This page explains what happens to the ground after a swimming pool is removed and the excavation is filled, why some of that ground settles and some of it does not, what the soils of the Tucson basin add to the question, and what an engineered fill and a lift-by-lift compaction test actually establish. It is written for owners who may build on the spot one day, for the designers and inspectors who will read the record later, and for anyone comparing a permitted, tested removal against a bid that leaves these questions unasked.
Terms used on this page
- Fill. Soil placed by people, as opposed to native soil that was deposited by nature and has never been disturbed.
- Engineered fill. Fill whose material was tested and accepted before placement, and whose placement was controlled and tested to a written standard.
- Lift. One layer of fill placed and compacted before the next layer goes on. Structural fill is placed in thin lifts, commonly about eight inches loose, because a compactor only densifies the top several inches of what it rides on.
- Maximum dry density and optimum moisture. For a given soil there is a moisture content at which a standard compaction effort produces the highest density. The laboratory Proctor test (ASTM D698 standard effort or ASTM D1557 modified effort) finds both numbers. Field results are reported as a percentage of that laboratory maximum.
- Relative compaction. The field dry density divided by the laboratory maximum. “95 percent” means the fill in the ground reached 95 percent of the density the laboratory achieved on the same material.
- Hydrocompaction, or collapse. A sudden loss of volume in a dry, loose, low-density soil when water reaches it for the first time. The grains were held apart by dry clay or salt bridges; water dissolves or softens the bridges and the structure drops.
- Differential settlement. One part of a foundation moving more than another. Total settlement is tolerable when it is uniform. Differential settlement is what cracks slabs and walls.
- Bench. A step cut into the side of an excavation so that fill is placed against a horizontal surface rather than a vertical face.
- Caliche. A layer of soil cemented by calcium carbonate, common at shallow depth across the Tucson area. It ranges from lightly cemented to rock-like.
The ground of the Tucson basin
Tucson sits in a structural basin between mountain ranges, filled over millions of years with sediment eroded from those ranges. The basin fill is thousands of feet thick. What a pool contractor and a foundation designer care about is the top thirty feet, and in that zone the deposits are young alluvium: gravel, sand, silt and clay laid down by washes and sheet floods on alluvial fans and terraces. The mix changes over short distances, which is why a soil report from one lot is not a soil report for the lot next door.
Four conditions in these deposits matter for a filled pool.
- Collapsible soils. A 1990 study in the journal Engineering Geology, using site investigations from across the Tucson area between 1982 and 1987, located collapse-prone soils in the youngest deposits: skirt and apron fans at the mountain fronts, inset fans, river terraces and floodplains. These soils were deposited fast and dry, never saturated, and never loaded. They can carry a building for decades and then drop when a leaking line, a new lawn or a flooded yard wets them for the first time. The Arizona Geological Survey lists collapsing soils as one of the state’s two principal problem soils, and names monsoon storms, irrigation and plant watering next to foundations as the triggers.
- Expansive clays. The other principal problem soil. Clay-rich layers swell when wet and shrink when dry. They are less widespread in the Tucson basin than in parts of the Phoenix area, but they occur, and the Geological Survey publishes shrink-swell potential maps for both cities. The same alternate wetting and drying that triggers collapse drives expansion.
- Caliche. Cemented carbonate layers at shallow depth are so common in the Tucson area that their strength has its own literature. Caliche is good bearing ground when it is continuous and bad ground when it is not: a footing that sits half on a caliche lens and half on loose sand is set up for differential movement. Pool shells are often built down into caliche, and breaking the shell out disturbs it.
- Regional subsidence and earth fissures. Decades of groundwater pumping compacted the deep aquifer sediments under parts of the basin, and the surface has subsided. Where subsidence is uneven, the ground can crack open in long fissures; Avra Valley west of Tucson has mapped fissures, and the U.S. Geological Survey has assessed the potential across the basin. This is a regional condition, not something a pool removal creates or fixes, but it belongs in any honest account of Tucson ground, and a geotechnical engineer will note if a lot sits in a mapped zone.
