One-Thousand Year Storm Begets Epic Battle between Man and Muck on Hotel Bennett
Noah—so the biblical account tells us—was commanded to build an ark in preparation for catastrophic floodwaters. As tough as his task sounds, Noah might have had an even more difficult time saving an active construction site from torrential rains than shepherding two of each species onto a mammalian cruise ship.
Don’t buy the argument? Just ask our project team on the Hotel Bennett in Charleston, South Carolina. In October 2015, an unusual confluence of weather events culminated in what the National Oceanic and Atmospheric Administration dubbed a “1,000 year rain event.” For much of the state, and especially in the Lowcountry, the storm did indeed reach biblical proportions, with numerous rivers bursting their banks, washing away roads, bridges, vehicles and homes in the process. Charleston’s historic peninsula district, where residents could be observed kayaking through the streets just blocks from our jobsite, received a record 11.5 inches of rain in twenty-four hours alone. By the storm’s end, accumulations reached over 16 inches in downtown Charleston.
Although there is never an opportune time for a contractor to prepare for inclement weather—much less a natural disaster—the deluge couldn’t have hit at a more inauspicious juncture of the project’s schedule. A mere six months into construction, the team had begun preparations to lay the 208,000-square-foot boutique hotel’s foundation and was poised to pour the crane pad. The same rains that stalled cars saturated not just every square foot but every square inch of the 22,000-square-foot excavated site, setting up an epic battle between man and muck that would rage for several months.
The Calm before the Storm
Before the storm hit, the project team had driven 364 62-foot concrete piles punched 18 feet below grade throughout the site. At the perimeter, the site was lined with 40-foot corrugated sheet piles, essentially creating a box or retaining wall on top of which the team poured a concrete beam that would eventually serve as a tie-in for the first floor slab. Excavation work had also begun for the hotel’s below grade basement, which is the first of its kind not only for project developer, Bennett Hospitality, but also for the City of Charleston. With an elevation of a mere 20 feet above sea level, Charleston, like most coastal areas, has a high and fluctuating water table that deters below grade construction activities. The team had excavated down to 17.5 feet below grade, but this process exposed about 18 feet of sheet piles. Due to active lateral earth pressure, the project team—at the recommendation of the structural engineer—left four 45°dirt wedges inside the site to support the sheet piles. The plan was to leave these wedges intact until the center portion of the slab-on-grade was complete and ready for the team to install large, steel braces from the center slab to the cap beam to support the walls.
Parting the Waters
At least that was the plan…until the skies opened, dumping a hard, humid rain over Charleston that not only washed away the wedges but also filled the site with approximately six feet of murky, green water, virtually erasing weeks of work in the process. The water didn’t just halt the project’s critical path activities; had hydrostatic pressure triggered a sufficient shift in the sheet piles, instigating a structural failure, any of the four surrounding city streets could have collapsed. Swallowed cars, evacuated businesses, injured pedestrians—all were potential tragedies had the project team not quickly developed a strategic plan to 1) remove the water and 2) reconstruct the dirt wedges as quickly as possible.
In order to drain the precarious pool inside the hole, the project team was keenly aware that they had to simultaneously account for the pressure of the water originating from outside it. Because the storm had resulted in high tides, the city’s water table had risen approximately nine feet around the project site. As large diesel pumps churned furiously, siphoning the morass of moisture, the project team monitored all exterior well points to maintain a careful equilibrium of hydrostatic pressure forces. Throughout the initial dewatering phase, the project team also surveyed the tops of the concrete beam every two days to ensure the continued integrity of the sheet piles.
Through a committed and concerted effort on the part of both Balfour Beatty and project subcontractors, the team was able to return to the pre-storm schedule phase in 20 days. But the process of removing the stagnant water wasn’t nearly as simple as it sounds. The storm didn’t merely negate previously completed work; it also created a mountain of new problems—foremost of which being a sludge-like material formed from rainwater mixing with clay—that the team would have to tackle simultaneously along with contractual schedule activities in the months ahead. At an added cost of $125,000, this earth work wasn’t exactly small in scope.
Come Hell or High Water
The project team less-than-affectionately termed their subterranean enemy “gumbo” due to its thick, glue-like properties. After installing well points inside the excavated area, the team quickly discerned that the water problem in the sediment was going to require a more tactical solution. Much to their chagrin, each well point was only pulling water from a meager one-foot diameter around it.
If all politics is local, so the saying goes, dewatering a project site post-storm might have its own version of that maxim. Knowing that the only way to gain a competitive advantage over the water was to attack it in segments, the team launched a localized counter attack with a de facto assembly line of pipes. In approximately nine segments of the site, below the ground they were forming on, the team installed and buried corrugated pipes within multiple layers of sand and gravel. The corrugated pipes engaged in a continuous process of removing water even as the team built footings and dug the trenches for plumbing atop and around them. Water from the corrugated pipes was transported to a larger, 24-inch whistle pipe, also buried vertically onsite, and from the whistle pipe ultimately to street level where it was filtered back into the stormwater system. This wasn’t a data center project, but our need for redundancy might as well have rendered it one. To prevent any dewatering downtime, the team connected the pumps to a main generator, which was also supported by a backup generator.
The project team had a final ace up their sleeves when it came to absorbing the stubborn water: bentonite waterproofing. Bentonite, a type of clay known for its ability to bind, seal and thicken, is capable of absorbing seven to 10 times its own weight in water. When it comes into contact with moisture, bentonite expands up to 18 times its dry volume, thereby forming an impermeable barrier under the slab. The project team applied bentonite sheets pervasively and in excess of traditional building practices, primarily due to the fact that the owner had invested in a 10-year unlimited warranty for the building’s waterproofing. This solution helped the team meet both a typical inspection and a second, third-party inspection of the concrete seams.
“It seemed like we were scheduling waterproofing activities down to the second,” recalls Tim Spano, senior project manager. “It was very stressful at times, but it was also fun and interesting, both for us and our subcontractor partners. I’m willing to bet that Charleston will start seeing a lot more below grade parking decks in years to come not only because lack of street parking is a big problem here but also because the local industry has observed our success.”
Indeed, Balfour Beatty’s approach to The Hotel at Marion Square’s foundation and the floodwaters that threatened to derail it has not gone unnoticed in industry circles. Recently, the project team facilitated a tour for the Pile Driving Contractors Association. Visitors were as intrigued as they were impressed to hear Tim’s tale of round-the-clock pumping and out-of-the-box thinking.
Natural disasters have an uncanny way of bringing out the very best in the human spirit. Values like faith, friendship, community and connection inevitably rise to the surface when the chips are down. Though a flooded construction site in no way compares to the calamity of homes turned piles of rubble, this project team, just like the City of Charleston, emerged stronger for the struggle. When Hotel Bennett opens for business, only those who built it will know the true depth of brotherhood that was on display.