DESIGN OF GEOSYNTHETIC REINFORCED WALLS AND SLOPS BY TERRAM PROGRAM

This research deals with the design of geosynthetic reinforced walls and slopes subjected to series of static compressive loading tests by the present TERRAM program. The objectives of this study are suggesting an optimum geometry of reinforcement placement to lessen the width of the side slope of slopes. The effects of the following variables were taken into account: the angles of slope, wall or slope height, surcharge, strength parameters (cohesion and friction angle) and unit weight for all soils involved in the problem (fill, natural soil and foundation soil), friction angle for reinforcement-soil and fill-foundation interfaces, as well as internal and external factors of safety are calculated for different distributions of tensile force in the reinforcement layers according to the different arrangements of reinforcement layers in terms of number, length, and spacing


Introduction
Soil is a relatively inexpensive and abundant construction material, which makes it ideal for use in construction. Soil is capable of providing very high strength in compression, but virtually no strength in tension. In civil engineering applications, soil usually fails in shear. Like other construction materials with limited strength, soil can be reinforced with foreign material to form a composite material that has increased shear strength and some apparent tensile strength. Metal strips, steel meshes and bar mats, geosynthetics and even bamboo have been used to reinforce soil.
The first modern-day design approach for reinforced earth structures was developed in the 1960's, by the French engineer, Henry Vidal (Das, 1984). The first reinforced earth retaining wall constructed in the United States used metallic strips for reinforcement and was completed in 1972 (Mitchell and Christopher, 1990). The construction of reinforced soil structures, including both slopes and walls, has increased considerably over the last 20 years as the advantages associated with this construction alternative are more widely recognized.
Without reinforcement, a stable slope can be constructed with an inclination angle less than or equal to the internal friction angle of the soil. The reinforced soil mass relies on the tension provided by the reinforcement to maintain stability at steep inclination angles. The mobilization of tensile resistance occurs once the slope experiences some deformation. For static and dynamic loading conditions, excessive deformations of a reinforced slope can occur when the reinforcement stretches, yields, breaks, or pulls out of the soil.
Numerous methods have been developed to design reinforced soil structures for static loading conditions, but considerably fewer procedures for seismic design are available. As the number of reinforced soil structures constructed in seismically active areas of the world increases, and in response to the observed performance of existing reinforced soil structures during earthquakes, the need for development of methods capable of predicting seismically induced deformations has become increasingly apparent. Development of a practical, yet accurate, procedure has been the focus of the research described in this thesis.

MSE Walls and Slopes
Reinforced soil structures are commonly referred to as mechanically stabilized earth (MSE) structures. The soil is typically reinforced with relatively light and flexible materials, such as thin steel strips or geosynthetics that are extensible and have high tensile strengths. The reinforcement enhances the shear strength of the soil mass by altering the pattern of the soil stresses. During the construction of MSE structures, layers of reinforcement are placed within the soil backfill.
Dry, cohesionless soils are predominantly used as backfill because of their high strength characteristics and because they allow drainage, thus avoiding the generation of pore pressures in the backfill.
MSE structures can be constructed relatively fast and easily. Large equipment is not needed to install the reinforcement; however, proper installation by well-trained workers is extremely important. Reinforced walls and slopes are flexible and do not require a rigid foundation, thus further reducing construction costs. The reinforcement, however, may be susceptible to corrosion, creep and deterioration over time. Additional factors of safety on design are required to account for potential degradation of the reinforcement over time, which can influence material costs.

Geosynthetics
MSE structures reinforced with geosynthetics are called geosynthetic reinforced soil (GRS) structures. A geosynthetic, as defined by ASTM (1994), is a "planar product manufactured from a polymeric material." Geosynthetics can be used for separation, drainage-transmission, protection, filtration, fluid barriers and reinforcement. The primary role of geosynthetics in this research is as reinforcement in the soil matrix. Of the wide variety of geosynthetics available today, a principal category is that of geotextiles.
Geotextiles are permeable textile materials that can be divided into two major groups: woven and no woven. Monofilament, multifilament or fibrillated yarns, or slit films and tapes are woven together to create a woven geotextiles; synthetic polymer fibers or filaments are mechanically heat-bonded or needle punched to create no woven geotextiles. The primary function the geosynthetic determines what type of geosynthetic should be used. This research focuses on the use of geotextiles as reinforcement.

Data Entry
For analyzing the problem, needs to enter the geometric properties (wall or slope height and face inclination), surcharge, strength parameters (cohesion and friction angle) and unit weight for all soils involved in the problem (fill, natural soil and foundation soil), friction angle for reinforcement-soil and fill-foundation interfaces, as well as factors of safety for global and internal stability. External factors of safety are calculated and presented at the final screen.

Internal Stability
The program calculates the number (spacing) and length of reinforcement layers with basis on limit equilibrium. The spacing is considered as variable with height, in order to optimize the reinforcement distribution. The active thrust is calculated at the wall or slope face as: The inclination of the front slope with the horizontal is given by θ =180-α. In this expression the design friction angle for the fill material is calculated as: T.FS = horizontal force times the pullout factor of safety. σ v = vertical stress. δ sr = soil-reinforcement friction angle. H = wall height. z= layer depth. θ= face inclination. ρ= 0.5(θ+φ' d ).

External Stability
The external stability is calculated according to the following equations:

Overturning
As shown in Fig.3

Bearing Capacity
As shown in Fig.4.Where, according to Meyerhoff: The load eccentricity at the base is given by: The normal stresses at the base are given by:

The Examples
This section deals with many chosen examples Pre-design numerically by the present computer program TERRAM.

Example No.1
This example deals with design of the wall and slopes as variable face inclination and constant other data (q=0 kPa, H=5m) shown in Figs

Example No.2
This example deals with Pre-design of the wall and slopes as variable face inclination and constant other data (q=20 kPa, H=5m) shown in Figs The relationships of the angle of slope (β) versus the number of reinforcing layers for q = (0, 10, 20, 30, 40, 50) kPa are shown in Fig.(29). These figures indicate clearly that the number of reinforcement layers increases with the angles of slope at the same loading condition, also these figures indicate that at the same angle of slope the number of reinforcement layers increases with the loading condition. For the case that H=5 and the surcharge load is more than 50 kPa can not design because surcharge must be numerically lower than 10 times the slope height.

Conclusions
According to the results which are obtained by this research, the following points are concluded: 1. The program used in this research gives three types of results: a) A drawing for the reinforced wall, showing reinforcement spacing and length. At this stage it is possible to alter the reinforcement, and the calculation is redone automatically. b) A brief report containing information for each reinforcement layer (position, length, and load). c) Factors of safety for external stability (sliding, overturning and bearing capacity for foundation soil). These safety factors are presented in the last screen, where the user may    Fig.9 The Slope after Reinforcement    Fig.18 The Slope after Reinforcement.     Fig.27 The Slope after Reinforcement.  of Reinforcing Layers q=0 kPa q=10 kPa q=20 kPa q=30 kPa q=40 kPa q=50 kPa