Ecological and Bioengineering Studies for Stabilizing the Wad Medani-Sennar Roadside Slope Linking the Gezira and Sennar States

)e erosion of the highway embankment slope’s soil along the Wad Medani-Sennar road is a significant issue, as there are many traffic accidents on this road, with an average of 15 to 25 fatalities per annum. It was thus decided to investigate this issue to find a method to protect slope from erosion on this road and to provide new approaches to slope erosion knowledge gap in Sudan. An engineering survey was carried out, followed by geotechnical studies, experimental work and interviews with academic experts regarding native vegetation in the survey area. )ese include measuring the eroded parts of the road; studying crosssections of the road; soil experiments to check the strength, compaction and particle size distribution; and a native vegetation survey to check for suitable plants that could be used to control the slope erosion. It was found that an appropriate bio-engineering method to stabilize the slope soil against erosion due to rainfall was to cultivate the grasses Cynodon Dactylon and Vetiver on the slopes. In conclusion, that using native vegetation for eco -protection, was an excellent solution to the problem based on the climate, native vegetation, and type of soil in Sudan and it reduces the accidents.


Introduction
In order to meet the needs of a growing population and the emergence of new cities, new roads need to be built to meet the needs of the population and their right to travel and move from place to place and to transfer goods for both private and commercial enterprises.e budding economy in Sudan thus requires the construction of new roads; however, this has led to serious soil erosion problems around the roads [1].
e capital cities of the two most important states in Sudan, Gezira, and Sennar are linked by the Wad Medani-Sennar road.Sennar has a large dam for producing electricity and provides electrical power to all Sudanese states.Gezira is an agricultural state and has the largest agricultural scheme in Africa (Gezira Scheme) [2].e Gezira Scheme is found in the area between the two capital cities. e Wad Medani-Sennar road is thus extremely important as it is used to transfer crops from production sites to wholesalers and retailers.e road is also the national carrier between the cities of the south and the north.
e simplest way and most cost-effective method to stabilize soil surfaces are to use vegetation, especially native vegetation, in order to prevent soil erosion.In general, local vegetation is not expensive, is easy to maintain, and is in harmony with the surrounding landscape.Native vegetation is also adapted to the local environment, which is dry, and thus are naturally water conserving.e use of native vegetation results in lower maintenance costs, conserves natural resources, and increases the biodiversity of the region, resulting in benefits for the local wildlife [3].
e main objective of this paper is to study the effectiveness of the ecoprotection method in reducing erosion to protect the Wad Medani-Sennar road, with the added bene t of reducing the number of tra c accidents on this road.

Study Area.
is research was conducted between two states in Sudan, Gezira, and Sennar.e study focused on the road linking the capital cities, Wad Medani and Sennar, of the two states.e Wad Medani-Sennar road is part of the long national road in central Sudan.

Site Description.
e Wad Medani-Sennar road passes through most agricultural areas in Sudan, including the Gezira Scheme, and through Sennar State, which has Sennar Dam.A survey of the road shows that there is no surface water drainage system, neither main channels nor side ditches, to remove surface water from the road.
Figure 1 shows two cross sections of the Wad Medani-Sennar road.e engineering measurements showing the di erent angles of the slope are shown, as well as the variation in slope heights.e slopes range from 27 °to 45 °in di erent sections, and slope heights range between 2.5 m and 4.5 m.  2

e Problems of Wad
Advances in Civil Engineering situation was dire and that nding a suitable method to curb this erosion was very important [4].

