TECHNICAL NOTE 1 - Stone Column


ITEM 203.99870017 – STONE COLUMNS - BOTTOM FEED - VIBRO-DISPLACEMENT

(DRY) METHOD

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DESCRIPTION


This work shall consist of installing stone columns using a bottom feed vibro-displacement (dry)

method in accordance with these specifications and in reasonably close conformity with the lines

and dimensions shown on the plan or established in writing by the Engineer.

This work shall be accomplished by a Contractor experienced and properly equipped in the

installation of stone columns by the dry method. The Contractor shall submit evidence of having

satisfactorily installed stone columns on at least three projects to the Director of the Geotechnical

Engineering Bureau, who will evaluate this experience for approval or disapproval. This

submission shall identify the client, size and dates of projects, list of equipment, method of

installation, and key personnel from those projects to be assigned to this project.


MATERIALS


Backfill used in the stone columns shall be crushed stone meeting the requirements of §703-02

Coarse Aggregate. The gradation shall conform to Size Designation 2, as shown in Table 703-4,

or an approved equal. At least one month prior to commencement of the stone column

installation, the Contractor shall submit to the Department a minimum of three 50-pound bags of

crushed stone proposed for this Backfill Stone for testing to determine specific gravity, and loose

and compacted density.


CONSTRUCTION DETAILS


A. Equipment. The Contractor’s equipment shall be capable of producing and/or complying

with the following:

1. Contain a motor to drive an eccentric mass at a frequency of 1600 to 3000 RPM which

shall be capable of generating at least 100 hp and 20 tons of centrifugal force.

2. Produce a double amplitude (peak to peak measurement) of the probe tip a minimum of

0.6 inches in the horizontal direction when the probe is in a freely suspended position.

3. The probe and follower tubes shall be of sufficient length to provide stone columns

having a maximum length of 35 feet and have visible external markings at 1.0 foot

increments to enable measurement of penetration and re-penetration depths.

4. Provide for supplying compressed air to minimize clogging within the stone delivery pipe

and to assist in the placement of the backfill stone.

5. Be capable of compacting the backfill stone and forcing the stone radially into the

surrounding in-situ soil by means of displacement in combination with vibration.

6. Be capable of constructing stone columns in a manner such that dissipation of vibratory

energy to the surrounding soil occurs mainly from the lower portion of the probe.

7. Be capable of constructing stone columns in a manner such that the probe remains in the

unbackfilled part of the hole at all times.


ITEM 203.99870017 – STONE COLUMNS - BOTTOM FEED - VIBRO-DISPLACEMENT

(DRY) METHOD


Page 2 of 3 11/13/00 09/01/05

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B. Stone Column Construction


1. The Drainage Blanket shown on the plans shall be placed before installing any stone

columns.

2. The sequence of stone column installation shall be in accordance with the plans unless

otherwise directed by the Engineer. The upper dense soil may be augered to a point

where penetration of the remaining soil may be achieved by the probe tip to the required

column depth. The depth of pre-augering shall be approved by the Engineer. Soil

removed by augering shall be disposed of as approved by the Engineer. Stone backfill

shall be placed from the bottom up to the working surface in increments not exceeding

4.0 feet in depth. The use of water during installation will not be allowed.

3. Prior to the installation of production stone columns, the Contractor shall install five test

columns within the stone column layout pattern shown on the plans. Each specific test

column location shall be selected by the Engineer. The purpose of these test columns is

to establish optimum, site-specific, stone column installation and construction control

procedures and power consumption to be used in the production work.

4. Stone columns shall be installed so that each completed column will be continuous

throughout its length with an average effective diameter of 3.5 feet for every 2 foot

increment of depth. If the average effective diameter of a 2 foot increment of depth,

either during the installation of a test column or a subsequent production column, is less

than 3.5 feet for three or more measurements, and the workmanship, power consumption,

and materials are otherwise acceptable, the Contractor shall be allowed to complete the

column being installed. The Contractor shall then cease all further stone column

installation operations in the immediate area of this stone column to allow the Engineer to

determine if it is necessary to adjust the column size and pattern spacing, and to what

extent. The time period for the Engineer’s assessment will not exceed one full work day.

5. No stone column shall be accepted for payment that has an average effective diameter

less than 3.0 feet for any 2 foot increment unless approved by the Engineer. The

Engineer’s decision will be based on a review of the Contractor’s installation procedures

including power consumption used for installing the stone column.

6. The average effective stone column diameter shall be calculated by the Contractor using

the Department’s assigned in-place density of the stone and the weight of stone used to

fill a given length of the hole. For calculation of constructed column diameter, the

assigned in-place density of stone columns shall be equal to the density corresponding to

95% of the laboratory relative density determined by the Department.

7. The Engineer may also require that stone column installation work cease if State installed

instrumentation at this site indicates potential or actual slope movement. In this case, the

Engineer will determine if, and to what extent, the sequence and rate of stone column

installation is to be modified.


ITEM 203.99870017 – STONE COLUMNS - BOTTOM FEED - VIBRO-DISPLACEMENT

(DRY) METHOD

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C. Subsurface Obstructions. In the event that subsurface obstructions encountered during

construction of a stone column cannot be penetrated with reasonable effort, the stone column

shall be constructed following the specified procedures from the obstruction to the surface of the

drainage blanket. The Engineer may direct the construction of a replacement stone column at

another location.


D. Construction Records. The Contractor shall provide competent and qualified personnel to

prepare and furnish to the Engineer, daily recorded logs of the following data:


1. Stone column reference number.

2. Elevation of top and bottom of each stone column.

3. Peak power dissipation from the probe, by instrument reading, for each one foot of stone

column, or each increment of stone charge added.

4. Volume measurement and calculated weight of the total amount of loose stone used for

each column.

5. Time to penetrate and time to form each stone column.

6. The average effective diameter of every 2 foot of increment in depth for columns. This

information shall be provided to the Engineer as work progresses as well as at the end of

the work day.

7. Details of obstructions, delays, notations of artesian flow from stone columns and any

other unusual ground conditions.

8. Pre-augered depth, if any, for each stone column.

All records shall be of form and content to the satisfaction of the Engineer.


METHOD OF MEASUREMENT


This work will be measured as the number of linear feet of stone columns, including test

columns and obstructed stone columns, satisfactorily completed in-place in accordance with the

contract documents and as directed by the Engineer. Measurement will be from the bottom of

each column to the top of the working surface. Measurement will be to the nearest 0.5 foot.


BASIS OF PAYMENT


The unit price bid shall include the cost of furnishing all labor, mobilization and utilization of all

equipment and material for developing and backfilling stone column holes with backfill stone,

providing records and logs as necessary to satisfactorily complete the work, and for removal and

disposal of any excavated or augered material from stone column holes.

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