Compressor station gravel pad on soft subgrade
By Sanat Pokharel, Timothy Yii, Marc Breault and Meisam Norouzi
Infrastructure development in natural resources rich Alberta, Canada, often encounters very soft ground conditions that pose several construction and sustainability challenges to transportation infrastructure, such as roads, rails and pipelines. As the conventional construction practices may not always prove sustainable, developers look for cost-effective and environmentally friendly innovative options.
In this region, it is important to minimize the use of scarce and expensive virgin aggregates in construction. Situated in a cold northern climate, this region faces short construction periods and frozen soil for considerable time; outdoor construction activities come to a halt during winter. This makes the tight project time lines difficult to manage, and both cost and time overruns can happen.
The conventional practice in this region for unpaved roads and load-bearing soil structures, such as compressor station pads, is to remove the existing soft soil and backfill with competent imported fill. For good-quality gravel, haul distance can be as long as 62 miles (100 km). In a similar setting the authors were approached to recommend a cost-effective, innovative design for a compressor station pad near Otter Lake in Northern Sunrise County of Alberta. The pad, intended to support heavy operating traffic and a crane, was conventionally designed to remove the soft soil and build a gravel structure with multiple layers of planar geosynthetics reinforcement. An adjacent pad that was recently built using the conventional design had met with several problems during construction. This experience indicated that building the same type of structure on this pad was going to be expensive and the construction schedule would be overrun. Since the work had to be completed in time for project commissioning, it did not seem practical to use a conventional design. In addition, winter construction would be necessary to meet the tight construction schedule. In lieu of the conventional design, a hybrid geogrid and high-strength novel-polymeric alloy (NPA) geocell-reinforced gravel structure was designed that saved both a huge amount of crushed gravel and cost and construction time.
This article discusses hybrid geosynthetics reinforcement design to support a heavy-loaded gravel pad for the compressor station and compares it with the conventional design, highlighting the challenges faced during the construction and the performance of the structure.
Geosynthetic reinforcement for load support
Planar geosynthetics have been used as soil reinforcement for many years; three-dimensional geocells are comparatively new. Geocell reinforcement usually utilizes geotextile for separation. Geogrid improves the stiffness of the reinforced soil by interlocking, lateral restraint and tension membrane; it reduces the applied stress on the soft soil and increases the bearing capacity while decreasing settlement (Qian et al. 2013). The performance of geogrid depends on aperture size and shape, material stiffness at junctions, and shape and stiffness of ribs (Giroud and Han 2016). Ever since the U.S. Army Corps of Engineers used geocell for reinforcing beach sand in the 1970s (Webster 1979), numerous research programs, experiments and monitored applications have been carried out to further understand the geocell-reinforcement mechanisms (Han et al. 2013). Vertical and lateral confinement, wider stress distribution and beam/slab effect are identified as the main reinforcement mechanism of geocell. Higher tensile stiffness, strength and creep resistance of geocell material provide the reinforced base with improved bearing capacity, higher modulus and extended design life (Pokharel et al. 2010, Thakur et al. 2013 and Kief et al. 2015). High-strength NPA geocell reinforcement also improves the creep resistance of the reinforced structure, which is a very important factor in the repetitive loading conditions, as it significantly reduces the initial deformation and rate of creep of the reinforced material (Thakur et al. 2013).
A design method using NPA geocell reinforcement for unpaved roads was developed by modifying the Giroud and Han (2004) method for planar reinforcement for rut criteria (Pokharel 2010). This method has already been employed to design unpaved roads in various projects and has been verified by Pokharel et al. (2015). Design using NPA geocells has been successfully employed in paved and unpaved roads and other geocell-reinforced, load-bearing earthen structures (Pokharel et al. 2013, 2015 and 2017, and Norouzi et al. 2017).
The authors have encountered several cases where a hybrid design with geogrid and geocells make the project structurally sound and economically feasible. Sitharam and Hegde (2013) recommended a high-strength geocell and geogrid hybrid design as an alternative to ground improvement in soft soil that could improve the bearing capacity of the foundation by four to five times. Kief (2015) reported reduction in maintenance cycles and cost, showing the efficacy of the hybrid geosynthetic solution by use of biaxial geogrid and NPA geocells.
The project and hybrid design
The compressor station had two pads adjacent to each other. Pad A was constructed earlier in the summer and the pad under consideration is Pad B. Pad A had faced several challenges due to soft soil at the surface. The construction of Pad B was planned for summer of 2016 but was delayed as the existing soil on-site was found to be of very poor strength. There were uneven and differential settlements under previously placed wooden mats and the existing silty clay soil was not suitable to support the expected loads. After removal of the mats, the equipment and trucks were stuck on the clay surface; the depressions right under the truck tires were as deep as 35.4 inches (900 mm). To avoid the difficulties faced earlier, the owner decided to look for an alternative solution for Pad B.
