Lab 4 Geology Part c

7.0 Question And Discussion 1.) For some cases, give the recommended value of safety factors for the rock slope in civil engineering / construction industry with some justification. Factor of Safety (FS) Typical target design Factor of Safety (FS) values range from 1.3 to 1.5; however, based on engineering judgment, values outside of this range may be appropriate, depending on the circumstances. The minimum FS to be used in stability analyses for a specific rock slope depends on factors such as: 1. The degree of uncertainty in the stability analysis inputs; the most important being the amount of intact rock, rock mass strength, discontinuity spacing, discontinuity shear strength and groundwater conditions. 2. The level of investigation and data collection. 3.

Costs of constructing the slope to be more stable.

4. Costs, risks to the travelling public, risks to the roadway, and other consequences should the slope fail. 5. Whether the slope is temporary or permanent.

2.) Describe and explain the rock slope stabilization method. Engineering and structural methods for stabilizing slopes can be grouped into four categories: a) Excavation and filling techniques. This would include excavating the toe of an earth flow until successive failures result in a stable slope, removing and replacing failed material with lighter, more stable material, or recompacted debris, excavating to unload upper portions of a mass failure, and filling to load the lower portions of a mass failure (most likely in conjunction with other loading or restraining structures). b) Drainage techniques. This would include efforts to remove or disperse surface water (as discussed in Chapter 4), drainage of tension cracks, using rock fill underlain by filter cloth to prevent upward migration of water into the road prism, insertion of trench drains, perforated, horizontal drains, or drainage galleries, insertion of vertical drains or wells discharged by syphons, or pumps, and electro-osmosis (the use of direct current passing between well

points and steel rods placed midway between the rods to increase the drainage rate) for drainage of low permeability soils. c) Restraining structures. These include retaining walls, piles, buttresses, counterweight fills, cribs, bin walls, reinforced earth, and pre-stressed or posttensioned soil or rock anchors (Figure 98). Organizations such as highway departments and railroads have developed charts and tables giving earth pressures for the design of retaining walls that require a minimum of computation. Nearly all of these charts and tables are based on the Rankine formula which describes earth pressures as a function of unit weight and internal angle of friction of the backfill material. d) Miscellaneous techniques. Grouting can be used to reduce soil permeability, thereby preventing the ingress of groundwater into a failure zone. Chemical stabilization, generally in the form of ion exchange methods, is accomplished by high pressure injection of specificion exchange solutions into failure zones or into closely spaced pre-drillled holes throughout the movement zone. Heating or baking of clay soils can sometimes improve their strength, and, rarely, freezing of soils will help gain temporary stability. Localized electroosmosis can be used to form in situ anchors or tie-backs. Suppression of natural electro-osmosis can be used to reduce unfavorable groundwater pressures. Blasting is sometimes used to disrupt failure surfaces and to improve drainage.

3.) The differences assessment of the rock slope and soil slope. SOILS ON SLOPES :

Large-scale geologic structures provide the initial framework upon which landscape development proceeds. Finer details of landscapes (i.e. individual landforms) are usually determined by DIFFERENTIAL EROSION.. E.g. domed strata provide rock layers dipping away radially from a central high point; differential erosion produces inward-facing scarps (S), outward-facing dipslopes (D) and radial strike valleys (SV).

Strength and Stress In the context of geomorphology, strength refers to the ability to resist being moved by erosional processes, which normally operate in a downslope direction. The force exerted by erosional processes (including gravity) is a SHEAR STRESS directed downslope and causing a mass of rock or soil to shear over the underlying material. Controls On Soil Characteristics The characteristics of soil depend on: parent material; climate; vegetation; slope.

1. Parent material: influences; a. the rate of soil development (rate of weathering) b. soil composition e.g. shales produce a lot of clay; sandstone produces sandy soil c. physical properties of soil e.g. permeability/drainage (number, size and connectivity of pore spaces); shrink-swell potential (amount of expansive clay); cohesive strength (clay content - clayey soils are "sticky" - this aids cohesion).