Installing pools in different ground conditions

by Sally Bouorm | April 1, 2013 2:40 pm

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By Doug Johnston & Terry Brannon

Generally, expansive soils are clays that exhibit a disproportionate affinity for water and, as a result, the pores within the soil fill with water causing mass expansion. This effect is sometimes reported by the testing laboratory as potential vertical rise (PVR). The PVR, which can reach 152 to 305 mm (6 to 12 in.) in some extremes, is a worst-case scenario based on the thickness of the expansive layer, its plasticity, and the availability of water. Keep in mind, however, under normal circumstances, the saturation of the top few inches of clay will swell and seal off the clay below. Therefore, the maximum PVR is seldom realized in practice in pool construction.

Other laboratory tests that are usually performed ‘Minus 200 Series’ or Atterberg Limits[2], which include plastic limit (PL) and liquid limit (LL) tests. The difference in these two parameters is the plasticity index (PI).

Without going into how the tests are performed, all builders need to review the geotechnical report for their site. If one is not available, it is worth the money to have one prepared. For a typical pool installation site, this would cost approximately $3,000. A pool contractor who fails to refer to a geotechnical report likely assumes responsibility for the project soils.

Understanding soil plasticity

A granular soil (i.e. sand, gravel) has no plasticity, whereas heavy clay might have a PI of 60 or higher. Some clay can hold its own weight in water, while a moderately plastic soil may have a PI between 20 and 30. Anything above 30 is considered high for pool foundations.

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The engineering problem is soil tends to shrink and swell with changes in moisture content, which can occur as a result of climatic change, moving water tables, or leaking pools. If soils swell, a pool can be lifted near the wetted area; if the soil dries, the pool could fall where the soils shrink away from the pool shell. Expanding soil can also put phenomenal pressures on pool structures and crack shells.

When an engineer finds highly plastic soils within the depths of the intended excavation, or within 1.5 to 3 m (5 to 10 ft) of the subgrade, he/she must be prepared to recommend various options to the client. These options include:

Soil conditioning

Soil conditioning involves excavating the pool to subgrade and pre-wetting the soil to cause it to pre-swell. Another way to accomplish this is to fill the basin and let it sit full of water for a few months; however, since the contractor typically does not have this kind of time, the water is injected into the soil under pressure, or roto-tilled into the subgrade.

One problem with this method, however, is it turns the bottom of the pool into a swamp, making for a difficult working environment. As a result, many contractors ‘muck’ out the moisture-treated soil, which begs the question, “Why bother in the first place?”

An off-shoot of this method is electrolyte injection in which proprietary chemicals are injected into the soil to stabilize the clay. While most often used in pavement subgrades, it has its occasional use in pool construction.

Over-excavate and replace

Another stabilization method involves undercutting the pool subgrade by 1 to 1.2 m (3 to 4 ft) and replacing the excavated expansive soil with a less plastic material (between 14 and 20 PI) that, while somewhat impervious to moisture, is not so expansive. The new soil retards moisture migration to the expansive soils below and somewhat seals the soil against moisture loss. The issues with this method are availability of low-plasticity non-expansive clayey soil within reasonable hauling distances, and the installation of the soil in highly compacted lifts under moisture control conditions.

Pier supports

Geotechnical reports will sometimes have recommendations for deep piers or drilled shafts that reach to non-expansive soils way below the surface or at least to the bottom of the weather-affected zone wherein moisture contents vary seasonally. These piers can range between 0.4 to 0.7 m (1.5 to 2.5 ft) in diameter. If built through deep clay layers, a casing is necessary to keep uplift from occurring due to skin friction on the pier, or bell-footed piers are required. One complaint with this system is cost as piers can be more than 7.6 m (25 ft) deep. Therefore, this option leads to several variations:

1. Frame support. For this method, a frame comprised of horizontal beams is engineered and built on top of the piers to support the pool’s weight. Box forms (void forms) are installed between, and under, the frames to keep the expansive soil from lifting the frame itself or the pool it supports. The frame and pier arrangement is based on the pool’s shape.
2. Pool structural floor. For this method, the pool is engineered to support its own weight (plus water) by installing monolithic pool floors with integral beams instead of a separate frame to span the piers. Carton or void forms are still necessary to keep the expansive soil from contacting the pool shell.
3. Precast concrete or steel piles can be driven to support the pool or pool sub-frame.
4. Auger piles can be ‘screwed’ into the soil to support the pool. Augers must be designed to sufficient depth to resist uplift from clay soils.