One condition that does not usually matter here is groundwater. The water table under most of the Tucson basin is hundreds of feet down. A pool excavation stays dry, which removes one complication and adds another: fill placed in a dry hole in a dry climate has to be wetted on purpose to compact properly, and it stays vulnerable to collapse if it was placed too dry.
Why a filled pool settles
A residential pool in Tucson is commonly five to eight feet deep at the deep end, with a shotcrete shell about a foot thick under the plaster. Remove the shell and the hole is nine feet deep. Nine feet of fill is a significant embankment, and three things happen to it.
- It compresses under its own weight. Every layer carries the layers above it. In well-compacted granular fill most of this happens during placement and in the months after. On nine feet of properly compacted fill the remaining movement is small and, more to the point, it is predictable, which is what allows a foundation to be designed for it. In loosely dumped fill it is large and continues for years.
- It can collapse when wetted. Fill placed dry of its optimum moisture behaves like the native collapsible soils described above. Water reaches it, the structure drops, and the drop is sudden and uneven. This, not slow compression, is the settlement that damages foundations in Tucson. Moisture conditioning at placement is the control: the fill is wetted to at or slightly above optimum before it is compacted, so the collapse has already been taken out of it.
- It moves differently from the ground around it. The fill and the undisturbed native soil beside it have different histories and different stiffness. The boundary between them is a plane of differential movement. If the excavation walls are left vertical, that plane is vertical and sharp. Benching the walls into steps blurs it, and keeping any future footing entirely on fill or entirely on native, or carrying it through the fill to native, removes it from under the structure.
A shell left in the ground with holes punched in the floor is a fourth case. The rubble and the void spaces between broken concrete are not fill; fines migrate into the voids over time and the surface drops in patches. That method is appropriate for a yard that will stay landscape. It is not a base for anything with a footing, and no engineer certifies it as one.
What a lift-by-lift compaction test establishes
Compaction control is a chain of tests, each one meaningless without the one before it.
- The material is tested when it arrives. A sample of the fill goes to the laboratory for grain-size distribution, plasticity, and the Proctor test that produces its maximum dry density and optimum moisture content. A collapse or expansion test may be added for the soil types described above. Fill that fails is rejected before it goes in the hole. This step is the reason “engineered fill” means something: the standard is set for the specific material, not assumed.
- Each lift is tested in place. The geotechnical engineer’s technician measures the in-place density and moisture of each lift, typically with a nuclear density gauge (ASTM D6938), and reports the result as a percentage of the laboratory maximum. A lift that does not reach the specified percentage is reworked and retested before the next lift is placed. The test locations, results and lift record are logged.
- The engineer certifies the record. The stamped letter states what material was placed, what it tested at, what standard was specified, and that the fill met it. It is a record of what was done. It is not a design for what will be built.
The percentage matters less than people expect and the moisture matters more. The building code floor for fill under a shallow foundation is 90 percent of the modified Proctor maximum. Structural fill in Arizona is commonly specified at 95 percent. The difference between 90 and 95 is real but modest; the difference between fill placed at optimum moisture and fill placed dry is the difference between a predictable fraction of an inch and a collapse.
What the building code requires
Section 1804.6 of the International Building Code, adopted across Pima County, requires that where a shallow foundation bears on compacted fill more than twelve inches deep, the fill comply with an approved geotechnical report. The International Residential Code, at R401.2, requires fill soils that support footings to be designed, installed and tested in accordance with accepted engineering practice. Nine feet of fill under a casita is not a twelve-inch fill. Whoever builds on a filled pool will need the record described above when they apply for the building permit, and the Town of Oro Valley says so explicitly on its pool permit page: where the area will support a structure, the shell must be removed in full, the excavation backfilled with approved engineered and compacted fill, and a geotechnical report provided by a licensed engineer.
What Omni does on a removal prepared for the future
Most owners who ask about building on the spot do not yet know what they would build, or whether they will. The work is the same either way. On a removal the owner wants prepared for possible future construction, Omni Pool Builders:
- Removes the shell, floor and walls, in full, and hauls it off site. Nothing from the old pool goes back in the hole.
