A lone drilling rig hammers away on a muddy, puddle-laden suburban-Philadelphia jobsite on an unseasonably cold April morning. Future residents of the five-story, 248-unit Skye 750 apartment complex would someday shuttle back and forth to various amenities in this bustling community. However, no structure could rise from the ground until Compaction Grouting Services Inc. (CGS) performed its namesake service to tame poor subsurface conditions.

“This whole town is basically a big sinkhole,” quips Supervisor Steve Oxendine, referring to King of Prussia, Pennsylvania. The region is underlain by dolomite, a carbonate rock that’s particularly vulnerable to erosion by water collecting underground.

Luckily, the contracting firm has 20 years of experience correcting precarious subsurface conditions in Delaware, Maryland, New Jersey, and Pennsylvania. In addition to compaction grouting, the Media, Pennsylvania-based company specializes in micropiles (minipiles), sinkhole remediation, soil nail walls and shotcrete, and slabjacking. CGS was retained by Earth Engineering Inc. (EEI) and worked as a subcontractor to LECESSE Construction.

Why Compaction Grouting?

Oxendine’s learned that addressing subsurface issues during pre-construction is far more cost-effective than doing so post-construction.

EEI confirmed the site was prone to sinkholes, and thus a candidate for compaction grouting, via test bores in 2004 and 2016. A cost-effective alternative to deep foundation systems and underpinning methods, compaction grouting strengthens soil to withstand higher bearing pressures. Low-slump, low-mobility soil-cement is injected into the ground at high pressures to compact loose, coarse-grained soils; densify and/or displace soft, fine-grained soils; and fill voids. A grid-like pattern predetermined by a geotechnical engineer is positioned across the jobsite and grout injected into evenly spaced holes drilled within the pattern.

The apartment building complex will have a 65,000-square-foot footprint founded on spread footings with a partially underground parking garage. CGS drilled 18,061 linear feet at more than 626 holes and pumped in 820 cubic yards of grout. Some areas of the footing will rest directly on bedrock. Rock down to 6 feet was dug out and backfilled with lean concrete. CGS helped prevent excessive excavation by test-drilling in zones where it was suspected that more than 8 feet of digging would be required because of a rock drop-off. In those cases, compaction grouting was a better choice.

Determining the location of additional holes was a fluid process that involved ongoing test drilling and coordination with EEI. “It wasn't hard set,” says CGS Senior Project Manager Justin Terry. “We were working closely with and feeding the engineers a lot of drilling information to help determine if additional areas needed to be grouted.”

Navigating Crowded, Muddy Site Conditions

The nature of compaction grouting means CGS crews are accustomed to muddy jobsites, but an unseasonably cold start to spring with multiple Nor’easters turned this one into a mud pit. Crews were also working in areas of low relief, so they had to deal with most of the site's surface water runoff from rain, snow, and a tire wash.

Vice President Mike Miluski, a professional engineer who teaches a grouting fundamentals short course at the University of Texas at Austin every year, uses “lake,” “pond,” and “swamp” to describe the jobsite. He vividly remembers one of the firm’s trucks halfway immersed in one such body of water.

CGS used dewatering equipment to dry out the puddles and air compressors to blow off the snow. “We always deal with mud,” Oxendine says. “You never have ideal sites with compaction grouting.”

A large excavator dug a trench in one corner of the jobsite, an idling articulated hauler had its tires blasted with a pressure washer, and oversized truckloads with corrugated pipe rumbled down the road as crew members made their way back to the drill rig. Foundation work couldn’t start until CGS finished, but the jobsite was crowded with other trades trying to get a head start where they could.

A Multitude of Tightly Spaced Holes

The site was filled with more than people and equipment, though. The sheer number of holes in such a small space meant CGS crews wouldn’t be able to address every other hole. No hole within 8 feet could be touched within 12 hours, so crews had to grout every third hole. Most of the grout ports were clustered on the jobsite’s southern and northeastern quadrants.

Oxendine, who supervised the project from the front lines, says this was the job’s greatest challenge.

“The paperwork was daunting,” he recalls, noting it was critical to crosscheck the plans and survey stakes to stay organized. Keeping the stakes in place and undamaged was also difficult as the various trades navigated the obstacle course on foot and in machinery.

Drilling into Bedrock and Casing Holes

CGS subcontracted the lion’s share of drilling to Brubacher Excavating of East Earl, Pennsylvania, which supplied an Atlas Copco ROC D7 drilling rig with operator.

The first step was to drill 3.5-inch-diameter holes. The next step was to drive in 2-inch (inside diameter) temporary casing with a bullet point on the bottom. The tip was then intentionally knocked out by yanking a rope from ground level to avoid clogging the hose with grout. It remains in the ground indefinitely. The casing was then pulled out, cleaned, and reused repeatedly.

