Internal bracing
The waterproofing designer must contend with the inherent problems of internal bracing systems, as they penetrate the plane of the waterproofing applied to the soil-retaining walls and the membrane under the pressure slab. The following paragraphs focus on various systems’ penetrations through the membrane that must be detailed to maintain the watertight integrity of the foundation waterproofing system.
Driven sheet piling
Driven sheet piling consists of profiled steel or wood driven into the ground before excavation. Sheet piling was patented in the 1890s, with production beginning in the early 1900s. It is manufactured from rolled steel and can be purchased in various shapes.
Sheets are driven into the earth and interlocked in a number of ways to form a caisson and limit passage of water and soil particles. Sheets are usually set back from the foundation wall for post-applied waterproofing systems, but may also be covered with plywood to receive a blindside waterproofing membrane system (Figure 6).
Soldier piles and lagging
The most common shoring solution in urban areas, soldier piles are either round or H-shaped structural sections driven into the earth at least 1 m (3 ft) away from the face of the foundation walls. The piles are held in position by being:
- anchored with tiebacks into the earth behind them; or
- braced in front of the piles by diagonal beams (i.e. rakers).
Rakers are anchored to a concrete foot block in the excavation and secured to lateral beams spanning the piles (i.e. walers). Wood boards (i.e. lagging) are fitted between the piles or on one side of them as the earth is removed. This system can be used for post-applied and pre-applied (blindside) waterproofing systems, sometimes referred to as one- or two-sided formed construction (Figure 7).
Secant piles
Secant piles are tubes of cast-in-place concrete in drilled holes. They are designed as interlocking cylinders with alternate units reinforced by structural steel shapes or cages of reinforcing bars. The inboard faces can be cut back to the faces of the steel pile flanges or a concrete or sand wall installed over them. The reinforced piles are called ‘primary’ piles while the intervening piles are ‘secondary’ (Figure 8).
Drilled/concrete soldier piles
Drilled/concrete soldier piles are drilled steel tubular piles that can be filled with concrete. The cylinders are closely spaced; shotcrete or sand walls are installed to form a flat face on the inboard side.
Slurry and precast walls
Slurry or diaphragm walls are constructed of bentonite/cement for cut-off walls or cast-in-place concrete for structural walls. They consist of piles bored and filled with a soil/cement mixture (generally bentonite). The soil/cement is then replaced with concrete. These panels have interlocking ends that may incorporate waterstops. They can serve a dual purpose of shoring the site during excavation and acting as a permanent wall when construction is complete.
Precast concrete panels installed in excavated trenches are a similar form.
Soil nailing with shotcrete
Soil nailing consists of inserting slender reinforcing elements into the soil. The reinforcing is installed into pre-drilled holes and then grouted. Shotcrete is then pneumatically applied over the surface to act as a rigid facing. (The terms soil nails, rock anchors, soil anchors, and tiebacks are sometimes used interchangeably.)
Shotcrete mesh walls
Shotcrete mesh walls are constructed mostly in the western states. Heavy wire mesh is retained in place with soil nails and covered with shotcrete (Figure 9). The skill of the applicator is critical to its success. An inexpensive method sometimes used on the West Coast involves employing the shotcrete to not only retain the earth, but also serve as a form for the waterproofing. (Read “Waterproofing Below-grade Shotcrete Walls” by Daniel Gibbons and Jason Towle in the March 2009 issue of The Construction Specifier before specifying this dual-purpose method. The article describes some of the problems with this system. Visit kenilworth.com/publications/cs/de/200903/files/48.html.)
Other
Some of the many other less-common types of retention systems include timber shoring, churn-drilled soldier piles, wet-set soldier piles, and cylinder pile walls. (For more, see Alan Macnab’s Earth Retention Systems Handbook [McGraw-Hill, 2002].)
