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Pre-stressed high-strength concrete pipe piles (PHC pipe piles) have been widely used in actual soft foundation treatment projects due to their reliable quality, fast construction, assembly line production, and environmental friendliness. However, large-scale slip damage still occurs in construction projects. In order to reduce and avoid such accidents, a highway in Guangdong (section K31+100~K31+388) was taken as an example for this study. Plaxis 2D software (V22.01.00) was used to establish a PHC pipe pile composite roadbed model and investigate the effects of tie beam form, pile lengths, pile spacings, pile verticality, and embankment filling loading modes on the settlement and stability of the composite roadbed. The results show that the original treatment plan, which had the form of a PHC pipe pile with caps, had a low horizontal bearing capacity and a poor anti-disturbance ability, leading to the occurrence of a landslide accident. A comparison of different structural forms revealed that the longitudinal and transverse tie beam form was the most stable, followed by the transverse tie beam form, longitudinal tie beam form, PHC pipe pile form with caps, and PHC pipe pile form without caps. Compared to the structural form of PHC pipe piles with pile caps, the stabilities of the transverse tie beam form and the longitudinal tie beam form were improved by 42.47% and 38.61%, respectively, while that of the longitudinal and transverse tie beam form was improved by 50.87%. The application of longitudinal and transverse tie beams effectively reduced the settlement of the composite roadbed, as confirmed by both measured data and finite element analysis. This structure achieved the desired vertical settlement control and lateral anti-slip effects.

Acquiring soil’s critical load is very important when designing soil foundation and soil subgrade with overloading method. Mohr-Coulomb strength theory is combined with slip-line field theory, then a theoretic method which is practicable to analyze critical load of soil come out. Analyzed theoretically and compared with others, the theoretic method is applied to describe the relationship between internal friction angel, internal cohesion, δ, lateral pressure and critical load, which express that this theoretic method is rational. Numerical analysis is adopted which make it clear that soil subgrade’s yield zone is small when it is under the critical pressure, which approve that this theory is very close to lower limit of soil’s critical load. The theoretical method can provide useful information to engineering designers and researchers.