State Space Reconstruction of Suction Surfaces for Compacted Unsaturated Soils

The Monash-Peradeniya-Kodikara (MPK) framework interprets the volumetric behaviour of compacted unsaturated soils in the void ratio − moisture ratio − net stress space without requiring suction as an independent constitutive variable. Nevertheless, explicit knowledge of suction distributions within this volumetric space can provide additional insight into hydromechanical behaviour and facilitate comparison with conventional suction-based frameworks. This paper reconstructs continuous suction surfaces on and inside the Loading Wetting State Boundary Surface (LWSBS) within the MPK space by combining suction topology inferred from published experimental datasets with a constrained mathematical reconstruction methodology consistent with MPK principles. The reconstruction is divided into three regions relative to the Line of Optimums (LOO). At the wet side of the LOO, suction contours on the LWSBS are reconstructed using effective stress relationships and semilogarithmic compression relationships. At the dry side of the LOO, suction contours are reconstructed from Soil Water Characteristic Curves (SWCCs) extended towards the dry condition using cubic Bézier curves. Suction contours inside the LWSBS are reconstructed on constant net stress planes using fifth-order Bézier curves to generate smooth, continuous, and nonintersecting suction surfaces. Application to kaolin and Merri Creek soils produces suction profiles consistent with reported experimental trends, including suction reduction during constant moisture loading and a distinct transition in contour curvature across the LOO. The reconstructed suction surfaces preserve the fundamental structure of the MPK framework while providing the first explicit continuous hydraulic representation of suction within the MPK volumetric state space. They enable estimation of suction at intermediate soil states where direct measurements are unavailable, facilitate comparison with conventional suction-based interpretations, and establish a foundation for future constitutive, numerical, and coupled hydromechanical modelling within the MPK framework.