and prism specimens. It is recommended to use another tests method because we have used only FFR method in this research.