Exercise #1 – Hillslope Stability: Analyses and Applications

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GEO347 – Climatic Geomorphology Exercise #1 – Hillslope Stability: Analyses and Applications

Handed out: February 19, 2020 Write-up due: March 6, 2020

Purpose: This exercise explores more deeply some of the mass wasting concepts covered in lectures. In Part (A) we will make observations and collect some data for use in Part (B). General Geologic Setting. Montara Mountain and the Devil's Slide area are located within the California Coast Range Geomorphic Province, which consists of a series of northwesterly trending ridges and valleys formed by compressional tectonic forces. The geologic units located within the study area consist of Cretaceous Montara Mountain granodiorite overlain with Paleocene age sedimentary rock. Granodiorite is a coarse-grained plutonic rock consisting of quartz, plagioclase and potassium feldspar, biotite, hornblende, or, more rarely pyroxene. It is in fault contact with the overlying sedimentary rock consisting of sandstone, shale, and conglomerate. The sedimentary rock within the study area is steeply dipping, folded, faulted, and further disturbed by repeated episodes of landslides. The fault separating these two units is an inactive, oblique fault that descends northwest across the cliff face. Colluvium and shallow slide debris are found throughout the slide, above and below the roadway. Source: U.S. Department of Transpiration and The State of California, Department of Transportation, 1986. Part A (40 points) – To be worked on in class and written up outside class: 1. For this question we will examine a long-standing problem stretch of highway #1 along the California coastline north of Half Moon Bay. The general geologic setting is described below, and a photograph is provided separately (also posted on UBlearns). After reading about the general geologic setting, look carefully at the left side of the photo. Find the bedding (layering) of the sedimentary rocks. Don’t be confused by the ravines eroded by water. Which way does the bedding dip? Explain why this makes the area more vulnerable to landslides. (Hint: See Table 1, reproduced from Ritter et al. 2011) 2. For this question we will measure slopes at Devil’s Slide. The method for calculating slope from topographic maps is shown in Figure 1. Figure 2 shows a portion of the Montara Mountain USGS 7.5- minute Quadrangle. A scale is provided as this image does not retain the original 1:24K scale. Points (A) and (B) show locations where landslides have occurred several times of the past 30 years. Determine the average slope of the land surface from the shore to the highway at (A) and (B). Repeat for an equal horizontal distance above the highway. Be sure to get your group’s numbers written down for later calculations. Compare the slopes at (A) and (B) to those on the headland (C). Discuss why the headland denudation rates are lower than those at (A) and (B).

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tan 𝛼 = !!

!!

Figure 1. Physical meaning of elevation contours (left) and method for calculating slope (α) from horizontal (dX) and vertical (dZ) distances.

Table 1. Field classification of rock strength. Reproduced from Ritter et al., 2011.

α

dZ

dX

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Figure 2. Topographic map from the USGS Montara Mountain Quadrangle. Part B (60 points) – To be worked on and written up outside class: 3. For solid rock the slope stability depends in part on rock strength (see Table 4.4 on page 100 in your text), which depends on both cohesion (c) and internal friction angle (φ). Assume the rock has φ of 25°. From the Coulomb equation and factor of safety, at just the point of slope failure,

𝐹 = 1.0 = 𝑠 𝜏 = 𝑐 + 𝑔 ∙ ℎ 𝑝! − 𝑝! 𝑡𝑎𝑛𝜙𝑐𝑜𝑠𝜃

𝑔 ∙ ℎ ∙ 𝑝!𝑠𝑖𝑛𝜃

We can solve for the value of c under dry conditions using

𝑐 = 𝑔 ∙ ℎ ∙ 𝑝! 𝑠𝑖𝑛𝜃 − 𝑡𝑎𝑛𝜙𝑐𝑜𝑠𝜃 .

N

(A)

(B)

(C)

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Assuming all slopes are just at the point of failure, calculate c for each of the four slopes (θ) your group derived in class (at points A and B, above and below the highway) for the Devil’s Slide area. Assume ps = 2000.0 kg m-3, g = 9.8 m s-2, and h = 0.1 m. What factors might contribute to the variable rock strengths in different parts of the Devil’s Slide area? Although the fault at this location is stable there has been suggestion that it might combine with rainfall to promote landslides. Speculate on how. 4. When rock breaks down it first forms relatively large blocks, which pile up as talus. Voids between the blocks are large so that water drains freely and does not contribute any pore water pressure. The strength (s) and the angle of repose (θ) is approximately equal to the internal friction angle (φ), which is close to 35° for most unconsolidated materials. As weathering continues, sandy grus and silt develop and fill in the voids. The dry strength remains at a θ = 35°. However, significant pore water pressure may develop in the fine material if enough water is present. Suppose that a slope fails when the material is saturated. Then, for failure at depth h, the factor of safety

𝐹 = 1.0 = 𝑠 𝜏 = 𝑔 ∙ ℎ 𝑝! − 𝑝! 𝑡𝑎𝑛𝜙𝑐𝑜𝑠𝜃

𝑔 ∙ ℎ ∙ 𝑝!𝑠𝑖𝑛𝜃

find θ, the repose angle, for φ = 35°. Assume ps = 2000.0 kg m-3 and pw = 1000.0 kg m-3. Discuss to what extent pore pressure is or is not a factor in triggering debris flows in this particular area. 5. In this section we will apply what we have learned to a case about litigation concerning the 1956 Portuguese Bend landslide in Los Angeles County, California. Here is the case:

Homeowners Allege Negligence (AP) Homeowners in the exclusive Portuguese Bend neighborhood near Long Beach, CA have fi led a class action suit in state court against the County of Los Angeles. This suit seeks compensation for damages to 160 homes affected by a landslide encompassing an area of roughly 270 acres. The homeowners allege that this landslide was caused by road construction along Crenshaw Boulevard, a county highway that traverses the northern portion of the slide area. The lawsuit also alleges fraud and negligence on the part of the developers for participating with the county in road construction in a geologically unstable area.

The scientific basis for this complaint can be found in the following article (on UBlearns):

Merriam, R. 1960. Portuguese Bend Landslide, Palos Verdes Hills, California. Journal of Geology, 68(2), 140-153.

What is the scientific basis for the homeowner’s lawsuit? What natural conditions in this area are conducive to landslides? What specific type of mass movement is likely to occur in this geologic setting? To what extent is it possible that the 1956 Portuguese Bend Landslide was triggered entirely by natural causes? What human action(s) could have contributed or triggered this landslide? How would the construction of Crenshaw Boulevard differ from the earlier construction of houses in terms of its ability to trigger a landslide? What do the successes of various corrective actions taken here suggest was (were) the cause(s) of the landslide?