Constructing basements to receive waterproofing

by Katie Daniel | December 13, 2017 2:32 pm

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by Justin Henshell, FAIA, FASTM
Auditoriums and computer rooms that do not require daylight now occupy basements that formerly housed mechanical plants and vehicle parking. These sensitive occupancies require the spaces be waterproofed to prevent water from migrating through the basement walls and slab in contact with the wet, adjoining soil. (This article has been extrapolated from the second edition of the author’s The Manual of Below-Grade Waterproofing [Routledge, 2016].)

Basements are defined as habitable spaces constructed below the existing grade or a grade that will be increased in height to cover it.  (Some building codes define a ‘cellar’ as a habitable space entirely below grade and a ‘basement’ as a habitable space partly below grade.) They must be designed to resist soil pressure, as well as hydrostatic forces occurring from the accumulation of stormwater or from a water table varying in depth above the lowest level of the basement or perched above it on a watertight strata.

Where adjoining construction or buildings do not encroach on the footprint of the building, the basement may be constructed above ground, and the walls buried with backfilled material or in an excavation.

Building a better basement
Excavated basements are built differently than the above-grade components of buildings. Generally, they can be constructed by one of two methods:

Bottom-up construction
The most common form of construction, the bottom-up method consists of first excavating the earth that will be enclosed by the foundation wall. Then, a soil retention system is installed and the concrete or masonry foundation walls are cast and braced against the soil pressure. The mat slab or pressure slab is then cast on grade at the bottom of the basement, followed by the foundation walls and interior columns.

Top-down construction
Few people would suggest building the roof first and then working down, yet, for some basements, this is an approved sequence of construction. In this method, the enclosing foundation walls are installed first, followed by the ground floor slab that is cast on the finished grade between them. The soil below the slab is then excavated and the subsequent lower basement floors are cast sequentially, finishing with the pressure slab at the bottom of the basement. Columns are inserted into drilled holes.

Generally, as the excavation progresses, the foundation walls are braced with cross-lot blocking or rakers and wales (or walers)—described later in this article—as the earth is excavated below the slab, exposing the foundation wall. Intermediate upper cellar floor slabs are cast as the earth is excavated to brace the foundation walls and columns. The pressure slab is cast after the excavation reaches the basement level (Figure 1).

Excavations and excavation supports
Where space is unlimited, a basement can be formed by simply excavating the soil, erecting foundation walls, casting the pressure (mat) slab on grade, and backfilling between the soil and the walls (Figure 2).

When space is limited to a few feet beyond a basement’s footprint, the basement can be formed by excavating and installing soil retention systems. A system consists of a network of braces and tiebacks called support of excavation (SOE). The American Society of Civil Engineers’ ASCE Guidelines of Engineering Practice for Braced and Tied-back Excavations explains the purpose of SOE.  (Created by the Committee on Earth-retaining Structures of the Geo-Institute of ASCE, the 1997 resource is also known as Geotechnical Special Publication [GSP] 74.) An excavation into soil or rock causes a stress imbalance as lateral support is removed; deeper and steeper cuts create greater imbalance and risk. When the slope of the cut area is not flat enough to retain natural support, a SOE must be constructed to provide this (Figure 3).

If the width of an excavation is limited to the lot lines, the SOE is erected on them. The waterproofing is pre-applied to the SOE and the foundation wall cast against it. The earth-retaining systems can be used solely to retain the earth and act as deep water cutoff walls, or they may be designed to also serve as structural foundation walls to support the building above. (Excavating basements must be conducted in a relatively dry environment; this requires removal of groundwater and runoff from stormwater. Control of this water is accomplished by pumping called ‘dewatering.’ Site dewatering methods are thoroughly discussed by Daniel G. Gibbons, PE, and Payal R. Volra in their August 2016 article in The Construction Specifier, “Construction Dewatering[2].”)

The data in Figure 4 shows most soil-retaining systems are not suitable substitutions for a separate waterproofing system. Some shotcrete system manufacturers claim their systems combine the functions of soil retention, a structural foundation wall, and a waterproofing barrier. These one-size-fits-all systems have not proved to be a complete success despite attempts by (primarily) West Coast builders.

Soil-retaining systems must be restrained to resist soil and water pressure. The walls can be tied back to the earth on the outboard side or they must be braced internally where tiebacks cannot be installed outside the building footprint. As mentioned, bracing methods include cross-lot blocking, and a system of wales
(i.e. horizontal members connected to the inner face of the walls) and rakers (i.e. inclined struts connected at the top to the walers and braced at the bottom with heel or foot blocks). This is depicted in Figure 5.

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[5].)

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].)

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Selecting the right option
Sometimes, site restrictions dictate which of the systems to be used. For example, when piledriver vibration is undesirable adjacent to an existing hospital or a high water table is encountered, the primary foundation walls are often constructed by installing caissons of bored secant piles, diaphragm walls, slurry walls, or sheet piling. Alternate concrete piles are reinforced with steel H-shaped piles. This forms a 0.9- to 1.5-m (3- to 5-ft) thick wall that functions as the foundation wall.

With top-down construction, when the foundation is completed, the first floor slab is usually cast on either the grade at the ground level or the wall, which is braced with struts and walers as the earth is excavated, exposing the slurry wall. The slurry walls can then be scarified to produce a reasonably uniform surface or leveled off by casting a sand wall against it.

