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Steep Slope Optimization design by using Reslope 4.0

Abstract

Structures in civil engineering field have a unique character. Their design takes place with consideration of the location and purpose of the structure. According to American society for Testing and Materials (ASTM), Geosynthetic materials are identified as polymeric materials used to strengthen soil, earth, rock or any other geotechnical-related material. Same as concrete, soil forms an abundant construction material with a high compressive strength and nearly no tensile strength. A laboratory test was done using the ReSlope software to determine the design consideration of steep slope reinforcement. The results indicated that the bigger the angle of inclination the high concentration of geosynthetic materials is required.

Introduction

Structures in civil engineering field have a unique character. Their design takes place with consideration of the location and purpose of the structure. According to American society for Testing and Materials (ASTM), Geosynthetic materials are identified as polymeric materials used to strengthen soil, earth, rock or any other geotechnical-related material. Same as concrete, soil forms an abundant construction material with a high compressive strength and nearly no tensile strength. When using soil as a construction material, reinforcement is recommended in order to overcome its weaknesses and increase its tensile strength. Materials used for reinforcing soil are moderately light in weight and flexible, but they must possess extensively high tensile strength. Examples of materials used to reinforce soil are thin steel strips and polymeric material. An earth structure with increased strength allows engineers to construct on steep slopes. In addition, geosynthetic strengthened soils structures are less costly compared to steel structures. This has made it a common global mode of construction used today.

Karl Terzaghi and lacroix were the first users of geosynthtics. The two used filter fabrics, today referred to geotextiles, as flexible forms. The Mission Dam in British Columbia, Canada was constructed using geotextiles filled with cement group that formed a closure between steel sheet pilling and rock abutments. In addition, pond liners, also referred to geomembranes today, helped in keeping an upstream clay seepage-control liner that prevents desiccation. Since their discovery in 1960s, geosynthetics have been used in making ground modification materials because of their flexibility and cost-effectiveness. The first launch of geosynthentics was done in 1977 in Paris (Leshchinsky, Ling and Hanks 850-855).

Laboratory tests are to test the behavior of geosynthtics. Such laboratory tests include triaxial compression test (Yang 17) and plane strain tests (Mcgrown 26). Geosynthetic materials can work as cohesionless backfills because the backfill inclination has a lesser effect on the required strength and reinforcement layout. For cohesionless soils, horizontal tensile forces are well taken care of bearing in mind the required strength of geosynthetics. In cases involving reinforced embankments over soft soils, the inclination of the reinforced geosynthetic that is found at the bottom and backfill interface plays an essential role (φu=0; undrained shear strength, cu, is used). Moreover, inclination has little effects on manmade reinforced slopes because the long term value of cohesion used in designs is small (Moayedi, Huat and Kazemian 55-62).

The use of geosynthetic materials has increased over the past ten years in the field of geotechnical and environmental engineering. Contractors and designers have solved many engineering and construction problems using geosynthetic designs because convectional construction materials are either restricted or costly. In addition, geosynthetic manufacturers have contributed in technological advancement. More than 80 manufacturers of geosynthetic materials are present today who provide North American marketplace with construction materials. Most of these manufacturers are found in Southeastern U.S. moreover, the industry has offered more than 12,000 job opportunities in U.S. in areas of manufacturing, distribution and installation of geosynthetic products (Moayedi, Huat and Kazemian 98-100).

Geotechnical and environmental engineering presents a significant number of geosynythetic types and their applications. The most commonly found types of geosynthetic materials are geotextiles, geogrids, and geocells.

Geotextiles

These are made of continuous sheets of woven or non-woven knitted or stitched fibers. Geotextile sheets are flexible and permeable with a fabric-like appearance. See figure 1(a) below.

Geogrids

Geogrids have a uniformly distributed pattern of apertures forming between the longitudinal and traverse elements. The apertures make the material have a direct contact with soil particles on both side of the piece. Figure 1 (b) shows a geogrid element.