- Has the fill material sampled and laboratory tested when it arrives on site, before it is placed.
- Places the fill in lifts, moisture conditioned, each lift compacted before the next.
- Has every lift density tested by the geotechnical engineer’s technician to a minimum of 95 percent of the laboratory maximum dry density, with failed lifts reworked and retested.
- Delivers the engineer’s stamped certification of the material, the testing and the result, and keeps it with the demolition permit record.
Omni does not design foundations, does not specify or assume a structure, and does not represent that the ground will not move. The certification records what was placed and how it tested. When a designer or engineer is already involved before the removal, their requirements for the fill are followed and take precedence. Anything beyond the certified fill, such as deepened footings, piers to native soil, a stiffened slab, or a different fill material, is a decision for the engineer who designs the future structure, and Omni acts on it only when that engineer directs it in writing.
What the owner does afterward
Keep water off the fill. Finish grade should drain away from the old pool footprint. Do not run irrigation over it, do not place a water feature on it, and repair any leaking line at once. This is the same advice the Arizona Geological Survey gives for every foundation in the state, and it matters more over a nine-foot fill than anywhere else on the lot. Keep the certification with the property records; the Arizona Seller’s Property Disclosure Statement asks whether a pool was removed, capped or filled, and a buyer’s inspector and lender will want to see how.
Common questions
Will it settle? A small, predictable amount, most of it while the fill is being placed. That is what the testing and the moisture control are for. Dry, untested fill settles a large, unpredictable amount.
How much? The engineer’s letter is the document that answers that for a specific fill. Nobody should quote a number on the phone.
Can I build on it right away? Design can start right away. Whether footings bear on the fill or are carried through it is the engineer’s decision for the specific structure.
Is 95 percent enough? It is the common structural fill specification and above the code floor. An engineer may specify more in the zone under footings and may specify the modified rather than the standard Proctor. Whatever the engineer specifies governs.
Why not fill it with concrete? Flowable fill, a low-strength cement slurry, is an option some engineers choose when construction will follow within months. It costs more per cubic yard than compacted soil. It is an engineer’s call, not a default.
Does a partial removal work for a patio? A paver patio on grade over a landscape removal is common. A patio slab with footings, a ramada, or anything attached to the house belongs in the full-removal category.
Get a written quote
The build-over removal page describes the service and its sequence. The pool removal hub covers the standard options and prices. Run TerraSpec, the pool removal cost calculator, or call (520) 222-8503 for a site visit.
Sources
- Farmer, I. W., and Glynn, M. E., Location of collapsing soils in the Tucson Basin, Engineering Geology, 1990: https://www.sciencedirect.com/science/article/abs/pii/001379529090042Y
- Arizona Geological Survey, Problem Soils: https://azgs.arizona.edu/geohazards/problem-soils
- Arizona Geological Survey, Earth Fissures and Subsidence: https://azgs.arizona.edu/geohazards/arizona-geohazards-resources/earth-fissures-subsidence
- U.S. Geological Survey, Potential for aquifer compaction, land subsidence, and earth fissures in the Tucson basin, Pima County, Arizona: https://pubs.usgs.gov/publication/ha713
- U.S. Geological Survey, Stratigraphy and tectonic history of the Tucson Basin: https://pubs.usgs.gov/publication/sir20045076
- ASTM STP, Strength Characteristics of Caliche Soils of the Tucson Area: https://store.astm.org/stp28920s.html
- International Building Code 1804.6 and International Residential Code R401.2, as adopted (text via a published adopted-code copy): https://seattle.gov/documents/Departments/SDCI/Codes/SeattleBuildingCode/2021SBCChapter18.pdf
- Town of Oro Valley, Apply for a Pool or Spa Permit: https://www.orovalleyaz.gov/Government/Departments/Community-and-Economic-Development/Services/Apply-for-a-Pool-or-Spa-Permit
- Arizona Association of Realtors, Residential Seller’s Property Disclosure Statement, line 135: https://www.aaronline.com/wp-content/uploads/2023/01/20/Residential_Sellers_Property_Disclosure_Statement_SPDS_Feb_2023_.pdf