RELIABILITY OF THE DIFFERENT SOFTWARES TO DESIGN THE MECHANICALLY STABILIZED EARTH WALLS CONSIDERING THE RECOMMENDATIONS OF THE VARIOUS CODES

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1 6 th Asian Regional Conference on Geosynthetics - Geosynthetics for Infrastructure Development, 8-11 November 2016, New Delhi, India RELIABILITY OF THE DIFFERENT SOFTWARES TO DESIGN THE MECHANICALLY STABILIZED EARTH WALLS CONSIDERING THE RECOMMENDATIONS OF THE VARIOUS CODES SEYED MOHAMMAD MEHDI MADANI Islamic Azad University, Central Branch, Iran G. REZA MOHAMMADIZADE Professional Civil Engineer, Geosynthetics Expert, Iran MEHDI JALILI Technical and Engineering Faculty, Civil Engineering Department, Semnan Branch, Islamic Azad University, Iran ABSTRACT Geosynthetic Reinforced Soil Structures are widely deployed in variety of applications around the world. And the evaluation of these structures in special loading conditions such as point and linear loads over abutments and also special geometrical conditions such as the nearness to adjacent structures, for example soil nailed wall, may be frequently necessary. There are several guidelines and softwares that are normally being used for design of the above-mentioned structures, However, most of them (if not all) do not make any recommendation for critical conditions and miscellaneous details such as connection of reinforcing element to the wall facade and to the retained zone, while those can jeopardize the whole structure stability. In this article, in order to evaluate the above mentioned topics, results of some parametric studies for an assumed abutment structure are presented. These analysis are based on various guidelines such as FHWA (NHI , 2009), BS ( ), Iranian national Code (No. 308, 2005), and guideline of Road Congress India Also several softwares such as PLAXIS, FLAC2D, MSEW and ReSSA are used. Finally a comparison between results is made and the advantages and limitations of each code and software are addressed. 1. INTRODUCTION Historically soil reinforcing soil has been used since Bronze Age (around 3000 B.C.). Further increase in knowledge of engineers, capability of manufacturers and satisfaction of project owners, resulted in fast expansion in use of reinforced soil. In past engineers for increasing the tensile properties of soil deployed wood, reeds, or any other types of accessible material inside the compacted earth (Kerisel 1990). Today, 327

2 328 Seyed Mohammad Mehdi Madani, et al. earth reinforcement usually involves soil stabilization through the placement of pre-manufactured inclusions (reinforcing element) made from steel or polymer materials. In definition, reinforcing element is the one that is placed in soil to improve the soil internal shear resistance. The modern concept of reinforced earth was introduced by Henri Vidal (1969) in France, and in 1971 was brought to the U.S. for a major landslide repair on Highway 39 in the Angeles National Forest, CA. The important factors of using reinforcement in soil are the ease of installation, constant quality (due to use of man-made products), economy of project, different type of facings (such as gabions, precast concrete panels, cast-inplace concrete, and modular blocks) and the accessibility of all needed materials. In recent decades, many recommendations and standards have been published and used to design the reinforced soil wall. Due to the knowledge of the authors of this article, in some circumstances such as special loading conditions, approximately to other structures (like nailed and stabilized walls) and when we need to control and have details of the connections between reinforcing element with other parts of the wall, the existing codes do not have précised recommendation or do not mention the matter at all. For this reason, some designers use the modeling software for taking the special conditions into account while unfortunately most of them are not fully aware of capabilities and limitations of different modeling software. Therefore, we initially cover and describe several recommendation and standards then focus on different software that generally use to design reinforced soil wall and at last we compare different recommendation, codes and software in general and specific conditions. The interaction between soil and reinforcement leads the composite material to have better mechanical properties. The soil, when properly compacted, in general, has good resistance to compression and shear. The tensile strength, however, is low or zero. For that reason using reinforcement is spread widely as shown in Figure 1. A reinforced soil structure includes: reinforcement, facing, reinforced zone, backfill soil, and ground. Fig. 1: Different parts of MSE wall (EBGEO, 2011) 2. AVAILABLE GUIDELINES AND CODES To compare different codes, relevant information from governmental agencies from countries such as Iran, India, Germany, the United Kingdom, and the United States is gathered. 3. COMPARING STANDARDS AND SOFTWARE IN GENERAL CONDITION Table 1 [which is basically presented by Zorenberg and Leshchinsky (2003)] and completed here gives comparative identification by categorizing existing codes and sofware in 6 parts as:

