Contractual matters 217
of the upper 0.5 m. The engineer may specify a depth vibrator with a certain power rating, although this selection is better left to the specialist contractor and will be reflected in his proposal by the probe pattern, method
statement, and execution time necessary for the volume of works. For preliminary assessment of the time to compact sand from loose to medium density, an average production rate of about 7500 m 3 for a 10-h shift of a single
depth vibrator is reasonable. Production and unit price depend, of course,
on many parameters and conditions described in Chapter 3.
For vibro stone columns the proposed unit for measurement is lin.m of
stone column with specified diameter, length, and quality of gravel backfill, which the contractor has to identify with appropriate data acquisition,
calibrated before starting the work and at suitable intervals during the
contract. The specifications will indicate which method of execution (wet,
dry top or bottom feed system) should be used. It is advisable to specify a
minimum power rating for the depth vibrator to be used to ensure proper
densification of the stone backfill. The decision to pre-bore through hard
layers should be left with the specialist contractor, provided their strength
can be described sufficiently accurately. For normal conditions, an average
production rate of 300 lin.m of 0.6 m diameter dry bottom feed stone columns can be assumed for a 10-h shift and one machine operating.
The main item for the execution of stone columns is occasionally split
in an item for the column construction in lin.m and a second item for the
delivery and placement of the stone backfill in cubic meter or per ton.
To underline the importance of the stone column diameter, the authors do
recommend always specifying the stone columns together with a minimum
diameter according to the design which has to be verified during execution
of the works. Price of the backfill stone can be decisive for the cost of a
vibro replacement project. It is therefore important to find suitable material
at economic prices. The use of sand or recycled material is often a worthwhile compromise, even if the stone column design needs amending for
reduced shear properties.
Vibro concrete columns represent a variation of small diameter in-situ
concrete piles and should, from a contractual point of view, be considered
accordingly. The establishment and on-site control of the load carrying capacity and settlement performance should generally be by standard load testing,
only in exceptional cases and with sufficient experience should dynamic load
testing be performed. Vibro concrete column foundations should only be carried out by experienced contactors using efficient data acquisition systems
closely controlling especially verticality and diameter of the column.
In all cases in which the works are being executed according to the specialist subcontractor’s design, the employer or turnkey contractor will base
his decision for the contract award on price, time of execution, and a warranted design. Resolution of disputes on the extent of the treatment (probe
spacing and depth, column diameter, depth, and distance) can only be on
the basis of a comprehensive geotechnical report.
of the upper 0.5 m. The engineer may specify a depth vibrator with a certain power rating, although this selection is better left to the specialist contractor and will be reflected in his proposal by the probe pattern, method
statement, and execution time necessary for the volume of works. For preliminary assessment of the time to compact sand from loose to medium density, an average production rate of about 7500 m 3 for a 10-h shift of a single
depth vibrator is reasonable. Production and unit price depend, of course,
on many parameters and conditions described in Chapter 3.
For vibro stone columns the proposed unit for measurement is lin.m of
stone column with specified diameter, length, and quality of gravel backfill, which the contractor has to identify with appropriate data acquisition,
calibrated before starting the work and at suitable intervals during the
contract. The specifications will indicate which method of execution (wet,
dry top or bottom feed system) should be used. It is advisable to specify a
minimum power rating for the depth vibrator to be used to ensure proper
densification of the stone backfill. The decision to pre-bore through hard
layers should be left with the specialist contractor, provided their strength
can be described sufficiently accurately. For normal conditions, an average
production rate of 300 lin.m of 0.6 m diameter dry bottom feed stone columns can be assumed for a 10-h shift and one machine operating.
The main item for the execution of stone columns is occasionally split
in an item for the column construction in lin.m and a second item for the
delivery and placement of the stone backfill in cubic meter or per ton.
To underline the importance of the stone column diameter, the authors do
recommend always specifying the stone columns together with a minimum
diameter according to the design which has to be verified during execution
of the works. Price of the backfill stone can be decisive for the cost of a
vibro replacement project. It is therefore important to find suitable material
at economic prices. The use of sand or recycled material is often a worthwhile compromise, even if the stone column design needs amending for
reduced shear properties.
Vibro concrete columns represent a variation of small diameter in-situ
concrete piles and should, from a contractual point of view, be considered
accordingly. The establishment and on-site control of the load carrying capacity and settlement performance should generally be by standard load testing,
only in exceptional cases and with sufficient experience should dynamic load
testing be performed. Vibro concrete column foundations should only be carried out by experienced contactors using efficient data acquisition systems
closely controlling especially verticality and diameter of the column.
In all cases in which the works are being executed according to the specialist subcontractor’s design, the employer or turnkey contractor will base
his decision for the contract award on price, time of execution, and a warranted design. Resolution of disputes on the extent of the treatment (probe
spacing and depth, column diameter, depth, and distance) can only be on
the basis of a comprehensive geotechnical report.