Long-Term Stability of Clay Slopes - Essay Example

LONG -TERM STABILITY OF CLAY SLOPES LONG - TERM STABILITY OF CLAY SLOPES Clay slopes have undergone changes which are either short-term or long-term in nature. However, it is quite difficult to draw a distinction between short-term and long-term stability. It certainly depends on the geologist, soil scientist or the engineer's subjectivity. Long -term stability could be ten years or even more. The biggest question to answer here is "how can long-term stability of clay slopes be analysed" Perhaps to answer this question one must be very conversant with soil science and slope behaviour and stability. In order to describe "the long term stability of clay slopes", this paper will look at factors that make slopes become unstable overtime; the measurement of the instability; impact of long-term stability; and the prevention of instability of clay slopes. To begin with, long term stability of clay slopes is influenced by several causes which as are embedded in two major factors. These main factors, according to Price (1984) and Ritter (1986, p 121) are shear stress and shear strength. Shear stress refers to the forces that cause movement of materials down slope while shear strength is the force that resists materials from moving down the slope. These forces oppose each other in the sense that, while shear stress wants movement of material to commence, shears strength refuses. This means that movement will only occur when shear stress exceeds shear strength. Otherwise, like Sparks (1964, p 56) puts it, in uniform material, shear stress and shear strength may remain comparatively uniform. It is worth noting that shear stress has several intricate components that make it provoke movement of material down slope. Ritter (1986, p 121) outlines material internal friction characteristics; material normal stress and material normal cohesion as factors that determine stability of shear stress. Any drop or increase in these factors may alter shear stress. Internal friction is further broken down into plane friction (produced when one grain of soil slides past another) and interlocking friction (which originates when particles are required to move upwards and over one another. Secondly, Ritter (1986, p.122) further states that effective normal stress has the capacity to hold the material together, thereby increasing internal resistance to shear. It acts perpendicular to a shear surface and is absorbed by the underlying slab at the point of contact between grains. It should also be noted that some of the shear surface is usually occupied by openings which are filled with air or water. And since pore pressure exists in these interstitial spaces, it tends to support part of the normal stress. Thirdly, Ritter (1986, p.123) further states that clay soils have cohesion, which comes as a result of ions and water by clay minerals, thereby creating a binding structure among particles. Unfortunately though, cohesion decreases with increased water acquisition in the soil material. Clay cohesive strength very much depends on attractive forces between the particles and the lubricating action of the interstitial liquid. The more the clay acquires water, the more the slope becomes unstable. However, it should be noted that the rate at which the slope gains more water and the water drains away determines the time the slope should become unstable. For example, fast gaining and slow draining of water on a particular slope will make the slope fail very fast. On the other hand, slow gaining and fast draining of water will make the slope remain stable for some time. Where clay soils remains in an undisturbed normal cohesive strength, long-term slope stability will be evident. Sparks (1964, p 57) agrees with the above statement and adds that cohesive strength increases with depth thereby exceeding shear stress, hence surface mantle (slope material) is the one to be more unstable. As more water is added , cohesion decreases and when all pores are filled, any further input of water results in complete destruction of the internal fabric, hence changes occur to the clay and finally to the slope stability. Ritter, (1986, p 124) quoting Skempton (1964) agrees by stating that the clay might expand and become plastic or fluid hence induce slope failure. According to Embleton (1979, p 136), Ritter (1986, p 128) and Price (1984), shear strength is increased by factors such as removal of lateral support (unloading), addition of mass (loading), earthquakes and tremors, volcanoes, regional tilting (plate tectonics), removal of underlying support and lateral pressure. The removal of material (load) from clay surfaces by excavating or erosion may lead to a progressive series of internal changes including lateral expansion of newly-formed slopes, opening up of fissures and tension cracks, increase of mass permeability, long-term softening and reduction of strength. Unfortunately, the removal of load on stable clay slopes may have effects in the long run. For example, unloading reduces overburden pressure