E:\Suraj\Improvement of Different Grounds.doc
Improvement of Different Grounds
Based on a grain size analysis a soil can be judged for its compactability:
Soils in zones A and B can be compacted by the deep vibratory compaction method Vibro Compaction (also called “Vibroflotation”), while soils of zones C and D cannot be compacted by vibration alone.
Soils in zone C are often found on sites where liquefaction due to earthquakes is of concern. These soils can be compacted during the installation of Stone Columns.
Soils in zone D are not compactable by vibration, but can be substantially reinforced, stiffened and drained by installing Stone Columns.
Requirements for the soil to achieve good compaction by vibration :
· The soil must be permeable enough to allow rapid drainage of the pore water during the compaction process. The permeability is high enough for all granular soils with less than 10 % fines smaller than sieve #200 (0.074 mm) AND less than 2 % clay.
· The friction angle of the soil must be high enough to permit the passage of the compacting shear waves. This requirement is usually satisfied if the soil is well graded.
· The sand or gravel should not be easily crushable (carbonate content in form of shells) or contain very platy mica minerals that would increase soil compressibility.
Vibro Compaction
Effects and Test
Compaction of granular soils by depth vibrators is known as Vibro Compaction. The method is also known as “Vibroflotation”. Natural deposits as well as artificially reclaimed sands can be compacted to a depth of up to 70 m. The intensity of compaction can be varied to meet bearing capacity criteria. Other improvement effects such as reduction of both total and differential settlements are achieved. The risk of liquefaction in a earthquake prone area is also drastically reduced.
The following diagrams illustrate the compaction process :
The principle of sand compaction (Vibroflotation):
The compaction process consists of a flotation of the soil particles as a result of vibration, wich then allows for a rearrangement of the particles into a denser state.
Effects of Compaction
• The sand and gravel particles rearrange into a denser state.
• The ratio of horizontal to vertical effective stress is increased significantly.
• The permeability of the soil is reduced 2 to 10 fold, depending on many factors.
• The friction angle typically increases by up to 8 degrees.
• Enforced settlements of the compacted soil mass are in the range of 2 % to 15 %, typically 5 %
• The stiffness modulus can be increased 2 to 4 fold.
Test Pattern
On large projects the optimal compaction grid spacing has to be determined by test grids.
The compaction effect in the test grids should be as close as possible to the treatment in the later production areas.
In order to achieve this it is advisable to arrange the test grids close to each other.
The distance between grid A (3.10 m) and grid B (3.40 m) should be
Stone Columns
Dry Stone Columns
Dry Bottom Feed Stone Columns were invented in Germany in the early 1970’s. They are particularly useful if washout of soil to the surface is to be prevented or where handling of process water for the Wet Top Feed method is problematic.
Dry Bottom Feed Stone Columns have been successfully used on large infrastructure projects like earth dams, highway embankments, airport runways, port facilities and under large industrial structures such as oil tanks and silos. They are a common choice for foundations in liquefiable soils in earthquake prone areas.
The V-RexThe V-Rex is a state-of-the-art custom built machine for Dry Bottom Feed Stone Columns rigs.
Some of the advanced features include:
· Built in data acquisition
· Easy mobilisation/demobilisation
· Modular leader extensions
· Process control computer, combined with electronic winches, drives rig during column installation on “autopilot”.
Foundation for a bridge in Leipzig,
Germany. /
/ The Vibro
Stitcher ®
Stitcher installing Dry Bottom Feed Stone Columns in Crosby, UK
The need for a fast and very efficient method of forming shallow to medium depth dry bottom feed stone columns led to the development of the stitcher.
Advantages of the Vibro Stitcher:
• Simple operation. No high tech gravel transport system involved.
• Vibroprobe can be pushed down with force to preload the column while producing it and to speed up the process.
• Verticality of the Vibroprobe can be controlled and corrected by the excavator, manually or automatically.
Stone Columns and Liquefaction Mitigation
Loose sandy soils below the water table liquefy during an earthquake. To prevent this, stone columns can be installed and have a threefold effect:
• They drain the soil.
• They compact loose sand and gravel layers.
• They reinforce layers that cannot be compacted and facilitate drainage (mainly very silty sands to sandy silts)
Stone Columns
Offshore Stone Columns
For many years there has been a need for the installation of high quality stone columns in an offshore environment. Previous attempts to install offshore stone columns often relied on the assumption that a mattress of gravel dumped on the seabed could be worked into the soil by moving a vibroprobe up and down.
No adequate means of quality control were available to demonstrate the integrity of columns installed in such a way. Proper documentation involving monitoring of column diameter variation with depth derived from measured batches of gravel placed at defined depth intervals was totally out of reach. With the new Marine Gravel Pump technology the problem of installing high quality offshore stone columns has been solved. The Marine Double Lock Gravel Pump ® guarantees integral columns by continuously pressurised stone discharge. Offshore platforms or dams under cyclic loading or earthquake loading can now be founded cost efficiently and reliably on stone columns.
/ Port of Patras, Greece, construction of a seawall on loose liquefiable sandy and silty sediments.
The stone columns for the foundation of a breakwater and quaywall in Patras serve as drainage for excess pore pressures that build up during construction of the seawall and also provide additional strength under earthquake loading. The 1.0 m diameter stone columns in a 2.7 m to 3.3 m grid extend up to 20 m into the soft silty and clayey marine sediments. The water depth at the treatment location reaches up to 32 m. Both the breakwater and the quaywall are treated with stone columns, as detailed below :
Breakwater : 4830 No. stone columns, 16 m average length, 77280 linm, 60665 m3 (1 m diameter), average square spacing 2.7 m.
