Term Paper on Ground Improvements and Effects 20151214 RCCadiz

8/19/2019 Term Paper on Ground Improvements and Effects 20151214 RCCadiz

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12/14/2015 Ground

Improvements on

Liquefiable SoilsA Partial Fulfilment of the Course

CE 263 Soil and Rock Dynamics

Rodora C. Cadiz2012-79606


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I.

Introduction to Principles of Mitigation

Liquefaction is a geohazard that threatens safety and security of lives and assets. It has two

mechanisms that must be triggered simultaneously to occur: increase of excess pore water

pressure and decrease of effective pressure. Although, not all liquefiable soil is a risk.

A risk is composed of a hazard, exposure and vulnerability. All three must be present for a risk

to be warranted for mitigation. Afterall, ground improvement is not an easy task and it can

be expensive.

The objective of mitigation is to reduce the risk of liquefaction by preventing the development

of excess pore water pressure and increase effective stress. Ground engineering alleviates the

hazard while exposure and vulnerability are factored by social elements.

Figure 1. Factors of Risk

II.

Objectives of Mitigation

a.

Reduce void ratio

Smaller void ratio means that the soil mass is denser and sand grains won’t be able to

move during shearing.

b.

Reduce degree of saturationShould the soil mass be compressed, a small degree of saturation means the void is

composed mostly of compressible air and will not produce excess pore water pressure.

Volume change is allowed.

c.

Increase permeability

Excess pore water pressure will not be able to develop if the fluid will be drained quicker

than the applied load.


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III.

Ground Improvement Techniques

a.

Sand Compaction Pile (SCP)

Installation of Sand Compaction Piles or SCPs uses vibration to rearrange the soil particlesto lessen the void ratio and increase the effective pressure. The second mechanism of the

SCP is the increased permeability of the soil. By installing the SCP at the design interval,

groundwater is easily and quickly dissipated even before excess pore water pressure

develops.

The effects of SCP are permanent. Once installed, the density and permeability of the soil

is increased.

The procedure of installation is discussed by Towhata in his book, Geotechnical and

Earthquake Engineering. The drilling equipment inserts a pile casing, usually of diameter

equal to 700mm. This casing is perforated and releases air at a pressure up to 500 kPa.

This air pressure pushes the immediate soil in contact laterally. Simultaneously, gravel

comes out of the bottom of the casing.

The spacing, X, is determined depending the degree of improvement desired. Degree of

improvement using SCP typically ranges from 5% to 20%.


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b.

Dynamic Consolidation

Dynamic Consolidation is a method proposed by Menard, a ground improvement

specialist. It is done by having a weight, made of concrete or steel, of 50-300 kN free fall

from a height of 20-30 meters. The impact on the ground releases an energy thatcompacts the soil particle. The procedure is repeated until specified energy is transferred

to the ground.

The effects of compaction is maximum near the surface and addresses only the increase

of density and effective stress and to lessen the void ratio.

Towhata discussed the summary of the design procedure.

1.

Determine the depth of effective compaction, D

2.

The points of impact are in a square configuration. Spacing, L is determined as

3.

The required impact energy per area is

Where Ev (kNm/m3) designates the energy per unit volume of soil required to

achieve desired SPT-N value.

4.

No. of impact=


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c.

Compaction Grouting

Compaction grouting pushes highly pressurized grout into the soft ground and

compresses the surrounding soil in the lateral direction. This procedure is ideal for

retrofitting as the equipment is manageable for use in the basement and the vibration

and noise is minimum. Existing and adjacent structures are not affected by the groundimprovement.

A steel pipe, 5 inches in diameter, is bored into the ground. Grout is injected at a

maximum pressure of 6 MPa. While the grout design mix can reach 3 MPa at 28 days.

d.

Gravel Drains

A less expensive method that uses gravel instead of sand to improve the hydraulic

conductivity of soil and dissipates the excess pore water pressure. Typical installation of

500mm diameter gravel drains is spaced at 1.5 meter.


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Gravel drains differ from SCP as it does not increase the density of the soil, the soil is

removed in place of the gravel drain. It only uses the mechanism of hydraulic conductivity

instead of addressing also the increase in density. This procedure does not produce

harmful vibration to nearby structures hence it is safe to use in populated areas.

Gravel drains cannot always maintain the excess pore water pressure less than 100%

during a level 2 design earthquake (return period of 500 years) so it has become less

popular in Japan.

e.

Grouting and Deep Mixing

For a larger scale ground improvement, grouting is combined with deep mixing. Used in

new developments, where there are no existing structures. Grouting develops stiff

bonding among the sand drains. Deep mixing is carried out by mixing soil with a cement-

like material. Deep jet mixing or DJM mixes cement powder with soil and ground water is

used to start solidification in saturated soil while cement deep mixing or CDM mixes

cement slurry on dry soil.


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.

f.

Blasting

Compaction by blasting is found to be economical and time saving. The energy released

by the explosion is similar to an earthquake, producing cyclic straining of the soil therefore

inducing liquefaction and forces the soil grains to rearrange themselves until pore water

pressure is completely dissipated. This results in denser soil.

Dynamites are installed and ignited from one side of a site towards the opposite side.

Although, the order of blasting is not yet well investigated.


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Its applicable to isolated sites where existing structures could not be affected by the

ground vibration and noise produced by blasting. A work around to isolate the site was

tested by Sato Kogyo Company in Tokyo where they installed sheet piles to close off the

site. This effectively reduced the ground vibration transferred to nearby structures.

IV.

Effect of the improvement on the site ground response

A study presented by O. Mavituna and B. Teymur in 2008 during the 14th World Conference

on Earthquake Engineering in Beijing shows that ground improvement by densification has a

significant decrease in ground acceleration. The study was conducted in Turkey and was based

on the 1999 Kacaeli earthquake and its aftershock with magnitude 7.4 and 5.8 respectively.

The peak ground acceleration in Adapagami was 0.41g. The site was composed of alluvial plain

on Quaternary Age alluvial deposits. The ground water level was found at a shallow depth

ranging from 0.5 m to 3.0 m.

Another study by Duzceer and Gokalp in 2002 showed that stone columns increased the SPT-N value of the site. Calculating the shear wave velocities using the improved SPT N-value were

also higher. Time-acceleration graphs showed that denser soil has smaller acceleration.


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V.

Effect of the improvements on the soil mass frequency

Generally, denser soil tends to have higher frequency. It is desirable to have a soil with higher

frequency because it cannot be moved by relatively small earthquakes.

VI.

References

Duzceer, R., Gokalp A. Improvement of foundations of oil tanks with stone columns, 9 th

National Conference of Soil Mechanics and Foundation Engineering. 2002.

Kramer, S., Geotechnical Earthquake Engineering. 1996.

Mavituna, O., Teymur, B., Effect of Improving Soil as a Countermeasure for Liquefaction, 14th

World Conference on Earthquake Engineering. 2008

Towhata, I., Geotechnical and Earthquake Engineering. 2007.

Term Paper on Ground Improvements and Effects 20151214 RCCadiz

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