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International Journal of Enviornmental Science and Technology, Vol. 2, No. 4, Winter 2006, pp. 381-385 Slope stabilization with Gleditshia caspica and Parrotia persica 1*G. H. Bibalani, 2B. Majnonian, 3E. Adeli and 4H. Sanii 1Department of Agriculture, Islamic Azad University, Shabestar Branch, Shabestar, Iran Received 5 August 2005; revised 25 October 2005; accepted 25 November 2005; available online 22September 2005 Code NUmber: st05051 ABSTRACT: The stabilization roles of two species, Gleditshia caspica and Parrotia persica, were studied on slopes in northern part of Iran. Landslides developed in this area because of incorrect land use and clear cutting of forest to change to agriculture land. Spread planting of Gleditshia caspica and Parrotia persica can help to control instability of soil in this area. Bishop’s method was used to calculate the safety factor of slopes. This calculation was studied for the following conditions with vegetation cover of Gleditshia caspica, with Parrotia persica and without vegetation cover. Parrotia persica helped to stabilize slopes with 45-60% grades and Gleditshia caspica helped for slopes with 25-40% grades. Key words:Gleditshia caspica, Parrotia persica, slope, forest, landslide INTRODUCTION In recent years, the increased population in the world has led to an increase in exploitation of renewable natural resources. This has caused the destruction of forests, especially over the last ten years. Numerous natural disasters such as floods, droughts and the rise of the sea level affect human activities and welfare. Iran has a long history of landslides and these landslides cause significant damage to houses, agriculture lands, roads and installations and loss of human life. It is very difficult and expensive to remediate slopes following a landslide due to the particular conditions of geology, topography and climate. Furthermore the occurrence of landslides is a geomorphologic phenomenon that is influenced by numerous factors such as climate, ups and downs, vegetation cover, geology and tectonic activity. (Ahmadi, 1993) In Gilan Province landslides can be prevented rather well, because it has special condition for topographical and geological of landslides producer. (Habibi, 1999) This order, and with due attention to that, landslides in Gilan has not been studied now, we have taken a decision to study on part of landslides in Roudsar township, and hoping to make a use of the results on the same part of province, and propose different solution to use decrease landslides in this area. Therefore we have studied the effects of landslides producer. But another aim of this research specifies roles of vegetation cover on stability of soil on the slopes. This study have been done in Roodsar of Iran in 1999. MATERIALS AND METHODS This area is in northern part of Iran from latitude 36°, 15′N to 37°, 30′N and longitude 50°, 15′E to 51°, 10′E. The area is about 228 ha. In this area, 32 ha are covered with Gleditshia caspica and 28 ha Parrotia persica. The entire area is sloped. The aide of analysis of slope is to calculate the safety factors on the critical landslide surface, defined by: (Baker, 1981 and Brunsden, 1987)
General analysis of soil stability on a slope can be based on the following procedure: (Drannikov, 1963).
There are various methods for the analysis of slope stability such as Bishop’s method; Fellenius’s method, Morgenstern’s method. Bishop’s method is used in the study reported here. (Baker 1981). Software using this method was used to analysis slope stability of area. Safety factor in Bishop’s method can be calculated from the fllowing equation: (Behniya, 1993 and Baher, 1994)
In which: F= safety factor, α= angle of surface with horizon (degree), w= weight of soil on the slope (N), c= adhesive coefficient of soil (KN/m2), L= length of chord of slide (m), u= pore pressure (KN/m2), b= width of area (m), φ= angle of internal friction (degree). In this research, random sampling was used. Plot was rectangular form, with 8×10 m dimensions and 3.5% of area was sampled. Of a total of 78 plots, 43 were in Gleditshia caspica and 35 in Parrotia persica forest. In addition to ground cover measurement, soil profiles were dug. In these profiles, one undisturbed soil sample was taken for soil mechanic test and numbers of roots in one square meter were counted (in one, 0.8 and 0.4 cm diameter classes). Characteristic of soil mechanic can be obtained from the soil samples of the area. The triaxial test is used to determine soil characteristic such as angle of internal friction and cohesion of the soil. Bioengineering is used to stabilize soil landslides. Bioengineering is a composition of biology and soil mechanic sciences, and is alive and changeable. It can assist in increasing slope stability. The following equation is used to determined increase of shear resistance of soil with vegetation roots: (Pedram, 1992 and Ovnagh, 1993) Where TR= tension resistance of root, AR= total surface area of roots in A, A=surface of soil. On the basis of amount ∆SR, it is possible to calculate the angle of internal friction of soil (φ2) and safety factor (F2) with vegetation roots. Root tensile strength was studied in one, 0.8 and 0.4 cm diameter classes for determination of increase soil safety factors. RESULTS F1 (safety factor without vegetation) was calculated with soil properties (u, φ1, c, and dan.) and area characteristic (b, α, l and h) with equation 2, and additional soil resistance with roots (∆SR) was calculated with equation 3, then angle of internal friction with vegetation (φ2) was calculated. F2 (safety factor with vegetation) was calculated with soil properties (u, φ2, c, and dan.) and area characteristic (b, α, l and h) with equation 2 (Tables 1 and 2). Relationship between angle of internal friction (φ’) and safety factor of soil without roots (F1) was shown for angle of slope 15, 18, 23, 27, 30 and 33 degrees (Fig. 1). Relationship between angle of internal friction (φ’)
and safety factor of soil with different ground cover of Parrotia sp. (F2)
was shown for angle of slope 15 degrees (Fig. 2). Effective of different
roots of 20-40% vegetation on F2 was compared with F1 on angle slope
60% (Fig. 4). Finally, the least ground cover of different 15 degrees
(Fig. 3). Effective of different slopes (18, 23, vegetation could be
stabilized soil on slope with 27, 30 and 33 degrees) on F2 was compared
with F1roots different amount of angle of internal friction (φ’)
and of Parrotia sp vegetation with ground cover abovedifferent
angle of surface with horizon, was shown in Tables 3 and 4.
DISCUSSION AND CONCLUSION Although there are several factors which may affect slope stability and lead to landslides in the study area, it is clear that the main factor is the removal of the natural forest cover of the slopes, to create tea plantations and citrus orchards. In order to increase the F of slopes in this area with gradients greater than 25%, the least F that can be established for different vegetation cover with attention to the angle of internal friction of soils (φ) and the gradient of the slope (α) in order to the least safety factor (F=1.3). The main contributory factors which affect slopes stability are as follows; (Watson, 1995)
These factors have not been considered in this study. In addition to angle of intern al friction, cohesion and pore pressure of soil, other factors were studied such as soil density and angle of surface with horizon. Data from the 78 test plots were preparation to distinguish the lowest Gleditshia caspica and Parrotia persica ground cover for slope percent and angle of internal friction (degree) to stable slope. Parrotia persica helped to stabilize slopes with 45-60% grades and Gleditshia caspica helped for slopes with 25-40% grades. Decrease ground cover and increase heavy precipitation in one day decrease soil safety factor on slope above 60% grades for Parrotia persica and 40% grades for Gleditshia caspica and occur landslides in these slopes. This study has revealed and quantified the effect of the Parrotia persica and Gleditshia caspica trees in the Roudsar township forest lands, a phenomenon by means of which the tree stabilizes the slopes in the Gilan State and probably also in other areas where the Parrotia persica and Gleditshia caspica are growing. It is a pioneer study, and the results have given estimations of the root force of these trees for the first time in Iran. The findings and methodology of the study may be applied in other areas and to other trees. REFERENCES
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