Geocells

These materials are moderately thick, with three-dimensional networks made of polymer strips. These polymer strips are joined together to create interconnected cells that may be filled with soil or concrete. On the other hand, 0.5m to 1m wide strips of polyolefin geogrid might be linked with vertical polymetric rods to form geocell layers also known as geomatresses

Applications of Geosynthetic

These materials find many uses in transportation, environmental, geotechnical and hydraulic engineering for roadways and railways stabilization and reinforcement. In addition, they are used for strengthening pavements, stabilizing retaining walls, embankments and landscaping. Geosynthetic materials are also used in landfills and waste containment, differential settlement solutions and offering solutions to steep slopes.

Roadway and railway stabilization and reinforcement

Most road and railway lines are reinforced using geosynthetic materials. Geosynythetics form strong and stable grounds for construction of roadways and railways. For example, geosynthetics were used in Ontario, Canada for widening a railway embarkment in order to offer support for heavy-loads transported to and from Canada. As seen on figure 2(a), the steep slope has been perfectly leveled and strengthened using reinforced soil as shown in the operations of figure 2 (b). To ensure a desired reinforced fill slope was constructed and achieving the needed geogrids reinforcement embedment lengths, the existing embankment slope was excavated and eventually nailed to give the required stability. Bulldozers and compactors were used to excavate and stabilize the ground respectively. In addition, the reinforced fill slope was moved into the excavated, nailed embankment slope using primary geogrid products. Tests done on NanYou highway in China by Zheng, Zhang, and Yang in 2009 proved that geosynthetic materials form an effective and cheaper solution to the treatment of expansive soil cut slopes. In addition, they provide strong reinforcements on embankment soils.

On the other hand, geosynthetics help in reducing vertical and lateral strains of ballast used in railway line contraction. A combination of ballast and geosynthetics increase the railway line stability and minimizes maintenance costs. In addition, geocomposite are also used in railway line stabilization and reinforcement. Fully instrumented and comprehensive field railway trials conducted in Australia indicated that field observations matched with numerical predictions. Moreover, Caverson and Lowry did a successful case study using whereby they used retaining wall made of a combination of steel fascia and geosynthetic reinforcement to widen rail embankments of Lakeshore West line in Mississauga, Canada. All these studies tried to prove that geosynthetic materials form an alternative to concrete reinforcements and not only are they stronger but also cheaper.

Pavement reinforcing

Geosynthetics are used to reinforce base layer of pavements. There are specific conditions and mechanism that make geosynthetic materials capable of reinforcing pavements. The use of geosynthetics in pavement reinforcement assists in overcoming tensile stresses to inhibit extension strains within soil particles. An effective pavement reinforcing material should have string mechanical properties in order to have an extended life. Such properties include retarding reflective cracking and impermeability. For example, woven geotextiles shown in figure 3 have been used to improve the sub-grade road at the intersection between Hwy 59 and Hwy 169 in Garnett, Kansas. On this situation, the geotexctile provided both higher tensile strength at low strains and an essential separation required for high water table profiles that are capable of causing failure in the pavement (Leshchnisky, Ling and Hanks 880)

Placement of a geosynthetic reinforcement at the asphalt concrete interface assists in reducing fatigue strain by 46%-48%. In addition, it decreases rutting strain by 16%-34% that occurs because of placing geosynthetic reinforcement at a height of a third the base thickness from the concrete. Novel polymetric alloy geocells reinforcements improve the performance of unpaved recycled asphalt pavement sections through widening the stress distribution angle while reducing the rut depth. This occurs when base courses have equal compaction in the unreinforced and reinforced sections.

Application in retaining walls

On the other hand, the application ofgeosynthetic materials on temporary and permanent retaining walls assist in stabilizing the walls as well as adding aesthetic value. According to simac and Elton, the engineering designs, analysis and procedures used in constructing geosynthetic materials account for the induced lateral loads. In order to assess the strength of geosynthetic materials in construction of retaining walls, a test was conducted that measured the performance of geosynthetic walls after ten years of service. A geosynthetic-reinforced wall 9 m high and different tiered configuration was tested. The test discovered that seismic performance of multi-tiered walls was better than that of single-tired walls with the performance increasing with a decrease in reinforcement spacing.