The footprint of Pad B under consideration was about 291,000 square feet (27,000 m2). Geotechnical investigation supplied by the owner had recommended 2 psi (15 kPa) as the characteristic undrained shear strength (Su) of the soft silty clay subgrade. The seasonal water table was found to be close to the existing subgrade in the southern side of the pad. The project needed solutions to support a 237-ton (215-tonne) crane and loads from trailers carrying 154-ton (140-tonne) loads, with individual axle loads not exceeding local highway limits. A 51-inch (1,300-mm) thick planar geosynthetic-reinforced granular structure was conventionally designed with three layers of nonwoven geotextile (grab tensile strength 160 pounds [712 N]) and triaxial geogrid to build the pad. This solution would have required 31.5 inches (800 mm) of existing soft soil excavation and 51 inches (1,300 mm) of reinforced crushed gravel fill. Once excavated, the material would need to be hauled off-site and replaced with imported material. All these activities would have contributed to a huge increase in the project cost and the extraction of a lot of virgin crushed gravel aggregate that would have made it impossible to meet the project deadline. Thus, an innovative design with a hybrid geosynthetics solution was presented that not only was sustainable in terms of cost and environmental indicators but also would enable winter construction.
The alternative design that was proposed in lieu of the conventional design featured a layer of biaxial geogrid perched between two layers of high-strength NPA geocell reinforcement and a geotextile as a separation layer. The design was tested for safety against bearing capacity, rut criteria as mentioned in Pokharel (2010), and the hoop strength of the geocell wall. The bottom layer of NPA geocell-reinforced gravel served a dual purpose: first as a construction platform where construction equipment could operate and second as a structural base layer during operation.
A nonwoven geotextile was selected to act as a separation layer between the granular fill and the subgrade. A layer of NPA geocell reinforcement installed on top of the nonwoven geotextile and was filled with 11.8-inch (300-mm) thick, 3-inch (75 mm) maximum size pit run gravel. Above this layer 5.9-inch (150-mm) thick, well-graded, 1.6-inch (40 mm) maximum size crushed gravel was reinforced with a layer of biaxial geogrid (Figure 1). Finally, the top layer of NPA geocell was installed and filled with 11.8 inches (300 mm) of the same crushed gravel material. Figures 2a and 2b provide a sketch of the structure alongside the conventional design for comparison.
At the time of design, the existing ground was thought to be 19.7 inches (500 mm) below the final design grade, which would have required 9.8 inches (250 mm) of soft soil removal. After removal of wooden mats and debris at the time of construction in January 2017, the subgrade was found to be 29.5 inches (750 mm) below the final grade, so there was no need for further excavation and removal of the existing soft soil before installing the bottom layer of NPA geocell. It would have been ideal to have 27.6-inch (700-mm) thick gravel reinforced with two layers of NPA geocell and one layer of biaxial geogrid for the design, but since the subgrade was already at 29.5 inches (750 mm) below the finished grade, the structure was designed to exactly match the final grade. The 29.5 inches (750 mm) of structure was checked for all three design criteria. A factor of safety of 3 chosen against the bearing capacity of the subgrade of 16.5 psi (113.8 kPa), due to the presence of dynamic crane loads, was satisfied. A short stretch of access road for heavy traffic load entering the pad was designed with only two layers of NPA geocell reinforcement.
NPA geocell, biaxial geogrid, and woven and nonwoven geotextiles were used. The nonwoven geotextile had a grab tensile strength of 269.8 pounds (1,200 N) and the woven geotextile had a grab tensile strength of 179.9 pounds (800 N). Less expensive 3-inch (75-mm) minus pit run gravel was used as infill at the construction (bottom) layer and 1.6-inch (40-mm) minus crushed gravel was used for the upper structural layers.
Construction and performance of the pad
Construction of the gravel structure started in January 2017 and was completed in February that year. The nonwoven geotextile was first laid on top of the subgrade; the bottom layer NPA geocell was stretched above the geotextile and filled with pit run gravel. A single lift of 11.8 inches (300 mm) was allowed at the construction layer for the compaction, so as not to damage the subgrade and to create a safe layer for construction equipment. The gravel brought in was in a nonfrozen state with natural moisture about 3% less than the optimum moisture content. Freezing temperatures during construction introduce complications when the moisture within soil freezes. However, given the time line, a control volume/weight compaction method was used for the granular reinforced fill to the specified degree of compaction.
Figure 3 shows the stretched NPA geocell and nonwoven geotextile. The gravel was end-dumped and pushed by dozer into the geocell pockets. Any frozen soil in lumps and large chunks were deemed unsuitable for construction and were rejected. Figure 4 shows the top layer NPA geocell being stretched and filled with gravel. Pokharel (2010) emphasized the importance of compaction in geocell-reinforced structures, so the right degree of compaction was always the top priority in order to achieve the required strength. In subzero temperatures, the nuclear densometer compaction testing did not seem to give accurate results. Therefore, a controlled volume/weight method was employed to guarantee that the required degree of compaction was achieved. The pad was divided into separate blocks and for each block the required weight of the material for a specified degree of compaction was calculated, and compaction continued until the exact compacted thickness of the fill material was achieved. This method was utilized for each subsequent layer of gravel fill to good success and helped ensure that adequate compaction was achieved. The structure performed as expected when the crane was brought to the site during spring thaw, when soil conditions are typically expected to be in their poorest state.
