by David Giles
Temporary dewatering and groundwater treatment systems have been installed for a highly anticipated mixed-use development in Reve, Boulder, Colorado. The Reve project is located at 30th and Pearl Street, across from the new Google complex. It has been four years in the making, and projected occupancy is January/February, 2021. This is Southern Land Company’s first development in the Boulder market. The Reve community will offer multiple lifestyle choices including micro, studio, efficiency, one-, two-, and three-bedroom town homes, and live/work residences. The plan contains 244 residences, 2276 m2 (24,500 sf) of retail space, 10,099 m2 (108,700 sf) of office space, and ample green space and bike paths.
With the dewatering system up and running, the excavation of both the north and south parcels has reached its target subgrade of 8 m (26 ft) below ground surface, despite having to perform the work throughout Boulder’s winter months. The estimated eight-month-long maintenance and operation phase of the dewatering continues and is expected to be complete by summer 2019.
Winter temperatures required constant monitoring of the deepwells. According to Southern Land project manager Jonathan Harel, a technician monitored the wells 24/7 either remotely with camera images every 15 minutes or onsite to be certain the low nighttime temperatures did not freeze the pipes and disable the pumps.
“Crews were very responsive to any situation that came up. Whenever a situation emerged, they were always willing to put in the time to resolve it, even late in the night,” said Harel.
Crews kept concrete blankets over the main that connected the wells and added heat trace to ensure nothing froze.
Why so deep?
As available lands become scarce, underground parking has become more and more appealing, especially in growing urban areas like Boulder. Additionally, city planners’ and developers’ desires to maintain views and not block sunlight further the advantages of underground parking over above-ground. For these reasons, Southern Land Company chose to build two levels of subterranean parking beneath the entire development.
Digging deeper, however, can be a difficult and costly task if improperly planned out. One of the most important considerations, especially for excavations extending below the groundwater table, is effective control and lowering of the groundwater prior to excavation. This process is known in the construction industry as dewatering.
The high water table, located just 3 m (10 ft) below ground surface, along with complex geological conditions at the Reve site, created special challenges for the dewatering firm, with important cost considerations for the developer.
“The site of Reve is situated over 1 to 3.3 m (2 to 11 ft) of man-placed fill, underlain by water-bearing, naturally occurring sand, gravel, cobbles, and even some boulders,” explained Bairn Leonard, the dewatering firm’s superintendent. “Making dewatering even more of a challenge, bedrock was encountered between 4 and 9 m (12 and 28 ft) below ground surface. With a planned excavation extending to approximately 8 m (26 ft) below ground surface, well below the top of bedrock, meant lowering the water table to the top of the confining layer. Committed to having two parking levels below ground, we proposed multiple possible dewatering methods for the developer to deliberate over. These were supported by the geotechnical engineering report conducted by Terracon, and the Groundwater Characterization Study by Ayres Associates.”