Void bridging with specialty geosynthetics and thin covers

By Brian Baillie

Introduction

Geologic or hydraulically formed cavities and sensitive utility trenches are sometimes a concern on construction, mining, and environmental containment sites. Naturally formed or man-made openings such as these can complicate the design and performance of overriding structures or create restrictions with site access. They can also create significant safety issues for personnel as equipment is moved over these less stable zones.

High-strength geosynthetic materials have been used for temporary and permanent void bridging designs and their use has been cost-effective and easy to construct. However, the majority of these designs have involved large embankment covers that utilize thick soil covers and soil arching to transfer loads over expected void spaces. The soil arch efficiently transfers a portion of the loads beyond the void perimeter while a geosynthetic reinforcement provides further stability and load transfer within the arch.

The problem for most construction operations is that a thin cover is usually the only available option; whereas, soil arching requires a thicker cover. Site cost constraints, available cover supplies, and limitations on how much soil may be disturbed are common reasons that the typical soil arching methodology for bridging voids with high-strength geosynthetics cannot be used on these sites.

In lieu of soil arching, the entire load must now be transferred to the geosynthetic reinforcement. Design modification can be accomplished while still ensuring that the solution is just as cost-effective and easy to construct.

Choosing the geosynthetic

To safely modify soil-arching designs to specific applications, the controlling variables must be understood. These variables will help the designers choose the right geosynthetic materials to stabilize the site. Controlling variables can include: live and dead loads, predicted void size, load interval or time, permissible cover depth, cover material parameters, desired deformation, or indication depth.

The larger the load, void dimension, and time interval under load, the stronger the reinforcement must be. The required reinforcement strength must also be evaluated for strain performance, which controls reinforcement elongation and the resulting surface deformation depression depths. Various polymers behave differently under load with respect to time. As polymeric reinforcements experience tensile stresses, behavior under load with respect to strain or elongation differs based on ultimate capacity to design load ratios and polymeric type. The allowable strain in the reinforcement will ultimately control the reinforcements’ loading ratio and the intensity of the surface deformation under load.

A reinforcement’s tensile behavior should be analyzed here through the stepped isothermal method (SIM—ASTM 6992), which is a state-of-the-art analysis with time-temperature superposition (TTS) reports. One is able to generate creep-rupture curves and isochronous stress-strain curves out to 114 years in accelerated time with SIM, thus yielding significant data for the designer in regard to load, as a percentile of ultimate strength (UTS), time under load, and stain.

This data is critical in the design and predicted performance of sensitive void bridging applications.

Overlaps & anchorages

Analysis for overlap and anchorage lengths is important and will differ based on the type of reinforcement being used.

An isotropic reinforcement, one with equal tensile capacity in both directions and commonly called biaxial, will be used under the design assumption of carrying tensile capacity in both directions over the void. The design for overlaps and anchorages will follow similar requirements based on the need for sufficient anchorage lengths, but in both directions. These lengths must provide equivalent pullout capacity to the imposed load over the void and are controlled by overburden loads and frictional interactions between the reinforcement and cover materials.

The analysis for isotropic reinforcements relies heavily on proper anchorage lengths in both the machine and cross machine directions, which are most likely assumed to be carrying equivalent loads with equivalent strain responses.

The other type common to void bridging applications is an anisotropic, commonly called uniaxial, reinforcement. An anisotropic reinforcement will carry the tensile load in one direction over the void. The design of end-to-end overlaps will follow the same design principles as the isotropic reinforcements that are controlled by tensile loads—allowable strain and deformation—and frictional interactions.

But since the cross-machine, or tensile members parallel to the roll width, strength is not being used in the design, the analysis for side-to-side overlaps is much easier. A general rule is to use a 2-3ft cross-machine overlap, but the designer should still analyze this length based on stain and allowable deformation distances. As the allowable deformation increases, more overlap will be required to ensure proper tensile uniformity and decrease the chance of panel separation.

Geogrids & geotextiles

Geosynthetic manufacturing technology has enjoyed significant advances in strength for geogrids, geotextiles, and geogrid-geotextile composite materials.

While this creates opportunities for designs with a wide range of options, the decision about whether to use high-strength geogrids (1a) or geotextiles (1b) in void bridging designs may be influenced by more than strength and project service life. For example, the use of an open structure reinforcement (e.g., standard geogrid) can provide an early indication of a void forming, as finer cover material may begin to fall through. It can offer an early warning sign that is particularly useful for sites with a wider range of design loads and the designed depression depth cannot be assumed for the entire range. A woven fabric, however, might be specified for the reinforcement if the ability to hold cover materials in place is of greater importance to the site.

These materials possess different interface friction characteristics so the embedment depths, based on pullout efficiencies, will be different, and those requirements may also influence the selection of the reinforcement.

Plane & wrapped methods

There are two designs for the reinforcement’s placement with thin cover over void-prone areas: plane method and wrapped method.

