GEOL151 Fall 2016: Lab for Week #4 Gooseneck Bend Slump

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1 GEOL151 Fall 2016: Lab for Week #4 Gooseneck Bend Slump Overview This week, we will investigate a recent example of dynamic hillslope processes: a slump (Figs. 1 and 2) that has caused damage to a road in Weybridge and is becoming a significant (and expensive!) problem the town is being forced to confront. The slump has been active off and on for several decades, but a heavy rain event on June 9 of 2015 triggered a slope failure that necessitated the rerouting of Gooseneck Bend Road. Any further slump activity will cause additional damage to the road and will potentially threaten the cemetery nearby. At present, the town is working with FEMA to assess future hazard and to develop a plan for remediation. We will work in groups to assess the slump in detail, taking measurements and performing calculations. Our ultimate goal is to determine the Factor of Safety of the hillslope in order to predict whether the slump will continue to be active. Gear Be forewarned; you will get dirty! Wear work clothes and close-toed shoes that you don t mind covering in mud. Work gloves may also be helpful for protecting your hands. In addition, each group will need the following items: Autolevel Tripod Stadia rod Shovel Torvane Background: The Factor of Safety The stability of a slope (Fig. 3) can be assessed by the Factor of Safety (Fs), where Fs is the ratio between the strength of the slope (i.e. the forces resisting motion) and stresses on the slope (i.e. the driving forces, acting to disturb a slope): Fs = C + (ρ s ρ w m) g z s cosθ tanφ ρ s g z s sinθ C = Cohesion of sediment (kg m -1 s -2 ) ρ s = Density of sediment (kg m -3 ) ρ w = Density of water (kg m -3 ) m = Proportion of soil slab that is saturated (unitless) g = Gravity (m s -2 ) z s = Slab thickness (m) θ = Slope of slide plane (degrees) Φ = Angle of internal friction (degrees) 1

2 Figure 3: Schematic diagram showing hillslope forces (borrowed from Figure 5.7 in the textbook). Pay particular attention to the yellow box (resisting forces) and red box (driving forces) since those are the two elements that are needed for Fs, where Fs = SS/τ. As long as the resisting forces exceed the driving forces (i.e. Fs >1) the slope should be stable. However, when the driving forces become greater than the resisting forces (i.e. Fs <1), the slope is likely to fail. Hence, knowing the Fs for a given slope is a useful approach for predicting future stability and planning accordingly. Note that the resisting forces and driving forces are in units of Pascals (kg m -1 s -2 ) while Fs is a unitless ratio. It is important to note that the addition of water can often lead to slope failure because it increases the effective weight of the sediment. It is not surprising, then, that many slides and slumps often occur in coincidence with or shortly following a rainfall event. For our purposes this week, we will assume that the only water input into the system is rainfall (i.e. there is no preexisting groundwater) and that all rainfall during the storm of interest stays within the system and does not percolate out from below the slab. Further, we will assume that there is no concentration of rainwater within the slab (i.e. that it does not preferentially flow into and gather in topographically low areas). 2

3 Field Plan We will make two Fs calculations during our field investigation: the first for the slope that failed on June 9 and the second for the current slope to determine whether another failure in the same location is possible. For the first, collect your data on the stable areas flanking the slump as they are most likely to give an accurate representation of what the pre-slump surface looked like. For the second, collect your data on the face of the slump itself. In order to calculate Fs, you will need to measure a number of important parameters. Those parameters, as well as instructions for their measurement, are listed below. Remember that you will need to make these measurements twice, once on the pre-slump surface and once on the post-slump surface. Record your data in the table on the following page. Cohesion (C) Dig a small pit along the slide surface. In the wall of the pit, use the torvane to measure the cohesion of the material. Make sure you are taking your cohesion measurements from the undisturbed material below the surface. Densities (ρ s and ρ w ) Since density is difficult to measure in the field, you can assume density values of 1600 and 1000 kg m -3 for sediment and water, respectively. For the sake of simplicity, we will assume that the pre-slump surface and the post-slump surface are composed of the same type of sediment. Proportion of soil slab that is saturated (m) In order to determine how much of the slide slab is saturated, you will first need to determine the height of the water table above the slide surface at the time of the failure event. The height of the water table above the slide surface (h) at any given time will be a product of both the recent rainfall and the amount of pore space in the sediment available to hold water. This relationship can be expressed by h = rainfall/porosity, where rainfall refers to the total rainfall in a given event (in meters) and porosity describes the available pore space in the sediment (unitless, because it is a ratio). See the next part of the lab handout for insight about calculating rainfall associated with the June 9 event. For porosity, assume 35%, which is a reasonable estimate for this type of sediment (but use it in its decimal form of 0.35). Next, to determine what portion of the slab is saturated (m), divide the height of the water table above the failure plane (h) by the total thickness of the slab (z s ), which you will measure in the next step. You can do these calculations at home and will not need to collect any additional field data. Slab Thickness (z s ) In order to determine the vertical distance from the surface to the slide plane, look along the edges of the slump where you will be able to see the original surface. In a location that seems representative of the slump as a whole, measure the distance from the slump surface to the original undisturbed surface. This is the vertical distance for the slump that occurred on June 9. Since we are unable to guess the vertical distance for a slump that may happen in the future, use the same z s value for both sets of calculations. 3

