Rapid in situ test and determination of dam foundation weak mudstone bearing capacity

Abstract

In a hydropower station project in the middle reaches of the Minjiang River, the dam foundation rock mass is the upper cretaceous Guankou formation (K2g) brick red weak mudstone. In previous investigations, the single-pipe core drilling technology was employed, which greatly disturbed the cores. Most of the cores were broken, and the mudstone cores quickly lost water, cracked, and disintegrated after being taken out, making it difficult to obtain several columnar cores. Indoor uniaxial compressive strength tests were conducted using only a few short columnar mudstone cores, but the obtained mudstone uniaxial compressive strength was extremely low, and the converted bearing capacity of the dam foundation mudstone was only 0.25 Mpa, which does not meet the dam foundation bearing capacity standard. Large-area pile foundation treatment is required for the dam foundation mudstone at a high cost, and the project construction period is delayed. To obtain the accurate bearing capacity of the dam foundation mudstone, we used a self-developed “self-anchored dam foundation rock mass bearing capacity rapid test device” and excavated the dam foundation ship lock section to the elevation of the foundation surface. In situ tests were conducted on 12 dam foundation mudstone rock masses in 15 days, and the longitudinal wave velocities of the rock masses were tested. Reliable bearing capacity parameters of the weak mudstone of the dam foundation were obtained, and the relationship between the bearing capacity of the dam foundation mudstone and the longitudinal wave velocity was established. In situ tests showed the minimum and maximum bearing capacities of 0.986 and 1.972 Mpa, respectively, for the dam foundation weak mudstone. After excavating the dam foundation mudstone, it showed high bearing capacity without obvious softening and relaxation. The in-situ-measured minimum bearing capacity (0.986 Mpa) is consistent with the recommended bearing capacity of dam foundation mudstone, which is much greater than the maximum load of the dam foundation (0.5 Mpa). The value meets the bearing capacity requirements for dam foundation rock masses without engineering treatment. This study obtained the real bearing capacity of mudstone for dam foundations, thereby overcoming the problem of the low bearing capacity of weak mudstone obtained through laboratory experiments. The method not only saves cost but also lays the foundation for scheduled pouring of dams.