INTRODUCTION OF VIBRO STONE COLUMN TECHNIQUE
History of ground treatment was establish by Keller for almost 150 years of excellence in ground engineering.
The Ground improvement for soil have been developed from different techniques :
- Keller invented Depth Vibrator and developed Vibro Compaction Technique in 1930’s
- Keller developed Vibro Replacement (Vibro Stone Columns) Technique in 1950’s
- Patent granted for Keller’s Bottom Feed Vibrator in 1972
The Vibro replacement ( Stone Column ) will improve the condition of soil
- Increase overall stiffness of the compressible soils
- Increase shear strength
- Allow rapid consolidation by providing radial drainage
Type of Vibrator
Choice of vibrator depending on type of soils and treatment depth. Available type of vibrators:
- 15 tons vibrator
- 23 tons vibrator
- 30 tons vibrator
- 70 tons vibrator
Method Statement of Installation
Standards and References.
To compliance with British Standard BS EN 14731: 2005
– Execution of Special Geotechnical Works – Ground Treatment by Deep Vibration
Method of Installation of Vibro-Replacement Stone Column have 2 type
a) Wet Method Installation
b) Dry Method Installation.
Access and Site Preparation
b. Check and verify that the following requirements are provided by the Contractor for the areas on which the ground treatment is to be conducted:
i. All existing utilities, cables and other buried services shall be relocated or their exact location properly
ii. The installed Temporary works is adequate, safe and suitable for the proposed type of ground treatment.
iii. All working platforms meet the design and construction requirements in terms of thickness, material type, slope and degree of compaction.
iv. Proper drainage is provided for all working platforms and other access and storage areas.
v. Sufficient area is provided for stockpile of material and machineries needed for the Works within the areas.
Resources Requirement during Installation
- Refer to the Stone Column method statements detailing the type of equipment to be used for the Works in which Vibro-replacement equipments are to be used. It shall be emphasized that the PMA of such equipment shall be valid.
- Bearing Plates: a rectangular steel plate 1200 x 1200 mm for 800mm dia. Stone Column and 1500 x 1500 mm for 1000mm dia. Stone Column shall be used as bearing plate.
- Settlement Recording Devices: four dial gauges are required for the measurement of the test plate settlement. The accuracy shall be at least 0.25mm.
- A Site Engineer to ensure technical compliance of the test
- An experienced Supervisor to ensure smooth and timely completion of the test
- An experienced Mechanic
- Sufficient skilled operators and/or semi-skilled workers to carry out the works
- The stone shall be to joint sampling and to be comply with the specifications requirements.
- Pre-Treatment CPT for the area shall be conducted at the proposed area of stone column.
- Referred to the result from CPT, The design layout and depth of stone column at particular to finalized
Please note, in the process of installation of stone column , using vibro replacement , the following steps are to be followed:
- Prepare working platform for the access of crawler crane / excavator with Vibro replacer and trucks to transport stone .
- Lay sand blanket as per the specification.
- Mark out stone column positions
- Shift Vibro – replacer to the marked position and lower down the vibro replacer . The Vibro – replacer is to be connected by follower tubes measured 310mm in diameter hoisted by a 50 ton crawler crane with the power pack mounted on a rack behind crane, in the process of vibro replacement , the vibro replacer is penetrated into the ground to the specified depth with the aid of water/ air pressure and vibro replacer self weight. Bottom water / air discharge jets come through the nozzle via hoses connected to high – pressure water / air pump. Once the required depth is reached , the downward water jet will be turned to minimal flow while the horizontal jet nozzles will be turned on.
- Vibro – Replacer is then slowly withdrawn and crushed run stone can be fed into the hole formed by vibro replacer
- RE – insert the vibro replacer into the stone and compact stone backfill to achieve the specified stone column diameter and density using the pre defined operational parameters obtained from trial tests.
- Raise vibro replacer , dump crushed stone , re lower the vibro replacer and compact the stone to form another section of stone column.
- Repeat the procedure stated in step ( 7) until the specified top level of this stone column.
- Shift the vibro replacer to another stone column position and repeat steps ( 4) to (8).
It is to be noted that a computer output sheet will be generated for installed stone column which will cleary indicate the depth, degree of compaction and pressure .
A. Twin vibrator
B. Bottom Feed
Typical Installation record
Post-Treatment Testing -Plate Bearing Test
As mentioned earlier, this Plate Bearing Test will be conducted in accordance with the procedure given in Clause 29 of BS5930: 1981, at a rate of 1 test every 5000 m2.
In order to monitoring the settlement of the ground some Post-Construction Instrumentation Monitoring such as Rod settlement gauges and Inclinometers to be implemented.