strength tests were obtained—according to ASTM D2166- 06—using a strain rate of approximately 1 %/min for silt/biopolymer specimens. A falling head permeability test was carried out according to specification ASTM D5084-03 to determine the permeability of the soil/biopolymer specimens. Measurement results Fig. 2, Compaction characteristics for sand/biopolymer mixtures Fig. 3 Effect of viscosity on maximum dry density for sand/biopolym nixtures lower densities. Simultaneously, the optimum water content (OWC) was observed to increase with increasing gum con- centrations for all specimens. This is due to increasing amounts of water being absorbed through the biopolymer with increasing concentration. For better understanding of the biopolymer-density behavior, biopolymer solution viscosities were measured for all concentrations using a rotational vis- cometer at the testing room temperature of 25 °C. The maxi- mum dry density for sand/biopolymer mixtures is strongly related to the solution viscosity, which is seen in Fig. 3. The mixture dry density initially increased with increasing solution viscosity for the three sand/biopolymer mixtures. The mixture dry density reached its peak point at a viscosity of approxi- mately 40 to 50 mPa s depending on the biopolymer. After the peak density, the high-viscosity solutions shifted the soil Fig. 4. Compaction characterizations for silt/biopolymer mixtures The shearing strength parameters (i.e., friction angle ~, and cohesion c) of sand/biopolymer mixtures were determined using a direct shear test. The parameters were obtained by plotting the failure envelope in t-o diagrams as shown in Fig. 6 for sand mixtures with biopolymer concentrations of 2 % and curing periods of 5 weeks. The failure envelopes for all cases appeared to be linear with correlation coefficients R? Fig. 6 Failure envelopes of sand/biopolymer mixtures with a concentration of 2 % and curing time of 5 weeks Figure 7 shows the stress—strain curves of silt/biopolymer mixtures with different biopolymers with a concentration of 2 % and curing time of 5 weeks. The biopolymers had a notable effect on the undrained shearing strength of the silt mixtures, but the effect varied considerably depending on the biopolymer. The unconfined compression stress increased to Fig. 8 Effect of biopolymer concentrations on the unconfined compressive stress for silt/biopolymer mixtures after 5 weeks Fig. 7 Typical stress-strain curves of silt/biopolymer mixtures with different biopolymers with a concentration of 2 % and curing time of 5 weeks Fig. 9 Variation of modulus of elasticity for silt/biopolymer mixtures after 5 weeks The results from the unconfined compressive stress for silt/biopolymer mixtures with different concentrations (i.e., 0.25, 0.50, 1, and 2 %) after 5 weeks curing time are shown in Fig. 8. The compressive stress for xanthan gum/silt mixture increased from 75 kPa at a concentration of 0.25 % to 338 kPa at 2 % concentration (i.e., 450 % improvement in the com- pressive stress). However, the guar gum/silt mixture compres- sive stress increased from 76 kPa at 0.25 % concentration to a value of 840 kPa at a concentration of 2 % (i.e., 1100 % improvement). This logically means that increasing the bio- polymer concentration translates to a remarkable increase in the mixtures shear behavior. However, the rate of increase depends on the biopolymer. Increasing the concentration from 0.25 to 2 % increases the compressive stress by a factor of 4 in the case of xanthan gum, 6 for modified starch, and 11 for guar gum at the same concentrations. Fig. 11 Curing time effect on the cohesion stress of sand/biopolymer mixtures: a guar gum, b modified starch, c xanthan gum The observed improvement in the shearing parame- ters of soil/biopolymer mixtures can be further assessed through scanning electron micrographs. Figure 10a—c shows micrographs of sand mixed with the three different biopolymers. The micrographs reveal biopolymer accumulations inside the soil gaps, which also create linkages that connect the soil particles together. The strength of biopolymer-treated soil de- pends on both the strength and density of these linkages existing in the pore space. The linkages created by xanthan gum are thinner compared to modified starch, which clearly explains the higher shear strength for modified starch-treated mixtures than xanthan gum- treated mixtures. These linkages also increase the ability of the soil to withstand strain, which is clearly reflected in the soil elasticity behavior. The thicker linkages of modified starch could explain why the elastic modulus of the modified starch-treated mixture is lower than the xanthan gum-treated mixture. For guar gum, the accu- mulations are much wider and thicker, almost filling the spaces between the soil particles, which further reinforce the aforementioned concepts on shear strength and elasticity. Fig. 12 Coefficient of permeability for different soil/biopolymer mixtures with different concentrations after 5 weeks curing time: a sand, b silt Fig. 13 Effect of curing time on permeability: a xanthan gum-treated sand mixture, b xanthan gum-treated silt mixture Fig. 14 Scanning electron micrographs of xanthan gum/sand mixtures with concentrations of 2 % for different curing times: al h, b 1 week, and. 3 weeks where Y represents the desired mechanical or physical property such as cohesion (c) in kN/m”, unconfined compression strength (dun) in KN/m?, permeability (K in m/s); y is the mixture density in kN/m*; ¢ is the curing time in weeks; and con is the biopoly- mer solution concentrations in percent. The coefficients aj, a2, a3, and 6 listed in Table 2 were obtained using the best fitting technique (least square method) via the used software. To better understand the effect of curing time on perme- ability, scanning electron micrographs for the xanthan gum- treated sand mixture are shown in Fig. 14 for different curing times. It is clear from the micrographs that the shrinkage due to the time of the biopolymer cross-linkage inside the soil matrices directly increased the void ratio and increased per- meability. The end point of the shrinkage process of biopoly- mers is the transition from the rubbery state to the glassy one (Eichler et al. 1997). Figure 14 shows the transformation of biopolymer accumulations form the rubbery gel state (Fig. 14a) to the glassy state (Fig. 14c). Through this transfor- mation, the biopolymer gains more strength, which is reflected in the increased shear strength observed in Fig. 11, and the biopolymer volume decreases inside the soil gaps, which is reflected in the increase in soil permeability observed in Fig. 13. one of the most durable biopolymers (Funaoka et al. 1995). Another durable biopolymer that can be used in soil improvement is sporopollenin; sporopollenin is also a highly durable biopolymer, resistant to physical, bio- logical, and chemical attacks (Jardine et al. 2015; Ariizumi and Toriyama 2011). To increase the wetting and drying resistance of biopolymer-treated soil, a prior study introduced a thermo-gelation biopolymer as a con- struction material for improving the durability of soil submerged in water; the study proved that thermo- gelation has a significant effect on enhancing the dura- bility of treated soil (Chang et al. 2015b). Therefore, the durability of biopolymers as soil improvement materials can be more thoroughly evaluated with more research in this area; however, the economic point of view should be included in selecting the appropriate type of biopolymer. Fig. 15 Cross-plot of measured values versus computed ones from regression equations for shearing parameters: a cohesion stress for xanthan gum/sand mixture; b cohesion stress for modified starch/sand mixture; ¢ cohesion stress for guar gum/sand mixture; d compressive strength for xanthan gum/silt mixture; e compressive strength for modified starch/silt mixture; and f compressive strength for guar gum/ silt mixture decade, there was an 80 % reduction in the price of biopoly- mers, and further reduction will be possible with projected expanding demand (JEC Group 2008). Moreover, the pro- duction price of biopolymers can be reduced by more than half by removing the requirements for food or medical grade material (i.e., food and medical grade materials re- quire high levels of purification or filtering). The introduc- tion of new fields and broader biopolymer applications,