3 Reliability of the Different Softwares to Design the Mechanically Stabilized Earth 329 Walls Considering the Recommendations of the Various Codes - part one includes basic information such as name, years and country of origin of the codes and software - part two compares Principals of Design such as calculation and analysis, limitations - part three covers the fill material and relevant parameters such as cohesion, friction angle, plasticity index and PH - part four considers the reinforcement element including geosynthetics and metal reinforcement. Also in this part we focus on the important specification of reinforcing element and the capability of software in modeling such parameters. - Soil/Reinforcement interaction is described in part five. Most of the recommendation and codes refer to this as friction angle. - Design considerations that describe the main safety factors in overturning, sliding, bearing capacity, seismic condition and pullout are mentioned in part six. It is worth to be highlighted in the above mentioned table that, appropriate soil investigation to achieve following results is mandated by all recommendations and codes: o Ground conditions below and behind the retaining structure, o Location of the groundwater table, o Geo-Environmental condition, o Geological and topographic condition o Main soil parameters such as cohesion, density, friction angle, PI, Young modulus, Passion ratio, grading, etc. With the same level of obligation, below items also should be attentional by designers o Planned design working life, o Loading and application of the structure o Allowable deformations, o Properties of the intended materials. This means, in some cases material test by valid laboratories is of certain need. Table 1: Comparing recommendations, codes and software in design of reinforced wall in general condition Section RECOMMENDATION & CODES SOFTWARE CONSIDERATION 1.1. Name FHWA AASHTO BS8006 EBGEO NCMA IRC:SP: ,101 FLAC 2D MSEW ReSSA PLAXIS 1. Base Information 1.2. Country USA USA Eng. German USA INDIAN Iran USA USA USA Netherlands 1.3. Publication Year , Analysis Method ASD and limit state Limit state Limit state Limit state Limit state Limit state ASD and limit state finite difference follows the design guidelines finite element 2. Principle of Design 2.2. Limitation No Metal --- No Abutment H 10m (308) 2.3. Investigation 2.4. Life Time Temp.36 months Per.75 y ---- * ---- Items are mention before Temp.36 months Per.75 y Category by 1 to 120 years Per.100 N.C. 100 years N.C

4 330 Seyed Mohammad Mehdi Madani, et al. Section RECOMMENDATION & CODES SOFTWARE CONSIDERATION 3.1. Cohesion Usu. zero Usu. zero Max. 5 kpa Usu. zero zero N.C. N.C. By user N.C. By user By user 3. Reinforced fill 3.2. Friction Angle φ 3.3. Plasticity Index Max 40(test) 34 (no test) Max 40(test) 34 (no test) N.C. N.C. 26 to 36 φ>25 φ>25 Some limitations considered by software PI 6 By PI 7 PI 6 PI 6 PI 6 N.C. N.C. N.C. N.C. agreement 3.4. PH 3 <PH< 9 5<PH<9,10 5 <PH< 9 3<PH<9 6 <PH< 9 3 <PH< 9 N.C. N.C. N.C. N.C Ultimate Tensile Strength S.F. 1.3 to 1.5 S.F. 1.1 BS9606 ISO DIN D4595 ISO S.F. 1.5 to 3 Must be control By S.F. N.C. Must be control RF creep 1.6 to 5 P Overall Coef RFd 1.1 to 2 Test More than 1.1 By Test By Test Mention in text by test or of Code 1.5 to 5 En/ISO S.F. ** 2 to to 2 By Test S.F. ** 1.1 to 1.4 Can be act by material prop. C. --- Can be act by material prop. C Geosynthetic RFId 1.2 to 3 Table 3-9 of FHWA Factor of Safety Test More than 1.1 By Test by test or at least 1.5 or 2 by soil type to to 1.4 by load case 1.1 to 3 By Test S.F. ** 1.1 to By Test N.C. Must be Calculate C. --- C. --- Must be Calculate Ultimate Tensile Strength S.F. 1.3 to 1.5 S.F. 1.3 to 1.5 S.F S.F S.F. 2.5 to 3 Must be control By S.F. N.C. Must be control 4. Reinforced Material 4.2. Metal Table Electronic Pro. 3.3 of FHWA Min. Galvanization Thick. 85 to 100 μm a BS According of AASHTO to P to 100 μm BS 1461 Min 70 μm Acc. to BS 140 μm N.C. By user By user By user By user N.C. By user By user By user By user 5.1.Geosy 5. Interaction Soil/ Reinforcement 5.2. Metal In Static According to friction angle of soil and type In and shape of material Dynamic According to FHWA N.C In Static By calc. Max = φ In Dynamic N.C. 2/3 of φ Simple modeling by defining Coe. R inter Calculate by physical test By Equation of Recommendation and code calculated and use several method to obtain minimum Model by interface & prop of reinforcement but need good physical test

