BEYOND ENVELOPE WATERPROOFING - BUILDING.

Besides preventing water infiltration, waterproofing systems prevent structural damage to building components. In northern climates, watertightness prevents spalling of concrete, masonry, or stone due to freeze–thaw cycles. Watertightness also prevents rusting and deterioration of structural or reinforcing steel encased in exterior concrete or behind facade materials.

Waterproofing also prevents the passage of pollutants that cause steel deterioration and concrete spalling, such as chloride ions (salts, including road salts used for deicing) into structural components. This is especially true in horizontal exposed areas such as balcony decks and parking garages. Prevention of acid rain contamination (sulfites mixed with water to form sulfuric acid) and carbon acids (vehicle exhaust—carbon dioxide that forms carbonic acid when mixed with water) is also an important consideration when choosing proper waterproofing applications.

Building envelopes also provide energy savings and environmental control by acting as weather barriers against wind, cold, and heat. Additionally, envelopes must be resistant to wind loading and wind infiltration. These forces, in combination with water, can multiply the magnitude of damage to a structure and its interior contents. Direct wind load pressure can force water deeper into a structure through cracks or crevices where water might not normally penetrate. It also creates vertical upward movement of water (hydrostatic pressure) over windowsills and through vents and louvers. Air pressure differentials due to wind conditions may cause water that is present to be sucked into a structure because of the negative pressure in interior areas.

This situation occurs when outside air pressure is greater than interior air pressure. It also occurs through a churning effect, where cool air is pulled into lower portions of a building, replacing warmer air that rises and escapes through higher areas. To prevent this forced water infiltration and associated energy loss, a building envelope must be resistant and weather-tight against wind as well.

Finally, and possibly most important, health issues of building occupants are now directly related to the success of a properly design and constructed building envelope. All types of mold require the presence of moisture for formation and growth. This moisture is almost always the result of leakage attributable to improperly designed and/or constructed building envelopes. Since mold can cause numerous health problems, this may be the most important issue necessitating a proper understanding of the building envelope and the 90%/1% and 99% principles presented throughout this part.

PREVENTING WATER INFILTRATION - BUILDING.

Considering that these two simple principles cover most leakage problems, it would seem that preventing water infiltration problems would be easy. Certainly, prevention of envelope failures must be a proactive process implemented before actual field construction activities commence. One of the first steps to implement this quality control process is to encourage preconstruction envelope meetings that include all subcontractors involved in the building envelope and cover the following topics:

● Review of the building facade components
● Review of the proposed waterproofing and roofing systems related to the building envelope
● Following the envelope barrier/drainage systems front line to ensure complete continuity
● Reviewing all transitions between envelope components to ensure effectiveness and compatibility
● Reviewing all termination details for waterproofing adequateness
● Instructing all attendees on the necessity of meeting the 90%/1% and 99% principles
● Assigning the responsibility for each termination and transition detail

The last issue is often the root of the 90%/1% principle, the fact that many leaks are directly attributable to transition details that are never installed because the general contractor overlooks assigning responsibility for this details in their subcontracts. For example, refer again to Fig. 1.9; whose responsibility would it be to install the reglet detail provided for the below-grade waterproofing-to-dampproofing transition? The general contractor might easily neglect assigning the completion of this detail to one of the involved subcontractors.

Since the waterproofing membrane would be installed first in most cases, it would be more appropriate for the dampproofing applicator to finish this detail. Although the masonry contractor as part of their contract often applies dampproofing, few masonry contractors understand the importance of this detail. What if the dampproofing used is a coal-tar-based product that is incompatible with the urethane waterproofing membrane? Further complicating the situation, an acrylic sealant might be used to finish the detail that is not compatible with either the membrane or the dampproofing.

FIGURE 1.9 A typical building envelope.

 Such situations continually occur during field construction activities and result in facilitating the 90%/1% principle failures.


Unfortunately, all too often waterproofing is considered an isolated subcontracting requirement, and few architects, engineers, general contractors, and subcontractors understand the importance of knowing the requirements of successfully designing and constructing a watertight building envelope. It must be clearly recognized that all components of a building exterior facade, from the backfill soil selected to the mechanical rooftop equipment, are integral parts of the building envelope and that all are equally affected by the 99%/1% and 99% principles.

THE SECOND MOST IMPORTANT PRINCIPLE OF WATERPROOFING - STRUCTURES.

The inattention to detail is often exacerbated by overall poor workmanship that presents
the next most important principle of waterproofing:

The 99% principle: Approximately 99 percent of waterproofing leaks are attributable to causes other than material or system failures.

When considering the millions of square feet of waterproofing systems installed, both barrier and drainage systems, and miles of sealant involved in building envelopes, it can be estimated that only 1 percent of envelope failures and resulting leakage is actually attributable to materials or systems actually failing. The reasons typically involved in failures include human installation errors, the wrong system being specified for in-place service requirements (e.g., thermal movement encountered exceeds the material’s capability), the wrong or no primer being used, inadequate preparatory work, incompatible materials being transitioned together, and insufficient—or in certain cases such as sealants, too much—material being applied.

Today, with quality controls and testing being instituted at the manufacturing stage, it is very infrequent that actual material failures occur. For example, it is rare to have an outright material failure of a below-grade liquid-applied membrane, as presented later. More often than not, the leakage would be attributable to improper application, including insufficient mileage, improper substrate preparation, or applying over uncured concrete, among numerous other possible installation errors. Furthermore, it is likely that the leakage is also attributable to the 90%/1% principle, with inattention to proper detailing of terminations and transitions with the below-grade membrane occurring.

