Pool construction in Bursa: what do frost, slopes and the Nilüfer plain change?

The highest temperature measured is 43.8 °C (July) and the lowest -20.5 °C (January). The annual 719.1 mm of precipitation is spread over 119.8 days; the wettest month is December with 93.9 mm and the driest is July with 14.3 mm. Annual sunshine is 2,026.8 hours and the annual average relative humidity is 69.4 percent.

From a pool engineering point of view, the two decisive rows of the table are January and August. In January the average low is 1.7 °C; this means night temperatures fall below zero for half the month, and still water in the pipework freezes. In August, a 25.2 °C average means unheated outdoor pool water stays around 26 °C; a comfort temperature of 28 °C is not reached without heating. In the four months from December to March there are 13 to 15 rainy days a month; the excavation and concrete programme avoids this window.

In Bursa the danger comes in two forms: night frosts in January and February, and multi-day freezes in extreme cold such as the record -20.5 °C. The first affects the pipework, the second the water surface and the overflow channel.

The pool design meets this load in three layers. The first layer is level: the pump and filter line and the skimmer, main drain and inlet pipes are laid below frost depth or with a fall that allows draining. The second layer is material: coping stones and joint mortar are chosen for freeze-thaw resistance; sprayed concrete combines high compressive strength, water-tightness and frost resistance, and the shell concrete is chosen by the same criteria. The third layer is operation: the pool enters winter with its water lowered below the overflow channel, its pipework drained and its surface covered; it is not fully drained.

On a sloping plot, the pool shell sits on the excavated slope on one side and on fill on the other. In this case the shell is also a retaining wall that takes the load of hillside soil and rainwater. If slope water collects behind the shell, the wall carries both earth pressure and hydrostatic pressure.

The specification meets this with three clauses: slope drainage behind the shell with perforated pipe in a gravel fill and a free outlet, the uphill wall calculated and reinforced as a retaining wall, and a toe extended under the wall on the fill side for overturning safety. On a hillside, pool construction usually also needs a garden terrace wall on the excavation side; this wall and the pool shell are separated by a joint as separate structures.

Under Archimedes' principle, when the water table rises above the pool floor, the shell is pushed upwards with a force equal to the weight of water it displaces; the formula is Fb = ρf · g · Vdisp. When full, the pool water balances this force; when empty, only the weight of the concrete remains.

Example: for a reinforced concrete pool with a 10 x 5 m floor and 1.5 m depth, when the water table rises 1.0 m above floor level, 10 x 5 x 1.0 = 50 m³ of water is displaced and the uplift is in the order of 50 tonnes. With a 20 cm floor and 20 cm walls, the same pool has a concrete volume of roughly 10 + 9 = 19 m³ and, at 2,400 kg/m³, weighs around 46 tonnes; there is no safety margin. On a plain plot, the solution is a hydrostatic relief valve, a base toe and continuous perimeter drainage connected to a sump. That is why in Bursa a pool entering winter fully drained is ruled out twice over: by frost and by uplift.

An air source heat pump and a solar cover extend the season from early May to late October. A January average of 5.4 °C means the heat pump will run at low efficiency in winter; for year-round use, a heat pump alone is not enough.

That is why indoor pools come up more often in villa and hotel projects in Bursa than in Mediterranean cities. In an indoor pool, humidity control is a larger item than heating: 69 percent annual humidity and cold outer surfaces increase condensation, and the dehumidification unit is sized to the pool volume. The thermal springs at Çekirge open a third option; in a pool fed with thermal water, the mineral content changes the choice of heat exchanger and stainless steel, and this is a separate thermal pool calculation.

Uludağ, the mountain rising above the city and known in antiquity as the Mysian Olympus, is the source of the slope and snow on its foothills, and so of the retaining and frost details. The plain of the Nilüfer River brings the groundwater and uplift calculation. The Ottoman-era baths at Çekirge show that the relationship between thermal water and pool engineering has been known in this city for centuries.

The short rule follows from this: pool construction in Bursa starts with a ground investigation, on a hillside the shell is calculated like a retaining wall, on the plain drainage is built before the shell, pipework is laid to frost depth, and the pool never enters a winter empty. If year-round use is wanted, the decision is an indoor pool with humidity control; a heat pump alone does not cope with a Bursa winter.