5 Reliability of the Different Softwares to Design the Mechanically Stabilized Earth Walls Considering the Recommendations of the Various Codes 331 Section RECOMMENDATION & CODES SOFTWARE CONSIDERATION 6. Design Consideration 6.1. Overturning N.C. N.C. S.F N.C. S.F. 1.5 to Sliding 1 S.F. 1.5 S.F. 1.2 S.F. 1.5 S.F. 1.5 S.F (Inter.) 6.3. Bearing Capacity 6.4. Compound and deep stability 6.5. Seismic Stability S.F. 1.2 S.F. 2 (3 geosy.) S.F. 1.5 (3 geosy.) One minimum specific Safety Factor that by engineer judgment can find out the failure mechanism By S.F. Inter. C. By S.F. One minimum Exter. C. S.F. 1.3 N.C. S.F S.F. 2.0 N.C. S.F. 1.4 S.F. 3 By S.F. By S.F. S.F. 1.3 to 1.5 S.F. 1.1 or 1.5 S.F. 1.3 S.F S.F. 1.2 to 1.4 S.F. 1.1 or 1.5 S.F. 1.3 to 1.5 N.C. N.C. S.F. 1.1 or (Static) 1.1 (Dyn.) 6.6. Pullout S.F. 1.5 S.F. 1.5 S.F. 1.3 S.F. 1.4 S.F. 1.5 S.F. 1.3 S.F. 1.3 to 1.4 N.C. By S.F. By S.F. S.F. 1.1 N.C. By S.F. By S.F. By S.F. By S.F. specific Safety Factor that by engineer judgment can find out the failure mechanism N.C.: Not Considered C.: Considered Inter. and Exter: Internal and External P.: Page Temp.: Temporary and Per.: Permanent * This software cannot control the settlement, bearing capacity and deflections. MSEW software is mostly used for internal stability and ReSSA is mostly used for external stability. ** According to Iranian Transportation publication Organization Meanwhile, creep reduction as a time dependent phenomenon and a very important factor on the lifetime of reinforcing element, has been significantly noticed. In particular, for Geosynthetics, creep factor could be calculated by couple of methods based on validity of certified laboratory creep tests. Accordingly, as could be noticed on table 1, some codes such as EBGEO 2011, suggest different values based on availability of the so called tests. In Table 2, recommended creep factor values for different raw materials are mentioned. Table 2: Creep Reduction factor in EBGEO, 2011 Material Acronym Common values for A 1 from product-specific analyses Minimum A 1 if analysis from to unavailable Aramid AR Polyamide PA Polyethylene PE Polyester PET Polypropylene PP Polyvinyl alcohol PVA VALIDITY OF CODES AND SOFTWARE IN SPECIFIC CONDITION There are many MSE jobs with their own unique conditions and executional issues. Most of the issues are being solved by engineering judgment; however, in some cases having a guideline seems necessary. According to the authors of this article, below discussed situations are the most frequent ones that engineers may face and it is important to have recommendation and/or software in hand for these conditions. 4.1 Use of Different Reinforcement Methods (Adjacency to Uailed/anchored Walls) It is frequent to have MSE wall adjacent to other structures such as nailed soil and piled earth. For these cases unfortunately most of codes mention a little. For example BS8006 says: If the reinforced soil

