Aspects of Design and Construction of a 30m high MSE wall: A Case Study

Reinaldo Vega-Meyer1, M.ASCE, and Carlos Zorraquino Junquera2

1 Senior Engineer, Tensar International Corporation, 2500 Northwinds Parkway, Suite 500, Alpharetta, GA. 30009; PH (770) 344-2090; FAX (770) 344-2084; email: rvegameyer@tensarcorp.com 2 Technical Office Manager, Isolux-Corsn Mxico, C/ Ro Jamapa 116, col. Cuahutemoc, 91060 Xalapa, Veracruz , Mexico; PH (52-228) 817-6034 ext. 305; FAX (52-228) 817-6034 ext. 312 email: czorraquino@isoluxcorsan.com

ABSTRACT: A large number and types of MSE (Mechanically Stabilized Earth) structures of various types have been designed and built around the world throughout the last thirty years as safe, reliable, and stable alternatives to conventional structures (e.g. cast in-place walls, over-excavation and material replacement, etc.). These structures have a successful record of performance with great success and economical advantages for the civil engineering works. This paper presents a case study related to design and construction of a 30 meter tall MSE vegetated wall built in Mexico supporting a section of highway. The structure is reinforced with two types of polymeric geogrids. This case study provides an overview of the analysis and design method, design parameters, and construction related aspects including revisions to internal drainage due to unexpected groundwater conditions. Observations on the performance during and after construction are also provided.

1. INTRODUCTION

Numerous MSE structures have been built worldwide in the last thirty years, using various types of facing and reinforcement. Their use has become commonplace today as they provide engineers with a cost efficient structure that, when properly designed and constructed, provide significant and extensive design life. Existing soils and fill materials vary, so it is very important to chose the correct reinforcement material in order to provide a stable engineered structure with long term resistance to degradation due to natural or added chemical content of the fill (e.g. pH concentration or changes) This potential chemical damage must be included in the designs in terms of Reduction Factor for durability (RFd) according to laboratory tests performed on the intended reinforcement material. High Density Polyethylene (HDPE) geogrids are manufactured from an extruded punched and drawn sheet and have been used for over 30 years as reinforcement

Page | 1

materials for MSE structures.

2. CASE STUDY

This paper presents a 30m high MSE wall which was designed and built with a flexible facing composed of geosynthetic wraparound using a biaxial polypropylene (PP) punched and drawn geogrid. The geogrid had included a high carbon black content (2.5%) as UV protection agent and will serve as surficial stability reinforcement for the retaining wall. In addition, the geogrid wraparound facing (or baskets) contained erosion control mat in order to provide temporary protection and time for any applied seed to germinate and grow. The verticality of the baskets was achieved by using welded wire forms (WWF) made out of black steel which serve as temporary form during construction and perform no structural function in the design of the wall. The grade separation project consisted of a 222 m long highway section supported by MSE walls of varying height. This paper focuses on a 22 m section of the project where MSE walls were 30 m high. This new highway project is located in the State of Veracruz within the City limits of Xalapa, East of Mexico City and West of the Gulf of Mexico (see Figure 1).

Figure 1. Location map

2.1 Site conditions

Part of the new highway design required walls that are located in the mountain chain known as the Sierra Madre Oriental. The subtropical climate in this region is due in part to the elevation which ranges from 1,200 to 1,500 meters above mean sea level (M.S.L). The wall presented in this paper is located on a mountain hillside where slopes reach up to 35 degrees and are more than 100m in height. When the semi-vertical cut was completed to provide the required space to begin the construction of the wall, some groundwater seepage was noted by field personnel. The in-situ soil is composed of a mixture of dense volcanic material and weathered

Page | 2

rock. It was concluded that groundwater seepage was part of the natural groundwater flow pattern from the upper areas of the mountain throughout the hillside. It was therefore imperative to utilize an appropriate sub-drainage system to permanently mitigate against any unwanted groundwater infiltration (Figure 2). This was done by placing a geocomposite drainage layer on the cut formation (behind the reinforced fill). During preparation of the site, including compaction of the formation, a saturated zone was encountered that rendered proper compaction impossible. Instructions were then given for deeper excavation whereupon further groundwater seepages were found at that lower level (Figure 3). The solution was to form a drainage ditch system beneath the affected area and providing outlets to conduct the water away from the foundation and reinforced fill zones (Figure 4).

