As temperature changes, red clay is prone to shrink and generate cracks, which weaken the structure and the stability of soil mass, leading to various engineering problems, such as damage and instability in engineering structures. To study the effect of environmental temperature on the crack evolution of red clay, Guilin Red Clay was taken as the research object, and the saturated mud samples were dried at 23, 40, and 60 °C respectively. During the drying process of the samples, the change of moisture content and the evolution process of surface cracks were monitored by high-definition automatic photographing and a weighing device, which were also improved. We used PCAS software to process the crack image, extract various geometric elements, observe, and analyze the change rule of the cracks during the drying process of red clay at different temperatures. The test results show that the cracking evolution of red clay at different temperatures is mainly divided into three stages: (i) the initiation of micro cracks; (ii) crack progress; and (iii) crack stability. With the increase of environmental temperature, stage (i) took less time. Meanwhile, the growth rate of the crack area increased. The number of final crack blocks of soil is significantly reduced. Moreover, the final crack rate is obviously increased. When the temperature is either 23 °C or 40 °C, the initial cracks almost happen at the same time in the samples with different diameters. While the temperature is higher than 60 °C, the cracking time will delay with the increase of the diameter. In addition, the decrease in water content leads to a decrease in the curvature radius of soil particles. Under the joint action of the surface tension and the matrix suction, the distance between red clay particles becomes shorter, so the time for red clay to start to generate cracks will be shorter, and the final crack rate will increase with the increase in temperature.
In this study, the damage regularity of saturated red clay under acidic conditions was investigated. The red clay was configured with different concentrations of acid solution (pH = 2, 3, 4, 5, and 7), and a consolidated undrained triaxial test (CU), scanning electron microscope (SEM) test, and mercury intrusion porosimetry (MIP) were conducted to derive the mechanical damage law of acid on red clay in combination with the Mohr-Coulomb damage criterion. The stress-strain relationship of red clay under different circumferential pressure and acid concentration was determined to be strain-hardening; the original cementing material eroded under strong acid, and the connections between particles were broken to form a "gully". The colloidal material in the red clay dissolved with the increase in acid concentration, destroying the original structure of the soil and producing a large number of pores, which increased the damage. The linkage strength between soil particles, therefore, gradually decreased, the soil particles became more dispersed, and the strength of the soil decreased, i.e., the damage to red clay increased.
Previous studies of transportation network design problem (NDP) always consider one peak-hour origin-destination (O-D) demand distribution. However, the NDP based on one peak-hour O-D demand matrix might be unable to model the real traffic patterns due to diverse traffic characteristics in the morning and evening peaks and impacts of network structure and link sensitivity. This paper proposes an NDP model simultaneously considering both morning and evening peak-hour demands. The NDP problem is formulated as a bilevel programming model, where the upper level is to minimize the weighted sum of total travel time for network users travelling in both morning and evening commute peaks, and the lower level is to characterize user equilibrium choice behaviors of the travelers in two peaks. The proposed NDP model is transformed into an equivalent mixed integer linear programming (MILP), which can be efficiently solved by optimization solvers. Numerical examples are finally performed to demonstrate the effectiveness of the developed model. It is shown that the proposed NDP model has more promising design effect of improving network efficiency than the traditional NDP model considering one peak-hour demand and avoids the misleading selection of improved links.
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