Geotechnical study recommends soil compaction for new prison site
The prior surfacing mining of the property where the proposed new Fayette County jail is to be located will require builders to take additional measures to prepare the ground surface before the foundation and walls are constructed.
The excavation and compaction work recommended by Geo-Mechanics of Elizabeth for the Dunbar Township project, is routine, because of the extensive coal mining that has taken place within the county and region over the past 150 years, said Dr. Javaid M. Alvi, company president.
“The recommendations are the most economical and feasible for the construction,” Alvi added.
Last year, a majority of commissioners agreed to replace the county’s current aging prison with a 115,000-square-foot, 480-bed correctional facility on property off of Mount Braddock to be purchased by the county.
Crabtree Rohrbaugh and Associates of Mechanicsburg and Sleighter Engineering Inc. of Uniontown have been hired by the county to provide architectural and engineering designs for the estimated $30 million project.
The county has yet to solicit bids for the construction.
Geo-Mechanics was hired earlier this year by Sleighter to conduct the geotechnical survey that would identify the subsurface materials in the proposed building, access road, parking lot and storm water management pond areas and offer recommendations as to what actions need to take place to ensure long-term stability.
The proposed construction site lies in an area that was extensively deep-mined, but the correctional facility is to be located “near the edge of the coal bearing area,” where a secondary mining operation took place in the early 1970s to remove the remaining Pittsburgh coal seam, according to the Geo-Mechanics study.
Alvi said that test borings done in the proposed construction area confirmed the strip mining and that it was backfilled with loose materials to a depth of about 70 feet, after the coal was mined.
For construction purposes, the top 10 feet of the backfilled material will be removed because of a potential problematic substance located within the upper layer, he added.
“(The material) contains potentially expansive pyritic, carbonaceous shale,” he said.
Pyrite, when interacting with water, can expand and cause foundation problems as was found to be the case at the former Uniontown Walmart site.
The analysis of the strata resulted in Alvi recommending that the deep dynamic compaction method be used by the county when preparing the site for construction.
The process includes the hoisting of a 10-ton metal drop hammer to a height of about 50 feet and then dropping the plate onto the area to be compacted.
“The (dropping of the weight to the ground) creates energy, collapses the material and it becomes densified,” said Alvi. “What you are doing is consolidating the existing looser material, so that it increases its density and strength and reduces the settlement.”
Afterwards, suitable materials will be brought in to complete the site preparation work.
Rob Sleighter, owner of the engineering firm, said that Geo-Mechanics and Alvi are well-respected in the field of geotechnology and was his first choice to conduct the geotechnical investigation that, when completed, would offer the best engineering solution for the 800-foot-long correctional facility.
“I wanted a company that was familiar with sites that had been previously backfilled with various materials where new construction was to take place,” said Sleighter, adding that Alvi offered several related circumstances during their initial conversation.
Geo-Mechanics was hired to provide geotechnical services as a Pittsburgh television station was preparing to construct a two-story building and satellite-dish farm in Pittsburgh.
The 40,000 square foot facility was to be built on property that was backfilled with stone and dirt and undesirable for construction.
“Extremely critical, was that the satellite dishes (when built) did not move,” said Sleighter, adding that the deep dynamic compaction was recommended by Alvi and employed by the contractor. “It has been nine years and they have not moved, not even a fraction.”
A chain stores in Wheeling, W.Va., were to be built on property with 200 feet of backfilled materials that were unsuitable and unable to sustain the large buildings. The engineering solution offered by Geo-Mechanics was the deep dynamic compaction, said Sleighter.
“There are many ways to address this type of problem, including the use of caissons and grade beams,” he said. “That would include drilling and pouring concrete in cylinders or shafts, with the structural grade beams carrying the load to the caissons.
“(The method) is very expensive.”
The dynamic compaction method is frequently used because it creates a suitable foundation area and can be done at a lesser cost for the builder, added Sleighter.
“(The method results) in the successful construction of a mat where the building will be located,” he said. “Afterwards, you can go to conventional construction.”
Sleighter estimated the size of the affected area to be about 2 acres of the total 62 acre property the county plans to purchase.
While other buildings, along with the historic Meason House, are in the general area, Sleighter said he is certain there will be no damage to the existing structures or water supplies.
Sleighter speculated that it will take two to three months for the excavation and compaction work to be completed, provided the weather is “cooperative.”
According to the county controller’s office, to date, the county has paid Sleighter a total of $28,019 for the geotechnical work.