Current state of practice in North America to evaluate the performance of slopes during earthquake loading is to use one of the following approaches:. Figure 1 below illustrates the approach. A slope is judged to be safe for a design earthquake if the factor of safety is equal to or greater than 1. By using this approach, you can evaluate the slope stability and the potential for any flow failure, however you cannot get a rough estimate of slope deformation. A wide variety of computer programs e.
The analysis procedure is straight forward, however, determination of a seismic coefficient that can appropriately represent the characteristics of design earthquakes is not easy. It is to be noted that seismic coefficient is not equivalent to the site-adjusted peak ground acceleration PGA because acceleration varies in depth of the sliding block and PGA occurs just at its surface.
Therefore, the equivalent seismic coefficient should be just a percentage of the site-adjusted PGA.
Stability of slopes
A number of recent publications have provided guidance in the selection of an appropriate seismic coefficient where the coefficient is a function of. The selection of appropriate seismic coefficient relies on both local code requirements and engineering judgement. The procedures outlined below are recommended for determination of seismic coefficient. A reliable design approach that you can use is the approach of Bray et al. The formulation is given as. If you are to just evaluate deformation of an existing slope, as mentioned above in the beginning of the article this method is NOT suitable.
For your convenience, I formulated the above equations in a spreadsheet. Figure 2. Although the procedure described above was originally developed for non-liquefiable site condition, it is also used for liquefiable site conditions. There are two main challenges when liquefaction is involved in slope stability analysis. Note that liquefaction generally triggers after the strongest motion pulses of an earthquake hit the liquefiable layer.
To this end, some percentage of the seismic coefficient calculated per the procedure described in the previous section should be used for slope stability 0. See Figure 3 to determine the residual shear strengths. However, it is to be noted that there is high level of conservatism incorporated into prediction of liquefaction triggering, peak ground acceleration PGAearthquake magnitude Mand residual shear strength of liquefied soil.Vegetation trees, shrubs, grasses, flower and ground covers helps stabilize slopes in numerous ways.
If you have indicators of potentially unstable slopes, call us. Vegetation management of urban, forested, coastal, marine shorelines, freshwater shorelines, and other riparian zones should conserve and maintain plant cover to reduce surface soil erosion and landslide styled movements of soil.
The relative effectiveness of vegetation in any site will be a function of quality of vegetation, topography, slope, hydrology, geology and soils. Erosion occurs when rainfall dislodges soil particles and carries them off a slope, forming rills and gullies that can trigger landslides. Raindrops hitting the soil surface can also seal the soil particles and make a crust that prevents infiltration and increases surface flow speeds, creating runoff. Of all these processes, retardation slowing of mobile surface waters and infiltration increased soil porosity both substantially reduce impacts to particle soil erosion.
Soil structure also plays a substantial role. Interception absorption of rainfall energy and transpiration soil moisture wicking reduce heavily in the winter when plants drop their leaves. Evergreen plants, as opposed to deciduous plants, do retain interception qualities throughout the winter, but their ability to wick moisture from the ground is still heavily reduced.
The primary factors of vegetation that affect mass-movements in slopes, particularly shallow sliding in slopes include adapted from Gray :. Root reinforcement, soil moisture modification reductionand buttressing and arching normally increase slope stability. Surcharge, and wind-throwing are having a net destabilizing effect.
My experience is that the net destabilizing effects of trees surcharge and wind-throwing are minimal compared to the benefits healthy plants and healthy root systems provide to surface soils. Gray and A. Restraint — root system physically binds or restrains soil particles while above ground residues filter sediment out of rain and stormwater runoff. Retardation — above-ground residues increase surface roughness and slow velocity of runoff. Infiltration — roots and plant residues help maintain soil porosity and permeability Transpiration — depletion of soil moisture by plants delays onset of saturation and runoff.
The primary factors of vegetation that affect mass-movements in slopes, particularly shallow sliding in slopes include adapted from Gray : Root reinforcement — roots mechanically reinforce a soil transfer of shear stresses in the soil to tensile resistance in the roots. Soil moisture modification — Transpiration and interception by the foliage limit buildup of soil moisture stress. Buttressing and arching — anchored and embedded stems can act as buttress piles or arch abutments in a slope, counteracting shear stresses.
Wind-throwing — destabilizing influence from turning moments exerted on a slope as a result of strong winds blowing downslope through trees. Courtney Bornsworth Courtney is a trained zoologist and marine biologist.
