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Building Envelope Design Guide - Below Grade Systems

by Mark Postma, PE
Carl Walker, Inc.

Last updated: 01-02-2007

Introduction

Many buildings constructed today extend one or several floors below grade level. These below grade areas provide functional spaces for uses such as storage, office space, mechanical/electrical rooms, parking, tunnels, crawlspaces, etc. While below grade areas in buildings provide important critical functions for the building, the subject of below grade building enclosure systems, although widely addressed during construction practices is not always wholly understood. The below grade portion of the building enclosure is seldom analyzed numerically in design. Acceptance of poor performance of the below grade building enclosure is typical and historically not questioned. Leaking into basement areas is a common problem for building operators and managers. Air quality, such as radon, and conditioning in terms of humidity levels are often a concern.

Durability of design and materials is mandatory with below grade enclosure systems. Unlike some other building components that might be designed to be replaced several times within the overall building service life, below grade systems need to be built to approximate overall service life. Below grade systems are often inaccessible for repairs and extremely costly if repairs or modifications are necessary. For below grade enclosure systems design and materials must not focus on the first initial cost but consider the life cycle costs of various design options. Also of great importance is to prevent damage during construction that could go undetected until the building is in service creating costly repairs or inadequate performance.

Description

These Design Guide sections describe the basic components of below grade enclosure systems and categorize its functions, provides guidelines for design considerations and provides details for adaptation for actual building projects. The scope of these sections is limited to the enclosure elements of the below grade portion of the building and does not consider interior elements. Below grade enclosures are typically comprised of 3 main elements:

For each of the three elements the following items are discussed:

  1. Introduction—Overall Discussion
  2. Description—Elements and Properties
  3. Fundamentals—Principles of Design
  4. Applications—Ranges or Uses, Climates, Risk
  5. Details—Construction Details with Commentary
  6. Emerging Issues—Research and Development
  7. Codes/Standards—Allowances and Limitations
  8. Resources—Documents, Associations and Web Sites

Figure 1 graphically illustrates the 3 main elements and the typical loadings for below grade building enclosure systems.

Below grade building systems schematic

Fig. 1. Below Grade Building Systems Schematic

Fundamentals

As with other building enclosure systems the functions of each of the elements of the below grade building enclosure can be grouped into four categories, as follows:

  1. Structural Support Functions—to support, resist, transfer and otherwise accommodate all the structural forms of loading imposed by the interior and exterior environments, by the enclosure and by the building itself. The below grade enclosure is often part of the buildings superstructure.
  2. Environmental Control Functions—to control, regulate and/or moderate all the loadings due to the separation of the interior and exterior environments; largely the flow of mass (air, moisture, etc.) and energy (heat, sound, etc.).
  3. Finish Functions—to finish the enclosure surfaces—the interfaces of the enclosure with the interior and exterior environments. Each of the two interfaces must meet the relevant visual, aesthetic, wear and other requirements.
  4. Distribution Functions—to distribute services or utilities such as power, communication, security, water in its forms, gas, and air conditioned air to, from and within the enclosure itself.

The four function categories, i.e. Structural Support, Environmental Control, Finish and Distribution, are expanded in general terms for each of the three elements of below grade enclosures.

Applications

The success of below grade building enclosure systems is largely dependent on the ability to control moisture. The control of moisture can only be achieved with proper investigation, design and construction practices. Also of importance is to understand where problems are likely to occur. Kubal suggests the following principle:

"The 90%/1% Principle: As much as 90 percent of all water intrusion problems occur within 1 percent of the total building or structure exterior surface area."

Therefore, attention to detail, particularly at penetrations is critical to the performance of below grade enclosure systems. Kubal suggests another principle that is equally important:

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

Failures typically occur from improper design or construction practices not material failure itself. Putting these two principles together Kubal states:

"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."

Recognizing that the performance of below grade building systems relies so heavily on design and construction aspects, it is critical to discuss the overall design and construction process.

The performance of the below grade enclosure systems is influenced by the design and construction process. The design and construction process can be divided into three (3) phases:

Through rigorous application of the criteria and design features presented the overall performance of the system for characteristics including moisture, thermal, air quality, fire safety, and acoustics can be achieved.

Investigation Phase

The investigation phase deals primarily with determining the site conditions prior to the design development. For virtually all construction projects, soil specimens are taken that provide basic information concerning soil conditions. These soil conditions once combined with water table information can identify the ground water characteristics in the vicinity of the site. It is these ground water characteristics taken as a benchmark that govern and contribute heavily to the success or failure of the below grade building enclosure system.

Interpretation of site boring (and other site related) information should be made cognizant of the time of year boring was taken. For example, evaluating soil-boring conditions with respect to ground water could produce one result if samples were extracted during the spring months and under conditions of heavy rain or run-off from snowmelt. Similar samples taken at a later time in the year such as August or September following a dry season could result in different ground water conditions.