Quaywall : 4500 No. stone columns, 10 m average length, 45000 linm, 35300 m3 (1 m diameter), average square spacing 2.85 m.
Quality Control
While in a land based operation a stone column can be assessed with load tests, or a borehole can be drilled into the column to check for continuity, such controls are not readily available under water.
This danger does not exist in a land based operation, where excessive spillage of gravel on the surface can be visually controlled.
Traditional data loggers are not able to generate the data required to provide adequate control. However, a new data logging method has now been developed which produces an output showing the stone column diameter, measured from
On the graph to the right the stone column diameter is plotted against depth. This is accomplished by the computer through a recording of the time and depth when each gravel batch has been sent through the Double Lock mechanism. /
Stone Columns
Wet Stone Columns
/ Wet Top Feed Stone Columns were invented in Germany in the early 1960’s. They are faster to install and need less sophisticated equipment than Dry Bottom Feed Stone Columns. However, the installation technique requires more experience than Dry Bottom Feed Stone Columns.Stone columns
West Kowloon, Hong Kong.
Where to Use the Wet Top Feed Stone Column Method ?
· Where the compaction of sandy and gravelly layers is required and those layers are located above the water table. Compaction is generally better accomplished with the wet method than with the dry method, as the flushing water assists in compaction of the sandy soil around the column.
· Where particularly clean stone columns are required. The flushing water cleans the columns during installation.
· Wherever there are no contaminants in the soil and the soil is not a highly plastic clay leading to the problem of handling the mud in the process water.
· Where space is available for a 500 m2 (= 5000 ft2) settling pond.
· Where the installation crew has sufficient experience in the more demanding installation methodology.
/ Hong Kong North Lantau Expressway, Tai Ho Section MTRC traction substation 8.000 m3 of stone columns.
A Word on Environment
The stone column is possibly the most “natural” foundation system in existence. Stone columns consist entirely of gravel, a substance that is found naturally in the subsoil. No additives are mixed into the stone columns. They are therefore not only environmentally neutral but also more durable than any other foundation system that would involve the use of cement or steel.
at Coco Beach, Puerto Rico.
Vibro Concrete Columns
Different types of Vibro Concrete Columns
/ Vibro Concrete Columns can be classified as cast-in-situ displacement piles. This type of pile has a high bearing capacity due to a high mantle friction and the enlarged base that can be installed when dry concrete is used.VCC constructed
using a truck mounted concrete pump
There are two main types of Vibro Concrete Columns :
- Vibro Concrete Columns (VCC) with pumped concrete.
- Dry Vibro Concrete Columns (DVCC) where the concrete is a dry mix.
Advantages of the DVCC Method over the VCC Method
The dry concrete is much less a liquid than the pumpable concrete that is used for the standard VCC method.
This allows the vibroprobe to transmit more vibration waves and thereby compaction energy through the concrete into the soil. The DVCC method is therefore advisable for projects where the load bearing capability of a pure stone column is not sufficient and compaction of the soil around the pile is desired (e.g. to prevent liquefaction in an earthquake). The DVCC pile offers the combined effect of high load bearing and soil compaction.
An enlarged gravel base can be formed to give the concrete column enhanced bearing capacity. It is also possible to build a partial stone and concrete column.
The advantage is to be able to build an enlarged base and to be able to vary the column diameter.
Possible Disadvantage of the DVCC Method
Although comparison tests in the same soil have not yet been carried out. Columns installed with pumpable concrete (VCC) may have a higher maximum internal strength due to the characteristics of the concrete mix. However, very often the internal strength of the pile is not the critical factor and the better compaction of the surrounding soil may lead to higher overall load bearing capacity of the DVCC columns
Installation of VCC
1. Locate rig over VCC point.
2. Vibrate down to depth.
3. Start the concrete pump.
4. Build the enlarged base.
5. Pull with constant speed while observing limit values for concrete pump pressure
Design and Quality
The Design
The design comprises the assessment of bearing capacity, settlement, stability and liquefaction potential of a soil after Vibroflotation Ground Improvement.
For Vibro Compaction the design is very straightforward :
Assess the required improved soil strength and stiffness after compaction and calculate otherwise as for any unimproved soil. The only difficulty lies in the evaluation of the degree of compaction possible in different soils. This can only be assessed with extensive experience in local soil conditions and the use of appropriate sounding techniques.
For stone columns the design is much more complex. Stone columns are a ground reinforcement, and their behavior is closely linked to the behavior of the soil surrounding the columns. Specific calculation methods for stone columns have been developed and calibrated using full scale tests, allowing stone columns to be designed with the same confidence as piles but with the advantage of better resistance to earthquake loads.
/ The graph shows the settlement reduction with different percentages of soil being replaced by stone columns.
Reduction of Settlements
Screenshot of the DC software.
Based on Priebe’s design diagram and assuming 25 % of the soil being replaced by stone columns (factor A/Ac =4) and a friction angle for the column material of 40°, it follows that n = 2.6 , i.e. a 2.6-fold reduction in settlements as compared to the unimproved ground. DC-Software and The Vibroflotation Group have jointly developed a Windows® program called DC-Vibro. This software is based on the Priebe method including the depth factor. A free trial version of this software can be downloaded from the Internet on DC-Software’s site www.dc-software.de. With DC-Vibro any arrangement of structures can be input and for every cross section an individual multilayer soil model and varying column diameter over depth profile can be assumed.