Embankment Stability

Geosynthetic materials also provide stability and minimize differential settlement for embankments. N example is an application of geosynthetic material in protection of a riverbank in Santa Fe, New Mexico. On this site, geogrids were used in reinforcing reinstated embankment and facilitated the reuse of onsite materials. On the back of the gabion facing units, geotextiles were installed to prevent structural backfill movement into the voids within the gabion, especially in times of high water flow in the river.

Application in landfills and waste containments

Secondly, geosynthetic materials are also applied to enhance drainage and filtration in landfills and waste containments. Over the past 25 years, the liner and liquid collection systems design procedures have evolved with development of improved design methodologies and geosynthetic materials. An application of geosynthetic materials was in a landfill at Shanghai Laogang Municipality. The landfill is located along the coastline and occupies 360 hectares with an estimated total capacity of 34 million tons of waste over a 20-years period. In order to ensure fast and controlled consolidation of the landfill foundation, engineers installed a drainage layer combined with prefabricated vertical drains across the landfill base. The drainage layer of the landfill consists of a woven monofilament geotextile filter placed directly on the soft clay foundation before placing the granular drainage layer. This geotextile layer has a combination of the properties of good tensile strength and an efficient filtering capability. In addition, electro-kinetic geosynthetics are used to extract water from slurry waste from a tunneling operation.

Costal and waterway construction and protection

Constructing and protecting projects along coastal areas and waterways is a challenging project to undertake. Coastal regions and waterways are characterized by uneven land contours, changing subgrades, continuous scour and many other harsh conditions. These characteristics make construction along these areas unfavorable but engineers have developed ways to adapt to these difficult conditions and give a durable cost effective solution. Geogrid and geotextile materials from innovative marine structures make Triton systems designed to be integrated with available fill vegetation. Triton systems perform the following functions:

Control of erosion and scour

Form foundations or cores for breakwaters, groins, underwater utility/pipeline installations, etc.

Building of high-strength fills built in submerged conditions or with weak fill materials

Channel linings and bridge scour protection

Causeways, levees, dikes and bridge approach projects

Under-layers for riprap in submerged and soft soils

In situ capping of contaminated sediments

Shore protection and sediment dewatering

Triton systems are flexible and resilient because they conform to land contours and site formations, hence protecting deformation of coastal and waterways. In addition, they are made of geogrids that are capable of resisting natural occurrences caused by chemicals, biological or environmental degradation coming from industrial runoffs and salty sea ocean water. Triton systems are designed mainly for such conditions because other materials cannot withstand high chemical reactions taking place in such places (Starrett 15-18).

Reinforcing steep slope

Geosynthetic materials offer a solution to people carrying out constructions on steep slopes. Reinforced slopes are mainly compacted fill embankments made up of geosynthetic tensile reinforcements in horizontal layers. This tensile reinforcement assists in holding the soil mass together across any possible failure place to promote slope stability. Geosynthetics enable engineers to construct structures to any heights on slopes and at any angle. In figure 7(a) shows an application of geotextiles and geogrids in providing primary reinforcement to a steep slope. The tallest geosynthetic-reinforced vegetated slope is found in Yeager Airport, Charleston. The construction was completed in 2007 and shows quality performance to date.

ReSlope 4.0

Introduction

Earth slopes are always designed using limit equilibrium whereby the equilibrium is calculated alongside assumed slip surface. A specification of a reinforcement layout that has the capability of satisfying prescribed loads against lateral failure is not an assurance of its stability. To overcome such misfortunes, geotechnical and environmental engineers developed a program referred to ReSlope 4.0 that provides the correct steep slope reinforce design layout. ReSlope 4.0 is a computer program used in designing geosynthetic reinforced steep slopes. This is an approved method requiring little input data, and the results are reasonable. The program is interactive and design-oriented and provides solutions to any problem. The method provides optimal layout, length and spacing, of a reinforced layers given the geosynthetic strength, design factor and the reduction factor. ReSlope concentrates more on design and not analysis of reinforced slopes that have a simple geometry because its design is specifically meant for geosynthetic materials. In addition, the program does not consider surface stability because the end design has the capacity of withstanding strength over a long-term (Leshchinsky 40-41).

