If the wet conditions are due to excess rain, you could just wait for the site to dry, but that can lead to additional costs and scheduling problems, especially if the equipment is needed elsewhere. If the problem comes from groundwater or drainage issues, waiting will not help.
There are limited solutions available. The traditional solution is overexcavation and replacement, but this is expensive, and there is often a lot of uncertainty about the quantities of material required. Because of this, contractors often turn to geosynthetics as a more cost-effective alternative. The geosynthetics most often used in site development applications are geotextiles and geogrids.
Geotextiles have often been used over soft soils, especially for construction site entrances, and can provide some benefits. The fabric prevents the loss of aggregate into the soil below, and, depending on the soil type, can also prevent soil particles from pumping up into the stone. This allows for the use of less stone than would be needed without the fabric.
In order for the fabric to provide additional support, it is necessary to place it into tension, like the surface of a trampoline. This is typically accomplished by placing the first lift of stone and then “rutting the fabric in”-running trucks or other equipment over it until ruts appear. The ruts are then filled in with the subsequent lifts of stone, and the layer is trafficked between each lift to further tension the fabric, until no new ruts appear. This solution typically saves some money compared with conventional methods, but it can still require a lot of stone. It can also be difficult to implement on sites with tough soil conditions, because geotextiles can’t provide support to bridge over soft, wet soils unless they can be placed into tension.
Geogrids work differently than geotextiles: Most geogrids are much stiffer than geotextiles and have larger openings (apertures) that interlock with the fill placed on them to create a stabilized composite layer. This allows you to quickly bridge over soft soils and to create a stable working surface with significantly less aggregate or other material.
A History of Success
Since their introduction in the 1960s, geosynthetics have proved to be versatile and cost-effective ground modification materials. Their use has expanded rapidly into nearly all areas of construction.
Geogrids form a distinct category of geosynthetics and can be divided into two general categories. Uniaxial geogrids, which are designed to carry high, constant loadings in the plane of the material, are used in mechanically stabilized earth designs for retaining walls and reinforced slopes. Stabilization geogrids, which are either biaxial or multiaxial, are used in applications where the loading is intermittent, and perpendicular to the plane of the material. Stabilization geogrids are stiff materials that distribute load over soft soil, like a snowshoe, and their apertures interlock with the fill material placed on them to create a stronger, more stable layer for a roadbed or other working surface.
Geogrids have a long and successful track record of stabilizing soft subgrades. On sites that can barely be walked on, workers can quickly roll out geogrid over the poor soil and push a layer of aggregate out onto the grid. The result is a firm, free-draining platform or a reliable access road. With geogrids, the challenges of bad sites and poor weather are greatly reduced.
On sites where subgrade stabilization is expected and included in the design, contractors can often increase the profitability of their projects by using geogrids to construct thinner fill layers than those originally designed using conventional approaches.
In addition to the stabilization of soft subgrades, geogrids can also be used to optimize flexible pavement designs. With the proper methodology, mechanical (geogrid) stabilization of the aggregate base layer of the pavement can be incorporated into the design to produce a more economical and longer lasting pavement. This can offer a value engineering opportunity on projects that include asphalt pavement.
Pavement optimization using geogrid can also make it easier to deal with some common challenges in road construction, such as fixed top grades, existing curbs, and shallow utilities. In each case, the use of geogrid in the pavement can allow for a thinner pavement section, which meets the design traffic requirements, minimizing the need for excavation or the relocation of existing utilities.
So whether the challenge is stabilizing soft soils or making flexible pavement sections more economical and efficient, the use of geogrids offers tremendous benefits to the contractor.
When equipment, production time, or valuable man-hours are at risk, geogrids can be the best investment for a project’s profitability. They are easy to install, use less aggregate fill, and require no skilled labor or specialized equipment. Geogrids help the crew get in, get out, and get the job done on time and on budget.
Different Solutions for Critical Results
When faced with weak or saturated soils on a construction site, contractors usually have a range of options for addressing the problem. Finding the best solution requires a thorough understanding of each available technology as well as the costs of all the materials (aggregate, select fill, geotextile, geogrid) that may be used. A common mistake is that of looking only at the unit cost of one component, such as geogrid, instead of looking at the total cost of each potential solution.
As an example, if the installed cost for aggregate is $20 per ton, then reducing the thickness of an aggregate layer by 1 inch provides a savings of $1 per square yard of area covered. So the use of a geogrid that costs $3 per square yard but reduces aggregate thickness by 8 inches would actually reduce the cost of the solution by $5 per square yard.