e Climate of Gezira.
e climate of Gezira State is tropical.Figure 3 shows the average temperatures for Wad Medani in Gezira, and climate characteristics for Gezira, based on o cial government data [5], are as follows: (1) Rainfall was measured at four monitoring stations: Al Shabarga, Algeria, Al Hosh, and Umm Al Qura.e south of Gezira is an equatorial rainy zone which has its dry season from December to January.e most rainfall was recorded between July and September.
e average annual rainfall is 350 mm to 373 mm.
(2) An average annual minimum temperature of 24.6 °C and an average annual maximum of 36.5 °C have been recorded.
(3) Bright sunshine on average of 7.5 h to 10 h per day.(4) e humidity in Gezira ranges from 70% to 80%.
is decreases in the dry season from 32% to 18%.Sudan is a high humid African country because it has two rivers, the Blue Nile and the White Nile, in the center of the country and the Red Sea in the eastern part of Sudan.e average humidity is 22%.
(5) Southerly and south-westerly wind predominate in summer, while northerly and north-westerly winds predominate in winter.is causes dust and reduces visibility during daylight hours.4. e intensity of the rainfall in autumn creates hazards on the Wad Medani-Sennar road as the soil erodes from the slopes [7].It is clear that there are no slope protections in places, such as side ditches or main channels, to remove surface water from the road.Gezira and Sennar states are both high rainfall zone regions, and usually, when constructing the ll slope for the road, mechanical protection is made.is lack of protection for the Wad Medani-Sennar road has caused the slopes to fail due to rainfall erosion.e ll slopes for road construction are categorized as man-made slopes, and advanced research in many developed countries has shown the e ectiveness of using native vegetation in stabilizing slope erosion [8]. is study has investigated the ecological method of using plant cover on the slopes to protect them from erosion.Various local plants were investigated as the best to use on the eroded section along Wad Medani-Sennar road. is will hopefully reduce human losses caused by tra c accidents as well as solve environmental problems due to soil erosion.Rehabilitation of eroded slopes is costly and is a natural disaster.erefore, it is necessary to consider new ways to protect the slopes and reduce the loss of life.ere is also  Advances in Civil Engineering a need for a cost-e ective and sustainable solution, particularly because Sudan is a developing country.Ecological protection, using vegetation cover, is one of the most appropriate and cost-e ective ways to solve the erosion problem of the Wad Medani-Sennar road.It is aesthetically pleasing, ll slopes are stabilized, and annual maintenance costs are low.

Objectives of the Study.
e main aim of this study is choosing the most suitable native vegetation and species to plant in the Wad Medani-Sennar ll slopes.Plants found naturally in the region of the road were studied, and those most suitable for use as an ecological and bioengineering method of protection against slope failure were selected.

Classi cation of Soil and Slope Types.
A study to classify the soil and soil types of the slopes along the Wad Medani-Sennar road was carried out using particle size distribution (sieve analyses) and Atterberg limit tests.

Methodology
A literature survey was conducted, followed by engineering measurements, and eld experiments were then carried out on the road site in order to classify the soil types along the Wad Medani-Sennar road.

Literature Survey.
e survey covered the related published work to clarify the methodology and to explain bioengineering methods and modern theories in the area of using vegetation to protect from and reduce erosion due to rainfall.

Determining the Native Vegetation.
For identi cation of local plant species and vegetation occurring around the slope areas, the help of experts was required, because some vegetation was di cult to identify.Also, the experts can easily check the best plants to grow for the study and the costs of these plants.

Vegetation Properties.
e properties of native vegetation, such as plant density, distribution, height, leaf shape, climatic condition, and soil surface covering, were investigated.All of this information is of use in selecting the most suitable local vegetation to use as ecoprotection on the Wad Medani-Sennar road.Advances in Civil Engineering

e Mechanism and Role of Vegetation in Stabilizing the
Soil on the Slope.e traditional mechanical methods of protecting slopes against erosion have an immediate effect, but are costly, both to construct and to maintain.Bioengineering, or ecoprotection, that is, planting local vegetation on the slopes, is both cheaper and easier to maintain.
One of the main benefits of using vegetation is that it is selfperpetuating [8].
Rainfall and runoff on the soil surface increase infiltration of pore water, and this is the main cause of erosion.Plant cover significantly reduces infiltration caused by rainfall, thus reducing erosion.

Slope Surface Protection.
e protection of the surface of the slope is a method of reducing rainfall infiltration during autumn, and this can be done using shotcrete plastering, or using blocked masonry or applying rep rap techniques.All these methods have less aesthetic value and are costly compared to using ecological methods such as using plants.Slope stability and vegetation are interrelated; plants that are growing on a slope hinder slope erosion and increase the stability of the slope.
3.6.Vegetation.Based on data from the past few years, the importance of vegetation in protecting against soil erosion and in controlling slope failure is clear.Vegetation also plays the basic role of preventing the soil surface from erosion that occurs due to rainfall intensity, sunlight, and wind [9].Vegetation covering a slope can control slope failure due to its root system mechanism, as depicted in Figure 5.It is clear that the roots penetrate quite deeply into the soil.
According to Singh and Rathod [9], there are four ways that vegetation influences slope stability.ese are shown in Figure 6.Protection is given against wind erosion, foot traffic, the impact of rain, and removal of water, due to the mass of vegetation and the mechanical reinforcement of roots, as shown in Figure 5.
Sediment generation is controlled by vegetation and maintains the biodiversity.e kinetic energy of raindrops is the main cause of soil particle disturbance [8]. Figure 6 depicts the use of vegetation in civil engineering and shows how vegetation protects the soil on the slope [8].e kinetic energy of the raindrops is dissipated by the vegetation. is is one of the advantages of using plants to prevent erosion.Another advantage is that plants can grow rapidly and cost less to maintain.

