Exercises for Land Stability, Including Subsidence

1. Read the newspaper carefully over the next few weeks and cut out any items that report landsliding or subsidence. For each event comment on what you consider caused the event, whether or not it could have been prevented or whether damage could have been avoided, and if so, how?

If you cannot find three or four articles or brief news items over about a month’s time span then make similar observations about historical events over the last 12 months.

2. Using the method of moments, calculate the safety factor of an embankment with the following characteristics:

Shear strength per unit of sliding surface = 2,470 kg/m²

Arc Length (L) = 35 m

Radius of arc (R) = 9 m

Unit weight of soil = 2,000 kg/m³

Volume of soil in slump = 39 m³

Horizontal distance between centre of arc and centre of gravity of slump is 4 m

The equation to use is: N =	Resisting Moment	=	TcLR
	Acting Moment		Wx

T_c = shear strength, L = length of failure surface, R = displacement of failure surface from arc centre, W = weight of soil and x = distance between centre of gravity of the soil mass and the centre of the circle that defines the failure arc.

See the figure labelled figure 4 (Stability). Use your browser back button to return to this page.

3. A large mass of rock and soil lies on the side of a steep mountain front, 1 km above the valley floor, and is gradually moving downslope. A town of 15,000 residents is located 2 km downslope from the mountain front and 200 m above the valley in line with the unstable mass. Using the information provided below, if the rock mass and soil fail suddenly, how far might the landslide material travel? The estimated volume of unstable material is 10 million m³. What should the town do?

Analysis of large landslides involves establishing parameters such as total vertical fall height (H), slide mass (m), its velocity of movement (V), its maximum runup (r), net vertical fall (h = H – r) and the angle of slope (α) over which the landslide moves.

The velocity for landslides can be estimated from the height the runup on obstacles or valley sides:

V = √2gr

Where g = acceleration due to gravity, 9.8 ms^-1. This method does not take into account frictional losses as the mass flows uphill against gravity so is only a minimum value. The average coefficient of friction (µ) can be estimated from Figure 5 (Large Landslides) following these questions. This is derived from a plot of landslide volume and calculated coefficients of friction for a large number of landslides.

Substituting µ and h into the following equation allows us to calculate the horizontal distance the landslide might have travelled.

4. Figure 13 shows the distribution of subsidence in the Santa Clara Valley, California. Answer the following questions about the map.

a. Where is the area of maximum subsidence?

b. What was the total subsidence between 1934 and 1967?

c. What was the average annual rate of subsidence for this area?

d. What problem is the town of Agnew facing as a result of subsidence?

e. What other problems might community face as a result of subsidence of this magnitude?

Figure 13.