Download property line wall substrate preparation

GN-015
PROPERTY LINE WALL SUBSTRATE PREPARATION
Introduction
CETCO's active waterproofing systems are well suited for property line (blind-side) applications. These projects
involve the installation of the waterproofing against a soil retention wall from inside the excavation of the
construction site. After the waterproofing, the concrete foundation walls are cast-in-place (or shotcrete)
against the membrane. Blind-side construction is most commonly performed with deep excavations and in
metropolitan downtown areas where construction abuts adjacent property lines.
Blind-side applications present major challenges for
waterproofing systems. Most notably, the substrate
upon which the membrane will be installed can
consist of several types shoring materials, each with a
wide variety of surface imperfections and variables.
Several common shoring wall construction techniques
include: H-Piles and timber lagging, corrugated sheet
piles, rock, auger cast caissons, and even cementstabilized soil. The purpose of these shoring walls is
to retain the earth. Attention is seldom paid to the
interactions between these materials and the
waterproofing membrane which is installed directly
onto it. The common result of this unfortunate
oversight is poor waterproofing installation or
damaged membrane resulting in water intrusion;
which can be severe in light of the high hydrostatic
pressures usually involved with these projects.
Rules for Wall Preparation
There are many kinds of property shoring line walls,
but in terms of preparation to receive a waterproofing
membrane, they need share the same primary
objective: to provide a monolithic substrate surface
upon which the waterproofing can be installed without
damaging it, both during installation and during
subsequent concrete placement. Any substrate to
receive a waterproofing membrane should be
inspected for conditions where this objective may not
be achieved, and additional preparation work should
be performed:
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Figure 2. A typical lagging wall installation
Figure 1. A lagging wall with large gaps requires a rigid
overlay prior to installing the waterproofing membrane
Smooth or remove all protrusions greater than 12mm (1/2”) in height which can snag, tear, or
puncture the membrane. This may alternately be accomplished by installing a protection course over
the irregular substrate.
Bridge large gaps between lagging timbers by placing drainage composite, rigid insulation, or cement
board across them, and fill any void behind the cover material with clean soil, gravel, concrete, sand or
other material which will support the membrane without significant deflection when the concrete is
poured. Rigid filler such as foam may be used to fill oddly shaped voids or cavaties such as those that
may exist between lagging and H-piles, where the lagging is installed “back lagged”.
Parge all small pits, holes, and cavities in the wall greater than 12mm (1/2”) in depth, or cover the
wall with a drainage layer or protection board.
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Lagging Walls
A common subgrade earth retention system is the soldier pile and timber lagging wall. These walls consist of
H-piles driven into the soil at regular intervals, with wooden timbers installed behind the flanges of the H-piles
(Figure 1). It is common for the timbers to be spaced to allow water to drain freely from behind the wall, as
shown in Figure 2. In this case, a layer of drainage composite such as Aquadrain can be placed over the
lagging before installing the waterproofing. The drainage composite directs water down the wall and also
bridges across the gaps to provide a uniform surface over which the membrane can be installed.
Other Types of Shoring Wall Construction
Figures 3-8 show other types of shoring walls that have been used in CETCO waterproofing projects. Site
conditions at these projects vary considerably, but the preparation requirements still remain the same. Some
subgrade wall construction methods require more preparation work than others, a fact that should be
considered during the design and specification process. CETCO's Product Manuals provide detailed
instructions on how the membranes should be installed on different substrate surfaces.
Figure 3. A rock wall, prepared with wire Figure 4. A lagging wall with steel
plates forms an excellent surface for a
mesh and shotcrete, but is unacceptwaterproofing membrane
able as large gaps still remain
Figure 6. Foam blocks are cut and
placed into the voids to address gaps
between H-piles and sheeted steel
Figure 7. A corrugated sheet pile wall
also provides a good work surface for a
waterproofing membrane
Figure 5. A shotcrete wall, adequately
finished to remove surface defects
Figure 8. A shotcrete wall
adequately finished
Alternative Preparation Techniques
In addition to the typical techniques of smoothing, bridging, and parging, a wall surface may be prepared by
using cement board, foam, or other materials that can be cut and fit into place to provide a uniform work
surface. The materials and means by which this is accomplished are secondary to the goal of achieving a
substrate surface which does not leave gaps behind the installed membrane.
Conclusion
It is very easy to specify the requirements for any wall surface; however, execution of these requirements in the
demanding environment of a below-grade shoring wall construction often proves to be difficult to obtain and
maintain along the project site perimeter.
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