ADAMA engineering was the first people to develop the first version of ReSlope program referred to ReSlope 1.0 that was used by the U.S. Army Corps of Engineers in 19995. Eventually, a commercial version, ReSlope 1.2 was introduced in 1996. ReSlope 4.0 is the most current program that is an upgrade of the original version (1.0) and is based on “Design Procedure for Geosynthetic Reinforced steep Slopes”. The version contains both computational and functional enhancements hence, more superior than the previous versions (Leshchinsky 44)

Tieback analysis

Tie back analysis was the common method used in calculating the required tensile resistance of each layer of a geosynthetic reinforcement. The tensile resistance ensures that the supported mass is safe from internal collapse because of its own weight and surcharge loading. In general, tensile resistance is the tensile force required to prevent the steep slope from sliding along developing slips that come out in the face of the slope. Tieback is a mechanism that gives capacity to the reinforcement tensile force. The reinforcement can develop tensile force at its front-end that restrains the soil from slipping outwards, if only particular types of facing, like wire basket or wrap around, are used (Leshchinsky, Ling and Hanks 778).

ReSlope analysis

Figure 8 shows the notation used while doing calculations using the ReSlope method. From the figure, primary and secondary layers for the reinforcement are used, but the ReSlope considered only the primary layers. Secondary layers are ignored because they offer better compaction on the front-end of the reinforcement reducing slippage chances. In addition, secondary layers have narrow spacing and only used where primary layers have wider spacing (more than 600 mm apart).

Figure 8: ReSlope notation

While analyzing reinforcements using ReSlope, steep slopes were referred to slopes inclined at angles for which they are unstable in the absence of a reinforcement. A granular backfill is considered steep if it has an inclination that is steeper than its angle of repose.

(i>φd); where I = slope inclination

Φd = angle of the slope/ friction angle

An unstable soil mass activates reinforcement layer in each steep slope forcing reactive forces in each reinforcement to restore a limit equilibrium state. The required reactive force in the form of log-spiral failure surface was selected to determine the location of the critical shear surface (Leshchinsky 46-49). The following mechanism is common in many geotechnical stability problems.

The computation system for estimating the tensile reaction in each reinforcement layer of a backfill was determined from the following steps:

Step 1: On this step, the soil mass acting against Dn was considered. Dn contributed to the lateral support of a soil mass when a reinforcement layer found on the face of the slope represents it hence, considered a facing unit. It restrains unstable soil above the reinforcement from moving outwards. In addition, the facing provides a lateral support when tied to reinforcement transferring the restrained load into tensile force in the geosynthetic. The moment equilibrium finds the critical log spiral producing max (tn). See figure 9 below. The tn value counterbalances the horizontal force against Dn signifying the reaction force in layer n. Hence, tn represents the force required to restore slope stability (Han and Daniel 133).

Step 2: Calculating the force against Dn-1. Dn-1 extends from layer n to layer (n-1). Using the equation of moment equilibrium, max (tn-1), was calculated. This is the force needed to retain pressure exerted by the unstable soil mass Dn-1.

A repetition of these two steps created a distribution of reactive forces in all reinforcing layers going down to t1. This resulted to a tieback forming a series of layers that have a long-term strength varying from tn to t1.

Figure 9: (Reslope 4.0-user manual, by ADAMA Engineering )

Calculating ReSlope

The following procedure was followed while calculating reinforcement design layout using the ReSlope method;

The tieback tensile force was calculated to determine the required length of geosynthetic materials needed to anchor the soft soil to backfill soil

The length of reinforcement geosynthetic materials was checked to ensure compound stability rotation

To prevent translational failure, the length of the reinforcement geosynthetic was checked. Translation failure occurs when the reinforcement soil mass slides with either the backfill soil or the slope foundation

The minimum structural force Fs against the circular deep-seated failure caused by weak bearing capacity of the reinforcement foundation was also determined.

