* Photo supplied. Changing times: Waterproofing below sea level looks set to become more popular in Bermuda as design teams make optimal use of the limited available land. The BUEI’s Wendy Tucker is pictured.
* Photo supplied. Changing times: Waterproofing below sea level looks set to become more popular in Bermuda as design teams make optimal use of the limited available land. The BUEI’s Wendy Tucker is pictured.
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As we continue to expand our built world, the financial equation that is addressed by owners and developers will seek to maximize the output of the project. One of the variables within the control of the investor and their design team is optimal use of the available land.

This can mean pushing the limits of our planning regulations on building height, as we are currently seeing in Bermuda, with the high-rise concept becoming more prevalent. What many people don't see is the simultaneous trend to build downwards. While most concerned environmentalists will find this trend more acceptable in terms of visual impact of development, it does introduce a different set of constructability issues than simply adding another floor on top of a structure.

Ground condition

The primary issue is ground condition, or the ability of the surrounding ground to support the desired structure. The makeup of the ground can be too soft from the existence of too much clay, soil or water, to name but a few. Most often in Bermuda, we encounter water from the water lens or from saturated soft terrain near the coastline.

In a commercial development, the element that extends furthest into the ground is usually the elevator shaft. As most people will appreciate, the existence of water in this structure could be problematic to the effective operation and longevity of the lifting equipment. More than a simple nuisance leak, this can have serious safety ramifications.

The design will be cognizant of this, so most shafts are built above the water table. There are instances, such as along Front Street and low-lying areas in proximity to Mills Creek, where the existence of water is unavoidable.

Specialist contractors such as Kaissa Limited are knowledgeable in providing waterproofing solutions for these critical elements. Projects such as Butterfield Bank's Traders Gate addition in the mid-1990's, BUEI, and Masters Limited at Bull's Head have employed a variety of different systems and approaches to construction.

At Traders Gate, the contractor utilized a double layer of concrete with a High Density Polyethylene (HDPE) liner sandwiched between. Once the excavation achieved the desired level of depth, the surrounding earth was shored up with timber. Pumps were placed in the excavation to maintain a minimal amount of water in the pit. Concrete was then pumped into the base of the pit.

If enough water is present, this is done through a tube extended to the base of the excavation that places the concrete below the water level without its dissipating in the volume of water. The concrete displaces the water, which is pumped out. The concrete is then allowed to cure.

Traders Gate

In the Traders Gate example, a layer of HDPE liner was laid continuously over the new concrete slab and up the walls of the excavated pit. This later tied into a continuous liner under the entire foundation slab of the development. Another layer of concrete was then placed over the liner in the pit, which eventually supported the block wall structure of the shaft.

At BUEI, the same contractor took a slightly different approach. A hollow concrete cube without a top was pre-cast on another part of the jobsite. Once fully cured, the cube was raised off the ground to permit the entire exterior to be sprayed with a rubber waterproofing membrane. The cube was then lowered into the excavated pit to form the base of the elevator shaft. The cube walls were designed high enough to be above calculated storm surges and tidal heights. Again, the foundation slabs were tied in to the shaft base, safely above water level.

It is critical for these installations to be executed with some precision to assure success.

Outside of structure

Waterproofing is always going to have the best chance of success when applied to the outside of the structure, and these areas will no longer be accessible once the construction of the building above is past a certain point.

Masters Limited presented a different issue, that of a failed, or inadequately designed waterproofing system, again in the base of an elevator shaft. Water was leaking through the block walls of the shaft, especially at times of seasonably highest tides and storm surges. The only way to treat this problem was by using a cementitious material that cured chemically rather than by evaporation. This enabled the contractor to plug the block joints that were enabling the ingress of water.

The work was done at low tide when the flow of water was at its lowest, thereby reducing the pressure against the new material while it cured. It was a slow process, executed with tremendous patience. A compatible membrane was then applied over the entire surface of the interior shaft walls up to two feet above the evident level of water ingress. The cementitious nature of the coating provided the best possible adhesion with the concrete block in order to resist delamination due to any residual water pressure.

Building down and its occasionally attendant necessity to waterproof below sea level may become more prevalent as we continue to expand our built world.

Want to know more? For further information on below tide level waterproofing, call Kaissa Limited at 295-4242.