Download FINAL APPENDIX A WITH PICTURES C REVISED

ATTACHMENT 4
CRITICAL VELOCITIES IN COHESIVE SOILS
There are no definitive data available for determining critical velocities in cohesive soils.
In an unpublished paper (Permissible Shear Stresses/Critical Velocities, 2005) Sterling
Jones, Research Engineer, FHWA, has collected and commented on various methods
available in the literature regarding this subject. The Office of Structures has conducted
limited tests of critical velocities in cohesive soils using the EFA Apparatus in the SHA
Soils Lab. On the basis of this existing information, OBD recommends the following:
1 For preliminary guidance on estimates of critical velocities in cohesive soils, use
the figure below developed from information in Neill‟s “Guide to Bridge
Hydraulics, Second Edition, June 2001” (Please note that there are two lines
drawn close together for the top two curves representing two different soil types.
The top line is comprised of straight lines drawn through the data points in Neill‟s
table. The lower line is a curve mathematically fitted to the data points.
2 For more refined estimates of the critical velocity of cohesive soil layers at a
bridge site, take Shelby Tube samples of the various soil layers and test them in
the EFA Apparatus in the SHA Soils Lab.
COMPETENT MEAN VELOCITY, (fps)
8
High Resistance - very stiff
to hard soils
7
6
Avg. Resistance - medium
stiff to stiff soils
5
4
3
Low Resistance soft or soft soils
very
2
1
0
0
5
10
15
20
25
FLOW DEPTH (ft)
APRIL 2011
Page 70