These two important principles of waterproofing work in unison to represent the overall majority of problems encountered in the waterproofing industry. By considering these two principles together, it can be expected that 1 percent of a building’s exterior area will typically involve actual and direct leakage and that the cause will have a 99 percent chance of being anything but material failure.

THE MOST IMPORTANT WATERPROOFING PRINCIPLE - STRUCTURES.

Each separate envelope trade contractor’s work, regardless of its being thought of as a waterproofing system or not (e.g., exterior mechanical apparatus), must become part of a totally watertight building envelope. Equally important, all individual envelope systems must be adequately transitioned into other components or provided with watertight terminations. Often the tradesworkers completing this work are not aware of, trained in, or supervised in enveloping a building properly. And this is the number one cause of water infiltration in all types of structures.

The resulting improper attention to details is responsible for countless problems in construction. Properly detailing a building’s envelope presents an enormous task. From inception to installation, numerous obstacles occur. Highlighting this interrelationship of various envelope systems is the most important principle of waterproofing:

The 90%/1% principle: 90 percent of all water intrusion problems occur within 1 percent of the total building or structure exterior surface area.

This 1 percent of a building’s exterior skin area contains the termination and transition detailing, as discussed previously with Fig. 1.9. This 1 percent area all too frequently leads to breaches and complete failure of the effectiveness of the building envelope and is the main cause of all waterproofing problems.

Industry members, including contractors, designers, and manufacturers, now are recognizing the importance of the 90%/1% principle first introduce by the author. Architects must recognize the importance of these termination and transition detailing, manufacturers must provide the appropriate details with their specifications, and general contractors must provide the coordination and oversight of the numerous subcontractors involved in a single envelope for the completed product to perform as expected.

The 90%/1% principle is the reason that despite continuing technological advances, waterproofing continues to be one of the major causes of legal claims in the design and construction profession. It is not the actual manufactured waterproofing systems or envelope components that leak but the field construction details involving terminations and transitions.

BASIC BUILDING ENVELOPE DESIGN.

To understand the complete enveloping of a structure, several definitions as well as their relationship to one another must be made clear:

Roofing. That portion of a building that prevents water intrusion (usually from gravitational forces) in horizontal or slightly inclined elevations. Although typically applied to the surface and exposed to the elements, roofing systems also can be internal, or sandwiched, between other building components.
Below-grade waterproofing. Materials that prevent water under hydrostatic pressure from entering into a structure or its components. These systems are not exposed or subjected to weathering such as by ultraviolet rays.
Above-grade waterproofing. A combination of materials or systems that prevents water intrusion into exposed structure elements. These materials can be subject to hydrostatic pressure from wind conditions and are exposed to weathering and pollutant attack.
Dampproofing. Materials resistant to water vapor or minor amounts of moisture that act as backup systems to barrier systems or an integral part of drainage systems.
Flashing. Materials or systems installed to redirect water entering through the building skin back to the exterior. Flashings are installed as integral components of waterproofing, roofing, and dampproofing systems. They also can act as divertor systems.
Diversions. Diversions redirect water being forced against envelope components and divert it elsewhere before it infiltrates or absorbs into the substrate. Examples include flashings, downspouts, sloped concrete decks, and drainage mats.
Building envelope. The combination of roofing, waterproofing, dampproofing, flashing, and divertor systems in combination with all exterior facade elements acting cohesively as a complete barrier to natural forces and elements, particularly water and weather intrusion. These systems envelop a building or structure from top to bottom, from below grade to the roof.

The entire exterior building skin must be enveloped to prevent water infiltration. It is important to recognize that every component used in the envelope or building skin must be waterproof. This would include many features that most people do not recognize as having to be waterproof to maintain the integrity of the envelope, including exterior lighting fixtures, mechanical equipment, signs, and all other types of decorative features.

Each item used or attached to the building envelope should be made waterproof and then appropriately connected to other envelope components to ensure that there are no breaches in the envelope’s integrity. All envelopes contain combinations of several systems, such as the building’s main facade material (e.g., brick), glass curtain walls or punch windows, and decorative features such as concrete eyebrows.

These main facade elements are typically barrier waterproofing systems (e.g., glass is actually a barrier system) or drainage systems, as in the case of brick. Installing divertors where necessary or appropriate for additional protection against water infiltration then completes the envelope.

Each individual system then must act integrally with all others as a total system for complete effectiveness as a weather-tight building envelope. Figure 1.9 illustrates the inter-relationships of the various components of a simplified building envelope.

FIGURE 1.9 A typical building envelope.

In Fig. 1.9, the horizontal roofing membrane terminates in a vertical parapet at the metal counterflashing that also transitions the parapet waterproofing into the membrane roofing. In this specific case, the flashing acts as a transition component between the roofing and parapet materials and ensures the watertightness of the envelope at this transition, enabling these two separate components to act cohesively.

A similar detail occurs at the coping cap. This flashing detail provides transitioning between the brick facade, water repellent on the brick, cavity-wall dampproofing,

wood blocking beneath the coping, and the parapet waterproofing. Note also that sealant in this case was added to protect against any hydrostatic water pressure or wind-driven rain from forcing water up under the flashing. Without this, transitioning and termination detailing the various independent systems involved could not function cohesively to provide building envelope watertightness.

On the vertical facade, vertical and horizontal control joints (not shown) finished with sealant allow for adequate space for the masonry to move during thermal expansion and contraction while maintaining a watertight facade. Note that the brick also has been detailed with through-wall flashing, diverting intruding water vapor and moisture that was collected by the dampproofing back out through the provided weep holes. Additionally, sealant at the window perimeters acts as a transition between brick facade shelf angle and the window frame. The window frame then acts as a watertight transition between the frame and glass, both being waterproof themselves.

