Influence of sand friction angle, grout stiffness, and confinement on the behavior of acrylamide and sodium silicate grouted sand

In the geotechnical engineering field, grouting is a commonly used option within the plethora of ground improvement methods. Yet, the practice of grouting relies heavily on empirically developed models to estimate strength and predict grouted soil behavior. Collectively, these models consider many factors including soil and grout properties, but do not incorporate the soil friction angle, which is also a commonly used soil parameter in geotechnical engineering. Thus, the objective of this study is to investigate whether the friction angle will influence the strength of a grouted soil – particularly 3 sand types of different densities, gradation, and shapes, and 2 grout types with different stiffness magnitudes – to predict the strength of a grouted soil mass. To achieve the stated objective, a series of tests were performed to characterize the friction angle of the different sands at the same void ratio/porosity. In a separate set of tests, friction angle values were controlled and grouped to investigate the influence of sand void ratio. Then, properties of the grouts at pre-gelation and post-gelation stages were determined. Finally, sand columns built with longitudinally split molds were permeated with grout and left to cure prior to compression or triaxial tests to determine the grouted sand properties. Following the procedures developed in the testing program, it is found that the prediction of grouted sand strength using sand friction is appropriate when the failure strain of grout exceeds at least 4 times the failure strain of ungrouted sand, due to strain compatibility. Additionally, acrylamide-grouted sand unconfined compressive strength models developed from 25 compression tests are presented using the grouted sand stiffness and complex modulus, along with ungrouted sand properties such as friction angle and gradation. Finally, the results showed that instead of using unconfined compression tests, confined compression tests on grouted sand will be more representative of grouted sand behavior in the field because it is demonstrated in this study that there is a crossover between the Mohr-Coulomb failure envelopes of acrylamide-grouted and ungrouted sands. Consequently, the efficacy of strength improvement of in-situ soils by means of acrylamide grouting is not constant with depth.