Effects of Extreme Scour and Soil Subgrade Modulus on Bridge Pile Bent Buckling

A common design/construction procedure for highway bridges that are over marshes, small creeks, and shallow bodies of water is to have the bridge superstructure supported on pile bents. During major flood events, the volume and velocity of flood waters can cause considerable scouring to occur at the bents. As the elastic buckling capacity and stability of these bents vary inversely with the square of the bent height, a scour of $4.6\mathrm{m}$ $\left(15\mathrm{ft}\right)$ at a bent that was originally $4.6\mathrm{m}$ $\left(15\mathrm{ft}\right)$ in height will reduce its elastic buckling capacity by a factor of 4. Also the correct buckling load of such bents is somewhat questionable because of the unknown degree of fixity afforded by the soil at the pile ground line. This paper summarizes an investigation of the effect of the soil subgrade modulus on bridge pile bent buckling and pushover capacity. Analysis results indicate that bent pile buckling and pushover loads are not very sensitive to the soil subgrade modulus, $k_0$ , unless the value of $k_0$ is very small, i.e., $k_0⩽1.36\mathrm{N}∕{\mathrm{cm}}^3$ $(5\mathrm{lb}∕{\mathrm{in.}}^3)$ . Also, the analyses indicate that complete pile fixity can be assumed at approximately $1.5\mathrm{m}$ $\left(5\mathrm{ft}\right)$ below the ground line unless $k_0$ is very small, i.e., $k_0⩽1.36\mathrm{N}∕{\mathrm{cm}}^3$ $(5\mathrm{lb}∕\mathrm{in}{.}^3)$ .