Fundamentals of hydrogeology

November 2013 » Features » PROGRESSIVE ENGINEERING
Groundwater sources and flow on a site impact waterproofing of below-grade structures.
Christine Diosdado, P.E.
Typical interior leakage in a below-grade basement space

Each year, uncontrolled water bypasses waterproofing systems, impacts interior occupancies, and damages interior finishes and contents of below-grade structures. A design professional can combat this problem by identifying all sources of water onsite during construction and during the anticipated life of the structure and by designing waterproofing for all potential wetted regions of the below-grade structure. Understanding the hydrogeology at the project site helps the designer meet both criteria.

Hydrogeology fundamentals
Hydrogeology explores the distribution and movement of water through soil and rocks within 1,500 feet of the ground surface. Immediately below the ground surface, soil pores typically contain both air and water. At the near surface, soil moisture forms disconnected water droplets with substantial air voids. Deeper within the soil strata and immediately above the water table, water is drawn upward by capillary attraction. Due to the irregularity in the size of the soil pores, capillary water does not rise to an even height above the water table. Instead, it forms an irregular fringe, known as a capillary fringe. In fine-grained soil, the capillary fringe can wet the soil as much as 12 feet above the water table.

Above the capillary fringe, moisture may partially coat the surfaces of rock and soil particles. However, if the water coating becomes too thick to be held by surface tension, a droplet will form and be drawn downward by gravity. This fluid can also evaporate and move through the pore spaces as water vapor. The entire zone, which extends from the top of the ground surface to the water table and includes the capillary fringe, is known as the vadose zone or zone of aeration. Water in the vadose zone has a negative pressure head that is less than atmospheric pressure.

The water table marks the boundary between the vadose zone and the saturation zone, a zone where all the soil pores are essentially filled with water. The water pressure at the water table surface, also referred to as the phreatic surface, equals atmospheric pressure where the gauge pressure is equal to zero. Below the water table, the fluid pressure increases linearly with depth forming a triangular horizontal loading pattern.

Unlike idealized mathematical models or water table readings on boring logs, the water table elevation is not static. For example, the water table fluctuates seasonally, and in some instances, daily and even hourly. During summer months in the Northern Hemisphere, the water table is normally at its lowest elevation. After spring thaws, the water table typically rises to its highest point. Aside from seasonal variations, the surface of the water table tends to undulate. In humid regions, the water table conforms to the shape of the surface topography, although the water-table relief is not as pronounced as the topographic relief. As the groundwater flows, the water table surface slopes from topographic high spots to topographic low spots. In the absence of groundwater flow, the water table will be relatively flat.

Although determining the uppermost elevation of the water table is important, a design professional also must identify all sources of water and understand how that water is stored and flows throughout the site. Aquifers – geologic units comprised of permeable rock or soils – store and transmit water. The intrinsic permeability of the soils around the aquifer determines the aquifer's classification. (Intrinsic permeability is the measure of the relative ease with which soil can transmit water under a hydraulic or potential gradient. As the name implies, it is an intrinsic property of the porous medium, independent of the nature of the fluid or the potential field.)

For instance, an unconfined aquifer consists of continuous layers of soil with high intrinsic permeability extending from the ground surface to the base of the aquifer. The groundwater table (or water table aquifer) described above, falls into this category. In an unconfined aquifer, rainwater is drawn downward by gravity through the unsaturated zone and replenishes the aquifer. Recharge can also occur through lateral groundwater flow or from upward seepage from underlying strata.

During recharge, the water table rises much like the water level in a bucket will rise as water is added to it. However, unlike an empty bucket, the aquifer is filled with rock, sand, and other geologic materials; water can only occupy the pore spaces. The rise of water within an aquifer can be approximated by the following equation:

Precipitation ÷ Porosity = Rise in aquifer

Where Precipitation and Rise in aquifer are expressed in inches and Porosity is expressed in percent. For example, an aquifer with a porosity of 5 percent will, theoretically, experience a water level rise of 20 inches from 1 inch of precipitation.

A confined aquifer is comprised of a permeable zone of soil between two aquicludes – confining layers of clay or silt with a low intrinsic permeability. Depending on the elevation, hydraulic conductivity, and rate of flow in the confined aquifer, the water contained within may be under considerable pressure. If a confined aquifer is breached, the water inside may rise a considerable distance above the top of the aquifer. An aquitard, known as a leaky confining layer, also consists of a layer of low-permeability soil, but unlike an aquiclude, it can store and slowly transmit water from one aquifer to another. A perched aquifer occurs when an impermeable lens of clay or silt blocks water from seeping downward. This intercepted water saturates the soil above the lens while leaving the soil under the lens unsaturated, resulting in an isolated water source disconnected from the main groundwater body. Figure 1 on page 50 shows a cross section through several types of aquifers.

Figure 1: Schematic cross section through several types of aquifers

Code analysis
We evaluated and summarized the current national building code requirements for the identification of site water and the selection and placement of below-grade dampproofing and waterproofing. As of the date of this publication, the 2012 International Building Code (2012 IBC) applies.

