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GEO (CEO) 于 (Wed Aug 22 20:47:38 2007) 提到:
为个两层的公共建筑提供基础设计参数，暂时没有确定是桩基还是浅基础。
粘土层的三轴试验是应该做固结不排水（CU）还是不固结不排水（UU）。
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trackbed (涮羊肉) 于 (Thu Aug 23 00:52:57 2007) 提到:
CU
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GEO (CEO) 于 (Thu Aug 23 10:18:11 2007) 提到:
can I ask why? any details?
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trackbed (涮羊肉) 于 (Thu Aug 23 14:16:05 2007) 提到:
not sure what is your concern. but for clay, typically settlement is a big
issue, and CU is used more extensively than UU for this purpose.
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wzg (Michael) 于 (Thu Aug 23 16:03:14 2007) 提到:
I also think it should be 固结不排水（CU）.
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GeotechPE (Dirt Guy) 于 (Thu Aug 23 19:42:12 2007) 提到:
I don't know what to say, man.
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GeotechPE (Dirt Guy) 于 (Thu Aug 23 19:42:58 2007) 提到:
why?
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GeotechPE (Dirt Guy) 于 (Thu Aug 23 20:08:58 2007) 提到:
UU is for sure
CU is optional (in case you have deep foundation or need surcharge the area)
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tootu (太土) 于 (Fri Aug 24 00:22:54 2007) 提到:
Both UU and CU are for strength testing. Consolidation test
is for settlement purpose. Normally UU yield less shear strength than
CU. So the answer for the original question is UU. The purpose is to
check bearing capacity.
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trackbed (涮羊肉) 于 (Fri Aug 24 12:53:15 2007) 提到:
sorry, I was wrong about consolidation, but I think UU is used for soil that
has high permeability, for clay, should use CU to get shear strength (c and
phi), isn't that right?
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myy (myy) 于 (Fri Aug 24 14:00:45 2007) 提到:
Is UU used for the soil with high permeability? why?
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tootu (太土) 于 (Fri Aug 24 15:39:56 2007) 提到:
Both UU and CU are for clay. Both UU and CU are for strength parameters (c
and phi). Neither of them has anything to do with permeability. UU test
yields phi=0 for saturated clay. UU test result is typically for short-term
strength analysis (undrained). CU test result can be used for either short-
term (short-term in the future) or long-term (with measured pore pressures)
purposes.
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trackbed (涮羊肉) 于 (Fri Aug 24 18:00:41 2007) 提到:
Thanks, man
This clarity the difference, but still why should be UU for this case? since
CU can be used for both short and long term?
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GeotechPE (Dirt Guy) 于 (Fri Aug 24 21:04:11 2007) 提到:
Think about when the failure could happen
Case 1.During construction
Case 2.Shortly after construction
Case 3.Long time after construction
Clay material has low permeability, so in Case 1 & 2, you can consider it is
undrained condition. Therefore, pore pressure can easily be built upon and
the pore pressure will reduce the effective stress of clay. Most of the time
, Case 1 & 2 are more critical because of the undrained condition.
For LZ's question, you need make sure that the building can be built first (
bearing capacity). It is Case 1 & 2 (undrained condition, so UU test)
You may need check long term stability too (drained condition, so CIU test).
For LZ's case (2-floor building), I don't think long term will be a problem
unless it is built on a slope or some other situations.
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xingguang (xingguang) 于 (Sun Aug 26 13:51:36 2007) 提到:
什么样的两层公共建筑？为什么要做三轴？
一般土的承载力，我认为做直剪就够了，主要原因是Buget和时间的考虑。如果非要做
三轴，那么首先考虑的是UU.原因是UU相对于CU成本低，试验需要的时间短，据此设计
偏保守。对同一粘土层，如果做20个直剪和做一个CU相比，你感觉哪一种更可靠！
做CU的目的是为了获得高的土的强度参数来减少设计成本，但作为岩土工程师，没有人
愿意承担高的风险。
如果从学术角度来看，施加土的上部荷载进行固结，然后在建筑荷重下做不排水剪。
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pandagao (熊猫高-趁年轻,轻裘快马天下行) 于 (Sun Aug 26 14:04:34 2007) 提到:
大多数情况下是排水，有些较特殊情况要考虑不排水，比如在施工阶段迅速增加的荷载
下，一般认为孔隙水没时间排出，这时就需要考虑不排水。
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myy (myy) 于 (Sun Aug 26 17:11:06 2007) 提到:
UU and direct shear should be enough. There is no need to do CU.
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GeotechPE (Dirt Guy) 于 (Sun Aug 26 17:56:47 2007) 提到:
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GeotechPE (Dirt Guy) 于 (Sun Aug 26 18:09:35 2007) 提到:
Copied from a textbook.
Disadvantages of the DS test include:
1. Difficult or impossible to control drainage, especially for fine-grained
soils.
2. Failure plane is forced--may not be the weakest or most critical plane in
the field
3. Non-uniform stress conditions exist in the specimen.
4. The principal stresses rotate during shear, and the rotation cannot be
controlled.Principal stresses are not directly measured.
Because the drainage conditions during all stages of the test markedly
influence the shear strength of soils, the direct shear test is only
applicable for relatively clean sands which are free draining during shear.
For clay soils, some unknown amount of consolidation could occur during
shear, which would give a larger shear strength than actual. Therefore the
test is not generally
recommended for cohesive soils.
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rongjiang918 (littlekitty0103) 于 (Thu Aug 30 22:38:46 2007) 提到:
I would say UU & CU.
UU for short term, CU for long term.
For normally consolidated clay and loose sand, short term is critical, for
over-consolidated clay and dense sand, long term is critical.
SHANSEP is pretty good test, only using CU, if you can accumulate your local
data, because fewer tests and lower total cost.