The original specification required CGS to drill 3 feet into bedrock, which ranged from 15 feet to 60 feet below the surface. Drilling was cut off at 50 feet if no rock was encountered. Excavation on one corner of the site turned up 5-to-6-foot boulders, some of which jutted out from stockpiles and embankments around the jobsite. CGS had to bore an extra 2 feet in this zone to ensure the drill was hitting bedrock.

Mixing and Pumping Grout

“It's tightening up the soils as we pump,” Oxendine hollers over the machine gun-like hammering of a drill. “The pressure surge (maximum injection pressure) on this job is 500 psi. That's a lot of pressure pumping all that grout down in there and compacting that soil.”

The site contained a mix of dense clay soils and softer karst conditions. A normal soil profile usually increases in strength with depth; but with karst conditions, the dense clays are up top and the weaker soils are just above bedrock because of water flowing above the rock formation.

There was no surefire way to determine exactly how much material a hole would take. The variables include depth to bedrock, soil conditions, and groundwater level. The refusal criteria required CGS to pump grout ranging from 54 cubic feet per foot at 50 psi or below, 27 cubic feet per foot between 50 psi and 150 psi, and 5 cubic feet per foot from 150 psi to 500 psi.

CGS crews pumped the grout in from 100 psi to 500 psi, although the average was 300. Locations that encountered unvarying terrain required about 2 cubic feet per foot, but challenging locations took 5 cubic feet on average.

“The higher psi tells you the soils are pretty good or you’re in bedrock,” Miluski explains, adding that a significant number of holes took less grout because the rock was higher than anticipated. “The soil has been relatively good and that helped our schedule.”

CGS generally pumped 20 cubic yards to 50 cubic yards per day using two to three crews. The grout was produced using a mobile mixer with sand and cement in the hopper and a 500-gallon water tank hooked up to a Putzmeister pump specifically designed for high-pressure grouting.

“We averaged 15 holes to 20 holes a day,” Oxendine says. “At one point, because some holes were 50 feet to 60 feet deep, we had three pumps and two mobile mixing trucks going at the same time.”

The mix of water, cement, and sand is crucial. The specification called for a relatively standard 300-psi compressive strength and 8% cement to 10% Type 1 portland cement, but mix gradation is what counts. The gradation of the aggregate in the grout has a great influence on its effectiveness. CGS used well-graded aggregate with sizes ranging from ½-inch to a maximum of 30% passing the # 200 sieve. #8 crushed stone (3/8- to 1/2-inch) was added to keep the grout from seeping into fissures.

“The more cement you have, the more it travels in the ground,” Miluski says. “You want the grout to be like a growing mass.”

Bidding Lump Sum Mitigates Risk

“This isn't a unit job,” Miluski says, looking out over the jobsite. “There's serious risk involved in giving a hard number.”

Skye 750 was a unit price project until CGS offered to bid it lump sum, as the company does on many projects. This helped mitigate risk and instill confidence for the general contractor, LECESSE Construction.

CGS Senior Project Manager Justin Terry agrees. Other subcontractors could have done the work, but he doubts they understand compaction grouting well enough to bid the project lump sum.

“It's a crapshoot,” he says. “How deep are we going to have to drill? How much grout are the holes going to take? It's scary for owners not familiar with the process to think they have the exposure of a possible $100,000 swing.”

It’s not uncommon to have 10% to 20% swings when comparing estimated drilling and grout volume quantities to actual quantities. Spread over a large magnitude (600-plus grout locations), the associated additions and deductions associated with unit costs can kill a project budget or sink a contractor. By providing a lump sum, CGS bet on its knowledge, experience, and production to eliminate the owner’s risk.

Finishing Ahead of Schedule

Project delays up front put CGS on the jobsite in February at the tail end of a cold winter that would bleed into a chilly spring. The crew geared up, working through snow, rain, and chilly temperatures amidst knee-deep puddles, mud, and the occasional snow mound.

In addition to schedule adjustments, tight working quarters, and excessively muddy conditions, a sinkhole opened where foundations were founded on rock in an area where grouting hadn’t been performed. Instead of waiting for a cement delivery, CGS immediately adjusted the mix to pump in high-mobility grout from its mobile mixers.

The pressure stayed on as the contractor that would be building the footers bore down on the CGS crews. Despite the challenges, CGS delivered the job a day earlier than expected. This was meaningful on a 40-day project with the aforementioned variables. CGS took on 13 additional holes at the engineer’s request and still completed the job on time.

“It's a big job, but we try to do it as fast as we can,” Oxendine says of the 12 workers that make up his crew. “We work long hours and we get it done.”