She has spent the last five years working in the fields of salmon habitat restoration, botany, and wetland ecology in both Southeast Alaska, and Western Washington. She now spends her time out in the field conducting surveys, designing restoration plans, and assisting with urban forestry consulting for Peninsula Urban Forestry. Courtney manages our native plant nursery. Related Posts. Invasive Plant Communities.
Benefits of Arborist Wood Chips. Rain Gardens. Trail Building.To understand the development and form of natural slopes and the processes responsible for different natural features. To assess the stability of slopes under short-term often during construction and long-term conditions. To assess the possibility of landslides involving natural or existing engineered slopes.
To analyze landslides and to understand failure mechanisms and the influence of environmental factors. To enable the redesign of failed slopes and the planning and design of preventive and remedial measures, where necessary.
To study the effect of seismic loadings on slopes and 4. Man made slopes - The sides of cuttings- - The slopes of embankments constructed for roads railway lines, canals etc.
Infinite slopes - The term infinite slope is used to designate a constant slope of infinite extent. Finite slopes - Finite slopes are limited in extent. Design and construction of earth dams. Slope failure can often be catastrophic, involving the loss of considerable property and many lives.
Erosion of the surface of slopes due to flowing water The sudden lowering of water adjacent to a slope. In slope stability analysis we determine the Factor of Safety as a ratio of resisting forces to driving forces.
Stability of Slopes
Theoretically, any slope with a Factor of Safety less than one will fail and any slope with a factor of safety greater than one will not. Factor of safety with respect to cohesion.
This is termed the factor of safety with respect to height. They have a common line of action which passes. The lateral forces must be equal and opposite and their line of action must be parallel to the sloped surface. The normal and shear stresses on plane AB are.Mod-05 Lec-40 Lecture- 1 on Stability of Slopes
The depth at which the shearing stress and shearing strength are equal is called the critical depth. Substituting these stress expressions in the equation above and simplifying, we have. This dimensionless number is proportional to the required cohesion and is inversely proportional to the allowable height. If in Eq. The friction circle method of slope analysis is a convenient approach for both graphical and mathematical solutions.
The total weight W of the mass above the trial circle acting through the centre of mass. The centre of mass may be determined by any one of the known methods. The resultant boundary neutral force U. The vector U may be determined by a graphical method from flow net construction. The resultant intergranular force, P, acting on the boundary. The resultant cohesive force C. If the slope angleheight of embankment H, the effective unit weight of materialangle of internal friction ', and unit cohesion c' are known, the factor of safety may be determined.
Taylor conceived the idea of analyzing the stability of a large number of slopes through a wide range of slope angles and angles of internal friction, and then representing the results by an abstract number which he called the "stability number".
This number is designated as Ns.The whole black earth is oppressed beneath the storm. All the rivers flow in flood, and many hillsides are furrowed deeply by the torrents, and They rush to the purple sea from the mountains, roaring mightily, and The fields of men are wasted.
The stability of these slopes should be thoroughly analyzed as their failure may lead to loss of human life as well as colossal economic loss. Such slopes are hypothetical in nature. In practice if the height of the slope is very large it may be considered as infinite slope.
Eg : Earth dam, embankments and cuts. It is the one with a base and a top surface and the height being limited. What is a Slope Failure? A slope failure is a phenomenon in which a slope collapses abruptly due to weakened selfretainability of the earth under the influence of a rainfall or an earthquake or any such factor.
Generally, a slope failure can be defined as a downward movement of a large amount of material. WHERE do slope failures occur? Triggered by Weather events, Geologic events, Human modification of the landscape, or Some interaction of all of the above. Therefore, slope failures occur nearly everywhere where slopes exist. Mountainous regions, hilly regions, and coastlines have the greatest occurrence of slope failures.
Active tectonic regions are prone to slope failures triggered by earthquakes or volcanic activity. WHEN do slope failures occur? Any season. They are more likely to occur in certain seasons if triggered by weather events such as rain, snow or freezing and thawing of soil water.
In many locations, both geologic and atmospheric processes may play a role in the movement of a slope. Rock fall 2. Slump 3. Rockslide or debris slide 4. Debris flow or mudflow 5. Earthflow B. Creep 2. When physical weathering ice wedging loosens angular boulders from rocky cliffs in mountainous terrain. The boulders break off and fall downslope producing an apron of coarse debris talus at the base of the slope.
Involves a mass of soil or other material sliding along a curved, rotational surface. Causes the formation of a small, crescent-shaped cliff or "scarp" at the upslope end. Slumps are sometimes seen along highways where the soil on the sides of the road is a little too steep. The slump may break into separate blocks, each with their own scarp surface.
The toe of the slump overrides structures at base of slope. Rockslides occur where sheets of rock move downslope on a planar sliding surface.
