DEVELOPMENT OF SCOUR IN COHESIVE SOILS BY NORMAL IMPINGEMENT OF A SUBMERGED CIRCULAR TURBULENT JET

Abstract

Scour is an important issue due to the potential great extent of loss and risks associated with scouring. The work described herein constitutes laboratory testing of the time development of scour holes in clayey soils produced by a submerged vertical circular impinging jet. Long term laboratory tests were performed on three types of manufactured pottery clays, Buffstone clay (50.3-51.7% clay), P300 clay (48.7-50.7% clay), and M370 clay (51.1-51.3% clay). Detailed measurements of the entire scour hole were performed on a 2 mm grid using a computer controlled laser optical profiler after scouring times of 5 min, 10 min, 15 min, 20 min, 30 min, 40 min, 50 min, 1 h, 1.5 h, 2 h, 4 h, 8 h, 16 h, 24 and then at every 24 h interval until the scour hole was considered to have reached equilibrium based on the criterion used by Mazurek et al. (2001). This resulted in long test durations ranging from 120 h to 384 h.

For the time development of the scour hole, a three-dimensional scour hole surface was produced using the data taken by an optical profiler. Thereafter, four cross sections of the scour hole were extracted from the three-dimensional scour hole surface. Dimensions considered for analysis were taken from both the three-dimensional scour hole surface and the cross sections of the scour hole. The volume of the scour hole, and the centreline and maximum scour hole depths were extracted from the scour hole surface. The section-wise maximum scour depth, radius of the scour hole, half-width about the jet centreline, and half-width about the section-wise maximum scour depth were extracted for each cross-sections. The growth of these dimensions were observed with time. For a significant portion of scouring, the centreline and maximum scour hole depths increased linearly with the logarithm of time. For the majority of the tests, the half-widths decreased with time. Temporary ceasing of the increment of the centreline and maximum scour hole depths was observed, called “plateaus”, in the time development plot. Scour hole dimensions for the cross-sections showed variability from the average scour hole dimensions. However, for most of the tests this variability decreased with time as the scour test proceeded.

To decide on whether the equilibrium state of the scour hole was achieved, all of the aforementioned scour hole dimensions were evaluated. The characteristic scour hole dimension to decide on the equilibrium condition was termed as the “critical equilibrium dimension”. Four of the scour tests reached an overall equilibrium state, the section wise maximum scour hole depth was the critical equilibrium dimension for three of those. The half-widths were the critical equilibrium dimension for one of the tests. However, previous studies did not consider the “side slope erosion” of the scour hole, hence neglected the section-wise maximum scour depth and half widths for identifying equilibrium condition.

Dimensionless scour hole profiles were developed using the centreline scour depth as the scale for scour hole depth and the half-width about the centreline depth as the scale for radial distance from the jet centreline. A general equation of the dimensionless equilibrium scour hole profile was developed by fitting a sine function to the equilibrium scour test profiles using linear regression analysis. Dimensionless scour hole profiles with time during the scour test were compared to the dimensionless profile at equilibrium. It was observed that while for some scour tests the equilibrium scour hole shape formed quickly compared to the time to equilibrium, for some scour tests the equilibrium shapes did not form until the scour hole stopped growing.