Slopes

B. Which material is cohesive? Material B.

C. How do I know? The y intercept is NOT at 0.

D. What is the cohesion value for the cohesive material? Using C=SS - ó times tanphi The slope is y (final) - y (initial) divided by x (final) - x (initial) is for Material B: 0.7171. Thus, choosing a point on the line, C = 8000 - 6000 times 0.7171 = 8000 - 4302 = 3697 Pascals!

E. Give a normal force of 4000 pascals, what will be the shear strength of the two materials?

Material A: 2900 pascals Material B: 6700 pascals

F. The data states that a slab's resistance to sliding down a slope is dictated by the density of the slab as adjusted by degree of saturation, the thickness of the slab, gravity, the cosine of the angle of the slope, and the tangent of the angle at which the slab is calculated to slide PLUS the intrinsic stickiness of the slab.

General formula: SS = C + [p_s – (p_wm)] times g times z times cosine of angle of slope times tangent of angle of internal friction, where C is the cohesion factor, p_s is the density of the slab, p_w is the density of the slab partially saturated by the value m, g is gravity and z is slab thickness. This may be simplified to: SS = C + ó' times tangent of the angle of the slope, where ó' is effective normal stress with water present in the slab. (I can't seem to import the correct Greek letters!)

Specific formula for Material A: SS = ó' times tan phi. C is zero.

Specific formula for Material B: SS = ó' times tan phi + C.

G. The angle of internal friction for A = arctan of the slope = arctan of 0.696 = 34.8 degrees.

H. The angle of internal friction for B = arctan of the slope = arctan of 0.7171 = 35.5 degrees.

Conditions under which the 2019 Riverside Avenue slide failed.

Parameters explored:

Slab thickness, h, (m): 1.0 - 1.5.

Failure plane angle (alpha, also theta in textbook): 30-38 degrees.

Cohesion (kg/m/s²): 50 - 150

Saturation depth, h'(m): 0.2 - 0.7.

Based on the sensitivity analysis, the ground was partially saturated when the failure occurred. The failure plane angle may have been mis-measured. The slide may have had a rotational component, which would change the angle. The material was minimally cohesive, due to lack of mature forest, and previous movement of material down slope and recent additions of fill.

My model results indicate that, given the assumed slope angle, the slope was unstable prior to the rain storm and already showing early movement. This is supported by the observation of extension cracks in the parking lot at the top of the slope prior to the rainstorm. Although the pavement at the top prohibited infiltration in that area, if the water was able to rise enough to prompt run-off down the slope, this would increase downslope movement of material, thereby decreasing slab thickness and thus decreasing the Factor of Safety. Prior dumping may have increased slab thickness, but likely added little cohesion. The area was deforested prior to the slide, thus there was minimal root cohesion.

The slab was likely quite porous, given recent dumping with no concerted efforts of compaction done. Thus, pores were numerous and readily filled with water, with the rainfall, thus reducing the shear strength of the slab.