The effectiveness of the use of waste fly ash (FA) and cement (OPC) in the stabilization of subgrade soils and the reasons likely to influence the degree of stabilization were investigated. Incorporating waste fly ash (FA) and cement (OPC) as additives leads to significant environmental and economic contributions to soil stabilization. This study involves laboratory tests to obtain the Atterberg limit, free swell index (FSI), the unconfined compressive strength (UCS), the California bearing ratio (CBR), and the scanning electron microscope (SEM). The test results for the subgrade soil illustrate that the Atterberg limit, plasticity index, and free swell index are decreasing with the addition of different proportions of fly ash and cement, i.e., 0%, 5%, 10%, 15%, and 20% and 0%, 2%, 4%, 6%, and 8%, respectively. The CBR value of untreated soil is 2.91%, while the best CBR value of fly ash and cement mixture treated soil is 10.12% (20% FA+8% OPC), which increases 71.34% from the initial value. The UCS of untreated soil is 86.88 kPa and treated soil with fly ash and cement attains a maximum value of 167.75 kPa (20% FA+8% OPC), i.e., increases by 48.20% from the initial value. The tests result show that the stability of a subgrade soil can be improved by adding fly ash and cement. While effectiveness and usability of waste FA and cement are cost-effective and environmentally friendly alternatives to expansive soil for pavement and any other foundation work in the future.

The stress state is the basis for evaluating soil deformation and strength. To study the three-dimensional (3D) stress state of the soil, a 3D stress device based on fiber Bragg grating (FBG) and Micro Electro Mechanical System (MEMS) was designed to measure the 3D stress state, principal stress, and direction of principal stress. First, the FBG one-dimensional earth pressure cell is developed and calibrated. Based on the calculation relationship between normal stress and the 3D stress state, and considering the device rotation, the 3D stress device was developed and verified in the large-scale triaxial test. The results show that the range of the one-dimensional earth pressure cell is 1 MPa, and the linearity is within 2.5% FS. The device can effectively measure soil’s 3D stress, principal stress, and direction. The measured results are consistent with the reference and theoretical results. The device provides a new method for effectively measuring 3D stress in soil.