Wednesday, April 30, 2014

Freezing soil movements

As soil freezes, the moisture changes from water to ice, it expands by 9.08 percent. Water density goes from essentially 1.0 to 0.9167 gm/cc on the old metric system notation, gm/ml now. When a saturated soil freezes, the free water expands. The attached plastic water depends on the temperature drop whether it freezes or not. If we have a silty sand at 30 percent moisture saturated. Gs=2.65, 1 cubic metre is composed of 1476 kg solids, 443 kg of water. If the free water was 25% (25/30) of the total water 369 liters, that new volume after freezing is 402.7 liters, or expansion of 33.7 liters. Since the only way soil can expand is upwards, that is 0.0337 of the depth of freezing from phase change only.

As freezing is occurring, water is drawn to the freezing front by thermodynamic forces. This can produce ice lensing, in addition to phase change. The amount of heave can be considerable, 150 to 300 mm in a season is not uncommon, I have often seen 100 in addition to phase change. For these to happen, all you need is a frost susceptible soil, a lower source of water, and a freezing front. These often show up around water line leaks and frequently cause water line failures as these start to mellow in spring, driving the frost front down further.

When a trench is insulated and drained, through a wet sandy silt deposit, you are designing in a seasonal settlement over the trench, in addition to any line settlement.

How many people with ADHD does it take to change a lite bulb?
           Look, ... there goes a rabbit. 

Saturday, April 26, 2014

Long term vs residual

Long term soil strength for slope analysis is akin to residual strength that we see with direct shear tests. Beyond a specific shear displacement, the shear strength drops to some residual value and remains at that level. This reduced level has a phi angle, as can be seen in the unit force vs unit perpendicular loading across the shear surface. Note that there is no such thing as shear strain, there is no distance to use for nominalization.

Slope failures start the rupture surface at the bottom toe, and the shear strength of the soil drops to residual once the displacement is adequate. The individual slices of simple shear analysis assumes that there is uniform distribution of the shear stress along the circle of analysis, which is oversimplification. The rupture surface starts at the toe, and later at the scarp, and proceed to where the dramatic happens. This may take years or days.

Flows, where excessively wet materials, loess, or liquafication occur, may happen seemingly instantly, with no warning. The hill is sitting there, and then it fails. Slopes and life around loess and liquifying soils is dangerous. As is life over carst formations, liquid mining (Fort Saskatchewan), and old coal mine workings. Ever geotechnical investigation should check these parameters.

All silts should be checked for loess condition. The first simple test is a "rolled down from natural moisture" plastic limit is conducted, and the results compared to a dried then plastic limit. Even a small difference is in question. Sensitivity is the second sign. Often in drilling, the material is observed as wet and soft, yet cone penetration shows a firm of hard condition. The data does not match, sensitivity is high, now what?          

Risk Bumpers

We engineers, from time to time, make a recommendation to do something, not to do something, to wait a period of time before building, or whatever.

Along come the owner, contractor, general engineer, and in the words of Dino, "you geotechnials are too conservative" and try to ignore our recommendations. I no longer care if you do not follow my recommendations. You may receiver a letter like this;

Our recommendations are not binding on you. If you wish to assume greater risk of movement, which is in fact the total risk, you may do so.

Engineered fill suffers from movement from a number of sources:
- time dependent elastic and plastic movements, compaction and consolidation of the underlying soils,
- stress changes and moisture changes in the engineered fill
- swelling or shrinkage brought about by changes in the water table changes due to a new moisture regime

If anyone thinks they can estimate these for the first year, which are the large portions of movements, just have at it and accept the risk.  Anyone who is risk adverse will wait the first year, which will account for something like 50% of the movements.

In summary, you can do as you like, but all movements are your problem.