RHCE Newsletter: NZ Special Edition Performance of Compass Pools in the Christchurch earthquake
Compass Fiberglass Pools 2: Concrete Pools In May 2012, Compass Pools Group Engineer Charles Rickard visited Christchurch to review the performance of the Compass Pool range in the recent major earthquakes. There he met with Mr Stuart Laing of Compass Pools NZ and Mr Jim Robinson of Southern Pools and Spas.
Stuart Laing explained that his company had been contacted by one particular insurer regarding the need to repair some 1800 swimming pools, the majority of which were made of concrete. The majority of the actual concrete shells were not structurally damaged but were out of level by typically up to 120mm. Stuart said that he had only had one Compass fiberglass pool damaged by the earthquake.
The damage to the fiberglass pool had been a buckle in the fiberglass floor. As a consequence of this, he had removed the coping and reinstalled the original shell since it was not permanently damaged. Further, he explained that there had been significant damage to fiberglass pools installed by other companies, generally because the fiberglass shell had not been adequately tied to the concrete coping.
So how does this record of performance allow us to assess the performance of a concrete and fiberglass pool in an earthquake? We suggest that there are two key causes;
Cause A; Force and movement due to the inertia of the swimming pool itself. A 9m x 4.5m concrete pool shell weighs approximately 40 tonnes. When full of water, the combined weight is approximately 100 tonnes. Earthquakes are measured in terms of acceleration . The higher up the Richter scale, the greater the acceleration. A moderate earthquake might have an acceleration of around 0.25 x gravity. Therefore, the force created by the earthquake would be 25 tonnes force. So what resists that force? The only thing available is the friction in the soil itself but at the same time, the soil is often being liquefied. So, the concrete pool will inevitably move, thought the strength of the concrete shell is such that it is unlikely to be damaged.
A fiberglass pool shell weighs less than 2 tonnes. A 600mm concrete coping on a 9m fibre-glass pool weighs 6 tonnes. When full of water, the combined weight is 63 tonnes. For the same level of acceleration, this creates a force of 13 tonnes i.e. approximately 50% of that of the concrete pool. Clearly one cannot guarantee that the pool will still not move under this particular cause. However, evidence from the Christchurch earth-quakes suggests that if the fiberglass pool is appropriately tied to its concrete coping, there is sufficient resistance to maintain a satisfactory pool level, no doubt assisted by the flexibility of the pool shell.
Cause B; Floatation forces generated on the pool due to liquefaction of the soil. The greatest damage to low level structures in Christ-church was caused by soil liquefaction. As you are no doubt aware, whole suburbs of properties adjacent to the river Avon will have to be demolished . The high level of water in the ground meant that suddenly buildings were floating in a liquid of density say 1200 kg/m3instead of being founded on firm material. The net result is that the dwellings were so badly distorted due to the liquefaction of the ground that repair was impossible. On the other hand, a tank set in the ground is now subjected to not only Cause A, but also uplift due to the buoyancy. This results in the tanks being pushed out of the ground as illustrated in the Picture A. A concrete pool full of water, when subjected to buoyancy forces from a liquefied soil, will become more or less weight-less and therefore more vulnerable to the inertia force (Cause A). However the pool shell is unlikely to be dam-aged. Alternatively, the fiberglass pool is not able to withstand that pressure and the floor will logically buck-led as occurred with the Com-pass fiberglass pool described by Stuart Laing.
Rectification – How do you rectify a pool which has been disturbed by an earthquake? A concrete pool shell is structurally sound but is sit-ting partially out of the ground. It has probably dam-aged all of the perimeter landscaping. Repair there-fore requires either the releveling of the concrete pool at a new level, perhaps utilising the ‘Uretek’ injection process, or the pool has to be demolished and rebuilt to the original level. If you utilise the Uretek injection method, then the landscaping must be remodelled to suit the new level which may be an unacceptable due to the position of the pool relative to nearby internal floor levels. Plumbing lines will need renewal either way.
The fiberglass pool on the other hand, requires demolition of the coping and removal of the fiberglass shell. The flexibility of the fiberglass material is such that the distortion is non-permanent and allows us to reuse the same shell to be reinstalled. Obviously, as above, new plumbing lines and a new concrete coping is required, but set within the original landscaping model.
Conclusion The additional inertia forces on a concrete pool in an earthquake leads to a much higher cost of rectification than with a fiberglass pool. Experience in the 2010-2011 Christchurch earthquakes have proved that the fiberglass pool is better able to withstand the impact of an earthquake than a concrete pool.
So, Compass Fiberglass Pools 2: Concrete Pools? 1. A fiberglass pool is less likely to be damaged than a concrete pool. 2. A fiberglass pool is less costly to repair than a concrete pool.