Pressure Drop: Hydrostatic Load and What it Means for the Building Owner and Developer
- John D'Annunzio
- Aug 2, 2018
- 4 min read
Hydrostatic [hī′drə-stăt′ĭk] pressure: The pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity. Hydrostatic pressure increases in proportion to depth measured from the surface because of theincreasing weight of fluid exerting downward force from above. - Dictionary.com
So, you do not have to worry about hydrostatic pressure unless you, someone you know, or something you know is underwater. So, hydrostatic pressure is one of those rare items which only scuba divers and architects are typically concerned with, right? Well, partially. If you are building owner, you need to know about hydrostatic pressure and what to do about it.
There are three key areas of design and construction where it pays handsomely to factor in hydrostatic loads:
1. waterproofing vs. dampproofing materials
2. uplift forces on slabs
3. pressure exertion on vertical below grade structures.
Waterpoofing vs. damproofing
The existence of hydrostatic pressure is the key determining factor for whether you need damp-proofing or waterproofing. The difference? waterproofing materials have water absorption rates below 4%, but preferably below 2% water absorption. If a material absorbs more H20 than that, it is damp-proofing and will break down under hydrostatic loads. Put differently, damp-proofing slows down moisture impregnation, whereas waterproofing prevents it.
Below-grade structures must be able to resist hydrostatic pressures that generally range from 30 to 62.4 psf per foot of depth, depending on a number of factors. The rate of hydrostatic pressure depends on the surrounding soils. Pressures are typically lower in dry, granular soils where water flow remains at the vertical subsurface and higher at wet soils where water flow is continual.
Hydrostatic pressure should be determined by a civil engineer prior to waterproofing design. A general rule in determining hydrostatic pressure is that it increases linearly with depth, which produces a triangular horizontal loading pattern where pressure is exerted from below the structure, above the structure and at the wet face of the structure. If calculations have not been made, you cannot reliably specify a material and you are opening the building up to truly expensive issues.
Hydrostatic pressure occurs at a building component anytime the water table raises above the component and it is directly related to ground water levels. Water rises in most soils by capillary action. The rise can be as extreme as 11.5 feet in soils made of small particles (clay and silt) to zero in granular (gravel) soils that have large spaces between the particles. When ground water levels are at the highest points – in the spring, after flooding from rains, heavy run-off from walls or from clogged drains – the hydrostatic pressure rises. These changes in pressure can occur on an hourly basis. The designer should specify waterproofing to meet the highest pressures under extreme conditions. This can be calculated by one of three methods:
P = wd on top slab
P = w(dth) lateral pressure at the base of the wall
P = w(dth) upward resistance on the slab-on-ground
Slabs-on-ground that are subjected to hydrostatic pressure must be properly designed to resist uplift from hydrostatic pressure. This can be accomplished in one of the following methods:
Increased slab weight (to counterbalance upward hydrostatic pressure)
Reinforcing the slab for flexural resistance and anchoring it to foundations and grade beams
Tying the slab to rock anchors
Installing under-slab drains and footing drains that are directed to the storm water system can also accomplish prevention of hydrostatic pressure.
The intensity of the hydrostatic pressure is also an important consideration in the selection of waterproofing materials. When subjected to continuous hydrostatic pressures, membranes with low moisture-absorbance rates (butyl rubber) tend to perform well. High moisture-absorbance membranes can be subjected to swelling, disbanding and wrinkling in these conditions. Wrinkled membranes are subject to an increased risk of puncture. Intense hydrostatic pressure can also force membranes in concrete voids exposing the membrane to cracks from stress created by flex allowing openings for moisture infiltration.
In summary, there are simple steps that you can take to avoid huge amounts of risk (trust us, you do NOT want to get below grade moisture intrusion - it is the beginning of the end for your building and perhaps your financial future!)
Make sure that hydrostatic conditions are known
Assess if your building is below the water table
Have a civil engineer calculate the maximum hydrostatic load your below-grade walls and slabs may be subjected to
Have a waterproofing consultant design your waterproofing (or damp-proofing) accordingly
Until next time,
John D'Annunzio
John D’Annunzio is one of the most respected voices and minds in the technical aspects of roof evaluation, roof design, and roof management. With over 25 years as a roofing consultant to Fortune 500 companies and a technical consultant to major architectural firms around the USA. He has served on projects around the world while writing four books on roofing and waterproofing, serving as an expert witness, and performing laboratory analysis of roofing materials. John is the Managing Editor for Architectural Roofing & Waterproofing Magazine and technical editor for Roofing Contractor Magazine.
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