6 332 Seyed Mohammad Mehdi Madani, et al. structure is adjacent to or is a part of any other structure, then possible interactions should be considered; for example, an adjacent bridge deck or piled structure can impose limits on acceptable lateral movement resulting from differential settlement. other codes remain either silent or little illustrative, thus in most cases, numerical software such as FLAC, PLAXIS, Geostudio, GGU should be applied to see the effect of the adjacency of different structure to MSEW, However, some issues like connection of MSEW reinforcing element to the nailing heads has remained unsolved yet (figure 2). Fig. 2: Adjacency of MSE wall to nailing 4.2. Performance of Facing & Connections It is of high importance to define how the facing is connected to the reinforcing element and indirectly to the body of the wall. Type of facing and method of connection that is considered in facing design, is in direct relationship with the physical and mechanical properties of reinforcing element. Figure 3 shows different shape of materials that can be plyometric or steel made. In BS8006, FHWA and AASHTO good description for the connection strength and some control measures are mentioned but other codes are not as descriptive. Meanwhile, convenient software just can calculate the connection forces. Some new connection and facing must be document and tested by vendor. It is important to say that mention. For example the recommendation and code does not mention strongly about the flexible structure (geosynthetic wall) and rigid facing (like concrete panel). Fig. 3: Type of reinforcement according to their shapes

7 Reliability of the Different Softwares to Design the Mechanically Stabilized Earth 333 Walls Considering the Recommendations of the Various Codes 4.3 Quality Control Success or fail of every single project is dependent on Quality control, which is vital to the Construction of infrastructure projects. All material that is used in construction of MSE wall must be tested and the main properties should be rechecked. Some codes (like BS, EBGEO and AASHTO) consider sampling and the quality control sequence, however, most of codes remain silent. 4.4 Rehabilitation In some cases rehabilitation may be require and some reconstruction may be necessary. In these cases, the codes and recommendation did not say the specific way of rehabilitation. 4.5 Deformation and Settlements In many geotechnical excavation the deformation (usually horizontal) and settlements are not limited because of the beside structures or devices. By this aspect that are used in many retaining wall the limitation of this parameters are various in different condition. It is important to say that reinforcement soil like geosynthetic are flexible wall and sometime large deformation and settlement take place during construction, by this condition it is important to codes and recommendation mention clearly about this topic. It very important that the limitation of deformation and settlement during construction and life time of MSEW wall be mention in special conditions such as using geosynthetic wall (flexible structure) with rigid facing (concrete panel). 5. Software Introduction There are several software such as FLAC2D, PLAXIS, MSEW and ReSSA that are normally used for designing MSE, however, that may be of help to have other software such as Geostudio, GGU etc. that are made by companies, in hand as well. In this section at first some explanation of mentioned software is given and after that the validation of each sofwares will be discussed by physical modeling based on research of Hatami and Bathurst (2005). In this section, we shortly introduce and compare the following softwares: 5.1 FLAC FLAC is a two-dimensional explicit finite difference program for engineering mechanics computation. The explicit, Lagrangian calculation scheme and the mixed-discretization zoning technique used in FLAC ensure that plastic collapse and flow are modeled very accurately [FLAC manual]. 5.2 PLAXIS Development of PLAXIS began in 1987 at the Technical University of Delft as an initiative of the Dutch Department of public works and water management. The initial goal was to develop an easy-to-use 2D finite element code for the analysis of river embankments on the soft soils of the lowlands of Holland [PLAXIS manual]. 5.3 MSEW MSEW (3.0) is an interactive program for the design and analysis of mechanically stabilized earth walls. It follows the design guidelines of AASHTO98/Demo 82, AASHTO02/FHWA-NHI , AASHTO , or NCMA97/98. MSEW (3.0) allows for global stability analysis using Bishop Method with circular arc. For more in-depth and efficient global analysis, MSEW(3.0) can export data files for analysis using ReSSA(2.0). 5.4 ReSSA ReSSA (3.0) is an interactive program used to assess the rotational and translational stability of slopes. It was specially developed to allow for convenient inclusion of horizontally placed reinforcement, thus

8 334 Seyed Mohammad Mehdi Madani, et al. enabling the design and analysis of mechanically stabilized earth slopes. This version includes an important enhancement allowing exploration of 3-part wedge mechanism, with and without reinforcement, using Spencer s method. ReSSA (3.0) can be used as a generic slope stability program considering circular slip surfaces (Bishop Method) and 2- or 3-part wedge slip surfaces (Spencer method). 5.5 Validation of Software On the first step we should be sure that each software could be deployed in modeling of MSE walls. For this, the research and physical modeling of K. Hatami and R. J. Bathurst (2005) has been modeled by different. Figure 4 shows a cross section and wall model of reinforced wall. Fig. 4: Cross section of physical modeling (Hatami and Bathurst (2005)) FLAC In figures 5 a) we can see the horizontal displacement of wall as the result of applying a 20 kpa load on the wall and in b) we can see the comparative results of modeling with FLAC2D and physical modeling. Graphs suggest good prediction and approximately between results. Both a) and b) are done with FLAC2D Ver. 7. (a) (b) Fig. 5: Modeling Wall in FLAC2D, (a) horizontal displacement of wall at the end of applying load 20 kpa of wall (b) Comparing physical and numerical modeling PLAXIS In the figure 6 the horizontal displacement of the wall at the end of construction stage is shown as noticed by Yan, Y., Damians, I.P. and Bathurst, R.J. (2015) we cannot accurately define the properties and specification data in PLAXIS. Inline, we also believe that using parameters of spring modeling