and total normal stress on the potential surface of sliding. This as Ritter, (1986, p 137) puts it, is accompanied by increase in shear stress as incisions (erosion) proceed - making the ultimate result to be slope failure. The removal of load may also cause clays to expand if these materials are relatively impermeable so that equilibrium pressure occurs. With time, clays can slowly transmit water and pore pressures recover equilibrium. Since there will be no further changes in the overburden press, shear strength will slowly diminish, as will the safety factor until a steady position is reached for the new slope (known as long term condition of the slope). On the other hand, Embleton (1979, p 138) states that loading or addition of material on clay slope, which Ritter (1986, p 128) says may be due to rainfall, talus or fills, mining or construction of buildings and roads, increases height and effective weight of slope forming material. .This situation increases shear stress on the potential surface of sliding. Embleton (1978, p 94) quoting Bishop (1960) further adds that such loading may increase pore-water pressure leading to a reduction in strength and safety factor, hence long-term stability becomes affected. Price (1984) brings a compromise by stating that loading can behazardous or not depending on the position it is going to occupy on the slope. If the load is placed at the toe end of the slope, there will be increased safety. If it is at the head of the clay slope, slope failure will occur. And if it is at a neutral point of the slope, there will be no harm to the slope. In other words, the slope will remain stable. Furthermore, earthquakes, tremors, tectonic activity and machinery operation on the slopes have great influence on long-term stability of clay slopes. Several geologistssuch as Stavridakis (2005) and Price (1984) agree that the afore-stated forces produce shocks and vibrations which create diverse problems to clay slopes. Amongst many, these shocks bring fissures, cracks and joints in the clay material. Sparks (1964, p 58) quoting Skempton (1964) says such flaws in the clay material decrease the strength of solid material and act virtually as stress concentrators. Thus failure occurs at such weak points immediately throwing increased stress on some other points which finally fails the material. When failure occurs, negative alterations occur in the pore space, pore water pressure, cohesion, internal friction and consequently disarm shear strength of the clay material on the slope. Stavridakis (2005), quoting Terzaghi (1936) and Skempton (1967), and Price (1984) reinforce the foregone argument by stating that the development of shears in clays accompanied by particle orientation depending on the nature of the clay minerals present creates clay slope instability because it reduces shear strength. Clay resistance decreases if shearing continues beyond maximum shear strength. What will follow is liquefaction, remoulding, fluidization, air lubrication and cohesionless grain flow. To some extent, like Buckle (1978, p 69) points out, the angle of slope and plants influence the stability of the slopes in one way or another. The absence of plants to hold soil material, encourage movement of the slope material. Also, the steeper the angle of slope, the more the shear stress, and the more vulnerable the slope material will be. The more construiction and mining take place using heavy machinery, the more the structural change occurs to the slope material and the more shear stress develops. On the other hand, shear strength is the ability of the slope material to resist movement which is induced by shear stress. According to Embleton (1978, p 89), in order for shear strength to be well analysed, one must know the properties of the slope material in details. Once the properties have been established, stability calculations can be made by engineers. It is fact, as Ritter (1986, p 82) puts it that clay development is usually controlled by the ions available in the parent material and many of these are highly mobile due to high reactivity. The mobility of ions makes the clays alter from one form to another. "Clay soils are made of different types," (Ritter, 1986, p 80). These include Kaolinite clays, Montmollironite clays, Vermiculite clays, Illite clays and Chlorite clays. These have different properties that define their shear strength. For instance, Kaolinite clays do not expand when wet because of their mineral arrangement. Again, cations and water do not penetrate between Kaolinite crystal units. This means there is low cation exchange capacity because ions exchange process is restricted to the external surface of the clay mineral. In addition, Kaolinite clays have low plasticity (capacity to be moulded) and low swelling capacity of the mineral. Such properties make Kaolinite clays to have reliable shear strength that can resist shear stress for some time if all other variables are dormant. On the other hand, Montmollironite clays have weak bonding between crystal units. Their mineral lattice