Structural Strength Deterioration Characteristics and a Model of Undisturbed Loess under the Action of Wetting and Freeze-Thaw Cycles

Undisturbed loess is affected by external environmental disturbances, such as wetting and freeze-thaw cycles, which cause microstructural changes that have an important impact on the structural strength of the loess. These changes in turn affect the stability of structures such as embankments, slopes, and guards. This article takes the Q3 undisturbed loess in Lintong District, Xi’an, as an example. The effects of wetting and freeze-thaw cycles on the loess expansion ratio and pore structure were studied by wetting tests, freeze-thaw cycle tests, and scanning electron microscopy (SEM). The changes in the compression index and compression modulus were studied by a confined compression test. The loess e-lgp compression curve was obtained according to the confined compression test, and the newly defined concepts of the loess structural strength, residual structural strength, and structural strength damage variable, in addition to the e-lgp compression curve, were combined with the experimental data to calculate the damage value generated by the disturbance during the sampling and preparation of loess. The deterioration of the structural strength and damage variable of loess was analyzed. Based on the microscopic statistical damage theory and Weibull distribution, the model used the volume expansion ratio as a variable to establish a statistical damage model under wetting and freeze-thaw cycles. Finally, on the basis of the test, the model parameters were determined. The models were verified by taking loess from a foundation pit in the northern suburbs of Xi’an and were in good agreement with the results of the test. Ultimately, the models have good practicability and can provide guidance for engineering design and construction.

DETERMINATION OF DIESEL COMPRESSION RATIO BY RESULTS BY THE PROCESS INDEXING

The calculation method for determining the volume of the compression chamber and the compression ratio in the cylinder is proposed according to the diesel’s indicator diagram. The point of the compression curve is used where rate of pressure rise is maximum. At this point, the second derivative of the pressure curve along the crank angle is zero. The equation for the second derivative of the compression curve in the polytropic form was fixed. In solving this equation the polytropic index has been reduced, which enabled to derive an expression for the volume of a cylinder at the specified point in the analytical form. As a result, from the known geometrical parameters of the cylinder and the data taken from the diesel’s indicator diagram, it is possible to determine the diesel compression ratio with engineering precision, which is especially important for modern low-speed diesel engines, such as MAN, Wartsila, the documentation of which doesn’t specify this setting.