Bearing capability of collapsible loess subgrade through cement-fly ash treatment

To investigate the reinforcement mechanism of cement-fly ash post-grouting on pile-net composite subgrade in collapsible loess areas, the static load tests in laboratory were carried out on the grouted cement-fly ash gravel (CFG) piles, the influence of post-grouting on the collapsibility coefficient of the soil samples around the piles was analyzed, and the changing rules of additional stress, pile side friction resistance and pile tip resistance in the depth direction of post-grouting pile-net composite subgrade under vertical static load were studied. Based on the Boltzmann mathematical model and load transfer function, the reinforcement mechanism of pile side friction resistance and pile tip resistance was investigated, and their calculation formulas after grouting were given. The influence mechanisms of elastic modulus of pile, post-grouting depth, pile-net replacement rate, and cushion layer thickness on the bearing capacity of pile-net composite subgrade were discussed by the numerical simulation method. Research results indicate that under the same static load, the collapsibility coefficient of the cement-fly ash post-grouting soil around the pile is less than that of the natural soil sample and less than 0.015. After post-grouting, the vertical additional stress of the pile top in the pile-net composite subgrade gradually decreases under the static load, the vertical additional stress of the soil between the piles decreases first and then increases, and the pile side friction resistance increases by about 1.54 times compared with the un-grouting pile. With the increase in post-grouting depth, the maximum stress in the depth direction of pile body increases first and then decreases, and the maximum stress is obtained at the depth equal to pile length. When the pile-net replacement rate is doubled, the stress and settlement decrease in the depth direction, among which the peak stress decreases by 24% and settlement decreases by 26%. With the increase in cushion layer thickness in the pile-net composite subgrade, the stress in the depth direction of the subgrade gradually increases. Therefore, the cement-fly ash treatment of collapsible loess subgrade can weaken the collapsibility of subgrade soil and improve the bearing capacity. In the construction process, the effects of elastic modulus of pile, post-grouting depth, pile-net replacement rate and cushion thickness on the bearing capacity of the subgrade should be considered.