Identification of site water – Section 1803.5.4 of the 2012 IBC requires the design professional to perform a subsurface investigation "to determine whether the existing groundwater table is above or within 5 feet" below the elevation of the lowest finished floor of a below-grade structure. (This requirement is waived if the below-grade portion of the structure is waterproofed in compliance with Section 1805.) The scope of the subsurface investigation also includes soil classification, soil engineering properties, and "the effect of moisture variation on soil-bearing capacity, compressibility, liquefaction, and expansiveness."

Section 1805 describes when and where waterproofing should be installed, along with waterproofing materials and methods of application. This section mandates waterproofing of below-grade walls and floors of occupied spaces, if those walls and floors are subject to hydrostatic pressure. However, if the water table is more than 5 feet below the lowest floor level of the below-grade structure, then dampproofing, not waterproofing, is required.

When the requirement for waterproofing is triggered, the code prescribes concrete floors and concrete or masonry walls, designed and constructed to withstand the hydrostatic pressures. The waterproofing options for the below-grade floor include "a membrane of rubberized asphalt, butyl rubber, fully adhered/fully bounded HDPE, or polyolefin composite membrane or not less than 6-mil polyvinyl chloride with joints lapped not less than 6 inches or other approved materials under the slab."

Placement of waterproofing on a below-grade wall is dictated by the maximum elevation of the groundwater table. According to section 1805.3.2, waterproofing should be applied "from the bottom of the wall to not less than 12 inches above the maximum elevation of the groundwater table" with dampproofing covering the remaining portions of the wall. Waterproofing options for below-grade walls consist of "two-ply hot-mopped felts, not less than 6-mil polyvinyl chloride, 40-mil polymer-modified asphalt, 6-mil polyethylene, or other approved methods or materials capable of bridging nonstructural cracks."

The code dictates a more conservative approach when designing the structural components of below-grade walls and floors. Section 1610 of the IBC states that "foundation walls shall be designed to support the weight of full hydrostatic pressure of undrained backfill, unless a drainage system is installed."

Code limitations – Although the code addresses the groundwater table, it fails to mention other hydrogeology features such as perched aquifers, confined aquifers, and water in the capillary fringe. Following are other limitations of the code:

  • As with all building codes, this code establishes the minimum standard for design and construction to provide adequate safety of life. These minimum criteria, however, may not necessarily align with an owner's expectation for a dry, interior below-grade space.
  • The overriding concern of the code is the structural integrity of the below-grade walls and floors. When designing the structural components of the below-grade structure, the code requires the engineer to conservatively assume that the walls resist full hydrostatic pressure for any undrained backfill, even if the historic water level does not support this assumption. However, no such requirement exists when considering the waterproofing demands of these below-grade walls and floors.
  • Although the code requires the identification of the existing groundwater table level, it does not explicitly require the geotechnical engineer to determine the highest anticipated level of the groundwater table. The groundwater levels may fluctuate significantly depending on the time of year and from year to year. Groundwater level recorded in a boring log is a snapshot of the water level at that time and may not represent the highest possible groundwater level.
  • The code does not consider how water flows on the site and around the below-grade structure and what water can conceivably be in contact with the waterproofing during the life of the project.
  • The code does not require any groundwater chemistry or soil contamination data to determine if the specified waterproofing materials are compatible.
  • The code is silent about water from construction activities and how that water can compromise the integrity and performance of the below-grade waterproofing.
  • The code requirement for waterproofing versus dampproofing is triggered solely by the existence of hydrostatic pressure from the groundwater table.
  • The code allows for a combination of waterproofing and dampproofing on the same below-grade wall. In practice, leaks tend to occur at the interface of these two systems.
Uncontrolled groundwater infiltration into a shored excavation

Recommendations
When selecting and designing waterproofing systems for below-grade structures, consider the following recommendations:

  • Do not assume that the water table elevation shown on the geotechnical boring logs represents the highest point of the groundwater table. Determine the "design" water table elevation by considering seasonal variations, historic climatological data, and historic hydrology records.
  • Identify all water sources on site and determine how that water flows (i.e., intensity, duration, and flow velocity) throughout the site.
  • Specify water control measures (i.e., dewatering wells, groundwater cutoff structures) during construction that capture, lower, dam, and/or redirect water away from the excavated region and the below-grade waterproofing.
  • Specify waterproofing on the entire below-grade wall surface regardless if dampproofing a portion of the wall is permissible.
  • Protect the waterproofing system throughout the construction process, including storage onsite, application, and post-application.
  • Determine the chemical and biological properties of the water and soil at the site and check that the waterproofing selected is compatible.
  • Retain a geotechnical engineer to review water control measures and any soil chemistry data.
  • Select a waterproofing system that is robust enough for the intensity, duration, and flow velocity of the water on site.
  • Consider the leak-risk tolerance of the owner, potential occupants, stored materials, and equipment.
  • Educate the owner about the useful life and periodic maintenance requirements associated with below-grade waterproofing systems.

Christine Diosdado, P.E., is senior project manager at Simpson Gumpertz & Heger, Inc. in San Francisco. Daniel Gibbons, P.E., is an associate principal at Simpson Gumpertz & Heger, Inc. in San Francisco.


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