**2. Ecological approaches to stabilize slope**

There are many types of natural and manmade slopes exist in Malaysia. Due to the nature of topography e.g. slope angle, length, aspect, gradient and curvature, and the weather condi‐ tions, these slopes become unstable and make serious geologic hazards. Around the world, landslide depends on the geological characteristics, hydrological condition and rainfall distribution [2,3]. In Malaysia, significant numbers of slope failure are reported on manmade and residual soil slopes especially at the time of high intensity rainfall due to rapid change of soil properties, particularly physical properties, such as bulk density, cohesiveness and shear strength [4]. However, there are three common triggering factors for slope failure in respect to Malaysia which are intense rainfall, water level change and change of slope loading (Figure 1), indicating hydrological condition gives infavorable impact on the slope stability. Apart from that, landslides also occurred due to the human activities such as cultivation, excavation for housing, foot paths and deforestation or construction works in hillsides and lack of plantation in hilly area [5,6]. In addition, improper slope design might cause manmade slope failure which has been acknowledged as one of the most reasons for frequent disaster around the world [7]. To overcome this problem, the slopes need to be proper designed, considering the geological characteristics*,* structural model, local weather and soil characteristics [8]. From eco-physiological point of view, the plant availability, so-called bioengineering systems, has become an alternative solution to minimize the impacts and stabilize the slope [1,9].

Bioengineering systems are extremely beneficial as it incorporates with geotechnical engi‐ neering, ecological and biological aspects [7,11]. In bioengineering techniques, less heavy equipments and live plants are usually used which are cost effective and provided long-term soil stability [12]. Generally, newly cut and manmade slope soils are unstable and expose to soil loss. Hence, it is needed to be made more resistant to erosion. In our study, Nordin et. al. [13], have discovered that different composition of root distribution and branching between two potential slope plants, namely; *Acacia mangium* and *Leucaena leucocephala* (Table 1) caused the differences in pull-out values. In tandem with *L. leucocephala*'s ability to establish on slope's harsh condition and reinforced the soil [14], it exhibited the higher force of pull-out, thus,

**Figure 1.** Common triggering factors which is related to slopes erosions [10].

**1.1. Objectives of the chapter**

520 Environmental Risk Assessment of Soil Contamination

**d.** Exhibit some potential slope plants.

**2. Ecological approaches to stabilize slope**

This book chapter will explore the attributions of vegetation to alleviate the most common slope problems in Malaysia; acidity and erosion. This chapter will also provide potential characteristics of plants to reinforce soil and the mechanism of Aluminium (Al) accumulator

There are many types of natural and manmade slopes exist in Malaysia. Due to the nature of topography e.g. slope angle, length, aspect, gradient and curvature, and the weather condi‐ tions, these slopes become unstable and make serious geologic hazards. Around the world, landslide depends on the geological characteristics, hydrological condition and rainfall distribution [2,3]. In Malaysia, significant numbers of slope failure are reported on manmade and residual soil slopes especially at the time of high intensity rainfall due to rapid change of soil properties, particularly physical properties, such as bulk density, cohesiveness and shear strength [4]. However, there are three common triggering factors for slope failure in respect to Malaysia which are intense rainfall, water level change and change of slope loading (Figure 1), indicating hydrological condition gives infavorable impact on the slope stability. Apart from that, landslides also occurred due to the human activities such as cultivation, excavation for housing, foot paths and deforestation or construction works in hillsides and lack of plantation in hilly area [5,6]. In addition, improper slope design might cause manmade slope failure which has been acknowledged as one of the most reasons for frequent disaster around the world [7]. To overcome this problem, the slopes need to be proper designed, considering the geological characteristics*,* structural model, local weather and soil characteristics [8]. From eco-physiological point of view, the plant availability, so-called bioengineering systems, has

become an alternative solution to minimize the impacts and stabilize the slope [1,9].

Bioengineering systems are extremely beneficial as it incorporates with geotechnical engi‐ neering, ecological and biological aspects [7,11]. In bioengineering techniques, less heavy equipments and live plants are usually used which are cost effective and provided long-term soil stability [12]. Generally, newly cut and manmade slope soils are unstable and expose to soil loss. Hence, it is needed to be made more resistant to erosion. In our study, Nordin et. al. [13], have discovered that different composition of root distribution and branching between two potential slope plants, namely; *Acacia mangium* and *Leucaena leucocephala* (Table 1) caused the differences in pull-out values. In tandem with *L. leucocephala*'s ability to establish on slope's harsh condition and reinforced the soil [14], it exhibited the higher force of pull-out, thus,

plant to alleviate soil acidity. The specific aims of this book chapter are to:

**b.** Outline the vegetation effects on erosion rate and carbon sequestration;

**c.** Elucidate the mechanism of Al accumulator plant to alleviate soil acidity and

**a.** Describe the ecological approach towards slope stabilization;

showing its capability to resist the uprooting forces. This feature helps the species anchorage and resists the slope against forces such as wind. Interestingly, the study also found that the pull-out force is much affected by the tensile strength (R=0.88; Figure 2). The tensile strength decreased with increasing root diameter. Thus, this property of higher tensile strength in the finer roots would ultimately yield an increase in shear strength of the root-soil composite in the field and provide better ductility to the root-soil composite with a higher capacity to withstand surface erosion and runoff [15].