But if the installed cost of aggregate for the project is only $10 per ton, the same solution would provide only $1 per square yard in cost savings. It is critical that estimating personnel and project managers understand how each material functions and are aware that they have access to design and estimating tools to allow them to accurately determine the total cost. These tools include design curves, cost estimating charts, and software and should be based on accepted design methodologies and full-scale, in-ground performance testing. However, never rely on a design tool if the company furnishing it can’t explain how it was developed.
Achieving Separation With Geogrids
A common misconception regarding the use of geogrids in subgrade stabilization relates to the ability to maintain separation between fine-grained soils and aggregate or other granular fill. The large apertures of a geogrid may seem to make it impossible for the movement of clay or silt particles to be restricted. And, in fact, if you simply lay a sheet of geogrid over saturated clay and walk on it, the clay will indeed push through the geogrid apertures. But geogrid is not used by itself. Instead, it is used with granular fill placed on top and that makes all the difference.
While geotextiles act as a separation layer between the subgrade and the granular fill, geogrids work differently; they act as part of the granular fill and enhance its performance. This is a function of the properties of both the geogrid and the granular fill, as well as their interaction, which creates a filter that restricts the movement of the fine soil particles.
The stiff, stable apertures of the geogrid confine the granular fill particles and prevent their movement, while the size gradation of the granular fill determines the space between particles. When the gradation is selected properly, filtration is achieved and fine soil particles cannot contaminate the granular fill. If the desired ratios are achieved, then the geogrid and granular fill, working together, will act as a filter at the interface with the subgrade and the pumping of fine soil particles into the granular fill layer will not occur.
In contrast, geotextiles rely on the openings in the fabric itself to achieve separation. The Apparent Opening Size (AOS) of the fabric quantifies the size of the opening and, therefore, the size of the particles that can be filtered by the fabric. Both nonwoven and woven geotextiles can be used in this way to achieve separation of fine-grain soils from granular fill materials.
It is important for the designer to remember that the particle size distribution of the subgrade soil directly affects the effectiveness of separation in this manner. Extremely fine-grained soils in fully saturated conditions will not be effectively contained by a geotextile with an AOS that is too large, so the geotextile must be selected with this in mind.
Placing the geotextile over the soil and walking across it can illustrate this problem. As a load is applied on the geotextile, water from the subgrade will flow up through the fabric. If this water is “dirty” (i.e. contains soil particles) then separation is not being achieved. The geotextile is still preventing the downward loss of granular material under these circumstances, but it is not preventing pumping.
It is also critical to take into account the potential for “blinding” of the geotextile, which occurs when a layer of cohesive soil material builds up on the fabric and prevents water from passing through. If this occurs, it restricts drainage, causing the structure to weaken due to the presence of excess water.
Whatever methods or materials are used, separation is critical to the long-term serviceability of structures constructed over soft soils. The contamination of granular fill with silts and clays reduces its strength considerably, and the gradual loss of granular fill into the soft subgrade when a geosynthetic is not used has the same effect. Always consider the potential need for separation when dealing with weak, wet soils.
Geogrid Technology in Practice
During a recent project in Wisconsin, a 3,200-foot-long berm was constructed to prevent further accumulation of sediment in a local lake. Constructing the berm required the use of Tensar’s most advanced geogrid product, TriAx Geogrid. The unique, triangular/hexagonal design and rib profile of TriAx Geogrid forms the MSL by confining aggregate particles within its apertures to create mechanical interlock. Once mechanical interlock occurs, the stiffened aggregate layer is able to increase the load bearing capacity of the soil with 360 degrees of load distribution.
The geogrid was particularly applicable for the project because the site consisted of a subgrade of saturated silty lake sediments with very low shear strength. Using the geogrid to construct a mechanically stabilized layer (MSL) allowed for the creation of a stable construction platform for building the berm embankment.
The geogrid design proved to be a more viable alternative than the conventional construction method of overexcavation and removal of lake sediments, since the variable depth of the sediment made it difficult to predict both the feasibility and cost of removal. Additionally, removal of the sediments would have created further environmental impacts, with larger construction limits required for overexcavation and replacement of the lake sediments. The ability to use the geogrid solution over the existing sediments saved both time and costs that would have been incurred through further excavation.
Tensar’s TX160 Geogrid was chosen because it reduced construction costs and increased the feasibility of the overall design. The use of the geogrid improved the slope stability of the berm embankment, reduced overall and differential settlement, and provided a separation barrier between the lake sediments and the granular fill used in the berm construction. The durable berm will benefit the lake restoration and wetland ecosystem for years to come.