Compound stability assessment

The tieback analysis provides the minimum required tensile resistance at each level of the reinforcement layer to create a stable structure with pore water pressure distribution and the slope angle. In addition, the active soil zone is defined on the outermost log spiral especially while designing retaining walls. The pullout resistance and the length anchored into the stable soil surface also determine the capacity of a reinforced wall structure to resist soil forces. A boundary defined by ‘active’ stable zone minimizes potential slippage occurring deeper into the soil mass compared to the outside. Such services turn into educed pullout resistance capacity producing an unstable system. To determine the required reinforcement length in order to prevent compound failures, convectional stability analysis was done. In the ReSlope analysis, surface stability was assumed to be zero. The internal stability leads into a long-term strength required for producing an internal stable structure at every reinforcement elevation. Figure 10 represents the compound stability analysis (Moayedi, Huat and Kazemian 97-100). The convectional factor of safety is:

Fs = tan(φavailable)/tan(φdesign)

=cavailable/cdesign

The specified minimum value for Fs (design0 for soil shear strength in ReSlope must meet all rotational slip surfaces.

Direct sliding analysis

Resistance against direct sliding is a crucial element in design and construction of slope reinforcements. The length needed to yield a stable mass, Lds, is obtained from a limited equilibrium analysis. Figure 11 represents the procedure used in determining the direct sliding force. Firstly, an assumption of the initial value of Lds was made followed by the value of δ. δ was specified between and the friction angle (φd) of the reinforced soil. Secondly, the maximum value of inter-wedged force (P) was determined by varying θ and at the same time solving the two force equilibrium equations for the active wedge A. The inter-wedge force represented the final lateral earth pressure exerted by the backfill soil on the reinforced soil mass. On the other hand, the vertical wedge force equilibrium for wedge B was determined. After getting NB values, the sliding resisting force, TB, along the base Lds was calculated. Fs –ds represented the factor of safety against direct sliding on the reinforced mass. ReSlope repeated the process for wedge A and B until the calculated factor of safety against direct sliding reached the prescribed value (Han and Daniel 122-124).

Figure 11: Direct sliding analysis

Deep-seated Analysis Using Bishop Method

The Bishop method was used to assess the minimum factor of safety against deep-seated failure. The method was combined with ReSlope to perform unreinforced slope stability analysis. The following analysis led into the bearing capacity of the foundation soil. The surface that renders the lowest factor of safety was selected from a number of circular slip services. However, there was a restriction on the selected circles to ensure they passed away from the bottom of the reinforced soil zone. In addition, the intersecting reinforcement layers were affected by the stability of the reinforcement. A maximum feasible circle penetration was set. The coefficient of seismicity was also included (Cs). In general, Bishop Method includes pseudo-static forces occurring due to self-weight and surcharge loads. Large numerical errors are likely to occur through a parameter referred to as mα. For mα < 0.1, the slide resistance of a slice is zero hence, avoiding large numerical errors (Moayedi, Huat and Kazemian 94-95).

Research analysis

Results

Fixed parameters

Height of the slope=30 meter

Slope Angle, i=60°

Surcharge load over the horizontal crest length 10 Kpa

Surcharge load away from backslope length 10 Kpa

Internal angle of friction ,Ø 34° for reinforced soil ,backfill soil & foundation soil

Cohesion Kpa 0.00 (zero)

Unit weight ,ϒ 20 Kn/m3

Factor of Safety on Shear Strength 1.3

Factor of Safety on Geosynthetic strength 1.3

Factor of Safety on Shear Strength Geosynthetic pullout 1.3

Factor of Safety on resistance direct sliding 1.3

Ultimate strength for Geosynthetic materials 20 kN/m2

Maximum allowable Geosynthetic spacing 0.6 m

Maximum allowable Geosynthetic spacing 0.3 m

Maximum allowable penetration into foundation when Bishops stability analysis is conducting

Factor of safety against deep-seated failure (Bishop stability) 1.1

Variable parameters

Horizontal Crest Length A , the following lengths were selected to study the effect of this factor:

(L/2), (L/3), (L/4) and L=zero, while all over factor are constant

Backfill slope Angle β , the following angles were selected to study the effect of this factor:

