Soil and Geotechnical

Soil Considerations in Swimming Pool Construction

by Ron Lacher, P.E.
Pool Engineering, Inc.

What’s important?

A. What are the site soil conditions and important characteristics of the soil

Is it Expansive? The Building Code establishes five classifications of expansiveness; non-expansive, low expansive, medium expansive, high expansive and very high expansive.

Will the soil compress under pressure? A second soil characteristic is density, those that can bear weight or those that will collapse or compress. If the site soil is compressible your pool may settle and/or crack.

Is the Soil Expansive?

Is it very hard, i.e. bed rock?

B. Components of soil

Salts – In seaside locations, salts lie in the ground and speed up the oxidizing of metal components of a project and can also adversely affect concrete.

Organics – Decaying organic matter normally rests at the surface but in some areas ir represents a significant portion of local soil especially in current or former swamplands or peat bogs. All organics must be removed to avoid settling and/or cracking of the pool.

Fill – When working in housing tracts, fill becomes a prevalent factor, usually as a result of large grading operations. Fill can have wide fluctuations in firmness depending on how or if it was compacted.

• Beware of areas where grading occurred without any compaction standards. For example, residential areas may contain uncertified fill if they were graded before 1963, when grading ordinances first came into effect.

Clay – In the range of soils, clay materials contain the smallest particle size and generally represent the most expansive of the soil types due to their limited drainage abilities. Non-expansive clays do exist, however.

• Clay’s small particle size draws moisture between particles by capillary action. The moisture forces the clay particles apart causing volume expansion. In freezing climates, the moisture freezes causing the potential for frost heave.

Sand & Gravel – With the largest particle size, sand & gravel provide the best drainage. Excavators generally have an easy time digging in this soil type, but if it should rain after the dig, the walls can cave into the excavation. Sandy soils often must be shorn up during excavation for the safety of crews and adjacent property

• When loosely compacted, sand provides poor bearing capacity and a high likelihood of future settling.

Silt – Silt stands between clay and sand in particle size and engineering characteristics. It has the properties of cohesive and granular material. Silt generally is not expansive and drains water better then clay but not as well as sand.

How to Lessen the Possibility of Structural Problems at Excavation:

A. What you may see if you look carefully at the excavation

• Critically important, the bottom of the excavation

B. The greatest percentage of problems are due to locating pools near or on descending slopes

• Precautions to take when building pools on or near descending slopes

C. Never play Soil Engineer or Structural Engineer

D. There are two key words we use to describe the soil conditions we want under the pool

• Uniform – that is the same soil condition under the entire pool
• Suitable – Soil that is suitable for support of the pool

Problem 1 (Non-uniform Soil)

A. Cut/Fill Transition

• Portion of pool excavation is in bedrock and portion in fill and/or soil
• Portion of excavation within bedrock will not experience settlement, and portion on fill and/or soil may experience differential settlement resulting in out-of level shell and significant cracks

B. Cut/Fill Transition – Extra caution is required when spas are raised or with shallow water features.

• Raised spa or shallow water feature may rests on different soil conditions than pool
• Spa or shallow water feature experiences differential movement at pool and cracks

Cut/Fill Transition – Here’s a variation near down slopes

• Descending slopes will frequently be constructed by benching of fill
• Descending slopes may be constructed with a “buttress fill”
• Result is pool constructed over cut/fill transition

Cut/Fill Transition – Extra caution is required when spas are raised or with shallow water features.

• Raised spa or shallow water feature may rests on different soil conditions than pool
• Spa or shallow water feature experiences differential movement at pool and cracks

Cut/Fill Transition – Here’s a variation near ascending slopes

• Ascending slopes will frequently be constructed with a “buttress fill”
• A shear key typically is placed at the base of a “buttress fill”
• Result is pool over cut/fill transition

Cut/Fill Transition – Extra caution is required when spas are raised or with shallow water features.

