Vibroflotation is a technique developed in Germany in the 1930s for in situ densification of thick layers of loose granular soil deposits. Vibroflotation was first used in the United States about 10 years later. The process involves the use of a vibroflot (called the vibrating unit).
The device is about 2 m in length. This vibrating unit has an eccentric weight inside it and can develop a centrifugal force.
The weight enables the unit to vibrate horizontally. Openings at the bottom and top of the unit are for water jets. The vibrating unit is attached to a follow-up pipe. The figure below shows the vibroflotation equipment necessary for compaction in the field.
The entire compaction process can be divided into four steps:
Step 1. The jet at the bottom of the vibroflot is turned on, and the vibroflot is lowered into the ground.
Step 2. The water jet creates a quick condition in the soil, which allows the vibrating unit to sink.
Step 3. Granular material is poured into the top of the hole. The water from the lower jet is transferred to the jet at the top of the vibrating unit. This water carries the granular material down the hole.
Step 4. The vibrating unit is gradually raised in about 0.3 m lifts and is held vibrating for about 30 seconds at a time. This process compacts the soil to the desired unit weight.
Stone column method for ground improvement is a vibro-replacement technique, where the weak soil is displaced using a cylindrical vibrating probe (i.e. vibroflot), thus creating a column that is then filled and compacted with good-quality stone aggregates.
With the inclusion of stone aggregates to the in situ soil, its stiffness and load-carrying capacity increases. It also helps to reduce the static as well as differential settlement of the soils.
Bulging action of the stone columns imparts lateral confinement to the surrounding soils and it also acts as a drainage path accelerating the consolidation of cohesive soils.
These stone columns are generally used for soils that are much more compressible but not weak enough to necessitate a pile foundation. Moreover, for the construction of low-to-medium rise buildings on soft soils, pile foundation sometimes becomes expensive. In such cases, stone columns are preferred.
Stone columns are very useful for the improvement of cohesive soils, marine/alluvialclays, and liquefiable soils. Stone columns have been used successfully for a widerange of applications from the construction of high-rise buildings to oil tank foundation, and for embankment and slope stabilization.
Stone column installation methods
For the installation of stone columns a vibrating poker device is used that can penetrate to the required treatment depth under the action of its own weight, vibrations, and actuated air, assisted by the pull-down winch facility of the rig.
This process displaces the soil particles and the voids created are compensated with backfilling of stone aggregates. The vibroflot penetrates the filled stone aggregates to compact it and thusforces it radially into the surrounding soils.
This process is repeated till the full depth of the stone column is completed. The lift height is generally taken as 0.6–1.2 m for the filling and compaction of the stone aggregates.
Depending upon the feeding of stone aggregates into the columns there are basically two methods for the installation of stone columns:
1- Top-feed method
In the top-feed method, the stone aggregates are fed into the top of the hole. The probe is inserted into the ground and is penetrated to the target depth under its own weight and compressed air jetting. However, jetting of water is also done especially when the soil is unstable. This also helps to increase the diameter of the stone columns and to washout the fine materials fromthe holes.
The top-feed method is suitable when water is readily available and there is enough working space to allow for water drainage. Moreover, the soil types should be such that it would not create messy surface conditions due to mud in water.
The top-feed method is preferable when a deeper groundwater level is encountered.
Stone Columns installation Top-feed Method
2- Bottom-feed method
The bottom-feed method involves the feeding of stone aggregates via a tremie pipe along the vibroflot and with the aid of pressurized air. The bottom-feed method is preferable when the soil is highly collapsible and unstable. However, the stability of holes will also depend upon the depth, boundary conditions, and the groundwater conditions. In areas, where the availability of water and space and the handling of mudin process water are limiting factors, the bottom-feed method can be implemented.
Due to limited space in the feeding system, a smaller size of aggregates is used inthe bottom-feed method compared with that used in the top-feed method. On the otherhand, the flow of stones to the column is mechanically controlled and automatically recorded in the bottom-feed method.
This book covers the design and implementation of ground improvement techniques as applicable to soft clays in the eyes of experienced geotechnical personnel, from academics to practitioners.
Soft soils and ground improvement have become major challenges to geotechnical engineers during the last three decades. Planners, architects, consultants, and contractors are now aware what soft soil is and the risks associated with development in areas where soft soil is encountered. They know that ‘good’ land is limited and, therefore, marginal lands need to be improved.
Table of Contents
1. Introduction (J Ameratunga, N Sivakugan & B M Das)
2. Engineering Geology of Soft Clay (I Shipway)
3. Basic Soil Mechanics (N Sivakugan and J. Ameratunga)
4. Geotechnical Testing (J Ameratunga & N Sivakugan)
5. Parameter Derivation (J Ameratunga & N Sivakugan)
6. Ground Improvement Methods for Soft Clays (J Ameratunga & N Sivakugan)
7. Replacement (J Ameratunga & N Sivakugan)
8. Preloading (S Iyathurai & J Ameratunga)
9. Preloading with Wick Drains (S Iyathurai)
10. Stone Columns (K Chan & B Poon)
11. Semi-Rigid Inclusions (Thayalan Nall)
12. Lightweight Fill (J Ameratunga)
13. Deep Soil Mixing (T Muttuvel, S Iyathurai & J Ameratunga)
14. Basal High Strength Geotextiles for Ground Improvement of Soft Foundation Soils (C Lawson)
15. Mass Stabilization (A O′Sullivan)
16. Observational Approach and Geotechnical Instrumentation (K A Dissanayake & C A Bridges)
Principales And Practice Of Ground Improvement Free PDF
By Jie Han
Ground improvement is popular in many countries to solve difficult geotechnical problems, especially when construction necessarily occurs in problematic soils and under difficult geotechnical conditions.
Many recent developments in equipment, materials, and design methods have made ground improvement technologies more effective, efficient, and economic. However, the state of practice for most ground improvement technologies is that the practice is ahead of theory. Some contractors have developed their proprietary technologies, design methods, and construction techniques for their competitive advantages.
Most of the existing books on ground improvement are focused on the concept, application, and case study. However, few books have been devoted to the principles and design methods of ground improvement. This book covers both theoretical and practical aspects in the design and construction of a variety of ground improvement technologies commonly used in practice.
