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Geotechnics 3 > 6 - Ground Improvement > Flashcards

6 - Ground Improvement Flashcards

1
Q

Why might ground improvement methods be necessary in geotechnical engineering?

A

Ground improvement methods are necessary to reduce excessive settlement and/or accelerate the rate of settlement, particularly when consolidation is expected to take decades, potentially delaying construction.

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2
Q

What is the main issue with consolidation in geotechnical projects?

A

In theory, an infinite time is required to achieve 100% consolidation (Uavg = 100%), which is impractical for construction. Thus, methods to reduce settlement or speed up consolidation are often required.

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3
Q

Why might accelerating the consolidation process be beneficial?

A

Accelerating consolidation can allow construction to begin years or even decades earlier than would otherwise be possible, making it economically justifiable despite the added cost of acceleration methods.

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4
Q

What are some methods of ground improvement used to accelerate consolidation?

A

Surcharge Preloading
Vertical Drains
Vacuum Preloading
Preloading by Groundwater Lowering

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5
Q

What is Surcharge Preloading in ground improvement?

A

Surcharge Preloading involves applying a temporary load to the soil surface to accelerate consolidation, which helps reduce future settlement during construction.

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6
Q

What are Vertical Drains used for in ground improvement?

A

Vertical Drains are installed to accelerate the dissipation of pore water pressure and speed up consolidation by providing a pathway for water to escape from the soil.

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7
Q

How does Vacuum Preloading work as a method for ground improvement?

A

Vacuum Preloading uses a vacuum to lower pore water pressure in the soil, which accelerates consolidation and reduces the time needed to achieve a desired degree of settlement.

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8
Q

What is Preloading by Groundwater Lowering?

A

Preloading by Groundwater Lowering involves lowering the groundwater table to reduce pore water pressure in the soil, thus accelerating consolidation and speeding up settlement.?

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9
Q

What are Shallow Surface Compaction and Deep Vibro-Compaction?

A

These are methods of compaction used to increase soil density, improve stability, and reduce settlement. Shallow surface compaction is done near the surface, while deep vibro-compaction is used for deeper layers.

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10
Q

What other ground improvement methods can be used aside from consolidation acceleration?

A

Blasting
Heavy Tamping
Grouting
Admixtures (e.g., Lime, Cement)
Reinforcement

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11
Q

What is Surcharge Preloading?

A

Surcharge Preloading is a ground improvement method where a temporary surcharge (weight) is applied to soft soils to induce consolidation, improving soil stiffness and strength before construction.

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12
Q

How does Surcharge Preloading improve soil properties?

A

The static weight of the surcharge causes soft soils to consolidate, enhancing the soil’s stiffness and strength. Once the desired consolidation is achieved, the surcharge is removed, and construction proceeds

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13
Q

What is the typical thickness of a surcharge fill in Surcharge Preloading?

A

Surcharge fills are typically 3 meters to 8 meters thick, producing settlements of 0.3 meters to 1 meter.

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14
Q

What are the advantages of Surcharge Preloading?

A

Only conventional earthmoving equipment is required.
Any grading contractor can perform the work.
Results can be effectively monitored using instrumentation (e.g., piezometers) and ground level surveys.
The cost is relatively low, provided the fill material is readily available.

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15
Q

What are the disadvantages of Surcharge Preloading?

A

The surcharge fill must extend at least 10 meters beyond the perimeter of the planned construction, which can be difficult in confined spaces.
Transporting large quantities of fill material to the site may not be practical or environmentally acceptable.
The surcharge must remain in place for months or even years, delaying construction

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16
Q

What is the required horizontal extension of the surcharge fill in Surcharge Preloading?

A

The surcharge fill must extend horizontally at least 10 meters beyond the perimeter of the planned construction.

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17
Q

How long must the surcharge remain in place in Surcharge Preloading?

A

The surcharge must remain in place for months or even years until the desired consolidation and soil improvement are achieved, delaying construction.

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18
Q

What is the general approach for specifying surcharge loading magnitudes and duration?

A

The approach involves applying a surcharge load that is heavier than the final structure, causing faster settlement. Once the surcharge-induced settlement matches the ultimate settlement due to the final structure, the surcharge can be removed, and construction can proceed.

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19
Q

How does the surcharge load affect the settlement rate?

A

The surcharge load is heavier than the final structure, causing the settlement to proceed at a faster rate. This allows the ultimate settlement to be achieved more quickly compared to waiting for the final structure’s load.

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20
Q

What happens when the surcharge-induced settlement equals the ultimate settlement due to the final structure?

A

At this point, the surcharge load can be removed, and the final structure can be constructed, resulting in some unloading and reloading of the soil but reaching near-ultimate settlement faster.

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21
Q

Why is the surcharge load applied heavier than the final structure’s load?

A

A heavier surcharge accelerates the consolidation process, creating higher hydraulic gradients and achieving ultimate settlement in a shorter period compared to using the final load alone.

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22
Q

What is the effect of a larger pre-load in surcharge preloading?

A

A larger pre-load results in quicker settlement, but it also applies greater vertical stress. This can lead to undrained bearing failure if the surcharge is too large before the effective stress increases.

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23
Q

Why might surcharge preloading need to be applied in stages?

A

The surcharge might be applied in stages to prevent undrained bearing failure. Initially, effective stress doesn’t increase, so the soil’s strength remains low. Over time, as excess pore pressures dissipate, effective stress increases, and the soil strength improves, allowing more load to be added..

24
Q

What happens to the effective stress during surcharge preloading?

