Tank Differential Settlement
Tank Differential Settlement
(OP)
I'm providing geotechnical recommendations for a 72 foot diameter water tank 20 foot height. Site at ridge top. About 3/4 of the tank footprint will bear on moderately to slightly fractured sandstone bedrock, expected settlement negligible per borings to 50 plus feet-the rock just gets more competent with depth. About 1/4 of the tank daylights out of the slope and will require engineered fill for support. The max fill height is about 11 feet this to include excavation and removal of unconsolidated colluvium overlying the sandstone. I am concerned about differential settlement of the portion of the tank on fill in relation to that on the rock. Calculated loaded tank settlement on fill by using Janbu and elastic modulus values from the literature commensurate well compacted gravels. At the edge of the tank, settlement of results are on the order of 0.4-0.75 inches. This is not taking into account OCR associated with the compaction effort-therefore arguably an overconservative assessment. Conversely, common rule of thumb for settlement of granular material under self weight is 1/2 percent of height, i.e in the same ballpark.
I will specify fill gradation, lift thickness, and compaction requirements i.e. Proctor plus field test strips. With little tank experience, I don't want to miss something obvious due to ignorance. Structural needs to know what they must design for or if site plan/tank size must be adjusted.
I will specify fill gradation, lift thickness, and compaction requirements i.e. Proctor plus field test strips. With little tank experience, I don't want to miss something obvious due to ignorance. Structural needs to know what they must design for or if site plan/tank size must be adjusted.





RE: Tank Differential Settlement
RE: Tank Differential Settlement
RE: Tank Differential Settlement
Cast in place reinforced concrete is proposed. I've informed structural designers of 0.5 inch potential differential settlement, awaiting reply.
Soil cement is an intriguing idea, I've seen mass production of soil cement, silty sand soil, using a pugmill. Can well graded gravel be mixed successfully with a "tilling" methodology? Wouldn't this technique prove less expensive than the mill?
JedClampett, A concrete slab 10'width will surround the tank, properly graded to drain this should prevent fill under the tank load from becoming wetted.
RE: Tank Differential Settlement
RE: Tank Differential Settlement
Elastic settlement using conservative modulus values with a FOS of 2 results in about 0.5 inches. I have never spec'd a fill this thick to support a structure, my experience with fills of 6-7' our local gravels and good construction control leads me to believe that the settlement will be 0.5 inches or less. A belief not strong enough to give it the title of engineering judgement. Yet.
RE: Tank Differential Settlement
RE: Tank Differential Settlement
RE: Tank Differential Settlement
Dik
RE: Tank Differential Settlement
Given a maximum of 0.05 feet of differential settlement over 20 linear feet can a concrete tank be designed to prevent cracking and consequent leakage?
If not then what is acceptable? How are the limiting values of deflection ratio and angular distortion determined?
EM-1110-1-1094 and other references familiar to me give ranges of values for acceptable limits of angular deflection for structures. I assume there is a reference for tanks-I just don't seem to be coming up with one via internet search and my references.
Seems we could limit the quantity of cement treated fill and or deep foundations to only areas required given a limiting value of angular deflection.
RE: Tank Differential Settlement
RE: Tank Differential Settlement
I concur with mr. Hookie66 9(structure)suggestion above.
Although it may look expensive but the whole tank foundation anchored on the rock you may eliminate differential setlemnt and other unknown which may occur in the future.
RE: Tank Differential Settlement
RE: Tank Differential Settlement
I am also exploring the idea of preloading with placed fill to simulate tank loading for two to three weeks. Fill would then be removed and concrete poured.
RE: Tank Differential Settlement