Zero-Lot-Line Building Envelope Protection and Redundancy
Construction of new buildings often requires the building owner to build below grade space to accommodate for parking or additional building use. In high density areas, such as many major cities, this also means that builders try to maximize square footage by building directly on the property line: Densely populated areas simply lack the available real estate space for above-grade additions. As a result, building teams look to zero lot line (ZLL) below grade construction.
What is a ZLL wall?
A ZLL wall is primarily used to maximize the building area by allowing the builder to build directly up – or extremely close – to the property boundary. These walls must be designed to resist soil and water pressure for the life of the building, and serve as the last line of defense against unwanted water ingress. This means these walls must be extremely watertight.
In ZLL construction, the process occurs from the outside in: First, the shoring wall is constructed. A shoring wall is most commonly constructed using two methods, commonly known as “wood lagging” and “shotcrete shoring.” Wood lagging involves placing treated wood (2” x 12”) between steel piles. Shotcrete shoring is achieved by temporarily securing the earth with soil nails or tiebacks and then applying gunite mixture for reinforcement. Upon completion of the shoring wall, the blind side waterproofing membrane is applied, reinforcement steel is placed, and the structural wall is put in place using cast-in-place concrete or shotcrete.
Using Redundant Layers for Protection
Many waterproofing consultants believe that creating redundancy with multiple layers will increase the chances for success in zero lot line construction. Redundant waterproofing is also used to decrease the amount of stress on the overall system itself, and it unites two groups of waterproofing solutions. Historically, these materials fall under two categories: Passive and active. Passive, or inert systems seek to create a physical barrier between water and concrete. Some common examples include PVC, Modified Bitumen, or Plastic Sheeting with latent adhesive. Active, or reactive systems generally swell if they come into contact with water. They are most commonly made from bentonite clay. Both methods seek to achieve the same goal of protecting the building envelope, and while each can be effective separately, some manufacturers have decided to combine both active and passive methodologies. With this strategy, the waterproofing consultant can tailor the system to specific site conditions, allowing for more comprehensive protection.
Zero Lot Line Waterproofing Performance
Performance criteria for blindside ZLL waterproofing requires the product attributes, hermetic capabilities, adhesion to substrate (mechanical and/or chemical bonding), and intermittent hydrostatic pressure resistance. For a ZLL wall, assurance of performance can be tested by methods and means that include, but are not limited to:
● Continuous inspection by a certified third party.
● Capacitance testing by installing a copper line with current in vulnerable seams
● Injecting pre-formed panels to prescribed limits.
● Smoke testing.
The Waterproofing Consultant’s Job
Waterproofing consultants are hired to work closely with the owner, architect, and waterproofing manufacturer to provide the appropriate “elixir” as dictated by several site-specific factors; some of which include:
Shoring Methodology: The proximity of the adjacent property, site geology, and cost can be determining factors in selecting the appropriate shoring methodology.
Concrete Application: When constructing below grade foundation walls, concrete can be poured in place or it can be shot through a nozzle. These different approaches create efficiencies that help save time during the construction process, but can also adversely impact the waterproofing.
Hydrostatic Conditions: Sites with shallow groundwater or deep excavations require site drainage during the construction process and for the functional use of the building if local codes allow. However, in many high-density areas it is becoming cost-prohibitive to continue dewatering upon completion of construction.
Making it Work
Building on a zero lot line property also has its challenges as these areas are located in tight conditions with little or no room to provide adequate safety, production performance, and watertight systems. They also are generally located near or in the water table where most of our major cities were built.
To overcome these challenges, one should begin the waterproofing selection process by prioritizing material selection based on the structural wall type. Best practice would err on the side of redundancy, because redundancy provides a wider range of adaptability to the site-specific conditions. With budget concerns, redundancy may allow for cost savings for other scopes of work that interact with the waterproofing system. Finally, water leakage will always be a risk which design teams have to understand and try to mitigate. The prudent first step in a successful project is to retain a professional waterproofing consultancy to address the myriad of concerns, and ensure that the best, most effective decisions are made to protect each site’s unique conditions. Additionally, leading manufacturers have created more robust warranty offerings to provide assurance to building owners and enable designers to feel confident about their product of choice.
David Alexander Devine is president of Commercial Roof Management San Jose, Inc. and senior consultant in the Bay Area, California with 30 years of waterproofing, roofing, and building envelope experience. Mr. Devine’s work includes forensic investigation of water intrusion, design, and value engineering for historical structures and museums. He served on the board of directors for RCI and was a member for 20 years