The sliding surface is a suitably oriented bedding plane or a fracture surface. Rockslides generate relatively thin sheets of rock that are broken into smaller blocks as they move downslope. Rather than moving downslope as a coherent mass slump, rockslide the material flows downhill as a chaotic mixture. Commonly occur in volcanic areas, where they are called lahars. Mudflows generally follow established drainage patterns.The slopes formed due to natural process and exist naturally are called natural slopes.
Natural slopes are those that exist in nature and are formed by natural causes. Such slopes exist in hilly areas. The sides of cuttings, the slopes of embankments constructed for roads, railway lines, canals etc and the slopes of earth dams constructed for storing water are examples of man made slopes.
The slopes whether natural or artificial may be. The slopes formed by unnatural process. Artificial slopes are formed by humans as per requirements. The type of slope extending infinitely, or up to an extent whose boundaries are not well defined.
For this type of slope the soil properties for all identical depths below the surface are same. In the making of natural slopes, their is no contribution from our side. The slope that is of limited extent. We the engineers deal with this type of slopes. The term infinite slope is used to designate a constant slope of infinite extent. The long slope of the face of a mountain is an example of this type, whereas finite slopes are limited in extent.
The slopes of embankments and earth dams are examples of finite slopes. The slope length depends on the height of the dam or embankment. In recent years soil cement as a facing material for earth fill dams has been found economical where suitable rip-rap is not available near the site. A reasonably firm foundation is preferred so that deformation after placement of soil-cement is not significant; however, no unusual design features need be incorporated into the embankment.
Subscribe to our Newsletter. Stay informed - subscribe to our newsletter. Email The subscriber's email address. Search AboutCivil. Similar Articles. Purposes and Functions of Foundations. Pile Driving Equipment - Pile Hammers. Ultimate Bearing Capacity. Load Capacity Of Piles. Training Course. Geotechnical Engineering. Related Civil-Engg.A slope is a ground surface that inclines either may be natural or man-made. Each slope has its own soil characteristics and geometric features, in order to resist gravity or collapse.
Soil mass will move slowly or suddenly without any signage downward and outward when slope failure occurred. Slides usually begin from hairline tension cracks, which propagate through the soil layers Das, Slope failures have caused an unquantified number of causalities and economic loss.
However, in rural area and less populated less effect of mass movements, only being part of natural degradation of the land surface. In the case of coastal cliffs instability involving the destruction of property is often accepted due to the costs of resisting natural erosion process with cliff stabilization measures are prohibitive.
Figure 1, Tulane UniversityProf. Stephen A. Nelson, 6 oct Gravity is the main force for mass wasting. By looking at the figure 1, gravity acts downwards on the flats surface. Therefore the materials will not moving under the force of the gravity. It will be different case when a material is placed on a slope, by resolving the force of gravity to two component acting perpendiculars and tangential to the slope. Consequently, the down-slope movement is preferential by the steeper slope angles the stress also increase and everything that decreases the shear strength for example by lowering the cohesion among the particles or the frictional resistance.
In other words, it is often known as the safety factor, F sthe ratio of shear strength to shear stress. Due to the consequences of slope failure, the topic has received extensive treatment in the literature.
Several models and analytical techniques have been developed to describe a variety of geometric and soil characteristics. The majority of literature focuses on deterministic evaluation of slope stability, however, with the new technology nowadays slope stability can be determine or predict factor of safety of the soil strength just simply entering the parameters. For this project the programmed used to analysis the slope stability had used different method approached to solve the factor of safety required for the problems being analysed.
In Limitstate:Geo 2. Meanwhile, in Geostudio the method use was General Limit Equilibrium. Thereafter, the factors of safety equations were presented to highlight the importance of modelling assumptions.
This chapter presents a review of slope stability analysis methods, including determining the factor of safety for the soil strength and the designing the soil parameters. The variability within soil parameters is summarized in this review. Finally, several case studies of slope stability analysis are summarized. The slope stability can be analyse nowadays by using a computer program.
LimitState: Geo 2. Drained and un drained analysis is required to be analyse to find out the factor of safety of the soil strength such that the slope collapses with soil parameters. The parameters of the soil properties have to be design until the factor of safety obtained equal to one F.
For this project, there were two computer softwares used for modeling the slope stability problems named Limitstate: Geo 2.After you enable Flash, refresh this page and the presentation should play. Get the plugin now. Toggle navigation. Help Preferences Sign up Log in. To view this presentation, you'll need to allow Flash. Click to allow Flash After you enable Flash, refresh this page and the presentation should play.
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