Deep vibratory compaction
Deep vibratory compaction is usually restricted to granular soils; increasing fines content will reduce the compaction efficiency. It is often found that a fines content of more than 10% causes difficulties. Soils exhibiting inter-particle bonding due to cementation, suction or some other cause may not be suitable for this type of ground treatment. In some cases, compaction efficiency can be increased by using water flushing, or in combination with vertical drains. Compaction up to ground surface is only possible applying additional
Deep vibratory compaction of granular soils can be achieved by methods which use either a depth vibrator (as shown in Figure A.1) or a top vibrator. Methods using a depth vibrator are similar in principle to the methods described in Annex B, although a stone column is not always to be formed.
- 1 Eccentric weight (within)
- 2 Extension tube
- 3 Isolator
- 4 Water or air jets
- 5 Motor (within)
- 6 Fins to prevent twist
- 7 Nose cone
Where a top vibrator is used, it is connected to the top of a compaction probe, which is designed to transfer the vibrations to the soil as efficiently as possible. Several different types of compaction probes are available including the vibro-wing (Figure A.2) and other flexible probes. Conventional vibrators for sheet-pile driving can be used, but special vibrators have been developed. Although the top vibrator usually vibrates vertically, the probe will cause horizontal accelerations which may locally be greater than the vertical ones. The compaction increases when resonance is created between the vibrating system (vibrator and compaction probe) and surrounding soil. By means of vibration sensors placed on the ground, and a vibrator with adjustable frequency, the frequency can be adapted to amplify ground vibrations; this method is known as resonance compaction.
Compaction is achieved by inserting the probe at treatment points usually on a triangular or rectangular grid. Spacings are typically from 1m to 4 m depending on the type and size of the compaction probe and vibrator capacity. At each treatment point the probe is inserted into the soil to the depth to which compaction is required. The compaction is obtained during penetration or during penetration and extraction. The compaction time at each point varies typically from 5 min to 40 min and the time required increases with the fine content of the soil. Compaction can be effected using several passes, with closer spacings in the later passes.
Installation of vibrated stone columns
There are three principal methods of installing vibrated stone columns, dry top-feed process, wet process and dry bottom-feed process, and for each method the installation of a single column is described. With the vibrated stone column processes, column installation is repeated for further columns at a predetermined spacing to effect the desired treatment. All three processes use a similar type of depth vibrator, which is an eccentric weight assembly rotating rapidly within a heavy tubular steel casing. The general arrangement of the depth vibrator is shown in Figure A.1. The nose of the vibrator is tapered to aid penetration on the ground, whilst vertical fins prevent the vibrator rotating during penetration.
The following descriptions are given as typical. In practice small differences in detail may be noticed.
B.2 Dry top-feed process
In granular soils, this method is usually only possible above the water table. The whole assembly is suspended from a crawler mounted crane and the vibrator is lowered onto the ground. Penetration of the fill and/or underlying weak soil is effected by a combination of the weight of the vibrator, the high frequency vibration and compressed air. A compressor supplies the depth vibrator with air, which emerges from nozzles in the main steel housing just above the vibrator tip. The general arrangement is shown in Figure B.1. After reaching the required depth, the vibrator is held in the ground for a short time and then withdrawn. A small charge of clean, inert granular material is tipped into the hole and the vibrator is lowered again to compact the granular material and interlock it with the surrounding soils. By adding successive small charges of granular material and compacting each one to chosen levels of power consumption, a dense stone column is built up to ground level. Typically gradings for the granular material are within the range from 40 mm to 75 mm.
- 1 Stone column being formed
- 2 Stockpile of granular infill
- 3 Vibrator
B.3 Wet process
The wet process is used where the dry top-feed process cannot be used because of unstable ground. The depth vibrator is similar to that used for the dry process but is equipped with water flushing. The general arrangement is shown in Figure B.2. The depth vibrator is suspended from a suitable crane, lowered onto the ground and the water jets are opened. The vibrator penetrates quickly through weak soils under its own weight aided by the water flushing and vibrations. The vibrator is partially withdrawn and is sometimes surged to flush out the weak soils accumulating in and adjacent to the bore. Following formation of an open hole the vibrator is kept in the ground and the water flow reduced whilst clean inert granular material is successively heaped around the top of the vibrator bore at ground level. The granular material then passes down between the vibrator and the surrounding soils to permit the construction of a stone column in short lifts and repenetration steps. It is important that the water flow is maintained until the vibrator reaches ground surface. The vibrator compacts the granular infill and interlocks it tightly with the surrounding soil. The cycle is repeated until a compact stone column is built up to ground level. Typically gradings for the granular material are within the range from 25 mm to 75 mm.
The wet process has considerable attendant problems of water supply, drainage ditches, settlement lagoons and final disposal of the effluent in a manner acceptable to the statutory authorities.
- 1 Stone column
- 2 Stone stockpile
- 3 Water flushing
- 4 Vibrator
B.4 Dry bottom-feed process
As the vibrator remains in the hole during column construction, the process can operate successfully in unstable hole conditions and can be used instead of the wet process in most cases. The bottom-feed depth vibrator has a heavy duty supply tube located down one side and permanently fixed to the vibrator forming a fully integrated vibrator/granular material supply. The supply tube bends inwards at the vibrator tip to ensure a central location for the supply of granular material. The general arrangement is shown in Figure B.3.
The cycle of operations for this completely dry process is as follows. The vibrator is positioned on the ground at the treatment point location; and the whole system is charged with granular material. With the granular material in the supply tube acting as a plug at the tip of the vibrator, assisted as necessary by compressed air and under the combined action of the vibrations and its weight, using an additional pull down force if necessary, the depth vibrator penetrates the ground to the required depth. The stone column is then formed and compacted by lifting the vibrator, holding the lift for a short time to allow the granular material to run, and then forcing the vibrator down on the charge of granular material to compact and tightly interlock it with the surrounding soil. This is repeated, charging the system with granular material as necessary, until a compact stone column is formed up to ground level. Typically gradings for the granular material are within the range from 8 mm to 50 mm.
- 1 Pressure chamber
- 2 Vibrator
- 3 Stone stockpile
- 4 Stone feed bucket
- 5 Stone delivery tube
 EN 1097-1, Tests for mechanical and physical properties of aggregates - Part 1: Determination of the resistance to wear (micro-Deval)
 EN 1097-2, Tests for mechanical and physical properties of aggregates - Part 2: Methods for the determination of the resistance to fragmentation
 prEN 1998, Eurocode 8: Design of structures for earthquake resistance