To transition between the barrier waterproofing system used below grade to the drainage system (brick facade), a reglet is installed. This reglet provides the detailing nec- essary to transition between the two systems while maintaining the watertight integrity of the envelope. Additionally, sealant is installed in the reglet to allow the systems to move independently at this point but still remain waterproof.

Even the waterstop shown in the concrete foundation provides a very important transitioning and waterproofing detailing that is often overlooked. In this wall section, the waterstop effectively ties together the vertical waterproofing to the horizontal slab waterproofing, providing a watertight seal by prohibiting the lateral movement of water along the concrete wall to foundation joint.

The Fig. 1.9 wall section also details divertor systems by sloping of the adjacent soil or landscaping and installing a French drain system. Each system, while not in itself necessary for the waterproofing of the building envelope, quickly removes water away from the structure, eliminating unnecessary hydrostatic pressure against the foundation walls.

As illustrated in Fig. 1.9, each separate waterproofing material effectively joins together to form a watertight building envelope.

WATERPROOFING - Completing the Envelope.

Once the sources of water have been identified, the types of systems to prevent leakage chosen, and the materials selected to provide necessary aesthetics to the finished product, the envelope design must be carefully constructed and reviewed to ensure successful performance of the completed product. To prevent all possible water intrusion causes, a building must be enveloped from top to bottom with barrier or drainage systems, with divertor com- ponents added where appropriate to increase performance of the envelope.

These systems then must interact integrally to prevent water infiltration. Should any one of these systems fail or not act integrally with all other envelope components, leakage will occur.

Even with continuing technological advances in building materials, water continues to create unnecessary problems in completed construction products. This is most often due to an envelope’s inability to act as an integrated system that prevents water and pollutant infiltration. All too often several systems are designed into a building that have been chosen inde- pendently and are acting independently rather than cohesively.

Detailing transitions from one component to another or terminations into structural com- ponents are often overlooked. Product substitutions that do not act integrally with other specified systems create problems and leakage. Inadequate attention to movement characteristic of a structure can cause stress to in-place systems that they are not able to withstand. All these situations acting separately or in combination will eventually cause water intrusion.

WATERPROOFING - Designing to Prevent Leakage.

Once a complete understanding of the potential sources of water and forces that can move this water into an envelope is gained for a particular structure, the design must incorporate effective systems to prevent such intrusion. Expected conditions for a particular geographic area that will affect the above-grade envelope are available from the national weather service at www.nws. noaa.gov. Below-grade water tables are determined by testing actual site conditions.

It also should be understood that substrate water penetration and absorption do not nec- essarily cause leakage to interior spaces. Water absorption occurs regularly in masonry facades, but the masonry is either large enough to absorb the penetrating water without passing it on to interior finishes or this water is collected and redirected back to the exteri- or by the use of dampproofing systems. Water penetration also occurs at the microscopic and larger voids in the masonry mortar joints, but again, the masonry absorbs it or the water is redirected back out through the dampproofing system.

For definition purposes, water infiltration and leakage are used interchangeably in this book because each is not an expected outcome in envelope design. All envelope compo- nents are designed to prevent leakage or infiltration by one of three systems:

1. Barrier
2. Drainage
3. Diversions

Barrier systems are, as their name implies, effective and complete barriers to water infiltration. They include actual waterproofing systems such as below-grade urethane membranes and other envelope components such as glass. They completely repel water under all expected conditions, including gravity and hydrostatic pressure. Refer to Fig. 1.6.

FIGURE 1.6 Barrier waterproofing system.

Drainage systems are envelope components that might permit water absorption and some infiltration through the substrate but provide a means to collect this water and divert it back out to the exterior before it causes leakage. Examples include masonry walls with cavity-wall dampproofing and flashing to divert penetrating water and water vapor back to the exterior. Refer to Fig. 1.7.

FIGURE 1.7 Drainage waterproofing system.

A new term in construction design is being used, namely, rain-screen system. This is simply another term for drainage system. Rain-screen systems use cavity-wall systems in curtain-wall and similar construction techniques, where the air space in the cavity wall is used to prevent air pressure from permitting water to enter the initial barrier facade com- ponents into the interior portions of the building.

Diversions actually redirect water being forced against envelope components and divert it elsewhere before it infiltrates or absorbs into the substrate. These might include sloping of roof decks and balconies, vertical drainage mats applied to below-grade walls, gutters and downspouts, flashings, and wind screens. Refer to Fig. 1.8 for typical examples of diversion systems.

FIGURE 1.8 Diversion systems.

Building facades usually contain combinations of these systems, each preventing water infiltration at their location on the envelope. However, regardless of how well the individual systems function, if they are not properly transitioned into other envelope components or terminated sufficiently, leakage will occur. These situations become the major issues preventing effective building envelope and waterproofing functioning and are the cause of most leakage that occurs in all structures.

WATERPROOFING - Water Sources.

Water likely to penetrate building envelopes is most commonly from rainwater on abo grade components and groundwater intrusion below-grade. Other sources also should considered as appropriate, such as melting snow, overspray from cooling towers, la scaping sprinklers, and redirected water from such sources as downspouts and gutters

The presence of any of these water sources alone, though, will not cause leakage; for l age to occur, three conditions must be present. First, water in any of its forms must be pres

Second, the water must be moved along by some type of force, including wind and gravity for above-grade envelope components and hydrostatic pressure or capillary action for below- grade components. Finally and most important, there must be a breach (hole, break, or some type of opening) in the envelope to facilitate the entry of water into the protected spaces.

