Extensible geostrip or strap reinforcement has been a fascinating topic in reinforced soil discussions, particularly over the past five years. It’s an older reinforcement technology, with inextensible metallic strip reinforcement dating back to the early 1960s. The geosynthetics industry’s development of extensible, polymeric reinforcement strips has found plenty of use around the world, but a number of major markets have been slower to adopt these high-strength designs despite plenty of case study and design support.
As part of Geosynthetica’s GeoAmericas series ahead of GeoAmericas 2020, we publish this addition to the proceedings from GeoAmericas 2016. Giulia Lugli, Moreno Scotto, and Matt Miller’s paper, “High-Tenacity Polyester Linear Strips in Non-Standard Reinforced Soil MSE Retaining Structures,” provides design methods and case studies to bolster the case for more MSE wall designers to embrace this technology.
High-Tenacity Polyester Linear Strips in Non-Standard MSE Structures
MSE retaining structures with polymeric strips reduce project cost and environmental impact compared with traditional methods. High tenacity polyester strips represent an alternative solution to soil reinforcement in MSE walls.
A wide range of strips tensile strength, width and LLDPE coating thicknesses allow the optimization of the design and the use of backfill materials such as lime treated soils, untreated ash fills and cohesive soils. This paper presents three case studies of MSE wall projects, recently realized in Texas and Florida, with non-standard soil materials.
Design method, tests results on high tenacity polyester strips in non-standard soils and installation considerations are described. The Texas case studies present designs based on high void ratio, low unit weight of the backfill soil and the use of onsite non-plastic clayey sand. The Florida project consists of the partial reconstruction of I-10 in Escambia County using “Sand Backfill”.
MSE retaining walls with concrete facing panels are well-known systems used in the United States and abroad to support or enable the construction of infrastructure in tight urban corridors, forming retaining walls, abutments and wing walls.
Polymeric geosynthetics have been introduced in the wall market representing an evolution and a significant advantage for both cost-effectiveness and performances. Geostrip-reinforced MSE walls are recommended in case of non-conventional retained soil such in case of poor subsoil conditions, chemically aggressive environment or in warm climates, where steel could represent an issue.
The general behavior of the MSE structure depends on the interaction between the soil reinforcing elements and the surrounding soil, which are linked to the properties of the materials used and the construction methods adopted. As alternative to traditional systems, high adherence polymeric soil reinforcing strips have been introduced in order to increase the design life of the wall even in highly aggressive environments, reduce the overall project costs and provide design flexibility.
MSE walls with polymeric strips are popular structures all over the world but fairly recent developed in the US.
Several walls have been instrumented in the past years to study and demonstrate the behavior and the performances of those structures.
2. MSE WALL SYSTEM
The structures presented in this paper are comprised of precast concrete facing panels and discrete high adherence polymeric soil reinforcing strips.
2.1 Concrete facing panels
The MSE wall system utilizes precast concrete facing panels that are usually either square in shape (5’ wide x 5’ tall), or rectangular in shape (10’ wide by 5’ tall or 7.5’ wide by 6’ tall). The typical, and minimum, thickness of the concrete panels is 5½ inches. The front face of the concrete facing panels can be finished with a broad range of textures to satisfy a wide range of aesthetic and architectural requirements. The precast concrete facing panels are designed based on project specific requirements or DOT special provision. The typical concrete compressive strength of the panels at 28 days shall meet minimum 4,000 psi.
2.2 Reinforcing strips
The soil reinforcing strips, which are the key structural components of the system, utilize high tenacity coated polyester strips. These geostrips consists of discrete bundles of closely packed high strength polyester filaments, lying parallel to each other, encased in a tough and durable polyethylene (LLDPE) coating.
This reinforcement has been used for many years all over the world in MSE walls as a geosynthetic alternative to steel reinforcements, due to its chemical stability and its low creep characteristics.
The strips are mechanically connected to the facing panels through a structural connection. In this mechanical connection, the polymeric reinforcing strips are wrapped around an anchor embedded into the concrete facing panel. Several connections systems are available on the market.
Strips are available in a wide range of tensile strengths and dimensions (thickness and width) to fit the specific project needs and design requirements. Furthermore the polyethylene coating allows the use of geosynthetic strips in highly alkaline environments (pH up to 11). High alkaline environments in MSE wall applications may include recycled concrete or lime-treated backfill materials.