The bottom layer improved the bearing capacity and served as the construction layer for the construction traffic, which otherwise would have needed layers of wooden mats to pass the truck traffic. After installation of the top structural area in some portions of the pad, a pile-driving rig was allowed to operate on the pad while installation was completed in other parts. During this time no significant rutting or differential settlement of the pad was observed. The site condition at the compressor station Pad B was working out great, and the operating 303-ton (275-tonne) crawler crane did not even make a small rut when moving around the site, so the hybrid geosynthetic design worked very well (personal communication with the contractor’s project manager, Cord Roberts, July 26, 2017). Figure 5 shows the condition of the pad on May 8, 2017, while lifting the compressor. After two freeze-thaw cycles and one and one half year of service on July 27, 2018, the pad and access road were both performing well without any maintenance requirements (personal communication with the contractor’s program manager, Mark Bonnell, July 27, 2018).
The design was successfully implemented to withstand the anticipated loads. Initial concerns had been brought up regarding the weak subgrade shear strength as well as the potential of subgrade and base course softening during the following spring after the pad was constructed. The design had anticipated that the mechanisms of cellular confinement and gravel interlocking would increase the stiffness of the entire structure and thereby reduce stress applied to the subgrade during spring thaw. The pad held firm under the applied loading, which validated the designed structure and construction methodology. The structure was basically designed for load support purposes, hence possible subgrade settlement was not examined; however, the designers had anticipated no appreciable differential settlement would occur at the pad under the design load.
A comparison of costs and quantities was conducted between the hybrid reinforcement design and the 51-inch (1,300-mm) thick three layers of planar geosynthetic-reinforced conventional design for a pad area of 291,000 square feet (27,000 m2). The conventional design would have required the removal of 21.7 inches (550 mm) of existing subgrade soil, which is equivalent to a total of 524,423 cubic feet (14,850 m3) of soft soil removal and hauling out. Additionally, it would have required 1,239,545 cubic feet (35,100 m3) of crushed gravel. The hybrid design, however, completely avoided the excavation; the crushed gravel required was only 429,073 cubic feet (12,150 m3) and 286,049 cubic feet (8,100 m3) of less expensive pit run gravel. The average haul distance for the gravel was 37 miles (60 km). The hybrid design, therefore, reduced the total gravel quantity by 524,423 cubic feet (14,850 m3) and haul by 19.54 million miles-cubic feet (891,000 km-m3).
The cost analysis showed the following results from using the hybrid reinforcement structure: 22.8% savings in initial project cost, 100% savings in excavation, 42.3% savings in granular fill volume, and 45.7% savings in granular fill costs. The reduction in the volume of aggregate contributed to environmental benefits; the reduced number of trips to haul material and construction duration all contributed toward lower CO2 emissions, as suggested by Norouzi et al. (2017).
A hybrid geosynthetic structure was designed to support a 237-ton (215-tonne) crane load and regular and construction traffic on a compressor station pad. The design was based on mechanisms of combination of planar geosynthetic (geogrid and geotextile) and three-dimensional NPA geocell reinforcement to improve the modulus of the structure, create a slab effect, and distribute the loads to a wider area to meet bearing capacity requirements of the subgrade and rut criteria at the surface for moving wheel traffic. The hybrid design eliminated the need to excavate existing soft subgrade soil, reduced the thickness of the structure by 42.3% and enabled the construction activities to be completed in freezing conditions. Overall, this design saved 22.8% of the project costs and potentially reduced maintenance cost during the pad operation. The structure performed well under the design loads and fulfilled its design requirements. In conclusion, this design paved the path and directed a future course for combining different geosynthetic material for the design and construction of a heavily loaded gravel pad on very soft subgrade soil in a sustainable way.
Sanat Pokharel, Ph.D., P.Eng., is principal engineer at Stratum Logics Inc. in Acheson, Alb., Canada.
Timothy Yii, P.Eng., is project engineer at Stratum Logics Inc. in Acheson, Alb., Canada.
Meisam Norouzi, MSc., P.Eng., PMP, is project engineer at Stantec Consulting Ltd. in Vancouver, B.C., Canada.
Marc Breault is president of Paradox Access Solutions Inc. in Acheson, Alb., Canada.
The authors would like to thank Anthony Ramos from TransCanada Pipeline Ltd. and Mark Bonnell and Cord Roberts from Strike Group for feedback on the project performance.
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Compressor station gravel pad on soft subgrade
LOCATION: North Sunrise County, Alb., Canada
GENERAL CONTRACTOR: Strike Group Limited Partnership
SUBCONTRACTOR: Paradox Access Solutions Inc.
DESIGN ENGINEER: Stratum Logics Inc. (Sanat/Pokharel/Timothy Yii)
GEOSYNTHETICS PRODUCTS: NPA Geocell, Biaxial Geogrid, Woven and Nonwoven Geotextile
GEOSYNTHETICS MANUFACTURER/SUPPLIER: PRS Mediterranean Ltd. (Geocell)/Geogrid and Geotextiles Supplied by Paradox Access Solutions Inc.