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1. can be used to mitigate structure offsets to a slope face (IBC 1808)
2. must achieve appropriate bearing or bedding below a wall line.
3. can be incorporated to achieve the 2-ft min embedment (APE required) of the pool structure at the perimeter of a shallow pool shell.
The pool geometry matters. The site matters.
A drop-stem construct provides confinement and containment of the subgrade below the pool floor and equally important, the drop-stem construct may need to provide enhanced bearing for additional loads (from the pool floor) at the perimeter wall line. Total loads may vary from 1500 to 5000 plf or more.
Drop-stem pools do not usually have the benefit of an external soil mass to resist lateral loads - the drop-stem must mitigate the loss of passive resistance of the a typical at-grade pool: the mechanical translation of the force to the soil mass below the pool floor.
Resistance to lateral loads: When a trough or basin is set below a weir-wall (vanishing edge) a large "key" is introduced to check lateral translation of the global model. Not too much effort goes into the force model when pools have adequate embedment on all sides, or an embedded basin. A pool next to a slope that has no physical key cannot always rely on bottom friction to service lateral loads.
Without positive confinement, the bedding area of the floor may need to be benched (stair stepped) down the grade toward the drop-stem. This element must be evaluated by the geotechnical engineer at the time of excavation. Some math (no load factors) for a primitive sample:
Sliding Shear (V), or lateral force, has two components in the drop-stem.
Vc for concrete (12" wide stem, 3000 psi concrete) plus
Vs for steel (assume #4 vertical rebar Gr 40 12" cc).
Total V resistance is (Vc + Vs).
Consider a 1-foot wide horizontal strip (shear plane) through the drop-stem.
Vs = (0.20 sq.in)(40 ksi) = 8000 lb resistance
Vc = (12")(3000 psi) = 36000 lb resistance
Vc+Vs = 44000 plf total resistance to lateral movement
Seismic Load. Lateral force on small pool - say 15'x20' could be 2500 plf.
Resistance (44k klf) >> Lateral Load (2.5klf)
It appears the drop-stem capacity far exceeds the lateral demand, however, this primitive case is deceptive. The full-section capacity of the drop-stem section cannot be used because the drop-stem is not infinitely rigid or fixed to the grade below. Significant capacity reductions need to be factored to consider actual soil conditions at the site. Actual drop-stem resistance may be only 10% of the calculated value.
Some of the seismic load will be serviced by friction between the pool floor and the excavation.
If the pool floor was not benched down toward the drop-stem, piers may be required to service seismic lateral loads. The geotech must assess the global conditions at the site during excavation.
Soil to Drop-stem interface will require additional prep work in the trench (glove-out, vacuum extraction, slurry with mechanical consolidation, rock-pack?, perhaps stage 1 concrete - stage 2 shotcrete. Prep varies with site conditions. The scope of work is indeterminate until the excavation exposes the subgrade.
Drop-stem trench - 24" depth? prep with glove-out? vacuum? slurry or rock prep?
Drop-stem trench - 36" depth or more? with double curtain? out-of-reach. how do you clean that?
You won't know the actual depth of required engagement for a drop-stem until the geotech directs the excavation.
Drop-stem projects should be assigned with care. Drop-stem projects should not be a standard plan detail, too many moving parts.
Hillside pools should always be carefully screened by the geotech to assure that drop-stem mitigation is adequate. When in doubt, "piers make partners."
* * * * * * * * * *
and this... from a central valley builder (I suspect this story gets ample recycling, author unknown):
A builder, surveyor, and a pool engineer drove out to see a job at a remote site. The truck broke
down during their trip and there was no cell service. They hiked a few miles back to a farm they'd seen on their journey and found the farmer. They explained their trouble, and the farmer apologized, no phone at the farm... The farmer offered them shelter for the night, however one of them would have to bunk out in the barn with the pig and the cow; the farmer explained these two animals could get a bit frisky. After dinner, the surveyor offered to sleep in the barn.
Off they went to bed.
10:pm - all seemed well, and then there was loud knocking at the rear porch. The farmer answered the door. The surveyor was standing there. He was very distraught and he complained that the pig and the cow were constantly making a ruckus: kicking, banging, snorting, whooping, squealing, mooing, running about and causing general mayhem. And the stench! The stench was unbearable! He could not settle the animals and he could NOT sleep in the barn. "Humans should not be exposed to these animals!"
The builder heard all the commotion and came downstairs. "LAND SAKES! I was reared on a farm, I'll sleep in the barn."
Off they went to bed.
12:am - all seemed well, yet again then there was loud knocking at the rear porch. The farmer answered the door. The builder was standing there. He was ashen, terribly distraught and he exclaimed that the pig and the cow were constantly making a ruckus: kicking, banging, snorting, whooping, squealing, mooing, running about and causing general mayhem. And the stench! The stench was unbearable! He could not settle the animals and he absolutely could NOT sleep in the barn. "Humans should not be exposed to these
animals!" The pool engineer heard all the commotion and came downstairs. "JUMP'N JUNIPERS! How bad can it be?, I'll sleep in the barn."
Off they went to bed.
12:15 am - all seemed well, and yet again there was loud knocking at the rear porch. The farmer's hospitality was thinning, however he answered the door anyway; it was the pig and the cow...
I get that kind of love everyday.
Have a good week.