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The method of soil improvement whereby granular soils are compacted using depth vibrators is known as Vibroflotation.
Natural deposited soils as well as artificially reclaimed sands can be compacted to great depths.
The current depth record lies at 70 meters for reclaimed sands and at 53 meters for naturally deposited sands. The intensity of compaction can be varied in order to achieve the desired effect depending upon the foundation or ground improvement tpurpose.
Vibro Displacement (Stone Columns and Concrete Columns)
The utilization of Stone Columns can be split into two distinct areas; as foundation elements (stone piles) or as ground reinforcement.
As a foundation element (stone piles), stone columns can be used for a wide range of building types from multi story buildings to oil tank foundations. They can function under multiple foundation types including strip-, pad-, slab- and single footings.
As a ground reinforcement technique, Stone Columns can perform liquefaction prevention, embankment stabilisation, slope stabilisation, as well as other ground improvement applications via reinforcement and drainage.
Here shall come introductory text about what we try to explain. Also a hint to go to articles (link) if someone wants 'method statements' which are not to be found here but under articles.Top of Form
Vibro CompactionVibro compaction is a ground improvement technique that densifies clean, cohesionless granular soils by means of a downhole vibrator. The vibrator is typically suspended from a crane and lowered vertically into the soil under its own weight. Penetration is usually aided by water jets integrated into the vibrator assembly. After reaching the bottom of the treatment zone, the soils are densified in lifts as the probe is extracted. During vibro compaction, clean sand backfill is typically added at the ground surface to compensate for the reduction in soil volume resulting from the densification process. The vibratory energy reduces the inter-granular forces between the soil particles, allowing them to move into a denser configuration, typically achieving a relative density of 70 to 85 percent. The treated soils have increased density, friction angle and stiffness. Compaction is achieved above and below the water table. The improved soil characteristics depend on the soil type and gradation, spacing of the penetration points and the time spent performing the compaction. Generally, the vibro compaction penetration spacing is between 6 feet and 14 feet, with centers arranged on a triangular or square pattern. Compaction takes place without setting up internal stresses in the soil, thus ensuring permanent densification.The use of clean sand backfill during vibro compaction allows the original site elevation to be maintained. However, on sites where the planned final grade is below the existing grade, lowering of the site elevation may be desirable. In these instances, the ground surface is allowed to subside during the compaction effort. Vibro compaction permits the use of economical spread footings with design bearing pressures generally of 5 ksf up to 10 ksf. Settlement and seismic liquefaction potentials are reduced. The required treatment depth is typically in the range of 15 to 50 feet, but vibro compaction has been performed to depths as great as 120 feet. Examples of previously performed applications include increasing bearing capacity, decreasing settlement and mitigating liquefaction for planned structures, embankments, railways and roadways. Vibro compaction rigs can be fully instrumented with an on-board computer to monitor parameters during vibro compaction. Monitoring these parameters allows the operator to correct any deviations in real-time during the construction process to keep the vibro compaction within project specifications. Data from the Data Acquisition (DAQ) system such as amperage and lift rate are recorded and displayed in real-time alongside specified target values on an in-cab monitor.
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How vibro compaction worksThe offset position of the eccentric weight housed in the vibroflot creates a horizontal vibratory action, which acts to compact loose granular soils (i.e. sand and gravels) into a denser condition, thus providing a significant improvement in the geotechnical properties of the treated ground. On reaching the required design depth water jetting from the nose cone is reduced, and the vibroflot is slowly extracted with pauses at regular intervals to ensure satisfactory levels of compaction are achieved at each depth. Additional side water jets are often utilised to assist with compaction and to encourage further erosion of the soils around the bore. The vibroflot is gradually withdrawn back to the surface where a zone of compacted ground is formed around the insertion point. Additional site won sand may also be added at the top of the hole to fill the cone of depression that is formed. The rate of extraction is varied to suit the conditions encountered on site and to ensure that the correct amount of densification is achieved for each project. Top of Form
Construction of an "hidden" dam (Photo)To stabilize the hinterland against landslides.
PenetrationThe vibroprobe penetrates to the required depth by vibration and jetting action of water and/or air
CompactionThe vibroprobe is retracted in 0.5 m intervals. The in situ sand or gravel is flowing towards the vibroprobe.
CompletionAfter compaction the working platform needs be levelled and eventually roller compacted.Vibro CompactionEffects and TestCompaction 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.
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
Effects and Test Offshore and Land Based
Home Company Projects Services Techniques Download Contact Us Vibro Compaction
Application of methodReduce foundation settlementsPrevent soil liquefaction during earthquakesIncrease in-situ density of land reclamation fillsIncrease shear strength to improve slope stabilityReduce water permeability to facilitate dewatering
Suitability of method
1.In cooperation with the architect, structural engineer, and the owner, define design objectives, such as admissible settlement, max. differential settlement, design earthquake (Mw and amax ), shear strength.2.Perform a site investigation. Select the field sounding and lab testing program so that it later can answer all design and QA/QC questions. 3.Calculate with unimproved soil parameters (Es , phi) to see what settlement (or stability against earthquake liquefaction, or slope stability) would result without treatment.4.From 3. above derive a sensible definition of what soil parameters need improvement, how much, and in which zones of the project such improvement is needed. Keep in mind that small differences in specified improvement level can mean huge differences in cost.5.For proper QC conditions in the Technical Specifications, translate the target soil mechanical parameters (Es , phi) into required sounding resistance values (NSPT, qc) that the contractor can work with.6.Set up the QA/QC plan to supervise the ongoing compaction works.7.On large projects: Plan a test installation to allow the contractor to calibrate is work method and grid spacing for the local soil conditions.