Available water is moved into the interior of a structure by numerous forces that include

● Natural gravity
● Surface tension
● Wind/air currents
● Capillary action
● Hydrostatic pressure

The first three typically are encountered on above-grade portions of the envelope, whereas the last two are recognized at grade or on below-grade areas of buildings or structures. For above-grade envelope components, horizontal areas are very prone to gravitational forces and never should be designed completely flat. Water must be drained away from the structure as quickly as possible, and this includes walkways, balconies, and other necessary “flat” areas. In building components such as these, a minimum  1 4 in/ft of slope should be incorporated rather than the  1 8 in that is often used as a standard. The faster the water is directed off the envelope, the less chance there is for leakage.

Consider the teepee, built from materials that are hardly waterproof in themselves; the interior areas remain dry simply because the design sheds water off instantaneously. The same is true for canvass tents; the material keeps the occupants dry as long as the water is diverted off the canvass immediately, but use the same material in a horizontal or minimally sloped area, and the water will violate the canvas material. Figure 1.1 emphasizes the importance of slope to prevent unnecessary infiltration.

FIGURE 1.1 Sloping of envelope components maximizes drainage of water away from the enve-
lope. The flat-roof design shown is often the cause for leakage problems simply because the water
stands or “ponds” on the envelope surface.

In fact, incorporating adequate slope into the design could prevent many of the common leakage problems that exist today. Simply compare residential roofs that incorporate a slope as high as 45   to commercial roofs that are designed with a minimum  1 8-in slope.

Although the materials used in the commercial application are more costly and typically have superior performance capabilities than asphalt shingles used on residential projects, the commercial roofs continue to have leakage problems at a far greater incident rate than residential roofing.

Surface tension is the momentum that occurs when water being moved by gravity approaches a change in building plane (e.g., face brick to lintel) and clings to the underside of the horizontal surface, continuing with momentum into the building by adhering to the surface through this tension. This situation frequently occurs at mortar joints, where water is draw into a structure by this tension force, as shown in Fig. 1.2.

FIGURE 1.2 Surface tension accelerates water
infiltration.

This is the reason that drip edges and flashings have become a standard part of any successfully building envelope. Drip edges and flashings break the surface tension and prevent water from being attracted to the inside of a building by this force. Some common drip edge and flashing details to prevent water tension infiltration are shown in Fig. 1.3.

FIGURE 1.3 Typical and common uses of “drip edges” to prevent tension
infiltration.

When wind is present in a rainstorm, envelopes become increasingly susceptible to water infiltration. Besides the water being driven directly into envelopes by the wind currents themselves, wind can create sufficient air
pressure that creates hydrostatic pressure on the facade that can force water upward and over envelope components. Again, flashing is used frequently to prevent this phenomenon from causing water penetration into a structure. This typical detailing is shown in Fig. 1.4.

FIGURE 1.4 Flashing used to prevent water under pressure from entering the
envelope.

Capillary action occurs in situations where water is absorbed into an envelope substrate by a wicking action. This situation is most likely to occur with masonry or concrete portions of the envelope at or below grade levels. These materials have a high number of minute void spaces within their composition, making them susceptible to capillary water intrusion. These minute voids actually create a capillary suction force that draws water into the substrate when standing water is present. This is similar to the action of a sponge that is laid in water and begins absorbing the water.

Materials that have large voids or are very porous are not as susceptible to capillary action and in some cases are actually used to prevent this reaction on a building. For example, sand is often used as a fill material below concrete slabs placed directly on grade to prevent the concrete from drawing water from the soil through capillary action. Typical methods to prevent capillary action in envelopes are shown in Fig. 1.5.

FIGURE 1.5 Preventing capillary water infiltration into envelopes.

Hydrostatic pressure most commonly affects below-grade portions of the envelope that are subject to groundwater. Hydrostatic pressure on an envelope is created by the weight of water above that point (e.g., the height of water due to its weight creates pressure on lower areas referred to as hydrostatic pressure). This pressure can be significant, particularly in below-grade areas, where the water table is near the surface or rises near the surface during heavy rainfalls. Water under this significant pressure will seek out any failures in the envelope, especially the areas of weakness—the terminations and transitions between the envelope components. This is why certain envelope substrates used below grade have to be better protected against water infiltration than those above grade. For example, concrete above grade is often only protected with a water repellent, whereas below grade the same concrete must be protected with a waterproofing material to prohibit leakage into the structure.

INTRODUCTION TO WATERPROOFING AND ENVELOPE DESIGN.

Waterproofing is the combination of materials or systems that prevents water intrusion into structural elements of a building or its finished spaces. Basic waterproofing and envelope design incorporates three steps to ensure a watertight and environmentally sound interior:

1. Understanding water sources likely to be encountered.   Water likely to penetrate building envelopes is most commonly from rainwater on abo grade components and groundwater intrusion below-grade. Other sources also should considered as appropriate, such as melting snow, overspray from cooling towers, la scaping sprinklers, and redirected water from such...

2. Designing systems to prevent leakage from these sources.   Once a complete understanding of the potential sources of water and forces that can move this water into an envelope is gained for a particular structure, the design must incorporate effective systems to prevent such intrusion. Expected conditions for a particular geographic area that will affect the

3. Finalizing the design by properly detailing each individual envelope component into adjacent components.  
Once the sources of water have been identified, the types of systems to prevent leakage chosen, and the materials selected to provide necessary aesthetics to the finished product, the envelope design must be carefully constructed and reviewed to...

THE BUILDING ENVELOPE.

The building envelope is equivalent to the skin of a building. In essence, a structure must be enveloped from top to bottom to prevent intrusion from nature’s elements into interior spaces and to protect the structural components from weathering and deterioration.