Figure 2. Subdrain material Figure 3. Water at base

Figure 4. Drainage at base

2.2 Seismicity

Mexico is divided in four seismic zones, ranging from A to D, with zone B being non seismic and zone D high seismic. There is also a soil characterization system which describes Type I (firm soil), Type II (transitional soil), and Type III (compressible soil). The city of Xalapa is located in seismic zone B with a soil type I, so according to this, the Peak Ground Acceleration (PGA) considered in the design was 0.14g.

Page | 3

2.3 Soils

The existing soils where the MSE wall related to this case study is located are composed of densified volcanic material that has been surficially altered by climatic conditions and transformed into a material called tepetate (local name), which is a sandy clay. The stratigraphy of the existing soil is shown in Figure 5 from a borehole log taken from the area around the wall ; the red horizontal line denotes the wall base elevation, blue area shows the rock quality designation (RQD), and yellow area corresponds to the SPT N value; moisture content (w) is also shown in the left hand side of the chart.

Figure 5. Boring log at MSE wall location

The cut soils material developed from the surrounding works were utilized as backfill in the reinforced zone of the retaining wall. This material which was analyzed and classified by a local soil laboratory and its characteristics are shown in Table 4 below.

Page | 4

Table 4. Backfill material characteristics

Ma xi mum Pa rti cl e Si ze

Si eve No.

4

40

200

Atterberg Indexes

L.L, %

P.I, %

USCS :

N.P

N.P

SM

3/4"

% Pa s s i ng

80.2

57.4

23.2

2.4 Design

The design of the MSE wall was undertaken in accordance with AASHTO Standard Specifications for Highway Bridges (2002) and FHWA-NHI-00-0043 design guidelines, which requires the determination of the geometric and reinforcement requirements to prevent internal and external failure using limit equilibrium methods of analysis. The software used for the analyses were MSEW, developed by ADAMA Engineering, Inc., Delaware, USA; G-Slope, developed by Mitre Software Corp., Edmonton, Canada; and ReSSA (3.0), developed by ADAMA Engineering, Inc., Delaware, USA. This software enables the internal, external, and overall stability to be checked. If the information about reduction factors (RF) is not available, the minimum RF recommended in Table 1 (adapted from the Federal Highway Administration, FHWA) should be used for design purposes.

Table 1. Durability Reduction Factors (FHWA)

Two additional reduction factors that must be included in order to define the allowable design strength of the geosynthetics are Installation Damage (RFid) and Creep (RFc). Again if no laboratory results are reported then the recommended RF

Page | 5

values from Tables 2 and 3 should be considered (adapted from FHWA-NHI-07- 092). Table 2. Reduction Factors for Installation damage (FHWA)

Table 3. Reduction Factors for Creep (FHWA)

2.4.1 Reinforcement

The primary geogrid reinforcement used in the analysis and design of the MSE wall is made out of high density polyethylene (HDPE) polymer resin and manufactured by Tensar International, forming a uniaxial (UX) geogrid poduced as a result of a punch and draw process resulting in a high tensile strength product appropriated for reinforcing of mechanically stabilized earth structures. The other type of geogrid material (UV stabilized) used to conform the wraparound facing wall facing was described in item 2.1 above. The maximum primary geogrid embedment length was 22.5m in this case. Figure 6 shows a sample of UX primary geogrid and figure 7 shows a sample of the UV stabilized biaxial geogrid.