9 Reliability of the Different Softwares to Design the Mechanically Stabilized Earth 335 Walls Considering the Recommendations of the Various Codes results in higher risk of errors when we want to have effects of interaction between reinforced soil and reinforcement element. Fig. 6: Modeling Wall in PLAXIS, horizontal displacement of wall at the end of construction MSEW and ReSSA As described on the manuals, MSEW and ReSSA are applicable to control the internal stability and the external stability of the wall. Meanwhile it is important to notice that by the MSEW and ReSSA designer cannot evaluate the displacement and the other relevant parameters of the wall. Accordingly we can only compare the results of the physical modeling of Hatami and Bathurst (2005) by MSEW and ReSSA software in static condition Therefore, for validation of these two software we just compare the safety factors that are estimated according to mentioned article with the results of FLAC and PLAXIS softwares. Also it is important to say that these software are approved by FHWA in its recommendation inthe Table 3, the safety factors of internal design of wall by MSEW software are shown. Table 3: Results of MSEW for Wall mention Figure 7 indicates different modes and outputs results along with minimum factor of safety that are achieved from ReSSA. (a) (b) (c) Fig. 7: Modeling Wall in ReSSA, (a) Spencer Translational, 2-Part WEDGE (b) Bishop Rotational Analysis Mode (c) Spencer Translational, 3-Part WEDGE

10 336 Seyed Mohammad Mehdi Madani, et al. 6. CONCLUSION According to the information of this article, following items are conclusion: - The important topic it should be noted that we cannot design everything by software. For example, by MSEW and ReSSA we cannot control displacement and with PLAXIS and FLAC we need extra design such as design of different connections. - According to Table 1 the recommendation, code and software are comparing to each other that shown using software beside the hand design is necessary for calculating displacement and some other control like bearing capacity and so on. - In some specific conditions the recommendation and codes some time did not have opinion that engineering judgment must be used for solving the problems. REFERENCES AASHTO LRFDUS-6. AASHTO LRFD BRIDGE, (2012), American Association of State Highway and Transportation Officials, Washington, DC, USA. ISBN: BS , (2010), Code of practice for strengthened/reinforced soils and other fills, BRITISH STANDARD, United Kingdom. ISBN EBGEO(2011), Recommendations for design and analysis of earth structures using geosynthetic reinforcements, German Geotechnical Society, Berlin, Germany. ISBN FHWA-NHI (2009), Design and construction of mechanically stabilized earth walls and reinforced soil slopes Volume I, U. S. Department of Transportation Federal Highway Administration, Washington, D.C., USA. GEOGUIDE6 (2002),GUIDE TO REINFORCED FILL STRUCTURE AND SLOPE DESIGN, GEOTECHNICAL ENGINEERING OFFICE Civil Engineering Department, Hong Kong, Itasca FLAC: Fast lagrangian analysis of continua. Version 7.0 [computer program]. Minneapolis, Minn: Itasca Consulting Group Inc; J.G Zorenberg and D. Leshchinsky (2003), Comparison of international design criteria for geosyntheticreinforced soil structures, Landmarks in Earth Reinforcement Ochiai et al (eds), ISBN Kerisel J., et al.(1990), Active and passive earth pressure tables. Balkema, Rotterdam K. Hatami and R. J. Bathurst. (2005), Development and verification of a numerical model for the analysis of geosynthetic-reinforced soil segmental walls under working stress conditions, Geotech. J. Vol. 42, , doi: /T PLAXIS. Reference manual 2D- version 8.6, PLAXIS. Delft, Netherlands: Delft University of Technology; Yan, Y., Damians, I.P. and Bathurst, R.J. (2015), Influence of choice of FLAC and PLAXIS interface model on reinforced soil-structure interactions, Computers and Geotechnics, 65: