RHCE Newsletter – Special Points of Interest
RHCE Newsletter: Special Points of Interest; Pool Bedding Retaining Walls
Retaining Wall Pressure on Swimming Pools
At the Cairns conference , and two or three afterwards, many of you will remember how I told you that a fiberglass pool is designed to carry soil pressure level with the top of the pool whilst empty of water. You may also remember that I explained that this design decision is to allow for water pressure externally to the top of the pool during normal operation. Without external water pressure, there is more or less no net pressure across the pool wall when one has water pressure inside and soil pressure outside of the pool.
The problems come with surcharge due to sloping sites or imposed loads such as cabanas and retaining walls. Subsequent to those early conferences, you may also remember how I said that you had to be particularly careful when building a swimming pool in front of a retaining wall. I explained how the wall creates horizontal and vertical pressure in front of it in order to withstand the soil pressure behind. I went further to say how that pressure created affects anything in front of the wall within a horizontal distance of 7 times the height of the wall. At the time, I remember someone coming up to me after the presentation and saying ‘Charlie, if we stay out of a distance of 7 times the height of a retaining wall, we will never sell a pool’. Clearly this is an issue for all of us.
Based on enquiries that I receive, retaining walls are becoming more and more of an issue. Within this newsletter, I am going to highlight two projects which I hope will be of interest to you all.
A NSW Pool We refer you to the enclosed site plan, engineering sketch and photographs (see Figures 1-3).
A pool was installed 2.5m from the base of a 2.5m high retaining wall. There is a roadway beyond the retaining wall. The retaining wall and road were already constructed prior to sale. The Compass Pools agent installed a pool at the base of the wall with a standard concrete coping. The customer later added stencil concrete between the retaining wall and the pool coping. We understand the retaining wall to be a crib type wall sitting on a strip footing. The structure is all treated timber. After 18 months, the customer noticed that a gap of 12mm had developed between his stencil concrete and the Compass coping.
The Compass agent came back and adjusted the paving on the concrete coping in order to disguise the movement. Another 18 months passed and there had been further movement of 10mm max. The agent asked me to inspect the pool. Referring to Figure 3, the retaining walls are pushing the pool towards the dwelling as indicated. I estimate there to be a horizontal pressure on the fiberglass pool of around 60KN/m2in excess of normal pressure which you might estimate to be 10KN/m2 . The overall deflection of the concrete bond beam is now 22mm which is still within AS1839 for a 10m pool. My inspection of the pool shell revealed that there was a very slight in-ward bow of the wall above and below the step ledge. It is to the great credit of the pool that it is withstand-ing this pressure. Further, if you look at the joints around the coping, the joint closest to the house has been compressed while the other has opened up.
For rectification, we have now strapped the Compass Pool coping to the stencil concrete behind and will reset the coping stones a second time. This is the most graphic demonstration that I have ever witnessed as to the lateral pressure exerted on the pool by an adjacent retaining wall.
So How Do You Avoid This Problem?
Refer to Figures 4 and 5. Clearly any solution will vary with soil type. This particular concept is for a clay site. One could build the wall and footing with its piers separate to the pool or attached to the pool depending on loading. One attaches piers to the foot-ing at spacings no more than 3 times the diameter of the pier. The reason for this is that it is proven that a clay soil will arch between piers at those centres. Thus the horizontal pressure will be taken onto the piers, projecting below the pool but also fixed to the concrete bond beam. There are other methods however the key point which I am trying to emphasise here is to focus on the additional pressure on the pool, once completed. This solution would not work for a sand site.