Pullout testing shall be performed to define the maximum tensile strength allowable at the connection between the panel and the reinforcement. The anchored connection element shall be corrosion free and prevent damage to the polymeric soil reinforcing strips during the construction or the life-time of the structure.
3. DESIGN PROCEDURES
The design of this MSE wall system is generally based on AASHTO LRFD Bridge Design Specifications (2012 and interims), and the design requirements, standard specifications and MSE special provisions of the DOT involved.
The Texas projects described hereafter represent an exception and have been designed, as requested, following AASHTO Standards 2002. The design of the MSE structure includes both external and internal stability of the MSE mass, including the facing and connection. In particular the Florida project discussed hereafter.
3.1 Design method
Internal design of the MSE structure requires the knowledge of the long-term and short-term strength properties of the soil reinforcements and the interaction parameters between the soil and the reinforcements. The design of this MSE wall system strictly follows the AASHTO LRFD bridge specifications (Chapter 11). Since this MSE wall system utilizes extensible geosynthetic soil reinforcements (polyester strips), the calculations of the internal stresses (vertical and horizontal) in the structure follow the AASHTO Simplified Method per AASHTO 188.8.131.52.1.
Regardless of the fact that AASHTO does not address polymeric reinforcing strips, the value of the coefficient of lateral earth pressure, kr, used in the internal stability calculations, is taken equal to 1.0 x ka based on AASHTO LRFD Figure 184.108.40.206.1-3 for geosynthetics (i.e., geogrids and geotextiles).
This design value was determined to be conservative for these soil reinforcing strips during the extensive experimental study conducted by DelDOT on the SR1 – I95 Interchange construction project, which included instrumentation and monitoring of a MSE wall section.
Published papers present the data collected from the beginning of the construction of the wall up to date and highlight the underestimation of the reinforcement loads (Rimoldi, et.al., 2013 and Luo, et.al., 2015). Since AASHTO does not provide recommended default or minimum values for soil-reinforcement interaction coefficients for polymeric soil reinforcing strips (i.e., CDS, Ci, F*, and α), the strip supplier relies on the results of product-specific interface direct shear testing and pullout testing to determine recommended interaction coefficient values for use in the MSE wall design.
The reduction factors (i.e., RFID, RFCR, and RFD) for use in calculating the nominal long-term design strength of the geostrips are confirmed by the NTPEP (National Transportation Product Evaluation Program) and BBA (British Board of Agreement) and several laboratory tests have been conducted to evaluate the strips behavior.
3.2 Reduction factor – Creep
Polymeric reinforcements in general are subject to long-term creep under sustained loading, particularly under high temperatures. Therefore, a strength reduction factor against creep, RFCR, must be applied to the short-term ultimate tensile strength in the internal stability design of MSE walls reinforced with polymeric elements. The polyester strips generally have higher tensile strengths and lower creep rates compared with other polymeric soil reinforcements (i.e., HDPE and PP). RFCR is obtained by conducting laboratory conventional creep tests, which generally involve testing a sample under a constant load maintained at a constant temperature. Extensive testing programs to evaluate the creep behavior of the soil reinforcing strips have been conducted. Testing in the US was conducted by NTPEP, in general conformance with WSDOT Standard Practice T925, at a baseline temperature of 20°C. Based on the results obtained, a strength reduction factor of 1.37 for a 120-year service life may be applied.
3.3 Reduction factors – Installation damage
To allow for the loss of tensile strength due to the potential mechanical damage that occurs during the construction of MSE walls, an appropriate strength reduction factor, RFID, must be considered. RFID may be determined by conducting full-scale installation damage testing, or laboratory installation damage testing. Extensive installation damage testing on various strip grades using different types of backfill materials has been conducted. The test results indicated installation damage reduction factor, RFID, values ranging from 1.0 to a maximum value of 1.10, considering all backfill types. A reduction factor equal to 1.10 is recommended to meet the minimum values of AASHTO LRFD specifications.
