Geosynthetic Reinforced Embankments Over Sinkholes

Construction of structures over areas prone to subsidence or sinkhole formation can be very challenging for geotechnical engineers and needs to be carefully analyzed and designed. Sinkholes formation generally forms underground over the course of many years or several decades/centuries (i.e. karst regions). However, they have the tendency to occur abruptly without warning, specifically when caused by artificial man-made ground modifications such as mining operations (i.e. shallow extractions). Conventional methods of stabilizing areas with sinkholes or potentially localized weak areas are grouting, dynamic stabilization or piling. A less time consuming and expensive alternative is the use of geosynthetics, (geogrid or geotextile) placed directly on top of the weak foundation.

Embankments, fills and pavements are essentially flexible structures. Therefore, the goal of the design of geosynthetic reinforcement of embankment foundations over areas prone to subsidence is to minimize damage by confining the vertical differential displacement of the structure within predetermined tolerances. Thus, it is essential to design not only the required strength of the geosynthetic to support the static load but also at what strain level the geosynthetic develops the required tensile strength. The design therefore includes the evaluation of the maximum allowable reinforcement strain and required tensile properties. The degree of acceptable surface vertical deformation generally depends on the importance of the structure. High cost structures such as highways are generally designed with a maximum differential surface deformation of 1%, while for non principal roads, the limit can be increased to 2% (BS 8006-1:2010). It is common practice to estimate the design void diameter based on experience of similar conditions, surveys and/or probabilistic approach. Another detail that needs to be taken into consideration is the design of the bond length, which is the length of the reinforcement that extends beyond the sinkhole area. This step is essential in order to develop enough bond between the soil and the geosynthetic and generate the required tensile load.

Figure 1. Conceptual role of reinforcement in limiting surface deformations due to subsidence (BS 8006-1:2010)

A common approach to determine the maximum allowable tensile load for the Serviceability Limit State design check and the post construction creep strain is to use the isochronous curves at a given temperature (stress-strain curves at different testing time).

Figure 2. Stress-strain isochronous curves for ParaLink

One of the most important economic forces in Union Bridge is a major North American cement and construction materials company. To secure its future productivity in the town, the company sought to expand its limestone quarry; but to do so, it had to tackle the risks that quarry would cause for the town itself.

Geogrids from Maccaferri have been used successfully for sinkhole mitigation and site remediation designs around the world. Work in the historic town of Union Bridge, Maryland, exemplifies this geosynthetic reinforcement approach. New development in Union Bridge is at times difficult for two reasons. First, the town is listed on the United States’ National Register of Historic Places, so a premium is placed on preserving the city’s traditional character and protecting the older structures and landscapes. Second, karst terrain is present, meaning that there is some surface instability and a heightened risk of sinkhole development. Sinkholes can develop in these soils gradually or suddenly, and are often caused by chemical dissolution of underlying carbonate rocks.

An existing roadway would have to be realigned, and that meant that the roadway would now have to pass over a potential sinkhole zone. A solution was needed to mitigate potential damage. Maccaferri was approached by the engineering consulting firm Hydro-Geo Services, Inc. to help solve the problem. With many years of international experience in sinkhole mitigation, Maccaferri worked with the project team to identify what would be the best solution for effective, economical results and safe, long-term performance. The embankment design of the new roadway leg needed to account for the possible development of a 15-ft.-diameter sinkhole.

A double-layer system design was selected, one that utilized Maccaferri’s ParaLink® 1000 kN geogrid in longitudinal and transversal layers for embankment reinforcement. 5ft overlap between adjacent geogrid rolls was necessary to guarantee reinforcement continuity. Maccaferri ParaLink® is a unique high strength geogrid in use since 1977 and with ultimate tensile strengths up to 1600 kN/m. It’s manufactured from high tenacity multifilament polyester yarns aligned and coextruded with polyethylene (LLDPE) to form polymeric strips.  

Figure 3. First layer installation along traffic direction

Figure 4. Second layer installation transversal to traffic direction

Concrete blocks were added along the perimeter to provide a uniform wrap and to increase friction between the bottom grid and the underlying soil. A minimum of 15’ anchorage bond length was installed as per design requirements.

Figure 5. Concrete blocks with wrapped geogrid

The grid was covered by a one foot layer of crushed aggregate. To ensure the integrity of the overall design, measures needed to be prescribed to prevent the intermixing of fines particles and the primary aggregate layer. A woven geotextile separation fabric was installed over the crushed aggregate to prevent this soil migration. This is commonly done to provide a drainage layer below the embankment/pavement.

Figure 5. Backfilling operations

Figure 6. Completed installation

For nearly 140 years, Maccaferri has provided innovative solutions to the construction industry and built environment. Renowned as the world leader in gabion retaining structures, Maccaferri has diversified significantly over the past 20 years and offering engineered solutions from reinforced soil structures, road stabilization, and natural hazard mitigation to hydraulic works and landfill construction. We are a privately held company with over 30 plants, more than 70 subsidiaries, local operations in 100 countries, and 3000 employees. Our US operations are headquartered in Rockville, MD and include a plant in Williamsport, MD, with offices and warehouses in nine states. We proudly offer local support and global experience.