Vegetation Survey.
Plants found on eroded sections of the Wad Medani-Sennar road were classified according to the kind of plant and general vegetation, their root systems, and other salient properties.Several different vegetation types were found on the side of the road, and it was found that the vegetation varied according to the height of the slope.Suitable plants were then identified according to their location on the road.Plants were photographed, and the common names, root systems, climatic conditions, ability to cover the slope, and so on were recorded.Residents assisted in the classification of some vegetation types.Important factors used to identify vegetations were the leaves, flowers, and whole plants.
In order to understand how local plants can stabilize embankment slope soil, information regarding slope stability as a function of the soil type, soil age, and soil compaction was required.
e collected plant and soil data were discussed with botanical specialists, and based on the plants and plant density around the slope areas, suitable vegetation was selected in order to carry out this research.

Classification of the Soil
Soil samples were collected from various sections along the Wad Medani-Sennar road.Figure 7 shows how soil density testing was carried out in the field, and Figure 8 shows how   Advances in Civil Engineering the washed sieve analysis and hydrometer test was carried out at Wad Medani Technological College (WMTC).Washed sieve analysis allows for an accurate picture of particle size distribution in the soil. is distribution was determined according to USCS standards and is summarized in Figure 9.
e di erent types of soils, classi ed according to grain size, are gravel, sand, silt, and clay.
e results have shown that there are no major di erences in soil types.is is because the two states lie in the same semiarid zone and have similar climates.After conducting the soil classi cation experiments, the quantities of the di erent soil elements (gravel, sand, and silt and clay) were calculated based on the Uni ed Soil Classi cation System (USCS).Conducting liquid limit and plastic limit tests were carried out for the samples, and the soil was then further classi ed based on both the USCS and the United States Department of Agriculture (USDA) standards [10].
e American Association of State Highway and Transportation O cials (AASHTO) also developed soil size limits for gravel, sand, silt, and clay.ese limits are all shown in Table 1.
As mentioned previously, the vegetation most suitable for experimenting was determined regarding their relevance to the climate and soil types.All vegetation information, including plant name, density, height, propagation rate, leaf shape, soil, and climate, was recorded in this research.

Results and Discussion
5.1.Soil Classi cation.In general, the soil is composed of varying amounts of gravel, sand, silt or clay, and is classi ed according to the size of the soil particles.Sieve results revealed that more than 50% of the particles are ner than 75 μm.According to the USCS, this is below A-line in Figure 10, and the soil is thus MH (silt with high plasticity).Table 2 shows the sieve analysis of a selected sample, classi ed according to USDA and USCS standards [11].e average particle size distribution in the soil was gravel 4.1%, sand 8.6%, silt 75.3%, and clay 12%.
e moisture content of the soil was determined to be WC 22.1%, PI 18%, PL 32%, and Ll 50%.Soil eld density tests showed a bulk density cd of 1.510 and a dry density cd of 0.939.Chemical testing and soil nutrient experiments have shown that the OM 0.4-0.5 less than 1%, and N is 0.03-0.4% nitrogen in the soil, P is 2-4 ppm phosphorous, and K is 0.5-0.6 m/100 g potassium/100 g of soil.
e soil pH was found to range between 7.5 and 8.5.
In this section, the study illustrates the soil analysis, along with a detailed description of the type of soil according to sieve analysis, moisture content, density ( eld density by sand cone method), hydrometer and (Liquid Limits) tests.
e geotechnical properties of soil, such as grain-size distribution, plasticity, compressibility, and shear strength, can be assessed using proper laboratory [10] is summarized in Table 3, which shows the sieve analysis results for slope soil found on the Al Hosh highway.
Using the water content results, liquid-plastic limits and shrinkage limits were determined.Applying the Atterberg limits to these results allowed for the determination of the soil type based on AASHTO Designation T89, which was modi ed for use by the New York State Department of Transportation.Figure 10 is a liquid-plastic limit chart for the various soil types.It shows a sample ' ow curve' that can be used to determine the liquid limit.e water content corresponding to 25 blows was reported and was read from the " ow curve" as the liquid limit.
e determination of the percentage of water content in soil is an important factor in soil classi cation.DAS 2002 presents a clear procedure for the calculation of moisture content.Table 4 shows the average moisture content for the Wad Medani-Sennar road, for some test samples.
Casagrande proposed the Uni ed Soil Classi cation System (USCS) in 1942, which was later revised by the Army Corps of Engineers.
is system is currently used in all geotechnical work [12].Table 5 summarizes the symbols used by the USCS to describe the di erent soil types.
Table 6 shows the results of di erent soil experiments, using DAS 2004 to classify the soil [13].A hydrometer was   Advances in Civil Engineering used to test the fine-grained soils.Table 7 shows the experimental results for some samples from the Wad Medani-Sennar road.
It is important to collect chemical data regarding soil nutrients in order to determine whether the soil on the slope is fertilized or needs chemical treatments when planting the vegetation.e soil chemistry was determined at the University of Gezira (College of Agricultural Sciences and Water Resources), and the results are given in Table 8.Based on these results, teachers at the University of Gezira collaborated with those at Wad Medani Technological College to decide on which vegetation and native plants to use for testing of the bioengineering methods that should be used to protect the Wad Medani-Sennar road.