Envelopes complete numerous functions in a building’s life cycle, including

● Preventing water infiltration
● Controlling water vapor transmission
● Controlling heat and air flow, into and out of interior spaces
● Providing a shield against ultraviolet rays and excessive sunlight
● Limiting noise infiltration
● Providing structural integrity for the façade components
● Providing necessary aesthetics
● Preventing of mold formation and growth

While the main purpose of any building envelope is to provide protection from all elements, including wind, cold, heat, and rain, this book concentrates on the controlling of water and leakage for all construction activities including the building envelope. Making a building envelope waterproof also provides protection against vapor transmission and serves to prevent the unnecessary passage of wind and air into or out of a building, assisting in the controlling of heating and cooling requirements. Before considering each specific type of waterproofing system (e.g., below-grade), some basic concepts of waterproofing and how they affect the performance of a building envelope are important to understand.

WATERPROOFING PRINCIPLES - THE BUILDING ENVELOPE.

Since our beginnings, we have sought shelter as protection from the elements. Yet, even today, after centuries of technological advances in materials and construction techniques, we are still confronted by nature’s elements contaminating our constructed shelters. This is not due to a lack of effective waterproofing systems and products.

Waterproofing problems continue to plague us due to the increasing complexity of shelter construction, a disregard for the most basic waterproofing principles, and an inability to coordinate interfacing between the multitude of construction systems involved in a single building.

Adequately controlling groundwater, rainwater, and surface water will prevent damage and avoid unnecessary repairs to building envelopes. In fact, water is the most destructive weathering element of concrete, masonry, and natural stone structures. Water continues to damage or completely destroy more buildings and structures than war or natural disasters. Water and moisture infiltration is also responsible for mold formation and the related health issues of building occupants.

Waterproofing techniques preserve a structure’s integrity and usefulness through an understanding of natural forces and their effect during life-cycling. Waterproofing also involves choosing proper designs and materials to counter the detrimental effects of these natural forces.

Site construction requires combining numerous building trades and systems into a building skin to prevent water infiltration. Our inability to tie together these various com- ponents effectively causes the majority of water and weather intrusion problems. Actual experience has shown that the majority of water intrusion problems occur within a relatively minute portion of a building’s total exposed surface area. An inability to control installation and details linking various building facade components that form the building’s exterior skin creates the multitude of problems confronting the design and construction industry.

While individual waterproofing materials and systems continue to improve, no one pays attention to improving the necessary and often critical detailing that is required to transition from one building facade component to the next. Furthermore, we seem to move further away from the superior results achieved by applying basic waterproofing principles, such as maximizing roof slopes, to achieve desired aesthetics instead.

There is no reason that aesthetics cannot be fully integrated with sound waterproofing guidelines.

Spread Underpinning.

Occasionally, due to site constraints, underpinning is achieved by spreading the foundation load over a greater area of ground, rather than transferring to a bearing strata at a lower level.

An example undertaken by the authors’ practice was in the restricted cellars of a series of large Victorian properties being redeveloped as office accommodation. The load-bearing walls sat on stepped brick footings, just beneath a cellar floor of compacted earth. In this case it was possible to cut pockets out of these footings, run reinforcement through the holes, and cast the whole of the cellar floor area as a reinforced concrete raft slab. This proved a very cost-effective and practical way of enhancing the load-bearing
capacity of the premises and providing a basement slab at the same time.

Discontinuous Underpinning.

Where the existing foundation has reasonable spanning capability it is sometimes possible to excavate and install piers in mass concrete or concrete and brick at a spacing  to suit the spanning capability of the original foundation.

The area of the base of this underpinning needs to be  capable of distributing the ground pressure from vertical and horizontal loading into the sub-strata without allowable limits being exceeded (see Fig. 15.13 for typical details).

Fig. 15.13 Typical discontinuous underpinning.

In other situations where good ground exists but the foundation is not capable of spanning, a pier and underpinned beam can be used, the beam being inserted in sections in a similar manner to that of the mass concrete underpinning.

This operation tends to be more tedious and more time  consuming, but where excavations are deep it can prove economic (see Fig. 15.14). As with continuous underpinning, the engineer must carefully consider the risk to site personnel before specifying these techniques.

Fig. 15.14 Typical pier and beam underpinning.

It is particularly useful for foundation jacking where subsidence or settlement requires re-levelling, the jacks being inserted between the soffit of the beam and the top of the piers. In some cases, particularly where the building to be underpinned forms part of the new construction, piles can be inserted on either side of the structure to support needle beams inserted through the existing structure to bear onto the piles. This is particularly useful where a basement extension is to be added to an existing building; the piles form the basement columns and the beams the framework for the ground floor structure. Typical pile beam under-pinning is shown in Fig. 15.15.

Fig. 15.15 Typical pile and beam underpinning.

Temporary lowering of the water-table by sump-pumping for underpinning operations requires careful consideration relative to the effect on new and existing foundations. As previously discussed, there is a danger that soils such as fine sands may suffer from loss of fines and may cause settlement of adjoining structures. There is also the possibility that in certain soils when the dewatering process stops, running sand or clay softening may occur. It is therefore important under these circumstances that the effects of  the temporary works and methods of construction are considered at design stage.

There are numerous ingenious piling systems available which minimize disruption of the existing structure, while maximizing economy and practicality of construction and a reputable specialist contractor should be approached at an early stage where appropriate.

Continuous Underpinning.

In this part the authors have considered only underpinning of existing buildings adjacent to new developments
and not underpinning required due to structural settlement or subsidence, which is a separate subject beyond the scope of this book.