Moisture level maintenance

Another theory is to maintain high moisture levels below the pool to prevent shrinkage caused by the soil drying out. To do this properly, the contractor must install two things:

1. A water source; and
2. A path for the water to saturate the pool subgrade.

Simply installing a crushed stone blanket beneath the pool does not work as it will cause the moisture content to rise and fall with the seasons. If a crushed stone blanket is specified, it should be drained to a sump (i.e. reservoir) or to ‘daylight’ to prevent water from accumulating beneath the pool. While this may be the least expensive alternative, this method is probably the least effective as well.

Resisting the pressure

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Engineers should design their pools to be stiff to resist pressures on pool floors and walls when built in expansive soils. A properly reinforced pool should resist soil uplift and loss of soil support due to shrinkage. As a result, it is often necessary to install ‘double matted’ reinforcing steel. If something has to move, it is always better if the entire pool can move and/or flex without fracturing the shell. After all, levelling a coping or even a gutter or vanishing edge is not impossible and costs far less than the preventative methods necessary to keep the pool absolutely level.[5]

Risk tolerance

The pool designer and engineer must also consider the pool type when recommending a stabilization method. Obviously, the tolerance for movement of a skimmer-designed swimming pool is much greater than it is for a vanishing-edge or perimeter-overflow pool. When choosing a method for building a swimming pool in expansive soils, the owner must be included in the negotiations. After all, neither the engineer or pool contractor selected the site; therefore, risk should not be transferred to them due to the owner’s decisions to save money.

If the owner’s tolerance for movement is small then he or she has to be informed that more costly building methods (e.g. piers and concrete frames) need to be employed. In some cases, foundation costs can exceed the value of the swimming pool. On the other hand, if the owner accepts some risk of movement, a less expensive method can be used.

The owner must be shown their options with regards to repair costs versus the expense of the original installation. For example, the cost of levelling a pool beam and replacing the coping could be a pittance when compared to a $100,000 foundation. In either extreme, the owner should be the one to accept the risk or pay the price for certainty. The pool contractor who fails to bring the owner into the decision-making process assumes liability for any failures, no matter how well-meaning his/her intentions.

Affects on pool plumbing

Expansive soils can also affect the pool’s plumbing. For instance, for pools built in expansive soils, plumbing should be designed to flex with ground movement and/or be easily accessible for repair. If a pool has bottom filtered returns, the filtered return loop under the pool should be fully encased in concrete out to the pool’s edges. In this manner, repairs can be made at the pool edge without the need to dig up the shell floor.

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While plastic pipe is extremely flexible, it will usually snap at a fitting; therefore, when transitioning from concrete to clay, or from select fill to heavy clays, one of the following two methods should be employed:

1. Install long pipe joints centred on the crossover point, rather than installing a rigid fitting or slip-joint at the interface. This method allows the pipe to bend at its point of maximum flexibility. It is also easier to repair.
2. A less common method is to install swing joints where the soil conditions change using a pair of National Pipe Thread Taper (NPT) offset elbows. Installing the pipe in an embedment and backfill with low to moderately plastic soil, instead of backfilling with highly plastic fill, will reduce the likelihood of pipe failure.

Although there are several options and building methods for installing inground swimming pools in expansive soils, in order to get ahead of the game, it is important for builders/contractors to understand the soils they are working with by having an engineered soils report performed before commencing the project.

 

 

Doug Johnston is the principle owner of Stone Crest Pools LLC, a swimming pool design/build firm in Keller, Texas. He is an internationally acclaimed environmental designer and has more than 20 years’ experience in designing and constructing outdoor living spaces and custom watershapes. Johnston, winner of the 2010 and 2012 GAVA Gold Award for global excellence, has been featured on Discovery Channel’s Epic Pools and in numerous publications worldwide. He can be reached via e-mail at doug@stonecrestpools.com[7].

 

 

Terry Brannon is the corporate president for The C.T. Brannon Corporation, an engineering and consulting firm based in Tyler, Texas. Brannon, who received his bachelors of civil engineering degree from the University of Texas at Arlington, is responsible for project design and construction phase engineering for all types of aquatic facilities. He has been an aquatics consultant to architects, landscape architects, and other engineers for the past 36 years and has been engaged in public works as an engineer and consultant for more than 42 years specializing primarily in design of hydraulic facilities. Brannon is an instructor for Genesis 3, a widely respected educational organization for pool builders and designers. He can be reached via e-mail at tbrannon@brannoncorp.com[8].

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