expands upon wetting. Therefore cations and water molecules easily penetrate the mineral interist where cation exchange takes place along surfaces of crystal units. This clay mineral type also has greater plasticity, swelling and cation exchange capacity. These properties make Montmollironite clays the weakest type of clay which is easily defeated by shear stress. In summary, Ritter (1986, p 81) states that the last type of clay to be made unstable by shear stress is Kaolinite. This means that if Kaolinite clays dominate the stable slope, long-term stability will be observed than if dominated by Montmollironite clays. Stavridakis (2005) and Ritter (1986, p 128) agree that shear strength is decreased by adsorption of water, increased pore-water pressure, shock, freezing or thawing action, loss of cementing material, weathering processes and structural changes of the clay material. According to Embleton (1978, p 94), increased weathering is associated with reduction of effective cohesion and effective friction angle. Weathering brings about development of soil regolith horizons with physical properties different both from each other and from the parent rock; increasing comminution of particles to smaller sizes and an increase in clay-sized particles; increase in water content; increase in pore-water pressure; increase in the number and size of voids and fissures (in coarse material, increase in void spaces and mass permeability, while in fine material porosity and permeability decline); collapse of original mineral structure and fabric; and changes in cohesion and angle of shear resistance. As clay masses weather, clay slopes become unstable over time. As if that is not enough, Price (1984) and Sparks (1964, p 58) agree that pore-water pressure, coupled with other factors such as loading, has effects on shear strength. The more the clay pores are filled with water, the more the soil becomes more none-cohesive and the more the shear strength weakens. This increase in water may be a result of rainfall, ice melting or flooding. This means rainfall, for example, may also cause failure of a once long-term stable clay slope. Embleton (1979, p 144), quoting Terzaghi (1925, 1960), states that water (rainfall) eliminates surface tension as air is driven out of the voids of fine grained cohesionless soils and reduce apparent cohesion; removes soluble cement; initiates softening, wetting and drying, hydration swelling and hydrolysis (weathering); increase in weight of soil and consequently, a decrease in shear strength. Perhaps this is why Sparks (1964, p 56) states that large movements of slope material are common in wet Equatorial or Humid zones and in active ice melting zones, where saturation of water in pores is very high, in addition to earthquake and volcano prone zones. It therefore entails that climates of specific regions have a strong bearing on stability of clay slopes. For example, some climates which have dry and wet seasons mean that the clay soil will be wet and dry at a certain period of the year. These conditions make the clay develop different characteristics. When wet, they might become plastic and have high pore water pressure, which will obviously lead to low shear strength and slope failure. Similarly, the abrupt drying makes the clay shrink and hardened (thereby increasing shear strength) and sometimes develops cracks (which bring shear strength to decline as explained above. Long-term stability of clay slopes has several impacts which might be beneficial to the human beings and the environment. Amongst many, long-term stability of clay slopes is more helpful to human beings. If the slopes remain stable for some time, the issue of landslides, mudflows, silting of rivers, demolition of constructions such as roads, railways, bridges and buildings due to clay slopes instability would be reduced. Secondly, long-term stability may also lead to a stable environment. For example, when there is little or no movement on the slope, the vegetation that is growing on the slope will not be disturbed. It is a fact that the movement of slope leads to the falling of trees, hence loss of the vegetation that, to some extent controls climate. Thirdly, if the slope remains stable, that means the properties of the clay soil such as permeability and pore pressure are balancing. That free draining reduces the incidence of flooding and silting (erosion), which in turn affect human activities. In short, these are just a few of the many impacts that can be availed on long term stability of clay slopes. Much as it is said that slopes develop instability over time, the rate at which different slopes do that differs. For instance, Natural Slopes, according to Price (1984) are slow to erode due to easy seepage that maintains average pore pressure. On the other hand, human made slopes erode quickly and have low permeability soils (clays). There will be inadequate time for construction period for the pore pressure to adjust to new conditions and the soil will then be undrained hence low shear strength will cause failure