Consolidation and 1D settlement

Question 1

What is important to know about consolidation?

Answer 1

• Drop-head is not uniform in the soil during consolidation.
• Hence consolidation and excess pore water is NOT uniform through the sample.
• Rate of consolidation is NOT constant
• Consolidation time also depends on the drainage path.

Question 2

What assumptions do we use to find the 1-D consolidation equation?

Answer 2

• Soil is saturated and homogeneous.
• Solid particles and water are incompressible
• Compression and flow are 1-D, occurring in the vertical direction only.
• Strains are small and Darcy’s law is valid at all hydraulic gradients
• Coefficients of permeability (k) and volume compressibility (Mv) are constant.
• The load is applied instantaneously and remains constant after.

Question 3

What is the coefficient of consolidation equal to?

Answer 3

Cv = (Eo’ * k) / gamma w. Units are m^2/s
Eo’ = stiffness modulus
k= permeability
gamma u = 9.81

Question 4

how to determine the average degree of consolidation
(U) ?

Uz = ?

Answer 4

U = Current settlement / Ultimate settlement

U = 1 - Sum( 2 * M^-2 * exp( -M^2 * Tv) ) m=0 to m = infinit
Tv = (Cv * t ) / d^2
M is just a intiga from 0 to infinity
Tv= time factor

For U < 63 % => U = 2 / sqt(Pi) * sqt(Tv)
For U

Question 5

Approximations for U?

Answer 5

For U < 63 % => U = 2 / sqt(Pi) * sqt(Tv)

For U > 63 % => U = 1 - 8/pi^2 * exp( (-pi^4 * Tv) / 4)

Question 6

What are Isochrones?

Answer 6

The distribution of excess pore pressure within an oedometer sample or field at a given time after loading is represented by a line called an isochrone.

Question 7

How is overall settlement calculated?

P=

Answer 7

P = 1/Eo' *  (intergral(d-0) of (/\sigmav' - ue) dz)
Eo' = stiffness of soil
ue= excess pore pressure
/\sigmav= increase in vertical stress at time t

Question 8

How does the drainage affect the depth?

Answer 8

impervious at the bottom the soil thickness = d

double drainage thickness of soil = d/2

Question 9

Talyor method (1948) how to find t90?

Answer 9

plot a graph of settlement against sqrt(time)

draw a line that is 1.15 times longer than x and where this line crosses the plotted line the x value is t90.

Question 10

Describe the Casagrande method.

Answer 10

plot gauge reading against log of time.
Extended the two linear regions till they intersect. This is a100
Calculate ao at the top of the graph.
a50 is then halfway between the two and find the point that intersects this.
find the corresponding time value at a50 and Cv is found using this.

Question 11

Name and describe 3 ways of speeding up consolidation.

Answer 11

1. Sand Drains - they reduce the drainage path length and so lead to accelerated consolidation.
2. Geotextile drains - work the same way as sand drains but made from a corrugated core with geotextile filter wrapped around it to keep the soil out.
3. Preloading - clays may be preloaded with surcharge fill which is left until consolidation settlement equals what will result under application of the final building.