34°, 20°, 10°, 5°and Zero, while all over factor are constant

Both factors : Horizontal Crest Length A and Backfill slope Angle β are interacting in the same time

Result analysis

From the results obtained by the ReSlope computer program, the height of the slope was 5 m. This value was recommended bearing in mind that the slope angle was very steep, 60o and higher heights might have caused high rate of failure. In addition, the horizontal crest length A was 2 m while the horizontal crest length B was 3m. With a back slope angle of 30o, it was possible for the geosynthetic material used for reinforcement to hold the soil mass together and prevent slippage. In addition, the slope at the bottom of the wall was zero an indication that no slippage could ever occur on that section.

Discussion

From the above results, the following can be deducted. Firstly, the geosythetic material used was strong enough to resist any kind of soil mass movement. In addition, the engineer designed a short system that was capable of controlling all sorts of movements. The material used allowed easier seepage of water to the bottom of the fill, but ensures no mass movement occurred. Secondly, the design took control of erosive forces that cause surface sloughing especially for steep slopes. The slope inclination was 60o, but the horizontal crest lengths A and B were small reducing the effects of erosive forces. In addition, geotextile materials were used on the steep side of the reinforcement to ensure a stable ground. The following material was selected because of the following reasons. It protects bare soils surface against erosion before the vegetation establishes. Secondly, it assists in minimizing runoffs in times of high water flow rates. Finally, it reinforces the rooting system of planted vegetation.

On the other hand, the above design took care of the factor of safety and shear strength factor. The geosynthetic material used had a force of 20KN/m2, while the factor of safety was 1.3. Given a unit weight of 20KN/m2, the geosynthetic material was capable of resisting any tensile force resulting from the soil mass supported by the reinforcement materials. Moreover, the tieback analysis acted as a perfect method of determining the required tensile resistance at each level of reinforcement layer. This was found to be 10Kpa.

Conclusion

The above laboratory test results indicate that geosyntetic materials are better alternatives to steel structures in design and construction of reinforcements. Soil reinforced with geosynthetic material offers a stable foundation where different construction works can take place as discussed above. From the research, the maximum inclination that a geosynthetic material can bear was 30o to the slope shear plane. At this inclination, the shear displacement of soil and mass movement was reduced to zero. Engineers should focus on constructing reinforced earth structures taking into consideration their orientation. The most recommended orientations presently are between 0-45o. In future, engineers should invest slope reinforcement systems that can offer stability even at highest inclination possible. When using soil as a construction material, reinforcement is recommended in order to overcome its weaknesses and increase its tensile strength. Materials used for reinforcing soil are moderately light in weight and flexible, but they must possess extensively high tensile strength. Examples of materials used to reinforce soil are thin steel strips and polymeric material. An earth structure with increased strength allows engineers to construct on steep slopes. In addition, geosynthetic strengthened soils structures are less costly compared to steel structures. This has made it a common global mode of construction used today.

Works cited

Han, Jie, and Daniel A. Alzamora. Geo-Frontiers 2011 advances in geotechnical engineering. Reston, VA: American Society of Civil Engineers, 2011. Print.Leshchinsky, D. “Software to Facilitate Design of Geosynthetic-Reinforced Steep Slopes”. Geotechnical Fabric Report, Vol. 15, No. 1, 1997; 40-46.

Leshchnisky, D., Ling, H. I. and Hanks, G. “Unified Design Approach to Geosynthetic-Reinforced Slopes and Segmented Walls”, Geosynthetics International, Vol. 2, No. 5, 1997; 845-881.

Moayedi, Hossein, Huat, B. Bujang and Kazemian, Sina. “Optimizing of Tension absorbtion of Geosynthetic through Reinforced Slope”. EJGE, Vol. 15, No. 1. 2010: 93-103

Starrett, Steve. World Environmental and Water Resources Congress 2009 great rivers : proceedings of World Environmental and Water Resources Congress 2009, May 17-21, 2009, Kansas CIty, Missouri. Reston, Va.: American Society of Civil Engineers, 2009. Print.