Digging to the Bottom of the San Antonio Botanical Gardens

One might think that the best thing for a garden is water, but in the case of the San Antonio Botanical Gardens water was causing damage. In the spring of 2007, Chamberlin Roofing and Waterproofing was charged with repairing and re-waterproofing the green roof at the Lucile Halsell Conservatory at the Botanical Gardens. It was a dirty job, but it had to be done. And Chamberlin was up to the task of uncovering and replacing the system beneath this green roof which doubled as a garden.

Technically defined, a green roof system involves a high quality waterproofing and root repellent system beneath soil and vegetation. Green roofs may be modular with drainage layers, filter fabric, growing media and plants already prepared in movable, interlocking grids, or, each component of the system may be installed separately. Green roof development involves the creation of "contained" green space on top of a structure. This green space could be below, at or above grade, but in all cases the plants are not planted in the "ground'’ per se, but rather above usable space that is protected with a waterproofing system. 

Upon arriving on site at the Conservatory, the first order of business would be to excavate the vegetation on the roof of the conservatory down to the existing membrane. This also proved to be the most challenging portion of the job.

“The most difficult and grueling part of this project was manually digging and moving the soil to expose the waterproofing,” Project Manager Justin Blair said. The depth of soil in large areas ranged from two feet to six feet. In many locations, Chamberlin had to spot excavate to a depth of 14 feet, which is more than twice as tall as the workers doing the digging.  

Excavation was done manually as the glass building structures within the garden were not able to support heavy equipment. As one might imagine, manual excavation can be an arduous process. But Chamberlin’s crews did a great job of dealing with the unfavorable conditions. Due to space constraints they literally removed soil one bucket at a time using a five-gallon bucket tied to a rope to physically lift the soil out of the small holes being made. Further, to protect the substrate most of this excavation was done without the use of a pick to loosen up the soil.   

Meanwhile, the garden stayed fully functional during the process. “At any given time, there could be a class of 2nd or 3rd graders on a field trip walking through the conservatory just below where we were doing repairs, so safety was a critical item,” Blair said. “We had to make sure that no material or debris would fall from where we were working down onto the visitors below.”

Crews were also challenged by summer rains. In the months of May, June and July rainfall totaled approximately 40 inches. Wet conditions slowed production as soil stability created safety concerns. Because the various excavation points were small in size, rain would literally fill the holes with water. The water had to be pumped out, and crews were forced to wait for the soil to dry so they wouldn’t sink into the mud.

“On the positive side, the rain was a good test for the waterproofing we had completed,” Blair commented.  

Once excavation was complete, Chamberlin crews were at home conducting more typical work processes. The ultimate scope of work included removing the existing roofing and waterproofing system and installing a new green roofing system. The new system featured waterproofing, soil, drainage, and vegetation.

Despite these challenges that could have significantly delayed the project completion, Superintendent, Sergio Lopez’s careful planning enabled Chamberlin to maintain the project schedule and keep safety top-of-mind. No accidents or injuries were reported.

“This unique project had many uncertainties due to the fact that almost everything we were to repair had been covered by dirt for 30 years. Some were predictable, but others were somewhat out of the ordinary,” Blair added. In all cases Chamberlin stayed true to form and devised workable solutions for the challenges at hand.