Large-scale model tests on the moisture and strength evolution of BT-SAP improved subgrade under full environmental cycle conditions

Subgrade moisture redistribution and strength degradation under coupled thermo-hydraulic cycles pose serious challenges for the long-term performance of transportation infrastructure. To overcome the limitations of small-scale tests and enable realistic assessment, an advanced all-weather environmental simulation platform was developed, integrating rainfall, snowfall, fog generation, temperature control, full-spectrum solar radiation, and real-time monitoring. Using this facility, large-scale trapezoidal subgrade models were constructed with and without bentonite-based superabsorbent polymer (BT-SAP). Two composite improvement schemes: layered and wrapped structures were tested alongside an unimproved control over 6 full environmental cycles. Key performance indicators (internal moisture distribution, surface compaction degree, deformation modulus, and settlement) were recorded. Results show that BT-SAP markedly reduces upward moisture migration, maintains surface compaction degrees within 6.25 % loss of initial values, and the deformation modulus within a 22.6 % loss of its original value, and preserves deformation modulus above 50 MPa after 6 cycles. Layered and wrapped subgrade structures exhibit comparable stabilization performance, with the layered design offering simpler constructability. These findings demonstrate BT-SAP’s capability to mitigate moisture-induced strength degradation and provide a scalable, full-section improvement methodology for enhancing subgrade durability under realistic environmental loading.