The plane method utilizes a single layer of reinforcement with long embedment lengths and overlaps beneath a thin geotechnical cover (2). This method involves many project-specific variables (Blivet, 2000) and relies heavily on reinforcement pullout capacity efficiency and strain correlations. As the geotechnical cover material thins, the decrease in overburden affects the amount of opposing reinforcement tensile capacity available per distance.

This correlation controls the required embedment length based on reductions for frictional efficiencies between the reinforcement and geotechnical material.

Another important design variable is axial stiffness, or strain performance. With a predetermined allowable deformation and void dimension, the designer can calculate the required reinforcement performance in strain. The reinforcement strain, calculated as a percentile of added length, will occur over the entire length. However, portions of the reinforcement closest to the load will incur greater tensile loads and greater strain deformations.

In the interest of establishing a conservative design, the loads and subsequent strain closest to the void can be used for the whole platform.

The wrapped method (3) has also been shown to be effective as well as potentially more efficient in cost and construction (Alexiew, 2008). A single reinforcement layer is wrapped back around a compacted geotechnical material with lengths designed to provide sufficient pullout capacity within a platform’s footprint. While it relies on many of the same design principles found in the plane method, the wrapped method differs in that the expected void area and the desired platform thickness will determine whether this method is a valid option.

Field study at mine site

A field study was conducted to evaluate bridging voids with geosynthetics and thin geotechnical cover materials. Prompted by a request from a mining facility, the field study was concerned with confirming the performance of a plane method design for managing a large void with a large live load. The mine in question had to halt operations in a particular zone because of questions regarding the subsurface stability.

The cause of the site’s instability was related to a former mine’s horizontal shafts located directly beneath an area of the operation intended for future works. The mine operators had considered stabilizing the subsurface shafts with on-site material, but doing so would have required the siting of a heavy drill rig over the void area. A working platform for the rig would make it safer, but that would require a large area and a thin cover. It would need to sustain a load of 90 kip over a 10–15ft void.

Another important requirement was the need to see deformations or void occurrence with smaller loads to safely remove personnel and equipment rapidly.

Conservative estimates were used to determine expected surface void dimension and direction. Void direction was decided to be vertical to the subsurface shafts (4). Because of the uniform properties in the subsurface strata soils, this was a logical assumption. The next was void size. Without local geologist knowledge to further inform the design, a 15 ft. void dimension was assumed.

A test section was created to evaluate the thin cover plane method (5).

Void bridging tests

A 9–10ft trench was created. The width and depth were enough for front-end loader tires. The tires would provide a platform for the construction of the reinforced platform. Chains were attached to the tires to pull them out after the section was fully constructed. Slick cardboard was placed on top of the tires to help with removal and a lightweight nonwoven fabric was placed below the reinforcement grid to hold the cover materials in place. The removal of the tires after construction did not adversely disturb the reinforcement and cover materials.

A front-end loader with a weight of 43 kips was then used to test the section and design. The design variables are shown in Table 1.

Initial loading with the 43 kips loader was done slowly, but once it was clear the test section had exceeded expectations, multiple passes and stops were implemented (6). The measured deformations due to test loads were smaller than expected because of implemented factors of safety.

The test then added more load and passes as well as greater analysis of the impact of distance of load from the void on the design. The loader bucket was filled and the loader was moved closer to the edge of the reinforcement. The section still did not fail and deformed within designed specifications `s(7)~.

Conclusion

The existence or occurrence of geologic or environmental subsidence or voids can happen at mining and construction sites. Using specialty geosynthetics can allow for increased safety and site access to otherwise difficult or impossible areas by creating a temporary bridging effect to allow for the safe removal of workers and machinery or a permanent bridge for structures constructed over areas of possible subsidence.

The design of these bridging systems can also account for the allowable depression depth and the allowable depression depth over an interval of time that will allow for other specialized applications such as utility and trench spanning. By analyzing project requirements and their relation to reinforcement capabilities, designers can solve a variety of specialized void applications.

References

Alexiew, D. (2008), “Reactivation of a geogrid-bridged sinkhole: A real life solution approval,” New Horizons in Earth Reinforcement, Otani, Miyata & Mukunoki, Taylor & Francis Group, London.

ASTM 6992–Standard Test Method for Accelerated Tensile Creep and Creep-Rupture of Geosynthetic Materials Based on Time-Temperature Superposition Using the Stepped Isothermal Method, American Society for Testing and Materials (ASTM), West Conshohocken, Pa., USA.

Blivet J.C., Khay M., Villard P., and Gourc J.P. (2000), “Experiment and design of geosynthetic reinforcement to prevent localized sinkholes,” Conference Proceedings, International Conference on Geotechnical and Geological Engineering, Melbourne, Australia, pp. 1–6.

Brian Baillie, P.E., is engineering/sales manager at Huesker Inc. He is based in Austin, Texas.