Significant difference between species at LSDp<0.05 = 0.73 (pull-out) and 18.9 (tensile strength)

**Table 1.** Pull-out test and tensile strength of *Acacia mangium* and *Leucaena leucocephala* (data are means ± standard error)

Knowing the importance of vegetation, however, the problem of vegetation establishment on freshly cut slope is a succession problem, in which naturally, the process takes a longer time. Fortunately, with the human helps and input, we can accelerate this process. Normaniza and Barakbah [1] proposed a concept for establishing vegetation cover on slopes which enhanced the process of natural succession (Figure 3). The introduction of pioneer species on slope is to initiate the succession process and later to accelerate the ecological restoration as a whole. Initially, the proposed pioneer species is to improve the quality of soil, be it acidic or infertile soil.

**Figure 2.** The relationship between pull-out force and root tensile strength [13].

In relation to the proposed concept, the selected pioneer must exhibit prominent characteristics which include high growth rate, good root profiles, water relations and high tolerance to a wide range of adverse factors with regard to soil quality, microclimate and mechanical stress [9,16,17]. It is anticipated that once the pioneer is established, the succession process would be enhanced through the changes of abiotic and biotic factors. Consequently, influx of other species will enrich the plant biodiversity of slopes. This plant community changes would not only hasten the process of natural succession but would also attract small animals such as insects and birds to the ecosystem. This fauna association would assist in promoting "florafauna" interaction *via* being the agents of seed dispersal, which would ultimately enhance the natural plant succession process.

This ecological approach towards slope stabilization has been proven on slope projects conducted by our team at several highways cut slopes in Malaysia [1,18] (Figure 4). Our study showed that legume trees which act as pioneers had increased the species diversity and slope stability parameters of the slope (Table 2). The legume tree plots exhibited higher root length density and resulted in a lower saturation level of the soil. In addition, penetration resistance was observed to increase with presence of legume trees. The increment of shear strength at 30 cm soil depth was prominent in legume tree plot. Overall increment of slope stability param‐ eters indicated the positive effect of legume trees in reducing the probability slope failures, since most surface failures occur at a depth of 20-50 cm [1]. In contrast, the grasses plots had almost saturated, 83.9-93.2%, a characteristic of failing slope.

In relation to the proposed concept, the selected pioneer must exhibit prominent characteristics which include high growth rate, good root profiles, water relations and high tolerance to a wide range of adverse factors with regard to soil quality, microclimate and mechanical stress [9,16,17]. It is anticipated that once the pioneer is established, the succession process would be enhanced through the changes of abiotic and biotic factors. Consequently, influx of other species will enrich the plant biodiversity of slopes. This plant community changes would not only hasten the process of natural succession but would also attract small animals such as insects and birds to the ecosystem. This fauna association would assist in promoting "flora-fauna" interaction *via* being the agents of seed dispersal, which would ultimately

enhance the natural plant succession process.

slope stability parameters of the slope (Table 2). The legume tree plots exhibited higher root length density and lower saturation level. In addition, penetration resistance was observed to increase with presence of legume trees. The increment of shear strength at 30 cm soil depth was prominent in legume tree plot. Overall increment of slope stability parameters indicated the positive effect of legume trees in **Figure 3.** A proposed concept for establishing vegetation cover on slopes and enhancing the process of natural suc‐ cession [1].

Figure 3. A proposed concept for establishing vegetation cover on slopes and enhancing the process of natural succession [1].

This ecological approach towards slope stabilization has been proven on slope projects conducted by our team at several highways cut slopes in Malaysia [1,18] (Figure 4). Our study showed that legume trees which act as pioneers had increased the species diversity and

reducing the probability slope failures, since most slope failures occur at a depth of 20-50 cm [1]. In contrast, the grasses plots had almost


3

**Table 2.** Slope stability parameters between the plots (grasses and legume trees)

In relation to the proposed concept, the selected pioneer must exhibit prominent characteristics which include high growth rate, good root profiles, water relations and high tolerance to a wide range of adverse factors with regard to soil quality, microclimate and mechanical stress [9,16,17]. It is anticipated that once the pioneer is established, the succession process would be enhanced through the changes of abiotic and biotic factors. Consequently, influx of other species will enrich the plant biodiversity of slopes. This plant community changes would not only hasten the process of natural succession but would also attract small animals such as insects and birds to the ecosystem. This fauna association would assist in promoting "florafauna" interaction *via* being the agents of seed dispersal, which would ultimately enhance the

This ecological approach towards slope stabilization has been proven on slope projects conducted by our team at several highways cut slopes in Malaysia [1,18] (Figure 4). Our study showed that legume trees which act as pioneers had increased the species diversity and slope stability parameters of the slope (Table 2). The legume tree plots exhibited higher root length density and resulted in a lower saturation level of the soil. In addition, penetration resistance was observed to increase with presence of legume trees. The increment of shear strength at 30 cm soil depth was prominent in legume tree plot. Overall increment of slope stability param‐ eters indicated the positive effect of legume trees in reducing the probability slope failures, since most surface failures occur at a depth of 20-50 cm [1]. In contrast, the grasses plots had

natural plant succession process.

522 Environmental Risk Assessment of Soil Contamination

almost saturated, 83.9-93.2%, a characteristic of failing slope.

**Figure 2.** The relationship between pull-out force and root tensile strength [13].

**Figure 4.** Topographic map of surveyed slopes in Peninsular Malaysia; a) Faculty of Science, University of Malaya; lati‐ tude 030 07' 28.5'' N, longitude 1010 39' 14.6'' E and b) Batu 38, Pusat Pengajian Luar, University of Malaya, Ulu Gom‐ bak; latitude 030 20' 45.27'' N, longitude 1010 46' 26.52'' E.
