The macroscopic mechanical properties of natural sedimentary soft soils, which are usually linked to their microstructure, are different from those of remolded soils. The interaction between soil structure and mechanical behavior is a manifestation of structural mechanics effects. It is essential to understand the effects of secondary compressibility to predict long-term foundation deformations. The effects of soil composition on secondary compression deformation are little studied, and the soil structure is rarely involved in the compression process. The sedimentary environment creates the initial composition and structure of soft soil, and it also basically determines its grain size and mineral composition, while different depths give soft soil different overburden pressures, and the soil composition and depth directly affect its yield stress during compression. So, natural sedimentary soft soils sampled at different depths and from different sedimentary environments (such as marine-neritic facies, sea shore facies and limnetic facies) were selected to study the influence of structure on the secondary compression coefficient C_α during pressure change and the relationship between soil composition and C_α. One-dimensional compression and consolidation creep tests were carried out on undisturbed and remolded samples. The undisturbed samples were obtained by the thin-wall samplers in rotary wash borings, and the quality of the samples met the test standard. Based on the concept of the void index I_v and the intrinsic compression line (ICL) proposed by Burland, the role of structure in the compression process was studied, and the influence of soil composition and structure on secondary compression characteristics was summarized. The C_α/C_c values are 0.031, 0.034, 0.030, and 0.036 for Shanghai, Tianjin, Suzhou, and Ningbo soft soils, respectively, within the range of inorganic clays and silts (0.04 ± 0.01) given by Mesri. According to the compression index C_c obtained by compression test, C_α/C_c can be used to estimate C_α. The yield stress of normal consolidated soil is near pre-consolidation pressure, while that of structural soft soil is greater than its pre-consolidation pressure. Natural sedimentary soft soils show over-consolidation characteristics due to the action of the structure; the soil structure resists the external load and hinders secondary compression. When the soil structure is almost destroyed, the pressure reaches the structure full yield stress P′. The tests of structural soft soils show that C_α changes with pressure before the structure completely yields, first increasing and reaching peak C_αmax near P′; the value of P′ is approximately 1.6–3.0 σ’_k, where σ’_k refers to the structure yield stress of soil obtained by the Casagrande method. After the structure disappeared, C_α gradually decreased and then stabilized, which is considered to be independent of the load. The C_αmax is positively correlated with the liquid limit, indicating that the peak value that can be reached by the C_α is related to the maximum content of bound water in soft soil, thus the soil composition has a significant influence on secondary compressibility, which contributes to the prediction of long-term foundation deformation.

Secondary compression studies can provide insights for evaluating the engineering potential and environmental impact of soil. The objective of this research was to investigate the effect of bound water content on the secondary compression of marine silty clay. To this end, a novel method was established based on thermogravimetric analysis (TGA) to determine the contents and limits of strongly bound water and weakly bound water for three typical marine silty clays including Tianjin mucky silty clay (TJ), Qingdao mucky silty clay (QD), and Weihai silty clay (WH). A total of 17 groups of uniaxial confined compression tests were performed for reconstituted samples at different absolute water contents under the condition of multistage loading to investigate their secondary compression characteristics. The results show that the initial dehydration temperature of strongly bound water (T_s) corresponds to the peak of the derivative thermogravimetry (DTG) curve. The values of T_s for TJ, QD and WH are 112.35 °C, 109.67 °C and 118.46 °C, respectively. The initial dehydration temperatures of weakly bound water (T_w) for TJ, QD and WH are 55.26 °C, 52.56 °C and 56.56 °C, respectively. The secondary compression coefficient C_α changes little before the strongly bound water limit and increases dramatically as weakly bound water content increases at the same vertical stress. A piecewise linear model and a quadratic polynomial model are established for calculating the average secondary compression coefficient from bound water content. Weakly bound water is the determining factor controlling secondary compression. Increasing the bound water content weakens the connection force and friction force among the particles and the viscosity of weakly bound water. The results will guide decisions on long-term settlement assessment and facilitate understanding of the secondary compression mechanism of marine silty clays affected by bound water.

This paper presents a new procedure for back-calculating the properties of a concrete pavement with two slab layers using deflection measurements from nondestructive deflection testing. The two slab layers are first represented by an equivalent single slab layer based on the principle of equivalent flexural rigidity. In the back-calculation analysis, the pavement deflection response under load is evaluated using an analytical model for a slab supported on an elastic foundation. Employing a closed-form back-calculation algorithm, it is shown that the back-calculated equivalent radius of relative stiffness and subgrade modulus are unique and independent of the characteristics of the two slab layers. It follows that the equivalent flexural rigidity for the two slab layers is also unique. The next step employs a trial-and-error algorithm to back-derive the moduli of the two slab layers by matching the equivalent flexural rigidity. An upper bound and a lower bound to the modulus can be established for the slab layer with the higher modulus. Similarly, an upper bound to the modulus can be found for the slab layer with the lower modulus. Four numerical examples are presented to demonstrate the use of the proposed back-calculation method.