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
B. The greatest percentage of problems are due to locating pools near or on 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
Problem 1 (Non-uniform Soil)
A. Cut/Fill Transition
B. Cut/Fill Transition - Extra caution is required when spas are raised or with shallow water features.
Cut/Fill Transition - Here’s a variation near down slopes
Cut/Fill Transition - Extra caution is required when spas are raised or with shallow water features.
Cut/Fill Transition - Here’s a variation near ascending slopes
Cut/Fill Transition - Extra caution is required when spas are raised or with shallow water features.
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
B. Extra caution is required when spas or water features are located near retaining walls.
Solution For Pool in Retaining Wall Backfill
Solution For Spa in Retaining Wall Backfill
Problem 3 (Non-uniform Soil)
A. Loose Soil in Bottom Not Cleaned Out
Solution For Loose Soil
B Error in over-excavating pool depth
Solution for over-excavated pool depth
C. Compaction requires proper methods
Problem 4 (Non-uniform Soil)
A Slope Creep
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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
a) Key or continuous footing
b) Install caissons of piles if required due to depth necessary
Especially Dangerous
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
Problem 5 (Un-suitable Soil)
A. Shallow Ground Water
Solution for high ground water
Copyright © 2012, Pool Engineering, Inc.
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