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Reinforced soil wall with geogrids

Using volcanic tuff as filling material helps in the widening of an important highway.

Features | August 1, 2022 | By: Guillermo Paz

FIGURE 1 Geographic location of the study area. Photo: Google Maps

Mechanically stabilized earth walls (MSE) are containment structures that have gained considerable popularity over the last few years due to their various benefits: constructive, environmental and economic. There are several types of reinforced earth walls, which vary according to the type of material used to reinforce the fill and the front facade system used.

In Honduras, the most widely used is the Sierra® Slope Retention System, which consists of uniaxial high-density polyethylene (HDPE) geogrids with a basket-type facade of electro-welded mesh. Constructively, this system presents execution times that adapt to the demands of the project, being able to work in extended hours, resulting in high construction yields. Economically, they are very competitive structures since they use filling material from borrow pits of various granulometries close to the construction zone. In turn, low-cost materials are used to form the facade of the wall, such as nonwoven geotextile or electro-welded mesh. They are structures that have the versatility to adapt to various topographic conditions, are environmentally compatible and built to high-quality standards, and have an extended life span. 

This document details the construction of an MSE 44 foot (13.5 m) high with an electro-welded mesh facade for widening a section of one of the most important land roads in Honduras. The road in question is located on the CA-11 highway in western Honduras and is a very important commercial highway in the country since it connects Honduras with Guatemala.

This road section was built in the 1970s when demographic, economic and commercial conditions were low in Honduras, so it was built with basic engineering standards such as a rolling surface with a double surface treatment and two lanes for circulation. Nowadays, when commercial relations have multiplied and all types of heavy-duty vehicles are circulating, a re-adaptation of the existing geometry of the road section is necessary, so the national authorities opted for improvement of the radius of curve of the section to improve the comfort and safety of the users.

Site conditions

The study area resides on a mountainous topography, has a tropical rainy climate and has an average height of 2,877 feet (877 m) above sea level. The geology of the site presents intercalations of tuffs, shales and ignimbrites. It has a difference of elevation of 20 feet (6 m) from the highest point and the lowest point, so the area drains a lot of surface runoff when it rains. This runoff has managed to infiltrate the different strata of the soil, weakening the mass of soil that has induced movements on the pavement. The solution to this condition that was applied when the section was constructed was to install small diameter PVC pipe perforated and wrapped in geotextile on a gravel bed as an evacuation mechanism for this runoff, now underground.

The road section was built at hillside on what appeared to be a deposit of an old, unconsolidated landslide. Land exploration work was carried out with 20 foot (6 m) deep open pits prior to the execution of the wall on an already excavated platform 13 feet (4 m) high, excavating a fractured, loose and saturated material of the same characteristics as the material of the cutting slope, which indicated the presence of a material with few geotechnical characteristics. Unfortunately, the study area lacked a relevant geotechnical exploration that faithfully characterized the terrain, so the information on the open pits was an added value to the project execution.

Containment system

The retaining wall system used is formed by a facade of electro-welded mesh baskets wrapped by a nonwoven geotextile and a triaxial geogrid, anchored to the uniaxial geogrid by a steel rod, which work together to resist differential seats and large external loads. These walls have the flexibility to vary the angle of inclination of the facade, generally using an angle of 70°. In turn, they work with any type of filler material since the geogrids used as reinforcement are chemically inert.

The system starts from the placement of a draining filter at the base of the wall. To do this, a drainage system is used, with a 6-inch (15 cm) diameter perforated PVC pipe wrapped in a nonwoven geotextile that is connected to a runoff collection system with Geodren on the retained soil. Depending on the saturation conditions of the foundation floor, a layer of crushed gravel at least 20 inches (50 cm) thick is placed in the first basket of the wall.

The reinforcement used is with uniaxial geogrids (HDPE), which resist large stress loads in one direction. In turn, its opening allows a good interaction with the filling material, allowing the interlocking between particles to be achieved, achieving a high performance of the structural filling. In Honduras, this system is commonly used for landslide stabilization, widening roads or stirrups in bridges. 

FIGURE 2 The filler material used from a nearby borrow pit

Reinforced fill

The filler material used (Figure 2) was a tuff, taken from a borrow pit located 7 miles (12 km) from the construction zone. Its geotechnical parameters are shown in Table 1. Various material banks with better geotechnical characteristics were evaluated but had the disadvantage of being too far from the construction zone or presenting poor borrow material.

Table 1 Geotechnical parameters of the filling material used

MSE wall design

For the design of the wall, the following types of analyses were performed:

  • Analysis of external and internal stability: Using the TensarSoil program, which applies the DEMO 82 methodology (FHWA 1997) and assesses the behavior of the wall against sliding, loading capacity and turning (external stability) and internal sliding of geogrids (internal stability), which takes into account factors such as tension, pullout, and pullout and breaking connections.
  • Global stability analysis: For this purpose, the TensarSlope limit equilibrium analysis program is used, which uses the simplified Bishop methodology to calculate the wall safety factor.

