Landslide Hazard Assessment Key PDF

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Online land hazard assignment key...

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Landslide Hazard Assessment You have been hired to complete a landslide hazard assessment for five potential development sites. The locations of these sites are shown on the map below. Your client, a housing contractor, is considering these for purchase and development. Your job is to determine the potential for landsliding at each of these sites and recommend any that are suitable for construction of single-family residences. This is a very common job done by environmental geologists. They use maps, field surveys, soil reports, and aerial photographs to accomplish this work.

Landslide Hazard Assessment Strategy Landslides do not occur at random locations. They occur in specific places for specific reasons. There are two basic reasons that landslides occur. First, a steep slope is needed so that the forces of gravity can act on the soil: landslides do not happen on level land. Second, soils with a low shear strength are the most susceptible to landsliding. These soils can flow like soft putty when they occur on a steeply sloping hillside. From the above paragraph, your conclusion should be: the places that have the highest landslide risk are where low shear strength soils occur on steep slopes. These are locations where both risk factors are present.

The landslide assessment matrix below shows the relationship between slope steepness, shear strength, and landslide hazard. As you can see, low shear strength soils on steep slopes are the most dangerous. High shear strength soils on gentle slopes are the most stable.

High Shear Strength

Low Shear Strength

Gentle Slopes

most stable

cautious site grading required

Steep Slopes

develop with caution

very dangerous

Your job is to determine the landslide risk for each of the five sites. After that is done, you will determine the suitability of these five sites for development using other information that can be found in a soil survey. You must be very careful not to recommend any sites that will put your client at risk. Sites with even a small amount of risk should be avoided. Topographic maps are maps that show elevation. This is usually done using color patterns or contour lines (lines that connect points of equal elevation). The topographic map below is of the area where your client's properties are located. Notice the brown contour lines that are used to show elevation

Two adjacent contour lines on this map represent an elevation change of 20 feet. This means that if you are walking along a road from the point where one contour line crosses it to the point where another contour line crosses it, you would have walked uphill or downhill a vertical distance of 20 feet. If you walk south on Route 857 in Riverside, you will be walking down a hill. You should know this because you can see from the contour lines that the elevation changes from 900 feet above sea level to 840 feet above sea level in just a short distance. The landscape in this area is moderately steep. You can tell that because the contour lines are very close together. However, if you walk south on Route 857 from Downtown to Walnut Heights, you will cross very few contour lines. This means that you are walking over a relatively flat landscape. Now, the key point for landslides: if the contour lines are close together (like on Route 857 in Riverside), the landscape is steep and the potential for landslides is high (ignoring other factors). However, if the contour lines are few and far-between (like on Route 857 between Downtown and Walnut Heights), then the landscape is flat and the potential for landslides is lower Soil Map -In an area the size of the typical U.S. county, there are many different types of soil. The U.S. government has made a tremendous investment in mapping these soil types because different types of soil have different characteristics. These characteristics determine the most appropriate agricultural and construction uses of the land.

Part of your job for this assignment is to identify the type of soil present at each of the five proposed construction sites. That can be done with this soils map. The colors on this map show the geographic distribution of soil types. To use this map, you just locate the property, get the code letters for the soil type, and then go to the Soil Properties chart to obtain information about the agricultural and mechanical properties of the soil.

Different types of soil have different suitabilities for agriculture and construction. The table below summarizes the engineering properties for soils present in the area of your client's proposed sites. You can use this table to help determine the landslide hazards of a site, and also to determine its suitability for construction. A description of each engineering property can be found beneath the table.

Soil Series

Map Symbol

On-Site Sewage Disposal

Alluvial Land

Ab

unsuited

unsuited

unsuited

variable

variable

Craster

Cr

unsuited

unsuited

fair

high

low to moderate

Dekalb

Dk

fair to good

good

fair

low

moderate to high

Findlay

Fn

good

fair

good

high

low to moderate

Lomax

Lo

fair to good

fair to good

poor to fair

low

Pope

Po

unsuited

unsuited

fair

low

low to moderate

Wilsondale

Wi

unsuited

fair

good

moderate to high

moderate

Homes with Lawns and Basement Landscape

Shrink Swell Shear Potential Strength

moderate to high

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On-Site Sewage Disposal: To pass state and local regulations, soils must have an infiltration rate that is fast enough to accommodate wastewater volume, yet slow enough to allow the breakdown of organic wastes. Soils are ranked as: variable, unsuitable, poor, fair, good, or excellent.



Homes with Basements: Below-grade construction requires a well-drained soil that is above the water table, and also a soil that will exert minimum stress on basement walls. Soils are ranked as: variable, unsuitable, poor, fair, good, or excellent.



Lawns and Landscape: Lawns and landscaping require a loose soil with adequate nutrients to support vegetation. Soils are ranked as: variable, unsuitable, poor, fair, good, or excellent.



Shrink Swell Potential: Some soils expand when exposed to moisture and shrink when they dry. This expansion and shrinkage exerts forces on foundations, basement walls, and floors. Soils are ranked as: variable, low, moderate, and high.



Shear Strength: Soils with a low shear strength are subject to landsliding and failure when oversteepened in excavations and embankments. Soils are ranked as: variable, low, moderate, and high.

Report (Turn this page in) Name__________________KEY__________________________ You have evaluated the five proposed construction sites and now it is time to report to your client. The first part of your report is on the landslide hazard at each of the sites. You are to categorize them as either a relatively "stable" or "slide-prone" site. "Stable" means that the site has little risk of landsliding, and "unstable" means that the site is either dangerous, or requires caution in grading or development. The second part of your report will be on the overall suitability of the sites for construction. In other words, which of them would you recommend for purchase and development. Your recommendation should be based upon a review of the landslide hazard, plus the other important considerations such as sewage disposal, basement development, landscaping, and shrink swell potential of the underlying soils. You are to give one site your "highest recommendation." Other worthy sites should be "recommended," and rejected sites should be designated as "unsuitable. Proposed Location

Landslide Hazard Rating

Overall Suitability for Development

Site 1

______ Stable ____x___ Unstable

_______ Highest Recommendation _______ Recommended ____x___ Unsuitable

Site 2

________ Stable ____x___ Unstable

_______ Highest Recommendation _______ Recommended ____x___ Unsuitable

Site 3

___x____ Stable _______ Unstable

_______ Highest Recommendation ___x____ Recommended _______ Unsuitable

Site 4

____x___ Stable _______ Unstable

_______ Highest Recommendation ____x___ Recommended _______ Unsuitable

Site 5

___x___ Stable _______ Unstable

____x___ Highest Recommendation _______ Recommended _______ Unsuitable

Your Name: _______________________________________________________...