Subgrade compactness is a key factor determining subgrade quality. However, defects exist in the traditional methods for subgrade compactness measurement. Based on the theoretical analysis of dynamic impact and vibration waves, the hydraulic compaction method was used to measure the layered compactness of loess subgrade in this paper. A full-scale test was then carried out to observe the variation of soil parameters after hydraulic compaction. The results show that the layered interface settlement (LIS) of subgrade has the characteristic of stage change bounded by 0.9 m of depth and 6–9 blow times respectively. The compaction effect is not obvious when the depth exceeds 1.2 m, and thus the improvement depth was introduced. The settlement curve has the characteristic of the Boltzmann function. The relationship between layered compacted compactness and LIS was obtained by the regression method. Finally, based on the theory of layered compaction, a measuring method for the layered compactness of loess subgrade based on hydraulic compaction was presented. The field application demonstrates that the results of the hydraulic compaction method correspond well with that of the sand-filling method.

The dam embankment (DE) is a highway structure used in the Loess Plateau to integrate the functions of a highway embankment and a dam. This paper studies the upper limit water level mechanism of the reservoir in the upstream of the DE to determine the criterion for setting upper culverts on the DE. A reservoir model is first established, and then the replenishment and loss of the reservoir water is simulated. The principle of water balance is employed to obtain water level formulas for the reservoir. Finally, an engineering example is used to verify the upper limit water level mechanism. The results show that the water level of the reservoir fluctuates near an ideal balance water level and an upper limit water level exists. Moreover, the upper limit water level has no relation to the water storage time, and is only related to the reservoir shape with big upper and small bottom, the small amount of water entering the reservoir each year, and the large water loss caused by evaporation and leakage. The upper culvert setting criterion is obtained through the upper limit water level mechanism, and it will provide important reference significance for the necessity of the DE culvert setting.

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