Where the site is fairly narrow and neither ties nor rakers are suitable, cross-lot blocking is the preferred system for bracing the soil-retaining system. Depending on the depth of the excavation, large pipes are installed to brace the walers at each floor or other practicable vertical spacing. Waterproofing is installed from the bottom up to each waler, and then the reinforced foundation is cast.

Pipes are relocated as required to enable the foundation wall to be constructed successively toward the top. They are removed as intermediate floor slabs replace them (Figure 10).

Basement walls
Basement walls (foundations) are designed to resist the lateral pressure of the soil and permanent or intermittent hydrostatic pressure, as well as support the exterior walls of the building above grade. They are generally constructed of reinforced concrete although in shallow basements they may be reinforced masonry.

Where positive side waterproofing is to be applied, the membrane should be covered with protection board or drainage composites and insulation. Drainage composites are unsuitable where the water table is above the footing. When there is a low water table and footing drains are provided, they may
be substituted for protection boards. Insulation is required where dictated by code. Where thermal resistance is required, insulation is generally extruded polystyrene (XPS), although oriented strand fiberglass could be used on shallow basements. Low-density (i.e. 1-pcf) expanded polystyrene (EPS) can be substituted for protection board. It is adhered to the membrane and becomes sacrificial when backfill settles and internally ruptures the boards.

Backfilling
The space between the foundation wall and the earth retention system should be backfilled with selected soil, sand, gravel, or low-density concrete.

Backfilling is one of the primary causes of membrane failure, with damage coming from large tree roots, boulders, construction materials, and similar refuse. Asphalt felt protection boards (ASTM D6506, Standard Specification for Asphalt Based Protection for Below-grade Waterproofing), generally 3.2 mm (1/8 in.) thick, offer sufficient protection, although they are no substitute for careful placement of selected backfill materials.

Sand is better in seismic areas, but generally more expensive than gravel. Backfill materials should be placed in 460- to 610-mm (18- to 24–in.) thick layers and compacted to 85 percent Modified Proctor. (The Proctor compaction test is a laboratory method of experimentally determining the optimal moisture content at which a given soil type will become most dense.) Lesser compaction will result in excessive settlement; greater than 88 percent will reduce the drainability. Where the SOE is in close proximity to the foundation walls and structural members project into the normal space to place and compact backfill materials, flowable fill is the preferred material.

Otherwise known as low-density cellular concrete fill, flowable fill is a lightweight concrete ranging from 20 to 60 pcf and 30 to 900 psi, but more often 16 pcf and 50 psi. It is pumped in place and can flow around obstructions. Its use should not substitute for the protection layer. Although it has some thermal resistance qualities, they are of little value when the flowable fill is saturated.

Flowable fill is sometimes used to pad SOE systems and rock-faced excavations to provide a surface on which to apply pre-applied membranes. In this use, it is often called a sand wall.

Where ground water is maintained well below the slab, footing drains and drainage composites or drainage backfills are required to relieve the pressure. The drainage composites and/or insulation generally provide sufficient protection for the post-applied waterproofing or dampproofing. However, where continuous hydrostatic pressure against the foundations exists, the drainage composite is unnecessary and the membrane requires protection only from the backfill.

Pressure (mat) slabs
The slab-on-grade is designed to resist the uplift pressure of the water when the slab is below the groundwater line. The slab can be increased in thickness to provide sufficient weight to counteract the buoyancy of the water or it can be anchored into position with rock anchors or caissons.

A 50- to 76-mm (2- to 3-in.) mud mat is usually provided on grade to receive the waterproofing membrane. It also serves as a relatively dry working slab. The mud mat is preferable to a tamped gravel substrate, because gravel compaction is never uniform, particularly at rising walls and penetrations. Moreover, penetrations and drains should be anchored with concrete to ensure they are held in position during the installation of the pressure slab. Special attention must be paid to the selection of rebar chairs if the tamped gravel alternate is selected.

Conclusion
This author offers design professionals the following recommendations when faced with a project.

1. Consult with the excavation contractor to ascertain the type of SOE system it intends to use. Determine if the components will require special protection from SOE components that encroach on the foundation wall.
2. Post-applied waterproofing membranes are less costly than pre-applied ones. Inquire if the SOE can be moved back from the foundation sufficiently far enough (about 760 mm [30 in.]) to enable its application.
3. Thicken the pressure slab to absorb piping and shallow pits to simplify waterproofing details.
4. Avoid systems using a wearing slab on gravel over pressure slabs to contain pipes and conduits.
5. Ensure backfill is deposited in shallow layers and properly compacted.
6. Sequencing the pressure slab to be cast prior to the foundation walls will simplify the waterproofing application.

Justin Henshell, CSI, FAIA, FASTM, is a principal at Henshell & Buccellato Consulting Architects, specializing in moisture-related issues in the building envelope, since 1974. His focus includes roofing, masonry walls, waterproofing, and condensation. Henshell has authored more than 40 technical articles and papers, presenting them in the United States, Canada, and Europe. He wrote The Manual of Below-grade Waterproofing (Routledge, 2016). Henshell can be contacted via e-mail at justin.henshell@verizon.net[11].

 

 

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