• Raised spa or shallow water feature may rests on different soil conditions than pool
• Spa or shallow water feature experiences differential movement at pool and cracks

Cut/Fill Transition Solution

A. Determine depth to bedrock in fill area

• Over excavate and re-compact entire pool floor (usually 30” to 36” deep) per the Soil Engineer’s requirements so entire pool rests on uniform fill
• Remove the fill and thicken the floor so the pool rests entirely on bedrock
• Extend footings through the fill to bedrock and construct a structural floor to span the fill

Solution For Spa or Shallow Water Feature

• Deepen spa or water feature to rest in same soil conditions as pool or provide deepened foundation.
• Option – Separate spa from pool

Problem 2 (Non-uniform Soil)

A. Pool placed within retaining wall backfill

• Shell placed in fill behind retaining wall (properly compacted?). Typical 1-2% retaining wall deflection allows movement of wall backfill and subsequently the pool. Possible cut-fill transition.
• Rotation and distress to shell, cracking and out-of-level

B. Extra caution is required when spas or water features are located near retaining walls.

• Floors of spas or shallow water features are typically shallower then the pool floor. Cracking occurs because the spa is structurally part of pool.

Solution For Pool in Retaining Wall Backfill

• Freestanding pool wall with pool bottom not located in retaining wall backfill
• Also, see Geologic Problem 1 Removing Cut/Fill Transition

Solution For Spa in Retaining Wall Backfill

• Deepen spa or water feature to rest in same soil conditions as pool or provide deepened foundation
• Option – Separate spa from pool

Problem 3 (Non-uniform Soil)

A. Loose Soil in Bottom Not Cleaned Out

• Bottom of pool/spa excavation not properly cleaned of loose earth materials
• Shell experiences differential settlement & floor cracks

Solution For Loose Soil

• Properly clean out all loose or disturbed materials to competent bearing material
• Not recommended to compact fill material

B. Error in over-excavating pool depth

• Over-excavated area typically backfilled
• Shell experiences differential settlement & floor cracks

Solution for over-excavated pool depth

• Properly clean out all loose or disturbed materials to competent bearing material
• Fill voids with gunite or other non-compressible material
• Not recommended to compact fill material

C. Compaction requires proper methods

• Requires that soil be at optimum moisture content.
• Optimum moisture content often requires that water be added
• Proper compaction cannot be obtained with too much moisture in soil
• Proper compactive effort requires suitable equipment

Problem 4 (Non-uniform Soil)

A. Slope Creep

• Affects swimming pools located close to descending slopes
• Leaves pool wall and possibly floor unsupported

• Slope creep is the gradual lateral and downhill ground movement of soils in proximity to the top of descending slopes
• Slope creep occurs mainly as the result of gravitational forces and is a process that is dependent on soil type, soil moisture content and topographic conditions
• Slope creep commonly results in downhill movement of soils in proximity to the top of slopes which causes trees to lean, fences to be displaced and tilted, and foundations of structures to be distressed

Solution for Possible Creep Damage

• Wall near descending slope built as freestanding
• Pool wall foundation system must extend below the creep zone

a) Key or continuous footing

b) Install caissons of piles if required due to depth necessary

Especially Dangerous

• Attached raised spas & shallow water features located near descending slopes

a) Raised spa or shallow water feature experiences loss of foundation support from slope creep

b) Spa or shallow water feature experiences

Solution for raised spa or shallow water feature located Near Descending Slope

• Deepen spa or water feature to provide additional foundation setback
• Option – Separate spa or water feature from pool, but expect rotation and separation from pool!

Problem 5 (Un-suitable Soil)

A. Shallow Ground Water

• Ground water near surface – either a temporary or permanent condition
• When draining for repairs, pool shell may float out of excavation due to high water table in surrounding earth materials. The pool will likely not float evenly and crack and require total replacement

Solution for high ground water

• Construct pool with rock pack and hydrostatic pressure relief valve and/or sub-drainage system. may require dewatering wells during construction to lower water table