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Is this something one must consider when constructing a pool beside a residential property?

posted May 8, 2022, 6:04 PM by jeffery jim
The answer is partially true and depends on the ground condition. As a professional engineer, I am less worry about the foundation. I am worry about the swimming pool instead. Let me explain.

The reason why it is highlighted that an excavation can trigger settlement or partial settlement to the property is due to bearing capacity of the soil. For those who understand geotechnical engineering, settlement issue is less likely to occur in most commercial properties in Malaysia. Most properties are resting on piles and hence there will be slight reduction for skin friction of the pile. In case the property is sitting on end bearing pile, this is less likely to be a worry at all. Issue about lack of soil and even overhanging pile will not be an issue.

For civil engineers with clear conscience, the real issue here is the swimming pool. If the property (single storey) is resting on pad footing, the bearing is a major concern as it carries and distribute load accordingly to the ground and not to bearing (harder ground). Assuming that the standard design is 100kPa, which is around 10 tons per meter square. The allowed imposed load or quasi permanent load will be able to perform accordingly since this particular load may only be around 2,500kg per meter square. This factor is more than sufficient.

In most constructions, soil treatment is one way to go forward for soil with extreme ground condition. Usually, excavation will take place until the depth of 3.0 meter. Geotextile (separator) is placed before backfill with suitable material. In this condition, the bearing would reach until 100kPa. How about soil strata below the treated zone? Say, it reaches 30kPa for peat or organic soil. What will happen in the long run? In this case, it reaches equilibrium where the soil's bearing is just sufficient to carry the quasi permanent load of the water.

The issue will arise when it comes to long run operation. Organic soil have high degree of compressibility as voids will fill the space and the reduction of water table. One only can forecast this state of settlement via Oedometer testing. In some instances, certain soils may reached 300mm settlement over a period of a month. Changes due to settlement can create issues.

1. Differential settlement which is triggered by different rate of settlement will lead to cracks to the swimming pool. This will further aggravate the situation if cracks of more than 0.3mm formed and water seepage is expected. When water seepage occurs, propagation of cracks will happen and cause uplifting and bulging. Areas which are not affected will remain or continue to settle and tilt depending on the rigidity of the swimming pool structure.

2. Water discharged and infiltration will lead to the weakening of the property's bearing. If the property is sitting on piles, then infrastructures such as stormwater drainage and sewer pipes could be deformed over a period of time. If the property is sitting on pad footing over the treated soil, the problem will be dreadful and could be hazardous. Similar to the swimming pool, the property resting of pad footing will start to experience differential settlement and over a period of time, this will trigger superficial cracks on architectural elements and it is matter of time before cracks start to plague on structural elements.

3. Only through Triaxial testing, one can only define the right diagnosis before coming out with the right remedial works. The outcome is similar to properties which experience industrial dewatering or trenching work parallel to the affected properties.

Therefore, it is less worrying when talking about an imaginary 45 degree isobar since this is not fully true and depends on the soil types which sits as bearing for property's pad footing since this may change the actual contour of the isobar. It is advisable that swimming pool issues should be first tackled or addressed before considering issues related to isobar and differential settlement which may affect the integrity of the property.