All foundation types may require underpinning when development takes place alongside or under an existing structure. The possible combinations of ground conditions, foundation details and levels is endless and complex. The basic methods and principles are quite simple. Where a new foundation or structure is to be constructed with  its foundation soffit below that of an adjoining foundation, underpinning is usually necessary. The exception to this is where the adjoining building is built upon a substantial ground strata such as hard rock.

The ‘traditional’ method of underpinning described below has been used extensively and effectively in the past and  is included here for completeness. However, the risks to site personnel involved in working beneath a temporarily supported section of superstructure of sometimes dubious integrity must be weighed against other underpinning options such as the use of mini-piles and needle beams  (see Fig. 15.15), which are now easily available and equally effective.

Fig. 15.15 Typical pile and beam underpinning.

Traditional underpinning is generally carried out in sequenced construction and in short lengths (commonly  1.0 to 1.2 m). The sequence is arranged to allow limited undermining of the structure at any one time. The limit of this undermining is dependent upon the structure’s capability of spanning over the undermined section and the  stability of the short section of unrestrained earth. In some cases beam underpinning may be provided to help the structure to span over greater distances. Typical underpinning is shown in Fig. 15.11.

plan
Fig. 15.11 Typical continuous underpinning.

The simplest and most common form of underpinning is  to remove a series of short lengths of sub-soils from below the adjoining building in a sequenced operation. As each section is excavated it is replaced immediately with mass concrete, which is allowed adequate time for curing prior  to the construction of the adjoining section. The top of the concrete is either cast with a pressure head so that it rises  to the underside of the foundation, or is cast low to allow wedging with dry pack or slate. Figure 15.12 gives a typical example of mass concrete underpinning.

Fig. 15.12 Construction methods for mass concrete underpinning.

In the authors’ opinion the preferred method of construction is to cast whenever possible with a pressure head. Concrete shrinks, and so theoretically this method encourages some slight settlement as the building above follows this shrinkage downwards. However, in the authors’ experience such settlement is usually negligible and is offset by the following advantages of the pressure head method:

(1) The underpinning is completed in one operation, rather than waiting up to seven days before dry packing. Also since concrete continues to shrink for weeks, even months, the logic of dry packing is inconsistent.
(2) The workmanship of the dry packing process is often of poor quality due to the difficulty of the technique. This requires increased supervision, and slows the whole sequence down even further.

It is rarely necessary to mechanically key mass concrete underpinning across the joints and the majority of mass concrete underpinning will perform successfully without  a key. The need for keying depends upon the requirement for vertical shear and/or tensile strength across the face, neither of which is usually necessary.

Slip Circle Example.

Slip circles have been mentioned previously and the following example, showing the calculation process for
deriving the factor of safety for a single slip circle with an arbitrarily selected radius, is included here for com-
pleteness. In order to find the most critical condition, i.e. the minimum factor of safety, a number of slip circle  calculations should be carried out using different radii. The engineer experienced in this field is able to identify the most likely mode of failure based on a knowledge of soil parameters and boundary conditions and hence reduce the computational effort required. This iterative calculation process is time-consuming and can be more cost-effectively carried out by one of the commercial software packages for slip circle analysis that are available.

A detached house, 9 m × 9 m on plan, is to be constructed on a sloping site; a section through the proposal is as shown  in Fig. 15.10.

Fig. 15.10 Slip circle design example.

Assuming an average value of  cu = 50 kN/m2, consider  a one metre wide strip for the case where Øu  = 0, i.e. the undrained condition immediately following construction

Next set up a circular arc using compasses, to pass through the edge of the excavation for the basement of slab/footing and close to the bottom of the new embankment. Measure the radius, and compute the arc length, r = 12.5 m. The angle subtended by the arc =α= 90°. Therefore

Deduct a length (arbitrary) disturbed by excavation and subsequent filling, i.e. 19.6 − 3.6 = 16 m.

Weight of ground is F1, assuming the small area of fill above the chord line equals the area omitted within the house.

Using 16 kN/m3 for existing ground and compacted fill
weight, F1 = 45 × 16 = 720 kN/m

The weight of a detached house of two storeys, including external and internal load-bearing walls, when averaged per metre run, equates to 170 kN/m. Therefore

By simple geometry, the centroids of the areas are located, and scaling their lever arms

Combining these gives

    2304 + 680 − 194 = 2790 kNm

Therefore

Since 3.6 is greater than 2, the factor of safety commonly adopted for slope failures involving buildings, then there  is an adequate factor of safety against slip circle failure.

However, as mentioned above, other circles should be checked in order to find the critical case.

Retaining walls and basements.

In commercial developments occupying congested city centre sites it has become common to utilize deep basements to provide accommodation for plant room, car parking and other areas. The depth of these basements requires careful consideration of the aspects of design and construction in order to achieve a satisfactory engineering solution. For the engineer requiring a full explanation of  the approach to design and the methods of construction  of deep basements, reference should be made to the IStructE publication where this topic is dealt with in comprehensive detail.

Retaining walls and peripheral walls to basements are subject to lateral (i.e. horizontal) pressure from retained earth, liquids or a combination of soil and water. They are normally made, in structural work, of concrete or brick (plain, reinforced or prestressed).

Basements are relatively expensive to construct (the cost per square metre is higher than for normal floor construction) so the client should be advised to carry out a cost  evaluation of, say, adding a further storey to the structure and eliminating the basement. However, basements can  be made cost-effective when they are used as cellular buoyancy rafts or where increased height is restricted by planning.

The walls are basically vertical cantilevers, either free or propped (at the top by a floor slab). Where the ground floor slab can be made continuous with the top of the wall (and not merely be propped) the basement can be designed as a continuous box. The walls can be constructed with either a base slab extending under the retained earth (see Fig. 15.1 (a)), which is generally the more economical form for  cuttings, or projecting forward (see Fig. 15.1 (b)), the more  economical form for basements.