Figure 6. Uniaxial HDPE geogrid Figure 7. Biaxial UV stabilized PP geogrid

Page | 6

Figure 8 illustrates the geogrid reinforcement in a typical wall section, showing the two types of geogrid and the erosion mat used to allow vegetation of the wall face.

Figure 8. Typical wall section showing geogrid reinforcement

2.4.2 Analysis

The soil parameters for the foundation, retained and reinforced soils considered in the analyses are shown in Table 5 below. The wall geometry includes a maximum height of 30.3m with a battered face of 11 degrees except for the lower 4.0m which has a vertical facing (only in the 30.3m tall wall section).

Table 5. Design soil parameters

SOIL TYPE (kN/m3) Reinforced12.5 Retained15.0 Foundation15.0

() c (kPa) 33.00.0 34.00.0 34.0 27.2

The maximum horizontal seismic acceleration coefficient considered in the analysis for internal stability was 0.14g, and 0.07g for external and overall stability. This is half of the peak value as stated in the AASHTO Specifications for Highway Bridges, 2002, Division 1-A, Articles 6.4.3 and 7.4.3, when performing a pseudo-static analysis (Mononobe-Okabe) assuming that no lateral restrain is provided.

As mentioned in section 2.2, the formation where the wall base would be located was affected by groundwater that potentially influenced the overall stability by reducing the calculated bearing capacity of the existing soil. Since the original analysis did not consider the presence of groundwater at the wall base, it was necessary to perform additional analyses including the new unexpected water

Page | 7

conditions. Following is an example of the wall bearing capacity analysis performed by using the MSEW software to examine the impact when groundwater at the wall base is considered. Figure 9 shows the analyzed wall section, and Figures 9a and 9b show the calculated factor of safety for bearing capacity without and with water at wall base respectively.

Figure 9. Analyzed wall cross-section

Figure 9a. BC_No water at wall base Figure 9b. BC_Water at wall base

As can be seen from the above figures, the presence of water at wall base was a critical factor with respect to bearing capacity and wall stability, therefore the construction of a the wide drain shown in Figure 4 was necessary. Following this remediation in the design reverted to the original analysis (Figure 9a).

A further variation from the original design involved the lack of wall embedment which occurred as the result of changes in the topographic survey information. A further analysis was performed considering no wall embedment and it was found that the overall stability was compromised, so additional geogrid layers and additional lengths were required in order to meet the minimum factors of safety for overall stability of 1.30 in static conditions and 1.12 in seismic conditions. This analysis was performed using G-Slope and ReSSA limit equilibrium software. Figures 10a &10b show the new analyzed wall section using G-Slope software and Figures 11a & 11b show factors of safety maps for the same wall section by using the

Page | 8

ReSSA software for static and seismic conditions respectively, both software analyses showing that the minimum required factors of safety were met.

Figure 10a. Overall wall stability analysis (Static) G-Slope software

Figure 10b. Overall wall stability analysis (Seismic) G-Slope software

Page | 9

Figure 11a. Overall wall stability analysis (Static) ReSSA software

Figure 11b. Overall wall stability analysis (Seismic) ReSSA software

After performing a set of analyses adjusting the reinforcement accordingly, it was found that a minimum berm of 1.5m was required from the wall base to the edge of the existing hillside slope. Also, it was necessary to build a 4.0m high compacted fill buttress in front of the MSE wall to also serve as an additional toe to protect the wall against potential erosion. This fill will be vegetated after completion of construction.

Page | 10

Another issue observed during a site visit was the proximity of the toe of the wall in relation to the existing top of slope. As a result of this geometric restraint the contractor was unable to provide the required wall embedment. Therefore, additional analysis for bearing capacity was required in order to find the minimum distance (berm) from the toe of the wall to the top of slope. The analysis confirmed that a minimum wall setback would be 1.5m. The calculation utilized to confirm this requirement was as follows:

Df = 0.0m b = 1.5m B = 22.5m H = 100.0m = 35 c = 85.0 kPa [SPT 26] = 15.0 kN/m3

Finding ultimate bearing capacity;

qu = c. Ncq + 0.5.( .B. Nq) qu = 1,023.5 kPa qall = 341.2 kPa > 340.0 kPa

(1)

O.K.

