Dominion Greensville County Power Station
Once complete, the Greensville County Power Station will be the largest and most efficient natural gas-fueled power station in Virginia. The power station, situated on a 1,143-acre site, will produce enough electricity to power 400,000 homes in Virginia at peak demand. In its first year of operation, it is expected to provide up to $8 million in property taxes for Greensville County and add 166 jobs.
Image: Courtesy of Dominion Virginia Power (Dominion)
Subsurface conditions consisted of soft Piedmont soils, elastic silts with sand and lean clays with sand, overlying weather rock (N>50 bpf). The Turbine and Steam Generators were supported on large mat slabs parallel to each other. The loading on top of the mats was a function of the equipment loads and varied from as much as 4 ksf to less than 1 ksf.
Overlapping stress influence also posed a problem with the loading on the large mats imparting stresses in the soils below adjacent mats which could contribute to mat settlement. Soil conditions under mats varied from weathered rock to over 35 feet of soft to stiff residual soils overlying the weathered rock. Settlement tolerances for the structures supporting power production were limited to 1-inch total settlement and 0.20 inches differential after connection of piping and duct work.
Overlapping Stresses from adjacent mat loadings contributed to the complexity of the project
In order minimize total and differential settlement caused by overlapping stresses and partial mat support on weathered rock a Geopier® Rigid Inclusion was used that transferred stresses to weathered rock layer below. This eliminated the effects of stress influence from adjacent mats and better matched the settlement performance of mats partially supported on weathered rock. Spacing of Rigid Inclusions was governed by mat loading conditions and limiting top of pier stresses based on the structural capacity, and minimizing the stress on the mat foundation while meeting the settlement tolerances for the project.
Rigid Inclusion Selection
Since drill holes would stay open, spoils were not and issues on this fill site, and groundwater was relatively deep, Cement Treated Aggregate (CTA) was used in the Geopier elements to create a high capacity Rigid Inclusion. Rigid Inclusion Elements were tested to over 95 ksf (approximately 300 kips). The actual compression at the top of pier design stress of 48 ksf was less than 0.1 inches.
To be able to install the CTA Geopier elements to depths of up to 30 feet beneath mat slabs and over 46 feet below demineralized water and service water tanks the Geopier X1® system was utilized.
The benefit of the X1 system was that a continuous feed of CTA materials could be made into the hole without removing the Geopier tamper. The tamper consists of an open-hole chained-mandrel powered by a high-capacity vibratory pile driving hammer and over 30,000 pounds of machine crowd force. The result is a very dense pier that can be installed to depths of 55+ feet quickly.