Since Honduras is a country with moderate seismic potential, both analysis situations were carried out using a horizontal seismic coefficient of 0.30 for dynamic analysis. In turn, the static analysis was performed.

The geometry of the wall was designed using the ratio of width and height of the wall, L / H> 0.70, and this value can be lowered to a ratio of 0.6. The dimensions of width and height were 31 feet (9.50 m) and 44 feet (13.5 m), respectively. Figure 3 shows the designed cross section.

FIGURE 3 Cross section of the MSE wall

The design parameters for reinforced padding, retained soil and foundation floor are shown below:

  • Reinforced filling: cohesion 0, friction angle of 34° and specific weight of 14 KN/m3
  • Retained soil: cohesion 5 KN/m2, friction angle of 28° and specific gravity of 18.5 KN/m3
  • Foundation soil: cohesion 15 KN/m2, friction angle of 24.6° and specific gravity of 18.5 KN/m3
  • Live load (traffic load): 12 KN/m2
  • Dead load (concrete pavement): 3.6 KN/m2
  • Minimum FS for static analysis of 1.3 and for the dynamic case of 1.1

The results of the analysis described above are shown in Figures 4, 5a and 5b.

FIGURE 4 Analysis of external and internal stability static and dynamic scenarios

Now, using the Sierra Bank and varying the values of friction angle (35.3°) and specific weight (21 KN/m3), the following results are obtained as follows in Figures 6a and 6b.

With these results, it can be inferred that with a heavier material like concrete (specific weight = 24 KN/m3), a failure due to global stability might happen if a concrete wall is used.

FIGURES 5a and 5b (a) Global stability analysis of static (minimum FS 1,430 > 1.3, OK) and (b) dynamic scenarios (minimum FS of 1,102 > 1.1, OK).

Construction process

The construction of the wall was complete in three months. To do this, the first step was improving the conditions of the foundation ground. In this phase, a ground improvement was made by placing two layers 20 inches (50 cm) of crushed gravel in two layers reinforced with triaxial geogrids with the aim of reducing possible differential seats and thus improving the bearing capacity of the foundation soil.

Once this stage was completed, the 20-inch (50 cm) thick draining filter basket was placed. On top of this layer, the crushed gravel is protected by placing a blanket of nonwoven geotextile and placing reinforced material baskets vertically spaced every 20 inches (50 cm). The reinforced filling is compacted with a compaction energy of 95% of the Modified Proctor with the maximum density values of 14 KN/m3 and an optimum humidity of 23.4%. The construction process is shown in Figure 7

FIGURES 6a and 6b (a) Global stability analysis of static (minimum FS 1,311 > 1.3, OK) and (b) dynamic scenarios (minimum FS of 1,051 < 1.1, failed).

Conclusions and recommendations

MSE walls reinforced with geogrids and, specifically, using the Sierra system, have good flexibility to be constructed in various adverse topographic situations. They also have reduced execution times that translate into economic savings.

The friction angle is a very important parameter for the correct interaction between the geogrid and the soil particles of the filler material, where values of friction angle greater than 30° are recommended to ensure that the interlocking between geogrids and filler material occurs.

The comparison between the filler material used from the 12 + 700 borrow pit and the Sierra borrow pit produce a significant effect on the calculation of the value of the safety factor due to the global stability of the MSE wall in the dynamic analysis. By not including the natural guard at the foot of the MSE wall, we eliminated a counterweight that could possibly reduce the destabilizing forces increased by the heavier specific weight of the Sierra material (21 KN/m3) in comparison with the volcanic tuff which has a lighter specific weight (14 KN/m3). Even though the Sierra material has a better workability for the compaction process, the volcanic tuff represented a better option in factor of safety terms. 

Comparing a conventional concrete wall, with specific weight greater than the Sierra material (25KN/m3 > 21 KN/m3), the better solution in this case was the MSE wall reinforced with geogrids and using a volcanic tuff as filling material.

FIGURE 7 Construction process of the MSE wall reinforced with geogrids

Acknowledgements

To the company ICA Inversiones and Mariana Stafford from Tensar International for the support and facilities to receive the field information and bibliography used in this document.

References

Federal Highway Administration, FHWA-NHI-00-043. (2001). “Mechanically stabilized earth walls and reinforced soil slopes, design and construction guidelines,” NHI Course No. 132042.

Federal Highway Administration, FHWA-NHI-07-092 (2008). “Geosynthetic design and construction guidelines,” NHI Course No. 132013.

Tensar International. (2010). “TensarSlope analysis program for reinforced slopes & walls user’s manual for version 1.10.”

Tensar International. (2010). “TensarSoil MSE design program AASHTO and NCMA wall design methods.”  

Guillermo Paz has a master’s in soil mechanics and geotechnical engineering, 2019 (CEDEX-UNED), Spain.

All figures courtesy of the author.


Project Highlights

Reinforced Soil Wall with Geogrids

Project: Widening of CA-11 highway

LOCATION : Honduras

Civil ENGINEERS: ICA Inversiones 

GEOSYNTHETIC PRODUCT: Sierra® Slope Retention System

GEOSYNTHETIC MANUFACTURER: Tensar

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