Ground Improvement A: Primary Consolidation and Vertical Drains

Question 1

What is primary consolidation?

What causes it

Answer 1

Permanent settlement of railway embankment foundations

Due to consolidation of the foundation soil (clay)

Dissipation of water causes consolidation

Question 2

What are the two solutions to primary consolidation?

Answer 2

1. Increase the rate of settlement (consolidation); settlement happens quickly so majority occurs prior to installation of railway
2. Stabilise clay foundation layer

Question 3

What are the methods of preventing settlement developing over a long period of time?

Answer 3

• Treating the soil (e.g. lime/cement stabilisation)
• Reinforcing the soil (e.g. piles)
• Dependent on the soils permeability

Question 4

What are the parameters that affect the rate of settlement development?

Answer 4

• Engineering characteristics of the soil
• Soil depth
• Layer boundary conditions (e.g. drainage)
• Embankment weight

Question 5

What can be done to improve the slow rate of consolidation in clayey soils?

Answer 5

Install sand drains

Question 6

How do vertical drains speed up consolidation?

Answer 6

They shorten the drainage path, hence making use of new horizontal drainage paths

The drain spacing is critical and the installation pattern is either in a triangular or square shape

Drainage layers are also installed on embankments that are not freely draining (ie. not fully compacted)

Question 7

What are the three types of drains?

Answer 7

1. Backfilled sand drain (200-400mm diameter)
2. Prefabricated drain (uses sand filled polypropylene sock)
3. Band drain (uses filter fabric around a plastic core)

Question 8

In what scenario is the longest drainage path the full depth of the clay layer?

Answer 8

If the bottom of the layer is impermeable, the drainage path is the full depth

If there is drainage at both the bottom and top, the drainage path is half of the depth

Question 9

How does the dissipation of water cause consolidation?

Answer 9

Before dissipation, the pressure in the water carries the load

After dissipation, the load is carried by the soil, causing it to compress

Question 10

What causes the clay to settle when an embankment is built?

What are the solutions to the problem?

Answer 10

• The water pressure increases to meet the applied stress from the embankment
• Over time the water pressure dissipates, causing the clay to settle
• Combined drainage solution (at the top, middle and bottom) is the most effective solution

Question 11

What does this show?

Answer 11

The equivalent radius for the triangular drainage pattern

This gives the same CSA, which can be used to solve the consolidation equation analytically

Question 12

For this average degree of consolidation (U) against vertical time factor (T_v), which curve should be used?

Answer 12

Assume the middle curve (1), for uniform excess pore pressure

This graph is used to estimate the consolidation U (vertical case)

Question 13

What do these equations represent?

Answer 13

The time factor for the vertical/horizontal consolidation

C_v/C_h are the vertical/horizontal coefficients of consolidation (in m^2 / year)

Question 14

What does this graph show?

Answer 14

This graph is for the radial consolidation against time factor T_r

Different values of n

Question 15

What is the relationship equation between average consolidation, vertical-only consolidation and radial-only consolidation?

Answer 15

Question 16

How to calculate the consolidation factor for this scenario?

NB. in question, only 40mm of additional foundation settlement can occur one year after construction

Answer 16

Question 17

What does this equation represent?

Answer 17

Consolidation (primary) settlement of a clay foundation

Question 18

What is the drain ratio equation?

Answer 18

Question 19

What are the solution steps to this sort of problem?

Answer 19

1. Calculate the primary consolidation
2. Calculate the consolidation factor U
3. Calculate R (in terms of n), using the drain ratio
4. Calculate T_v (NB. t is in years)
5. Use the average degree of consolidation curves (vertical) to find U_v
6. Now we have both U and U_v, so can estimate U_r using the relationship equation
7. Calculate T_r, which may be in terms of n (as we only know R; have to use the drain ratio equation)
8. Use the radial consolidation factor/radial time factor graph and different values of n, find the equivalent time factors, THEN substitute these time factor (T_r) values back into the T_r/n equation to find alternative values of n (see table)
9. Plot these values of n against each other to find the actual value of n
10. Look back at step 3, where we have R in terms of n (after using the drain ratio); we can now use the actual value of n to find the equivalent cylinder radius R
11. Plug R into the square pattern drain spacing equation to find the actual drain spacing