Aerial photo of Cayucos’ Hardie Park Pool shows where the original water leak was found by the restrooms [green circle at lower left]. The red stripes are where further digging found high groundwater saturating the underlying clay soil just 3-feet below the surface. Photo courtesy SLO County
Problems with Cayucos’ Hardie Park Pool are so serious that it may require the pool’s demolition and replacement, according to information obtained from County Parks.
The community pool, owned and operated by SLO County was closed for the summer in May after discovery last fall of a leaky pipe under the bathrooms. Subsequent investigations of the property and the underlying soil supporting the pool uncovered much more serious problems.
An engineering firm, MSD Engineering, Inc., was brought in last September to study the extent of the bathroom water leak. What they discovered is a serious issue with the very soil underneath the pool that apparently dates back to 1979 when the pool was built.
“During the initial on-site evaluation provided on Oct. 8, 2025,” reads a summary letter to the County from MSD dated May 14, 2026, “it was noted that the bearing and foundation support soils around the bathroom area were completely saturated.”
The company dug some hand-auger bore holes down 30-feet below the base of the pool decking to see what soils were underneath supporting the pool.
The bore holes, “identified what appeared to be expansive clay soils consistently present around the pool areas.”
Expansive clay soil, aka “shrink-swell” soil, can absorb water and swell, and then shrink back down when dry. The volume change can be more than 10% and exert immense pressure on foundations, causing cracked walls, and damaging underlying infrastructure. It is apparently not a good foundation for swimming pools either.
“Due to the risk of saturated expansive clay soils to existing structures such as pools and bathroom buildings as well as site improvements such as pool decking and supporting equipment, it was determined that additional sub-surface investigation was warranted to better understand whether the water identified was localized and simply due to the leaking pipe or an indication of more widespread shallow groundwater present around the whole park.”
The company dug four more holes around the outside of the pool enclosure and only went down 3-4 feet before they hit groundwater that filled the holes in as they dug.
“Shallow groundwater,” MSD wrote, “backfilled the excavations as they were dug (or shortly thereafter) with a consistent presence of clayey soils. Although the leaking pipe could have caused localized saturated soils adjacent to the bathroom, the uniform presence of groundwater at all four locations suggested a more widespread issue.”
MSD then told the County they needed to bring in a geotechnical firm to do further testing and GeoSolutions, Inc., was contracted to do that work. MSD explained the risks.
“The greatest risk identified during the preliminary evaluation was that if the shallow groundwater and expansive soils were present consistently around and below the pool, then during the draining process the pool could lift due to buoyancy forces.
“If the pool were to lift, this would result in a complete failure of the existing pool structure as well as any interconnected pressurized water piping.”
In March GeoSolutions went to work, first waiting a couple of weeks after the last significant rainfall, and discovered that the high groundwater was indeed everywhere.
“GeoSolutions, Inc. found that the soils around the entire pool property were generally classified as clays and fat clays, saturated starting at depths as shallow as 2-feet below existing grade, extending down through the soil materials below the deeper bearing depths of the pool.”
The lab results were not good. “The laboratory results demonstrate the soils on-site are well above the ‘high’ plasticity threshold and in the saturated condition identified, represent a need for significant care and consideration within the scope of the proposed project.”
Plasticity is the ability of a solid material to undergo permanent, irreversible shape change when subjected to external force (e.g., bending a metal paperclip or molding wet clay),” according to an internet search.
MSD said the lab tests confirmed their concerns that if the pool is drained, it could pop out of the ground from the water pressure. They are also concerned about what could happen if the County was to simply pump down the groundwater.
“Due to the nature of the soil conditions identified on site,” MSD’s report said, “it can be concluded that challenges associated with dewatering the property for the purpose of performing the pool maintenance work could include subsidence of sub-surface soils resulting in potential damage to pool equipment, piping, as well as adjacent private and municipal structures and features such as sidewalks, roads and buildings.”
The pool is bound on one side by a storage facility, and by roads on the other three. However, there are other structures nearby including a County owned tennis court, and Cayucos School is across the street.
MSD and County officials searched the records to see if there had been anything installed during construction to counteract the buoyancy and expansive soil situation. They couldn’t find the original documents, but did find some maintenance records.
“Although no formal original construction documentation could be found,” MSD reported, “reports of previous repairs and other issues with the pool strongly suggest the existing pool conditions represented a high risk of failure during the proposed repair process.
“Previous reported maintenance work on the pool included a 2019 fiberglass repair as well as maintenance on the leaky skimmers.”
Apparently, the County did know about the soils and that no new materials were brought in to counteract it. Also, they apparently don’t make ‘em now like they used to.
“It was found that no non-expansive import below existing pool decking or around the bathroom building was identified and the saturated clays were observed to be consistently present at shallow depths around the entire property.
“This strongly suggested that sensitivities to the clay soils that would be used on contemporary projects were not introduced into the original pool build.”
All this has led the County to begin talking about scrapping the repair project they had planned and replacing the entire pool, presumably built to modern standards with measures taken to deal with the clay soil and high groundwater.
“It should be noted,” MSD said, “that although no obvious damage was observed to the existing bathroom building as a result of the leaky water pipe, due to the relatively shallow footings and the saturated expansive clays supporting the structure, future settlement or heaving could occur as a result of potential moisture fluctuations to the bearing soils.”
Estero Bay News asked Shaun Cooper, the Assistant Director with County Parks & Recreation about the pool situation.
He said the original project they intended to do before all this was discovered was pretty standard maintenance.
“The initially planned improvements,” Cooped told EBN, “included re-plastering the pool, replacement of the pool deck, and ADA accessibility upgrades.”
In the notice of closure, the County mentions public safety concerns. What exactly are those concerns? “Protecting the health and safety of community members remains the County’s highest priority,” Cooper said. “Out of an abundance of caution, the pool will remain closed while the County completes a comprehensive engineering assessment and determines the repairs necessary to safely operate the facility.”
The County notice of closure also said it would take 9 months to complete its engineering study. Where is that process now?
“The County’s Public Works Department is currently preparing a request for proposals to solicit proposals from structural engineers to evaluate the full extent of the damage, identify repair alternatives, and develop associated cost estimates and timelines.
“That evaluation process is expected to take approximately 9 months.”
He added that once they get the full engineering report, the staff will “evaluate next steps, funding needs, and a future timeline for repairs and reopening,” Cooper said.
Meanwhile, he said the County’s other public pools are all open for summer with the nearest one to Cayucos being at Cambria’s Shamel Park. And for folks in Cayucos who want something closer, the Morro Bay Recreation Department runs several aquatics programs at Morro Bay High School’s pool.
See: www.morrobayca.gov/1019/Aquatics for the schedule.
In what could be considered a bit of irony, when Morro Bay High’s original pool, which had been there since the school was built in 1959, needed to be drained, it was also feared that high groundwater would pop the pool out of the ground and destroy it.
The School District decided to close it and build a new one when the school was to undergo extensive remodeling and upgrading. It took over 15 years for that to finally be realized.