Site work should also consider the conditions in the vicinity associated with surface characteristics; drainage slope, permeability, and other utilities or other features. Underground storage tanks, sewer lines, reservoirs, water ways or water shed paths need all be examined when considering the overall site conditions.

Structure Use Requirements—Following the site assessment, the structure use requirements concerning the need for absolute dryness or hundred (100%) percent waterproofing should be considered. Structures have variable demand for underground services. Structure utilization is a significant element of the original investigation process and should be fully considered during development of the criteria and the waterproofing, moisture capture and overall water management system.

Following the assessment of usage, a system for subsurface water management should be examined. There is both an internal water management techniques, such as sump pumps or dewatering systems in the basement, or external foundation drains that all need to be coordinated to achieve effective water management at the time of design. Once again, the requirements for specific needs and features of the building basement and sub-basement system must be fully considered in order to achieve the desired result.

Design Phase

The design of the waterproofing system must follow the selected structural design system. Until the structural system is selected and certain features of the design established, it is not practical to develop an effective waterproofing system or water management system other than at a conceptual level. This document develops the concepts, principals and standards as opposed to specific detailing for each condition. Common details are provided for features of water management, waterproofing protection and the basics of system installation. Sound engineering judgment and expertise is needed to apply these guidelines and procedures to a particular project.

Clear definition of the penetrations required in the structural wall or floor system must be established. Detailing of these penetrations to make them water tight under all conditions of use are paramount to the success of below grade enclosure systems.

Proper specifications to address the various issues associated with water management should be done consistent with the typical specifications format. Where special requirements occur to amplify or highlight certain needs for the structure in question, special notations or supplemental specifications should be drafted that address the waterproofing system characteristics and particulars. Such specifications are typically accompanied by detailing of the various aspects of the waterproofing installation. This detailing once established in concept and for intent for specification requirements (prescription vs. performance) can be supplemented by manufacturer details addressing the particulars of a given waterproofing system. These details commonly deal with such things as penetrations, joints, drains, sumps and moisture collection systems.

Construction Administration Phase

Adequate construction administration involving the review of material submittals and installation sequences are important to performance, as is on site observation by the design professional to verify that the construction is being completed in general accordance with the design document drawings and specifications. A well planned design improperly implemented will not provide adequate performance.

With the construction process underway, steps must be taken to properly coordinate the waterproofing applications or waterproofing requirements with the general contractors scheduling. It is frequently the case that schedules are compressed and the needs of waterproofing exterior walls once satisfactorily curing is achieved can produce kamikaze type efforts with respect to exterior wall waterproofing. The conditions under which waterproofing must be installed in order to be effective and meet the manufacturer specification for warranty must be clearly spelled out. Exterior wall drainage application and subdrain discharge must be complete at this time.

Coordination meetings at the time of construction that include the waterproofer and any sub-trades required to install the waterproofing such as installation work should also be identified and coordinated. Features and characteristics of each installation and system should be examined closely.

Troubleshooting

Troubleshooting of the system either at the time of construction or performed in a water test should be considered. A special interest for water testing or troubleshooting before the fact relates to overhead structures that must retain water but will be subsequently buried or backfilled following construction. All aspects of troubleshooting at the time of construction should be considered.

Detals

Details related to below grade building enclosure systems with commentary are as follows:

  1. Foundation Walls—CAD Details
  2. Floor Slabs—CAD Details
  3. Plazas/Tunnels/Vaults—CAD Details

Emerging Issues

The following are emerging issues in below grade building enclosure systems:

  1. System monitoring during service (waterproofing, protection, drainage grids or boards, soil separator/ filter fabrics)
    • Waterproofing
    • Water transmission rates vs. age—against physical variables head, membrane type, ground water chemistry
    • Elastomeric properties vs. age—against physical variables head, membrane type, ground water chemistry
    • Protection layer breakdown vs. age
    • Drainage layer effectiveness vs. age—against water table, applied loads, soil separator type
    • Soil filter/separator fabric type vs. permeability when used with various backfills
    • Leak detection systems
  2. Designing for long term system Maintenance
    • Injection grout systems for post construction leaks
    • Wall weep ports to discharge impounded water
    • Internal contained leak collection system at wall floor interface
    • Interior positive side water proofing protection/membrane systems
    • External secondary perimeter drain fields
  3. Service life prediction from below grade waterproofing
    • Getting the maximum service-life from exterior systems
  4. Integration of existing systems with new adjacent (newly constructed) facilities
    • Solving continuity problems
  5. Enhancing systems in place;
    • How to extend the existing system with targeted maintenance
    • What options are available
    • How can they be adapted to existing constructed systems
    • Data utilization considerations

Relevant Codes and Standards

Conform to local codes and ordinances with regard to groundwater situations except where provisions provided in this document are more stringent or complete.

Additional Resources

WBDG

Products and Systems

Section 07 92 00: Joint Sealants, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications (UFGS), DRAFT Federal Guide for Green Construction Specifications, MASTERSPEC®

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