What factors might contribute to the variable rock strengths in different parts of the Devil’s Slide area?

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).


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.





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 .






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