Fig. 15.1 Typical retaining walls.

While propped cantilevers (e.g. basement wall propped by ground floor slab) have a maximum bending moment (for a udl) of  pH^2 /8, compared to that of a free cantilever of  pH^2 /2, they are not frequently used in building structures.

This is because the wall must either be temporarily propped, or not backfilled, until the ground floor can act as the prop.

However, in the authors’ experience it can be worth considering the use of the more economical propped cantilever, especially for design-and-build contracts, where a close relationship is developed with the contractor from an early stage, and construction methods can be programmed into the design. It is important to provide a clear route for  the propping force through the substructure and to take account of any out-of-balance lateral forces, such as those resulting from sloping backfill on one side of the structure.

The bending moment diagrams for triangular pressure (i.e. no surcharge) for the three cases: free cantilever, propped cantilever and cellular (fixed), are shown in Fig. 15.2.

Fig. 15.2 Bending moment diagrams for retaining walls.

It can be seen that partially filling a basement with water can equalize the external earth pressure on the basement wall. The authors’ practice has used this method of temporary propping, raising the water level as backfill is placed. Where the basement is con- structed in waterlogged ground, filling the basement in this way can also be utilized to avoid flotation before the weight of the rest of the building is added.

Walls to swimming pools are a special case since they can be subject to reversal of stress. With the pool empty, the wall is subject to earth/water pressure on its earth face and with the pool full and earth pressure absent (either due  to shrinkage of backfill or water testing for leaks, before backfilling), the wall is subject to water pressure alone on its water face (see Fig. 15.3).

Walls to culverts can similarly be subject to reversal of stress under the two conditions of earth pressure acting alone or when the water pressure is acting alone. Service ducts, boiler houses, inspection chambers and similar excavated substructures can unwittingly be subject to internal water pressure acting alone, which needs to be designed for. This has happened when heavy rainfall during con- struction has flooded and filled the substructures with water before the backfill has been placed.

Fig. 15.3 Pressures acting on swimming pool walls.

Design Example: Piled ground beams with suspended slab

Previously design example  is to be reworked on the assumption that the building is now to be relocated in an area where the  5 m depth of fill is of a much poorer quality, and is con- sidered unsuitable for supporting a floating ground floor slab. The ground floor slab is therefore to be replaced by wide plank precast concrete floors, spanning 8 m parallel to grid lines A–E.

The additional loads due to this suspended floor are shown in Fig. 14.32, and the increased pile loads are indicated.

The increased loads could be catered for by increasing the  number of piles along each load-bearing internal wall  (parallel to grid lines 1–5). In this case however, it has  been decided to maintain the same pile and ground beam layout as in Design Example 3.

Pile capacities
As previously, the pile capacities given in Table 14.9 are derived from previously design Example (Fig. 14.29).

Piles of 450 mm diameter will again be used. Comparison with Design Example 3 indicates increases in length of between 0.5 m and 4.6 m.

Check on strength of pile cross-section
A check on the stresses in the pile cross-section, carried out in a similar manner to Design Example 3, indicates that grade C35 concrete is required.

Ground beam size
The ground beams are designed in a similar manner to previously design example, taking due account of the additional loading from the suspended ground floor.

The calculations will be found to indicate that the 600 mm × 625 mm deep ground beams in previously design example will need to be deepened by approximately 200 mm to accommodate this additional loading.

Fig. 14.29 Pile safe working loads for design examples.

Fig. 14.32 Piled ground beam and suspended slab design example.

Example: Pile cap design.

A pile cap is required to transfer the load from a 400 mm × 400 mm column to four 600 mm diameter piles, as shown in Fig. 14.30.

Pile caps can be designed either by the truss analogy or by bending theory (see BS 8110: Part 1: 3.11.4.1(5)). In this example bending theory will be used.

For a pile cap with closely spaced piles, in addition to bending and bond stress checks, a check should be made on the local shear stress at the face of the column, and a beam shear check for shear across the width of the pile cap. For more widely spaced piles (spacing  > 3  × diameter), a punching shear check should also be carried out.

Local shear check
The ultimate column load is Pu = 6400 kN.

Length of column perimeter is u = 2(400 + 400) = 1600 mm.

The shear stress at the face of the column is

Bending shear check
In accordance with BS 8110: Part 1: 3.11.4.3, shear is checked across a section 20% of the diameter of the pile (i.e. D/5) inside the face of the pile. This is section A–A in Fig. 14.30.

The shear force across this section – ignoring the self-weight of the pile cap, which is small in comparison – is given by

The corresponding shear stress is given by  vu = Vu/bvd, where bv is the breadth of section for reinforcement design.

In accordance with BS 8110: Part 1: 3.11.4.4, this must not exceed (2d/av)vc where  av is defined in Fig. 14.30 and  vc is the design concrete shear stress from BS 8110: Part 1: Table 3.8. Thus

For grade C35 concrete, from BS 8110: Part 1: Table 3.8, assuming six T25 bars, the minimum value of  vc is  0.4 N/mm2, giving

Thus, provided the average effective depth exceeds  d = 846 mm (the local shear check), minimum reinforcement  to satisfy bond and bending tension requirements will be adequate in this instance.

The necessary depth for the pile cap is
    h = d + 25(diameter bar) + 75(cover)
       = 846 + 100
       = 946 mm ⇒use h = 950 mm

Fig. 14.30 Pile cap design example.

Design of Foundations at Pile Head.

A general description of ground beams and pile caps is discussed in previously and restraints and cap/beam details are briefly mentioned.