The Selected Vegetation Types
According to engineering surveys and soil classification and chemical experiments carried out on the Wad Medani-Sennar road and vegetation around the road, some plant types had higher population densities than others.Cynodon dactylon was selected because it was noticed that its root structure and shear strength significantly decreased soil Advances in Civil Engineering erosion [14], enhancing the stability of shallow soil [15].
Other plants, such as trees or bushes, were also found near the project area.However, many of these had lowpopulation densities.Plants with lower densities were omitted, and only those with large densities, such as Cynodon dactylon and Vetiver [14,16], were used for the investigation.
Truong [12] found that Vetiver grass was fast-growing and cultivates profusely.Its root system helps to prevent erosion and for shallow surface movement in the soil [13].e root system is strong and has an average tensile reach of 75 Mpa. is is approximately one-sixth of the strength of mild steel.Truong argued that the massive root system also increased the shear strength of soil.Vetiver can also grow in soil with extreme pH levels and high temperatures such as those found in Sudan.Vetiver has been proven to help stabilize soil and control soil erosion [13].
Many of the plants observed can be analyzed easily.Mesquite is often found and has many benefits.It can contribute to soil conservation, environmental stability through stabilization of dunes, hedging, and windbreaks and provides shelter belts around villages and agricultural schemes.Cassia angustifolia can grow in low rainfall areas [9], and Striga hermonthica, or Striga lutea as known in Sudan and many other African countries, is a very harmful plant that has deleterious effects on corn, sorghum, and sugarcane crops [17].e local vegetation types selected are shown in Figure 11.
According to the plant density data, two types of grass, two shrubs, and one type of tree were selected.ese are summarized in Table 9. Surface coverage data for the native plants selected (grass or shrubs around the Al Hosh location where the studies in this research occurred) are summarized in Table 10.
e soil determines the growth of a particular plant.For soils with a unique particle size distribution, the plant best suited for the soil must be selected.e best particle size range for each plant type is known, and thus using the maximum and minimum percentages obtained from the particle size distribution charts of each selected plant, the best plants for the soils were chosen.e ranges of soil needed for both creepers and trees or bushes were thus defined.According to the USCS, the most common soil type that could be used for the selected plants were classified as MH or OH as shown in Figure 10.
At most of the locations, the range of the slope angles varied from 27 °to 45 °.However, at a few other locations, the slope angle increased to 60 °.According to the above analysis, Cynodon dactylon, Vetiver, and Cassia angustifolia can be recommended for erosion control of the soil slope surface and to improve slope stability.However, both the positive and negative effects that occur by growing these plants in filled and natural slopes need to be further analyzed.Both Striga hermonthica and Mesquite need to be excluded, as Striga hermonthica is extremely harmful to cultivated crops, and it can rapidly creep to farms, growing over and destroying crops.Mesquite grows quickly and can grow up to many meters in height, thus affecting visibility on the Al Hosh highway.
is could lead to an increase in traffic accidents.

Conclusion and Recommendation
Based on the results of the study, the following were found: (i) Although not all plants were considered, the study has proven through vegetation surveying, geotechnical

Figure 5 :Figure 6 :
Figure 5: Root system in the soil.

Table 1 :
Soil separated grain size and limits according to UCS, USDA, and AASHTO speci cations.

Table 3 :
Sieve analysis test result.

Table 4 :
Water content test.

Table 5 :
Symbols are used for identifying the soil types.

Table 8 :
Chemical test and soil nutrients.