3.4 Reduction factors – Durability
The reduced tensile strength due to chemical and biological degradation during in-ground use is calculated considering an appropriate strength reduction factor, RFD, in the design. It should be noted that the strips are encased in a tough and durable polyethylene coating (sheath), which acts as a chemical barrier that, provided it is not broken or damaged, will reduce the risk of chemical attack on the polyester fibers. The strips tested meet the requirements to allow the use of the AASHTO minimum reduction factor for long-term degradation in non-aggressive environment (4.5 ≤ Soil pH ≤ 9.0 and 30°C for permanent structures). Tests have been conducted also in aggressive soil and the reduction factors are below 1.30 even with pH up to 9 and soil temperature of 30°C.
4. CASE STUDIES
The projects presented hereafter each had developed designs based on the design method and reduction factors discussed above. The results of the design have been satisfactory and appear to be correct for all the projects.
4.1 Circuit of the Americas, Formula 1 (Texas)
This project consisted of construction of MSE retaining walls with concrete facing panels for the entry bridge to the Circuit of The Americas™, a 3.4-mile world-class motorsports circuit located near downtown Austin (Texas).
The project consisted of two false bridge abutment walls of approximately 8800 square feet total. The abutments were supported by drilled shafts within the reinforced zone of the MSE structure (see Figure 1). The maximum design height for the walls was about 23 feet. The dimensions of the concrete facing panels used in the project were approximately 7’-6” by 6’-0” (nominal), 5 ½” thick. The facing panels meet Class A requirements with a minimum compressive strength of 4000 psi. The geosynthetic linear reinforcing strips had the mechanical properties reported in Table 1.
The backfill material was characterized by 3” minus limestone rock with an average size of the aggregates of approximately 1 ½”. The peculiarity of this material was related to high void ratio and very low unit weight, about 90 pcf, compared to the typical 120 pcf expected. Pullout testing was performed to define the coefficient of interaction (Ci) in order to calculate the proper friction coefficient F*. The coefficient of direct sliding (CDS) was applied as per contract requirements.
Creep, installation damage and durability reduction factors took into account the onsite temperature and conditions.
Those values were calculated taken into consideration a structure design life of 120 years over a 75 years design life expected. NTPEP (National Transportation Product Evaluation Program) report is available. The input data assumed for the design are listed in Table 2.
L-Pile Analysis for the Light Pole Foundations was provided to define the mobilized soil reactions (lbs/in) and the shear force (kips) along the wall height. The pole foundations were centered horizontally along the MSE walls approximately 10 ft. behind the facing. Based on this geometry and on the 34-deg internal friction angle of the reinforced soil, the lateral forces were considered spreading over a horizontal distance equivalent to the width of two of the precast panels – 15 ft (see Figure 2).
The lateral forces were added to the horizontal pressure due to the backfill and to the live loads. A ¾” batter of the vertical face, over a 6’ high panel, was required based on the soil properties and on the residual batter noticed during the construction phase. The geostrip installation is typically less labor demanding compared to the placement of the steel strips. The installation rate for this project specific considering a crew of 4 workers was about 2 rolls (750 ft) of geostrips every hour. The construction was properly finished (Figure 3) and no installation issues occurred. TxDOT visited the site during construction.
4.2. Sam Houston State University – SHSU, Sycamore Avenue Site (Texas)
Two MSE retaining walls have been built for multi-family housing in the City of Huntsville, Texas. The project consisted of about 16,300 square feet total. The maximum design height for the walls was about 18 feet. The load surcharge applied in this design was the standard live load of 250 psf, the peculiarity for this project was representedy the reinforced material. The contractor of the Sycamore project requested to utilize the onsite material as MSE wall backfill. The fill consisted of onsite non-plastic light brown sand and a reddish brown clayey sand (Figure 4).
Laboratory testing evaluated the soil parameter of the backfill material. The light brown sand had a maximum dry unit weight of 102 pcf and 9.2% water content. The reddish brown clayey sand had a unit weight of 118 pcf and 12.1% water content. The plasticity index observed was 9. Friction angle testing was also performed for the non-plastic sand with a result of 40.9-deg. The input data of the design are shown in Table 3.
The friction coefficient along the soil-reinforcement interface (CDS) was determined in accordance with ASTM D 5321. Interface direct shear tests on the strips were performed using the backfill material to define a recommended value.