A

Initially, the effective stress does not increase with the surcharge, meaning soil strength doesn’t improve right away. Over time, as excess pore pressures dissipate, the effective stress and soil strength increase.

25
Q

What is the basic principle of vacuum preloading?

A

Vertical drains are installed in soft soil, and an impermeable layer is placed on top. By pumping water out through the drains, the soil undergoes consolidation and settlement.

26
Q

How does the pressure applied in vacuum preloading compare to fill surcharge?

A

80 kPa of vacuum pressure is approximately equivalent to placing 4 meters of fill, but vacuum preloading doesn’t require transporting large amounts of fill material.

27
Q

What is the key difference between vacuum preloading and fill surcharge regarding pore water pressure?

A

Under fill surcharge, excess pore water pressure increases and then dissipates, while in vacuum preloading, pore water pressure decreases to a negative value due to the applied vacuum pressure.

28
Q

Why is vacuum preloading more complex than fill surcharge in terms of pore water pressure changes?

A

Vacuum pressure causes more complex pore water pressure variations, especially when combined with fill surcharge. Monitoring pore water pressure is crucial during the process.

29
Q

What is the advantage of vacuum preloading over surcharge preloading?

A

Vacuum preloading is generally cheaper and faster compared to the surcharge method for an equivalent load.

30
Q

What does groundwater lowering do to the soil?

A

Pumping water out lowers the groundwater level, reducing pore water pressure, which increases effective stress, induces settlement, and improves shear strength

31
Q

How is groundwater lowering similar to vacuum preloading?

A

Both methods reduce pore water pressure, leading to increased effective stress and consolidation. However, groundwater lowering doesn’t typically achieve the same level of pore pressure reduction as vacuum preloading.

32
Q

What is a common unintended consequence of groundwater lowering?

A

Groundwater lowering often occurs accidentally when extracting water for irrigation, which can cause consolidation in nearby soils.

33
Q

Why is groundwater lowering used as a temporary method in underground construction?

A

It strengthens soil by increasing effective stress, which aids in construction. However, it carries risks if a pump fails, potentially causing soil collapse due to a sudden loss in strength.

34
Q

What precaution must be taken when using groundwater lowering?

A

Care must be taken to prevent damage to surrounding infrastructure from consolidation settlements, and the system must be monitored to avoid failure and subsequent collapse.

35
Q

What does the time for consolidation depend on?

A

The time required for consolidation is proportional to
𝐻^2

36
Q

How do vertical drains accelerate consolidation?

A

Vertical drains reduce the drainage distance for excess pore water to escape, allowing for horizontal flow, which speeds up the consolidation process.

37
Q

What is the typical impact of using vertical drains on consolidation time?

A

Vertical drains can reduce the time needed for consolidation from several years to just a couple of months.

38
Q

Why are vertical drains used in thick compressible soil layers?

A

In thick compressible soils, the time for consolidation can be excessive, so vertical drains are used to shorten the drainage path and accelerate the process.

39
Q

How is the required spacing of vertical drains determined?

A

The required spacing is determined by a radial drainage analysis, balancing construction cost and rate of consolidation.

40
Q

What is the typical spacing between vertical drains?

A

Vertical drains are typically spaced 1-3 meters center to center.

41
Q

What pattern is commonly used for vertical drains?

A

Vertical drains are typically installed in a triangular or square pattern.

42
Q

How are vertical drains arranged in a square pattern?

A

In a square pattern, the spacing between drains is
S, and the radius of each drain is rd
. The drains form a regular square grid

43
Q

Why is it difficult to analyze a square-shaped drainage zone?

A

Analyzing a square drainage zone is complex without numerical programs, so it is converted to an equivalent circular area for easier calculation.

44
Q

How is the radius of the equivalent circular area (R) calculated for vertical drains in a square pattern?

A

The radius is calculated using the formula:
𝑅=0.564⋅𝑆
R=0.564⋅S, where
𝑆
S is the spacing between the drains.

45
Q

How are vertical drains arranged in a triangular pattern?

A

Vertical drains are laid out in a regular triangular pattern with spacing
S between them and a radius r for each drain.

46
Q

What is the shape of the area drained by each PVD in the triangular pattern?

A

The area drained by each PVD is represented by a hexagon in the triangular pattern.

47
Q

Why is it difficult to analyze a hexagon-shaped drainage zone?

A

Analyzing a hexagonal drainage zone is complex without numerical programs, so it is converted to an equivalent circular area for easier analysis.

48
Q

How is the radius of the equivalent circular area (R) calculated for the triangular pattern?

A

R = 0.525S, where S is the spacing between the drains.

49
Q

What is the formula for the area of the hexagon (A_hexagon) in a triangular pattern?

A

A = (root3 / 2) * S^2

50
Q

How is the equivalent circular area (A_circle) related to the hexagonal area?

A

pi*R squared is set equal to area of hexagon.

51
Q

What is the governing equation for pore pressure dissipation in the vertical drains?

A
52
Q

What do cv and ch represent in the excess pore pressure equation?

A
53
Q

How are the average degrees of consolidation in the vertical and radial direction (Uv and Ur) related?

A
54
Q

What is the formula for the average degreee of consolidation for the radial direction, Ur?

A
55
Q

What is the equation for ų in the radial direction ?

A
56
Q

What is “smear” in the context of vertical drain installation?

A

“Smear” refers to the alteration of soil properties adjacent to the drain during installation, which typically reduces the consolidation rate.

57
Q

What is the 8 step process to solve a PVD problem?

A

Geotechnics 3 (8 decks)

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