of the slope over time. If long-tern stability of clay slopes is to be known, critical experimentation and observation of these slopes in different parts of the earth have to be done on a very large scale. Geologists and soil engineers have to humble themselves to conduct all sorts of experiments concerning slopes and clay soil behaviour. The scientists should not spend most of their time in Laboratories and simply developing theories. Perhaps they rise and go right into the field with all sorts of instruments such as tapes, shovels, testers, etc to check thing such as cation exchange capacity, mineral structure, loading and unloading, acidity, pore-water pressure, parent material, weathering rates, climate changes, structure, texture, slope angles, chances of regional rise, volcanism, earthquakes and tremors, name them! They have to be prepared to get dirty, wet, bitten by insects and pricked by thorns. The clay slopes should be their offices and their job. They have to read extensively on soil science, like Price (1984) states, to see previous experiments done, such as anchoring of soils. They should equip themselves with a lot of formulae. Above all, the scientists must be well versed in clay soil and slope behaviour in order to interpret their results with kin expertise. Prevention of any hazard needs strong-willed people. People who should be prepared to work the whole clock, day in day out. Embleton (1978, p 94) agrees with the statement above by stating that such people have to develop observation skills to increase monitoring of pore water pressure in slopes, especially those prone to landslides and all other potential instabilities in order to have better explanations. People must also start contemplating on where to construct what, and how, and when and with what. This paper has already said that the zealot construct different structures using heavy machinery has to some extent developed cracks in the clay, thereby altering the properties of that clay - a situation which increases shear stress. Random construction has, however, encouraged instability. Perhaps scientists should start advising people on where to construct roads, railways, and building using a set of given materials, in a method that is more safe, and with machinery that does not cause excessive harm to the slope. Furthermore, Embleton (1978, p 94) suggests that there should be frequent examination of the nature of climate controls in order to reduce the instability. Perhaps studying previous climate records will give direction to the scientists, who will therefore provide instant remedies where possible. Checking soil properties would also prevent the problems of long-term clay slope instability. Scientist with full knowledge of the soil mineral structure will apply that knowledge in different circumstances. For example, Buckle (1978, p 69) states that the Norwegian quick clays lose plasticity when salts are mixed into the flowing mass. Throwing bags of sodium or magnesium Chloride into the fluid mass makes the water clay bonds restored and stabilize on the Norwegian roads. If this scenario is applied to many potentially unstable slopes, then dangers of instability might be alleviated. With the description above, it should be concluded that ground conditions such as plastic weak material, sensitive material, collapsible material, weathered material, sheared material, jointed or fissured material, adversely oriented mass discontinuities, contrast in permeability and effect on ground water and contrast in stiffness of the material; coupled with geomorphologic processes such tectonic uplift, volcanic uplift, glacial rebound, fluvial processes, erosion, deposition loading on slope and vegetation removal; together with physical and man made factors such as rainfall, melting floods, earthquakes, thawing, volcanoes, shrinks, excavations, loading and unloading, irrigation, water leakage, vegetation removal, mining and constructions respectively will correctively increase shear stress and reduce shear strength. Such a situation will affect the long-term stability of clay slopes. Fortunately, the knowledge of Soil and Slope scientists will help in discovering possible dangers and provide possible remedies to such pending dangers so that the clay slopes remain stable and useful to human beings. BIBLIOGRAPHY Buckle, C 1978, Landforms in Africa, Longman, Hong Kong Embleton, C et al. 1978, Geomorphology, Oxford University Press, Oxford Embleton, C & Thornes, J (editors) 1979, Process in Geomorphology, Edward Arnold (Pub) Limited, London. Price, T 1984, "Long-term Stability of Clay Slopes", Geotechnical Reference Package, [Online] Available at: http://www.environment.uwe.ac.uk/geocal/SLOPES/SLOPES.HTML#SLOPES Sparks, BW 1964, Geomorphology, (2nd edition), Longman, London Starvidakis, EI 2005, "Strengthening the Soil Engineering Characteristics by Using Anchors during Earthquake Impact", EJGE PAPER, [Online] Available at: http://www.ejge.com/2005/pro0586/Abs0586.html-4k Read More

 

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