Stainless Steel Replacement
Community Swimming Pool for the City of South Gate, California
By Richard Hess, S.E., SECB, F. ASCE
This is about the design and construction of a swimming pool. The unusual circumstances and the problems encountered that the reader may find interesting include:
1) The pool's sidewalls are made of stainless steel covered by a PVC coating.
2) It is located inside an existing building on shallow foundations that were not constructed in accordance with the record drawings.
3) The original pool was designed in the 1970s as a conventional reinforced concrete wall and slab construction, and then changed to an aluminum wall and floor construction without corresponding construction documents.
4) The fill material between the 13-foot deep pool and the shall building foundations, less than ten feet distant, turned out to be uncompacted granular material that collapsed upon removal of the original pool, necessitating shoring for the building.
For the owner, the City of South Gate, the overall design of the pool was based on the following criteria:
1) The first priority for the South Gate pool was the recreational element - providing for non-swimmers and recreational swimmers, including wheelchair enabled persons.
2) The existing configuration, with 1/3 shallow water and 2/3 deep water was to be reversed, providing 2/3 shallow water.
3) Primary users include: recreational swimmers (largest group), lap swimmers and children of all ages that are not "water safe: and who prefer shallower water, four high school swim teams holding 25- meter competitive events, a growing water polo group, divers at the high school level, and swim teams at the club level.
4) Occasional users include: swimmers in competitive 25- meter events and aerobic class attendees.
5) Preferences for pool features included a large set of entry stairs, an accessible ramp, a diving well to meet high school standards, maintaining the 3- meter diving board, starting the shallow end at a depth of 36 inches to increase slowly to deeper water.
6) A lap swimming area that was at least 6 feet deep.
The dimensions of the existing building and requirements for access and viewing stands fixed the size of the pool. The pool building area is 200 feet by 111 feet 3 inches wide, and the pool is 164 feet long by 75 feet wide. The depth at the deep end is 13 feet and it is 3 feet at the shallow end. One of the key advantages of the type of construction chosen, which incorporates prefabricated wall panels, is that the side walls are not dependent on the surrounding soil. In fact, these pools can be constructed partially or entirely above ground, as was done for the 2004 Olympic trials held in Long Beach, California. In that project, the pool was temporarily placed on a large asphalt parking lot.
For the Long Beach project, varying the thickness of the concrete foundation, which was poured over a waterproof barrier to facilitate later removal, compensated for the slope of the lot. Since the foundation was not embedded, lateral forces on the pool walls were resisted by carbon steel straps laid on the lot surface, connecting one side of the pool to the other. The surrounding deck was a wood and steel platform built to the level of the top of the pool. With minor modifications, that could have been a permanent installation with the advantage of having all of the piping and electrical conduit readily accessible.
Most swimming pools built in California are constructed by excavating to the desired depth in natural or compacted engineered soil, placement of reinforcing steel and then shotcreting concrete walls. When this is not feasible because of the lack of adequate soil conditions, formed concrete walls are usually employed. Formed concrete walls were designed for the community pool in the City of South Gate, California, which is located approximately ten miles south of downtown Los Angeles. The original gymnasium and pool were built in the mid - 1970s. However, for reasons unknown at this time, the concrete floor and walled pool were not built and, in their stead, sheets of welded aluminum were placed on the bottom and aluminum walls up to approximately thirteen feet high were installed around the perimeter. Now there are no drawings or calculations available for the design of the original aluminum pool.
Swimming pools are contracted for on a design-build basis. When bids were solicited for this location, a concrete pool with gunite walls was questionable because of the unknown soil material that existed inside the perimeter of the building foundations. When the existing pool was removed, could the surrounding soil be counted on to make normal cut and shoring possible and leave the building foundations with adequate lateral support?
Competitive bids were returned utilizing three types of construction: I) Shotcrete II) Formed concrete; and III) Stainless steel wall panels. Since construction would take place inside the existing gymnasium building, which is supported on shallow foundations, the shotcrete option meant that, after the existing pool was removed and the building foundation shored, the area might have to be filled with compacted earth fill. The inner area would then be cut out and removed so that the shotcrete could be applied to the freestanding walls. This would require a very large volume of soil being brought in, and then being removed from the building.
Both the stainless steel and the formed concrete wall options would require a minimum amount of excavation beyond the existing pool wall. However, as would be expected, the prefabricated steel wall panels would be more economical and take less time to construct, and this option resulted in the lowest bid. In addition, there was less disruption to the existing facility and less potential damage to the building due to installing a series of three-foot wide panels rather than constructing formwork, installing reinforcing steel, and then stripping and removing formwork for the walls. The stainless steel pool was proposed by DWR Construction Inc., who has built many similar pools on the west coast. The prefabricated panels were manufactured by Myrtha Pools / A&T Europe S.p.A. in Italy to plans made and approved locally.
Arroyo Geotechnical of Anaheim, California, performed a geotechnical investigation in August 2007. Two borings to a depth of approximately 26 feet were made, near the southwest corner of the building where the deep end of the pool was located. Due to constraints posed by the existing gymnasium building, soil borings were made only on the outside. Therefore, the amount of soil compaction inside the building was unknown.
The soil at the borings consisted of primarily silty sand. The upper ten feet was fairly dense, underlain by approximately 15 feet of less dense material over more dense material. The water table was 24 feet below grade. When construction commenced, it was found that, after removal of the aluminum walls, the area between the pool and the building foundation was filled with uncompacted material. This material caved in, exposing the building footings on the south and east sides.
When it was discovered that the soil inside and adjacent to the building foundation was unstable, the contractor called the structural engineer to observe the condition and recommend a solution. The construction of the building foundation did not follow the construction drawings that were provided. The footings were not as deep as was shown on the plans, therefore their area of influence now intersected the foundations for the new pool. The solution consisted of installing diagonal strut posts bolted to the building foundation and to concrete footing pads, which would be incorporated into the pool foundations.
Stainless steel pools, and stainless steel liners for damaged pools, have been used in Europe and in Japan for many years. Their popularity in Japan is at least partly due to their ability to withstand displacement due to earthquakes, in which concrete and FRP pool linings have failed. In Europe, it has been reported that a stainless steel pool structure has been in service for over 40 years without sign of deterioration. The structural design of a prefabricated stainless steel pool typically receives more scrutiny from plan checkers because of what is often considered a novelty in both construction and in material, and also because it must be designed to stand alone with net forces in either direction. It must be designed to resist active or at rest soil pressure when the pool is empty, putting the support struts in tension. it must also support water on the inside with the possibility of inadequate or no soil support on the outside....