The values of the pullout interaction coefficients (Ci) were determined by conducting laboratory or full-scale pullout testing. The pullout testing performed suggested that the pullout behavior of the polymeric soil reinforcing strips is somewhat similar to that of ribbed steel strips.
It should be noted that the current AASHTO LRFD specifications do not specifically address pullout behavior, or parameters, of polymeric reinforcing strips. The current AASHTO LRFD specifications only address smooth and ribbed steel strips, steel grids/mats, geogrids, and geotextiles. Pullout testing on polymeric strips showed higher values for the coefficient of interaction at low confining pressures (i.e., at the top of the MSE wall), with the coefficient of interaction decreasing to a minimum constant value at a critical confining pressure (or at an equivalent critical depth).
The design values for the soil-reinforcement interaction coefficients Ci and F* for use in the MSE wall design, are based on a conservative minimum bounding envelope of all the test results. The AASHTO specifications do not provide guidance on the scale effect correction factor for polymeric strips. Based on pullout testing results performed to date with different reinforcement lengths, α factor for strips was considered 0.95. A sheep foot compactor was used during the placement of the backfill (see Figure 5). The lifts were approximately 8” thick for the entire depth of the structure.
The batter considered for this structure was about 1-deg every 10 to 15 inch. The construction was completed on schedule and no settlement issues were observed (Figure 6).
4.3 Sr 8 (I-10), Escambia County (Florida)
The project was part of the reconstruction works in Escambia County. The improvement consisted in the widening of the Interstate 10 (I-10) to six lanes from east of State Road (S.R.) 291 (Davis Highway) to the Escambia Bay Bridge.
Four MSE walls were constructed in 2014 for a total of 12,000 sft. Square panels, 5 by 5 feet nominal dimensions and 5.5 inch thick, were used and the maximum design height considered was 30 ft.
The MSE walls built on the I-10 were widening of existing abutments (Figure 7) and wing walls. The abutments were supported by square concrete piles within the reinforced zone. The soil reinforcements had to be designed and positioned to avoid any obstruction.
No specific testing was performed by the MSE wall supplier. The soil parameters accepted from FDOT and used for the analysis are presented in Table 4 hereafter.
The sand backfill used in this project is accepted statewide in Florida except in Miami-Dade and Monroe Counties.
The batter applied during the construction was about 1-deg every 10-15 inch. Architectural finishes were required for the structure. Some panels have been customized to meet specific requirements; other panels were cast using approved formliners (Figure 8).
Even if the project was classified as non-conventional project, no settlement issues occurred during and after the installation. The reinforcing geostrips did not present any secondary batter release; during the installation the contractor had to decrease the inward panel batter to meet the project specifications.
The author would like to thank Tricon Precasted Limited™ and TEG Engineering, LLC for the valuable data and photographs provided for writing of this paper.
ABOUT THE AUTHORS
Giulia Lugli is the Group Business Development Geosynthetics Unit – MSE Walls Manager with Maccaferri Inc. Moreno Scotto is Head of the Geosynthetics Business Unit for Maccaferri. Matt Miller is Vice President and an MSE Wall Expert at Corsair Consulting, LLC.
AASHTO LRFD Bridge Design Specifications, Seventh edition, (2014)
BBA Certificate n. 12/H191, (2012). Linear composites retaining walls and bridge abutments systems – ParaWeb™ straps for reinforced soil retaining walls and bridge abutments.
Greenwood J. H. (2007). Assessment of ParaWeb, ParaGrid and ParaLink to ISO TR 20432, Report Number: 2007-0242 Issue 5.
Luo, Y., Leshchinsky, D., Rimoldi, P., Lugli, G., Xu, C., (2015). Instrumented MSE Wall Reinforced with Polyester Straps, Transportation Research Board Annual Meeting , Paper #15-0985, Washington, DC, USA.
NTPEP Report No. 8508.1 (2010). Final product qualification report for linear composites Paraweb/Paralink and Paragrid product lines.
Rimoldi, P., Leshchinsky, D., Arrigoni, M., and Bortolussi, A., (2013). Vertical wall with concrete panels facing and geostrips reinforcement: Instrumentation and Data Reduction. Proceedings of the Design and Practice of Geosynthetic-Reinforced Soil Structures, Bologna, Italy.