In addition to providing restraint, the ground beam is also used to transfer loads from the superstructure to the pile and can be used with or without pile caps. For example, two alternative layouts are shown in Fig. 14.22 indicating a wide ground beam solution and a narrow beam using pile caps.

Fig. 14.22 Alternative beam/cap layouts.

Where the increased width of the beam needed to accommodate the pile diameter, plus the total of all necessary  tolerance, is only slight and where a reduction in beam depth helps to compensate for the additional concrete, a wider beam omitting the pile caps can be more economic.

Often the ground beam can be designed compositely with the walls above and by using composite beams a standard nominal size ground beam, dictated mainly by the practicalities for construction, can be used. This has the advantage of standardizing shuttering, reinforcement and excavation, making site construction simple, economic and quicker than the traditional solution. Many different beams designed ignoring the benefit of the contribution from the structure above can severely complicate the foundations (see Fig. 14.23).

Fig. 14.23 Composite action versus normal design.

When considering the use of composite action, consideration must be given to services which may pass through below ground level in these zones. It is often the case that in adopting composite beams the resulting shallow beams can be more easily made to pass over the services. The use of composite action should however be used with caution if there is a requirement to maintain flexibility of future layout. Any modifications involving the introduction of major openings in the walls would invalidate the design assumption that the wall and foundation act together.


A further help in standardizing a smaller and more economic section is that composite action often makes it
possible to precast the beams alongside the excavation and roll them into position, speeding up construction.

For building structures the basic alternative foundations for support on piles generally adopted consists of one or a combination of the following:

Type 1 Concrete ground beams with or without caps sup- porting the main superstructure load but with a floating ground floor slab between the main wall (see Fig. 14.24).
Type 2 Concrete ground beams and suspended in situ or precast concrete floor slabs (see Fig. 14.25).
Type 3 Flat slab construction (see Fig. 14.26).
Type 4 Suspended slab and beam foundations with voids or void formers (see Fig. 14.27).



The economic viability of the pile solutions for the above foundations will differ depending on many variables but, by applying the following basic principles, realistic cost comparisons can be made and piling options exploited:

(1) Minimizing pile numbers relative to pile length/cost and beam length/cost ratio.
(2) Maintaining axial loads on piles and ground beams wherever practical.
(3) Providing pile restraints from other necessary structures wherever practical.
(4) Standardizing on the minimum beam size which can accommodate pile driving tolerances, restraint stresses and pile eccentricity while exploiting any possible composite action.
(5) Minimizing the depth of excavations.
(6) Minimizing the required bending of reinforcement.
(7) Minimizing the shuttering costs by simple standard beam profiles.
(8) Use of simply supported design and simple beam cages wherever possible unless some small cantilever action can greatly reduce the number of piles per unit.
(9) Minimizing the need for pile caps wherever practical by the use of slightly wider beams.

Fig. 14.24 Piles and floating ground slab.

Fig. 14.25 Piles and suspended ground slab.

Fig. 14.26 Piles and flat slab construction.

Fig. 14.27 Piled suspended slab and beam
construction.

Pile Caps.

1 Introduction
The design of pile caps had at one time become a math-ematician’s delight – and a designer’s nightmare. Highly complex formulae with numerous empirical variants could result in expensive design and construction to save a couple of reinforcing bars. As in all design and construction the aim must be ‘to keep it simple’.

2 The need for pile caps – capping beams
It is frequently not possible to sit a superstructure column direct on to a pile because:

(1) It is practically impossible to drive piles in the exact position and truly vertical. Piles wander in driving and deviate from their true position. A normal specification tolerance for position is ±75 mm and for verticality not more than 1 in 75 for a vertical pile or 1 in 25 for a raking pile. A column sitting directly on a pile with
an eccentricity of 75 mm will exert bending as well as direct stresses in the pile.
(2) A single, heavily loaded column supported by a pile group will need a load spread (pile cap) to transmit the load to all the piles.
(3) A line of piles supporting a load-bearing wall will need a capping beam to allow both for tolerance of pile positioning and load spreading of the piles’ concentrated load to the wall.

3 Size and depth
Pile caps are usually of concrete but can be large slabs of rock or mats of treated timber. This discussion is limited to the more common use of concrete.

To allow for the pile deviation the pile cap should extend 100–150 mm beyond the outer face of the piles. The pile group centroid should ideally coincide with the column’s position (see Fig. 14.16).

Fig. 14.16 Plan on triple pile cap.

The depth must be adequate to resist the high shear force and punching shear and to transmit the vertical load (see Fig. 14.17). The shaded area of the pile cap plan in Fig. 14.17 is the area where the column load is directly transferred to the piles. For such a condition the shear stresses are generally small but bending moments need to be catered for.

Fig. 14.17 Load transfer from column to piles.

Alternatively, peripheral steel as a ring tension around a cone shaped compression block may be considered to be a suitable equilibrium of forces (see Fig. 14.18), however, full tension laps must be provided for the peripheral steel.

Fig. 14.18 Ring tension pile cap.

Single column loads supported on larger pile groups can create significant shear and bending in the cap which will need top and bottom reinforcement as well as shear links (see Fig. 14.19).

Fig. 14.19 Pile cap, typical reinforcement.

The heads of r.c. piles should be stripped and the exposed reinforcement bonded into the pile cap for the necessary bond length. Pile caps to steel piles can be reduced in depth if punching shear is reduced by capping and/or reinforcing the head of the pile, as shown in Fig. 14.20.

Fig. 14.20 Reinforced pile head.

Piles for continuous capping beams supporting load-bearing walls can be alternately staggered to compensate for the eccentricity of loading due to the 75 mm out-of-line tolerance (see Fig. 14.21).

Fig. 14.21 Continuous capping beam.