4 Slope of Slide Plane (θ) To measure the slope of the two surfaces, conduct a high-resolution survey using the autolevel and stadia rod. Create a cross-sectional profile of both the undisturbed surface next to the slump (to assess the slope of the surface that failed on June 9) as well as the current slump (to assess the slope of the surface that may fail again). Collect your survey data in the field, then plot them in Excel to calculate slope. Angle of Internal Friction (f) The angle of internal friction measures a material s ability to withstand shear stress. There are numerous tables available that correlate sediment type with f. For the purposes of our calculations, we will assume a f value of 30. Parameter (units) Value for Pre-Slump Surface Value for Post-Slump Surface C = Cohesion of sediment (kg m -1 s -2 ) ρ s = Density of sediment (kg m -3 ) ρ w = Density of water (kg m -3 ) m = Proportion of soil slab that is saturated (unitless) g = Gravity (m s -2 ) z s = Slab thickness (m) θ = Slope of slide plane (degrees) f = Angle of internal friction (degrees) Determining Rainfall and Slab Saturation (to do at home) The addition of water is often the trigger leading to slope failure because water rapidly increases the weight of sediment, which is a key component in the driving force. As described above, the proportion of the slab that is saturated (m) is a unitless ratio that is expressed as the water table height above the slide surface (h) divided by the total slab thickness (z s ), where the water table height (h) can be calculated by dividing rainfall by porosity. For example, if 10 cm of water is added to sediment with 50% porosity, the water table will rise 10 cm / 0.5 = 20 cm. This makes sense, since only 50% of the sediment s space can be occupied with water. If the total slab thickness is 1 m, then the proportion of the slab that is saturated is 0.2 m / 1 m = 0.2. Therefore, in addition to knowing the porosity, you must also determine how much water was added to the system during the rainfall event of interest. There is a datasheet for this lab posted on the class website that you can download. In that document, there is a sheet called Weather Data, which contains detailed weather data for June , the day the Gooseneck Bend Slump 4

5 occurred. Use these data to determine the total rainfall over the course of the day. Note that I have provided you with the raw data as downloaded from Weather Underground, so you will need to do some thinking and manipulation before you can use it. Remember that your rainfall data must be in units of meters before you can use it in the above equations. Data Reduction I have already created an Excel template (posted on the class website) that will guide you through the Fs calculations if you desire. For the slump that occurred on June , you will be able to calculate Fs based on the measured rainfall. For your consideration of future slump events, you should consider whether failure may occur over a range of possible rainfall amounts. This last piece is called a sensitivity analysis and is used to study the sensitivity of a certain system (in our case, this specific hillslope) to a parameter of interest (rainfall). Field Report Present your data and interpretations in a detailed, professional field report. Imaging that you are a contractor and you will be giving this report to the town of Weybridge. Refer to the How to Prepare a Successful Field Report handout for more ideas on organization, content, and presentation. In particular, please include the following: A detailed description of the slump s morphology augmented with sketches/diagrams A plot of the precipitation on June 9 that shows the CUMULATIVE rainfall over the course of the day A well-organized table of all the values you used in your calculations The Fs value you determined for the surface that slumped on June 9 (and discussion about whether this value makes sense or not) Thoughts about whether you can determine when over the course of June 9 the slump occurred given the sensitivity of the slope to different rainfall amounts The Fs value you determined for the current surface over a range of rainfall events (and discussion about how likely a future failure will be) A sensitivity analysis plot showing Fs of the current surface over a range of rainfalls Any other sketches, diagrams, or graphs you deem helpful Discussion about limitations and assumptions involved in our approach Recommendations to the town of Weybridge about how to proceed, given what you determine is the likely future of the slump As will be the case with many of our investigations, there is no right answer. I am looking for you to present your data clearly, then make logical, supported interpretations and recommendations. You may work together to develop ideas; however, each student must turn in his/her own field report. These reports are due at the beginning of the lab period on Wednesday next week (9/28). 5

6 Figure 1. Satellite image of the Gooseneck Bend Slump in Weybridge VT and surrounding area. Slump location: N, E 6

7 Figure 2. Satellite image of the Gooseneck Bend Slump along Otter Creek. 7

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