NAME_______________________
SLOPE STABILITY
Introduction
Mass wasting is the general term for the movement of material downhill in response to
gravity. This movement may be quick and catastrophic or gradual. Base level is the separation
between being stable and not stable. If the rock or soil is lying flat on a plain it is below base
level and stable. If it is above base level it can be unstable and ready to fall or metastable which
means that it is relatively stable for the time being but will become unstable.
Mass wasting takes on two forms depending on whether the topography of an area is
mature or immature. In areas of immature topography, mass wasting can be devastating.
Common occurrences include rock falls and rock slides, mudflows, sheet washes, and long slides.
These processes can be catastrophic causing death and destruction. In areas of mature
topography, creep is the dominant process of mass wasting. This slow movement of soil can
slowly knock over retaining walls, telephone poles, and even houses but damage is minimal and
there is no loss of life. The greatest problem in areas of mature topography is when human
activity disrupts the natural landscape. Removal of vegetation, piling dirt for construction
purposes, and cutting into rock and soil for roads and railroads essentially makes the local
topography immature and subject to the same problems of mass wasting. Rock falls on the sides
of highways are not uncommon and very expensive to remedy.
The controlling factor in the stability of a slope can be summarized as the angle of
repose. It basically means that a specific material, under a given set of conditions, will form a
pile to a specific maximum slope angle. If material is added to exceed that angle, the material
will fail and slide down the hill until the original slope angle is reestablished. It is the material
properties of the rock or soil that determines the angle. There are many factors that can affect
this angle like moisture content, vegetation (holds things together), and shaking (as in earth or
wind quakes). In this exercise, you will use some standard techniques to classify sediments and
determine if slope in certain areas will be stable.
Grainsize: very coarse (>2 mm), coarse (1-2 mm), medium (0.5-1 mm), fine (0.5-0.25), very
fine (<0.25).
Angularity: grain shape is angular (sharp edged), rounded, mixed.
Sorting: well sorted (all grains about the same size), bimodal (2 distinct sizes), poorly sorted
(mix of sizes).
Procedure
1) Angle of repose
Dump 1/2 of the sample of sediment provided onto a piece of paper and measure the
angle of the slope relative to the table using a protractor. Repeat the exercise for the entire
sample. Do the same thing for each sample. Look closely at the sample to classify it.
Sample A) Describe the soil (color, grain size, sorting, angularity of grains)_________________
______________________________________________________________________________
Angle for 1/2 ______________________________, 2/2 ________________________________
Now add enough water to your sample to moisten it and mix thoroughly. Repeat the procedure.
Angle for 1/2 ______________________________, 2/2 ________________________________
Now add enough water to saturate the sample. Repeat the procedure.
Angle for 1/2 ______________________________, 2/2 ______________________________
Sample B) Describe the soil (color, grain size, sorting, angularity of grains)_________________
______________________________________________________________________________
Angle for 1/2 ______________________________, 2/2 ________________________________
Now add enough water to your sample to moisten it and mix thoroughly. Repeat the procedure.
Angle for 1/2 ______________________________, 2/2 ________________________________
Now add enough water to saturate the sample. Repeat the procedure.
Angle for 1/2 ______________________________, 2/2 ______________________________
Sample C) Describe the soil (color, grain size, sorting, angularity of grains)_________________
______________________________________________________________________________
Angle for 1/2 ______________________________, 2/2 ________________________________
Now add enough water to your sample to moisten it and mix thoroughly. Repeat the procedure.
Angle for 1/2 ______________________________, 2/2 ________________________________
Now add enough water to saturate the sample. Repeat the procedure.
Angle for 1/2 ______________________________, 2/2 ______________________________
Sample D) Describe the soil (color, grain size, sorting, angularity of grains)_________________
______________________________________________________________________________
Angle for 1/2 ______________________________, 2/2 ________________________________
Now add enough water to your sample to moisten it and mix thoroughly. Repeat the procedure.
Angle for 1/2 ______________________________, 2/2 ________________________________
Now add enough water to saturate the sample. Repeat the procedure.
Angle for 1/2 ______________________________, 2/2 ______________________________
2) Application to a problem
The attached topographic map is of an area in northern New Jersey that has just had a
new housing development built on it. It is your job to determine which areas will be stable under
what conditions.
Step 1: Measure the average spacing of the contour lines (perpendicular to the strike of the
contour lines)from different spots on the map.
Step 2: Divide up the map into separate areas based on average contour spacing (slope) and label
them Area 1, Area 2, etc.
Step 3: Determine the average slope angle of each area (1, 2, 3, etc.). First draw a straight line
approximately perpendicular to the contour lines. Measure the length of the line using the
scale on the bottom of the map. Count the contour lines and determine the elevation
change along the line. Now draw a triangle.
Where: Tan (Slope Angle) = Elevation Change/Measured Distance or,
ArcTan (Elevation Change/Measured Distance) = Slope Angle
Slope 1 _______________________________________________________________________
Slope 2_______________________________________________________________________
Slope 3 _______________________________________________________________________
Slope 4 _______________________________________________________________________
3) Stability (Consider your map areas and answer the following questions):
a) Under dry unvegetated conditions which slopes would be stable if they were composed of
samples (from question 1):
A____________________________________________________________________________
B____________________________________________________________________________
C____________________________________________________________________________
D____________________________________________________________________________
b) After a rain shower, under moist unvegetated conditions which slopes would be stable if they
were composed of samples (from question 1):
A____________________________________________________________________________
B____________________________________________________________________________
C____________________________________________________________________________
D____________________________________________________________________________
c) After a hurricane, under saturated unvegetated conditions which slopes would be stable if they
were composed of samples (from question 1):
A____________________________________________________________________________
B____________________________________________________________________________
C____________________________________________________________________________
D____________________________________________________________________________
4) What measures could you take to stabilize the slopes against failure and mass wasting?