Term Paper on Ground Improvements and Effects 20151214 RCCadiz

12/14/2015

Ground Improvements on Liquefiable Soils A Partial Fulfilment of the Course CE 263 Soil and Rock Dynamics

Rodora C. Cadiz 2012-79606

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 saturation Should 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. Page 1 of 9

RCCadiz 2012-79606

III.

Ground Improvement Techniques a. Sand Compaction Pile (SCP) Installation of Sand Compaction Piles or SCPs uses vibration to rearrange the soil particles to 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 that compacts 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 ground improvement.

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 cementlike 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 SPTN 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, 9th 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.

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