Less Aggregate Means More Profitability
Geogrid brings a number of benefits that include simplifying construction by reducing labor time and the amount of necessary equipment. But most importantly, the utilization of the product over soft soil can, in many instances, save customers up to 50% in construction costs by requiring less aggregate, as well as increasing the lifespan of the surface and enhancing the reliability of trafficked surfaces. Additional cost savings on such projects come from the reduction of labor and equipment requirements, as contractors do not have to overexcavate or haul additional aggregate from offsite.
When an oil-and-gas company needed an unyielding and stable platform over 9 miles of weak access road soils, it turned to geogrids for an affordable and environmentally sustainable solution that offered superior performance.
The unpaved road supported year-round traffic volumes of up to 1,000 heavy vehicle trips per day, which greatly exceeded the loading for which it was designed. The increase in traffic, significant snowfall, ice lensing, and soft subgrades were causing the road to deteriorate during the annual spring thaw cycle.
The project owner selected a geogrid option that incorporated one layer of geogrid as the best solution for stabilizing the roadway for public and commercial use. The system allowed the contractor to bridge saturated and crumbling soils by placing the geogrid directly on the existing grade.
“TriAx is our first solution for soft subgrade issues,” says Jeff Osburn, project manager for Glenn O. Hawbaker. “It covers areas that can barely support equipment. It’s phenomenal how well it works.”
The crew repaired the road crown where needed. The workers then covered all areas of the roadway with Tensar TX7 Geogrid and 16 to 30 inches of dense graded aggregate. Reducing the aggregate requirements saved the project owner an estimated $1.2 million in material and installation costs.
Reshaping Environmental Projects
Beyond their use for stabilization of soft soils, geosynthetic materials are also used in mechanically stabilized earth (MSE) projects such as retaining walls and reinforced slopes. These are the applications where uniaxial geogrids are applicable. MSE designs are based on the in-plane tensile strength and creep resistance of the geosynthetic. In contrast to the subgrade stabilization applications discussed above, MSE structures use the geosynthetic material as reinforcement of the soil mass behind the face of the wall or slope.
An additional consideration for these projects is the type of fill that is to be used in the reinforced soil mass. The geotechnical properties of the fill, such as unit weight, plasticity index, etc., must be accounted for in the MSE design. Beyond this, the type of geosynthetic chosen for reinforcement of the structure can allow for the use of fill material that is less expensive and more environmentally friendly.
The Briarcliff City Apartments sit high on a bluff in Kansas City, MO. Part of the bluff that supports the complex’s parking lot and swimming pool was eroding and a special retaining wall was needed to structurally support it.
The project design was geotechnically complex, and work crews also faced significant logistical issues. There was no access from the front and not much space in the back of the bluff area to store materials. Because of the lack of storage, workers had to bring materials in as needed. In addition to the logistical challenges, an unusual amount of rainfall resulted in erosion behind the wall during its construction.
“This was a challenging project on many levels,” said Grego Randy, the retaining wall installer. “In addition to logistical issues, the cut at the back of the reinforced zone was unstable and intermittent sliding had occurred.”
Mike Stein, P.E., was brought in as the project’s engineer and wall designer. He pointed out another dilemma; the upper portion and lower portion of the wall were on different limestone rock formations.
Stein also noted that the area had been a borrow-and-dump site, requiring removal of a mixture of shale materials and soil in the excavation area.
Originally, a wall incorporating a polyester geogrid had been specified. However, the environmentally conscious owner/developer wanted to use recycled concrete within the reinforced zone. This backfill material is not allowed in retaining walls utilizing polyester geogrids, because the concrete material has a very high pH, which attacks the polyester material. Steel reinforcement straps were not an option either, for the same reason.
The contractor made some inquiries and eventually specified a wall system that incorporated Tensar uniaxial geogrids made from high-density polyethylene, which have the capability to withstand the concrete’s high alkalinity. These uniaxial geogrids are resistant to physical deterioration and loss of strength caused by aggressive chemical environments, permitting the use of a wide range of backfills, including recycled concrete.
“This system was the right solution for the Briarcliff project because of its proven track record,” said Grego. “It provided us with the right materials to make sure the wall was not only structurally sound, but also attractive.”
Despite the significant challenges, the project was a resounding success. Once the retaining wall was installed and construction of the complex completed, the apartment community opened successfully on its new, stable foundation.
Assured Long-Term Success
The use of geogrids in construction has increased dramatically since their invention by Tensar in the 1970s, and the technology of both the materials and their application continues to improve, with new and more refined uses introduced on a regular basis. This technology can save significant time and money on many types of construction projects when properly designed.