Nq = 4.0 Ncq = 4.1 b/B = 0.1 F.S = 3.0 v = 340.0 kPa

Where; Df = wall base embedment b = distance from wall base to the edge of the slope (berm) B = wall base width (width of reinforced zone) H = bottom slope height = bottom slope angle c = undrained cohesion (value includes a F.S=2.0) = unit weight of foundation soil Nq = Meyerhofs bearing capacity factor, (Fig. 411, Das, 2004) Ncq = Meyerhofs bearing capacity factor, (Fig. 4.12, Das, 2004) v = applied vertical stress from MSE wall (value from MSEW analysis)

2.5 Construction

The construction of the MSE wall started once the new analysis and design were ready and approved by the geotechnical engineer. The following set of photographs describes the process of building this 30m high MSE wall.

The construction of the highest section of this MSE wall began in March 2012 after the above-mentioned issues were resolved and release for construction was approved by the project geotechnical engineer.

Page | 11

Photo 1. Subdrain around reinforced zone Photo 2. Subdrain on cut surface

Photo 3a,b. Installing primary geogrid: first and second layer

(3a) (3b)

Photo 4a,b. Drain at wall base and finished compacted surface

(4a) (4b)

As menitioned before, the wall facing is composed of UV protected biaxial geogrid containing an erosion control blanket (NAG350), and the verticality of each 0.5m basket is reached by using a welde wire form (WWF) as a non-structural material (see photo 5.) Photo 6 shows constructin advance corresponding to a wall height of 19.5m.

Page | 12

Photo 5. Wall facing detail

Photo 6. Construction advance at an approximate wall height of 15.5m (4.0m of wall is buried)

Surveying data was supplied by the contractor showing that no lateral nor vertical movement has been detected during the construction of this MSE wall. Figure 12 shows a plan view of the surveying points in the wall area and figure 13 shows surveying data ploted in a cross section format (just one section is shown here).

Figure 12. Plan view-surveying points at MSE wall location

Page | 13

Figure 13. Surveying points plot showing no deformation on the MSE wall

3. CONCLUSIONS/COMMENTS

The analysis and design process does not end when it is submitted to the project management for approval and construction. It is important to confirm that all design assumptions meet with project requirements and these with existing site conditions. Site inspection previous the beginning of construction is a must, and if any uncertain event appears then appropriate remediation shall take place and additional analyses shall be done. This case study related to a critical structure clearly shows and confirms how different are the assumed conditions (on paper) compared with the site existing conditions (reality), so every partie involved in a project should make sure that all assumed parameters meet project and site conditions.

Page | 14

4. ACKNOWLEDGEMENTS

The authors appreciate the help and support of all the persons involved in this project from the private companies Isolux Corsan, Mota Engil, Inarmex, Triada, and Ecomex; and from the public sector to SCT (Communications and Transport Secretariat) agency, that made the elaboration of this paper possible.

5. REFERENCES

AASHTO (2002). Standard Specifications for Highway Bridges, 17th Edition, Code: HB-17, ISBN: 156051-171-0. Das, B.M., (2004). Principles of Foundation Engineering. Chapter 4, Figures 4.11 and 4.12. Fifth Edition. Brooks/Cole-Thomson Learning. Federal Highway Administration, FHWA-NHI-07-092 (2008). Geosynthetic Design and Construction Guidelines, NHI Course No. 132013. Federal Highway Administration, FHWA-NHI-00-043 (2001). Mechanically Stabilized earth walls and Reinforced Soil Slopes, Design and Construction Guidelines, NHI Course No. 132042.

Page | 15