The instabilities of the slopes are the direct reason of landslides, normally for a landslide, there are one trigger of landslide and one or more landslide causes. There is subtle difference between these trigger and cause, but they are different. The causes of the landslide are why the landslide happens in that place at the specific time, which includes factors associated with human activity, physical factors, morphological factor and geological factors. All the causes can be considered as the reasons for the instabilities of slope, while the trigger is just for initiating landslide. Usually it is easy to determine the trigger of the landslide after the occurrence, while it is not so easy to tell the specific reason for landslide, and it is not easy either to determine the nature of landslide trigger before the happening of movement event.
Considering about the geological causes of landslides, the earthquakes, rainfall and snow fall, material contrasts, permeability contrasts, adversely orientated discontinuities, jointed or fissure materials, sheared materials, and weathered materials are the major reasons for landslides, in which the rainfall and snow fall, permeability contrasts are relevant to the water content of clay as the materials contained in the flows.
And for the causes defined in the morphological part, erosion, vegetation change, slope loading, subterranean erosion,erosion of lateral margins, glacial erosion, wave erosion, fluvial erosion, rebound, uplift and slope angle are considered at thesame time, in which the slope loading and slope angle are relevant with the applied force on the clay.
As clay is one of the main components of landslide, its influence on the triggering of landslide is worthy of deeper investigation, such as its temperature, water content and the force applied are the main factors that need further consideration. Because as shown above, the velocity of landslides are normally decided by the water content, lower water content leads to higher velocity of landslides, which is due to the lower interaction force between the materials contained in the flow. As for the temperature, it may influence the activity of the water inside the flow, such as it is ice or water, the triggering of landslide may be influenced, and the velocity of the landslide is also influenced if it is triggered already. For the force applied, like been mentioned above in the classification of landslides, the angle of the slopes make major difference on the landslides, normally the steep slope causes higher possibility of triggering of landslide, because the applied gravity due to steep angle, and the velocity of landslide is also influenced by the angle of slope, because of the accelerate is dependent on the angle of the slope also, as larger angle of slope leads to higher accelerate of the flow.
Therefore, the effect of the temperature, applied force and water content within the clay is worthy of study, thus to help to prevent the happening of more landslides.
Effect of clay property
The type of soil affects the failure of slope a lot, it has been reported that the silty clays and clay lead to instabilities of slopes in most cases.This can be attributed to the fact that they are prone to absorb too much water, and the permeability of the clay is low. The distribution of water is also influenced by the silty clays and clay.
The sandy soils are not easily to cause the failure of slope because of their relatively high permeability, the pore-water pressures within the soil will not reach the critical levels, and they are able to maintain the relatively similar mechanical strength when they are dry. The instabilities arisen from the cohesion-less sandy soils are very shallow, thus the slopes with sand as bottom are easily to equilibrate as the grains are prone to roll downslope quickly. Talus slope is formed at the angle which offers little possibility of catastrophic failure, unless under the conditions of seismic shaking or high precipitation.
Nevertheless, the mechanism of failures caused by clays (cohesive soils) is quietly different with the sandy soils’. Curved surface or well-defined planar are where failures occur usually, the material within behaves more like a unit than the material within the sandy soil. The relative susceptibility to landslides in regions of well-developed, deep soils is determined by the structure and composition clayey layers. While for the regions where the soils are shallow, the critical plane of weakness is provided because of the clay seam in the underlying bedrock, thus the instability is prone to develop.
According to the report done by R. A. Roth(ROTH, 1983), the soil data collected for the San Mateo County was studied to dig more on the effect of soil type on landslide, thus more on the effect of clay property on landslide(Jennings et al., 1977).
Basically, there are different sources of applied forces on the clay.The most popular one is from the natural gravity, which is relevant to the angle of slope. Slope angle is surely a parameter that affects the landslides’ distribution. From the intuition, the increasing of slope angle leads to high likelihood of occurrence of landslide, which could be demonstrated mathematically in the case where the length of uniform slope is infinite, and the possible failure plane is paralleled to the slope. The shear stress on the possible failure plan is easily to be expressed as
No other force except the force from gravity is taken into account. The τ is shear stress, ρ is the density of material within the soil, g is the gravity caused acceleration, h is the vertical height of sliding plane, while the α is the angle of slope.
So as shown in the equation above, increasing the slope angle and increasing the weight of the column (such as surcharge or water saturation) lead to higher possibility of failure; while for the same material considered, the possibility for failure increases with increasing slope angle.
2.1 Effect of water content within clay
There are several factors that affect the water content of clay, like the soil type (silty clay or sandy soil)(Brabb, 1987), and the rainfall also affects the water content of clay. Indeed, these two factors have been studied over a long time, and the data obtained from different areas reveals that there is a positive relation between landslide occurrence and rainfall. Like reported byR. A. Roth(ROTH, 1983), the higher values of mean annual rainfall lead to higher landslide frequency, which is consistent with the previous studies that showed occurrence of slope movements that are significant when the annual rainfall reaches over 10-15 inches.
And the study on relationship between rainfall and landslide frequency was done with caring about the influence of other factors. The landslide frequency decreases when the mean annual rainfall reaches 36-40 inches, which means that there are other factors that influence the landslide occurrence. Because in theory, the increasing of precipitation should lead to the increasing of landslide frequency, so there must be some other factors that are caused by increasing rainfall influenced the landslide frequency at the same time. The factors like density and type of vegetation are related to rainfall, so they may play a role. The relevant study on the factors have been done by R. A. Roth also(ROTH, 1983), which shows that the greater extent of vegetation is caused because of the higher rainfall 36-40 inches, the vegetation helps to stabilize the soil thus to reduce the frequency of landslide, this could explain the reduction of landslide frequency in the areas receiving 36-40 inches rainfall. The sequence of storms and intensity of the rainfall influence the frequency of landslide also, there would be more runoff with the increasing intensity of storms(Legget, 1939).
Figure 7 Frequency of landslide vs. mean annual rainfall
The soil type is also relevant with the water content within the clay, like water dispersible clay is defined as the clay within which soil sample is dispersed with water and no pre-treatment is made to remove compounds as cements, like carbonates, iron oxides, organic carbon, and no existing of dispersing agent either. The water content of the clay is dependent on the proportion of natural clay to total clay, which could influence the stability of structure of the clay, thus the dispersion of clay particles in the water would be influenced by the mechanical energy(Christensen, 1992), the stability of the flow is influenced also, instability of the landslide is therefore affected.
Thus in order to study the effect of water content of clay on the initiating of disaster like landslides, the mean annual rainfall in the area studied needs to be take into account; the soil type also needs the attention paid, like the sandy soil contains less water, while the clay or silty clay contains more water, which performs differently also in the initiation of landslides. Therefore, in order to study on this effect, an appropriate area where landslides happen frequently should be chosen rightly; at the same time, the soil types of this area should be collected through official way or from the library; furthermore, the rainfall data should be collected also from the weather bureau, or from the library. With the data from all aspects, the effect of water content within soil could be studied deeply, thus to instruct the prevention of landslides or inhibit the damage of potential landslides.
2.2 Effect of applied force on clay
As has been studied already byR. A. Roth (ROTH, 1983), the slope angle has been proved to influence the frequency of landslide a lot, the data is from the digital database of San Mateo County(Aleotti and Chowdhury, 1999). As shown in the graph, the increase of frequency of landslide is as sharp as over 5 percent on 3 degrees of slope angle, and a leveling off higher than 15 percent on 9 degrees. From the data shown in the graph, all slopes that are steeper than 15 percent (9 degrees) are easily to initiate landslide at similar possibilities, which is in agreement with previous research done by other researchers. Radbruchetc. (Radbruch-Hall and Crowther, 1973)has concluded that the slopes of 15-35 degrees are prone to occur landslides for all of California, only very few happens below 5 degrees. While as studied by Cleveland and Blanc(Blanc and Cleveland, 1968), in San Clemente, California, there is no correlation between landslide occurrence and steepness for the slopes with angle higher than 10 degrees.