# Primary consolidation

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### #1 Calixico

Calixico

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Posted 10 March 2010 - 02:26 AM

Help with these two question for clarity.

1. In primary consolidation is the H (thickness of soil layer) always reference the hight of the clay layer only and not the sand layer at any time.

Is this true? In primary consolidation is the compressible soil strata always split into two layers with maximum of 15 feet for hand calculation?
Example given a load of 450psf sits on top of a sandy soil of depth 10 ft (sp-wt= 120 lb/cf )and immediately below that is 10 feet (sp-wt= 95 lb/cf ) of clay layer. The water table is also located 10 feet below the load. If I need to find the initial effective overburden pressure of the normally consolidated layer.

Is it always the norm to not incorporate the entire depth under the load (i.e. the entire 20 ft)

This is how it's calculated.

(120 lb/cf *10) + (95 lb/cf -62.4lb/cf )*5 = 1813lb/cf

### #2 chess5329

chess5329

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Posted 11 March 2010 - 03:55 AM

QUOTE (Spyderman @ Mar 9 2010, 07:26 PM) <{POST_SNAPBACK}>
Help with these two question for clarity.

1. In primary consolidation is the H (thickness of soil layer) always reference the hight of the clay layer only and not the sand layer at any time.

Is this true? In primary consolidation is the compressible soil strata always split into two layers with maximum of 15 feet for hand calculation?
Example given a load of 450psf sits on top of a sandy soil of depth 10 ft (sp-wt= 120 lb/cf )and immediately below that is 10 feet (sp-wt= 95 lb/cf ) of clay layer. The water table is also located 10 feet below the load. If I need to find the initial effective overburden pressure of the normally consolidated layer.

Is it always the norm to not incorporate the entire depth under the load (i.e. the entire 20 ft)

This is how it's calculated.

(120 lb/cf *10) + (95 lb/cf -62.4lb/cf )*5 = 1813lb/cf

This is what you use to calc for effective stress when the problem specify at the middle of the clay layer.

When you calc for primary settlement this is correct eventhough you have to consider the total thickness of clay in the formula.

Anybody agree with this?

### #3 jamie

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Posted 22 March 2010 - 07:28 PM

i agree with the above. for consolidation, since the pressure at the top of the clay layer will be less than at the bottom of the clay layer (due to the weight of the clay on that bottom portion), pressure is always calculated for the middle of that layer to get an average pressure for the consolidation. when considering the actual amount of settlement in feet or inches, you need to use the entire height of the clay layer. When looking at the time for consolidation, it depends on what is on the top and bottom of the clay layer whether or not you use the full height of the clay layer, or half the height. if it has a free-draining soil on top and bottom (sand or gravel for example), then the distance water from the center of the clay layer to the sand is half the clay layer height. if it has another impermeable soil on one side and sand on another, you will use the full height of the clay.

to your first question, from my understanding, primary consolidation is usually defined as the settlement that occurs over time due to the loss of water - water being squeezed out of the soil. because sand is considered free-draining, this water loss is somewhat instantaneous and therefore long-term consolidation is usually only in reference to clays. when clays are loaded, you will immediately get an increase in the pore pressure, which will decrease with time as the water drains out.

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