Improvement of fine-grained and cohesive soils 149
access to a working recipient. Thorough compaction of this layer is recommended before concreting work commences. Measurements and the above
parametric study reveal that this layer helps to concentrate foundation
loads into the stone columns. It helps to equalize the bearing stresses acting
on the footing. However, the load distribution layer itself does not increase
the improvement factor of the stone column-reinforced strata any further.
Footing and slab stiffness help to evenly distribute foundation loads on
the stone columns leading finally also to evenly distributed settlements.
stone columns should always be arranged directly below footings and foundation slabs. Only when other criteria dictate—bearing capacity, slip failure, or earthquake mitigation—do they also need to be placed outside.
For the prevention of liquefaction, it is also recommended to extend the
stone columns outside the structure over a width which is equal to at least
two-thirds of the soil improvement treatment depth.
4.4 QUALITY CONTROL AND TESTING
In addition to the quality control measures that were discussed in Section 3.4
for the vibro compaction method, concerning assurance of the geometrical
layout of the vibro stone column pattern and their treatment depth, verification of the column diameter d is of prime importance. This geometrical
relation is represented by the area replacement factor a c , which significantly
influences design and performance of the vibro stone column foundation.
Modern data acquisition systems for vibro replacement collect, store,
and display all relevant data of a stone column including column identification number and, as a function of real time, depth, AC current or hydraulic
pressure of the vibrator motor, stone volume (measured directly or deduced
from measured stone weight), air pressure, and installation duration. All
data are generally collected numerically and can be stored or used for graphical displays to support construction and supervision procedures or to serve
for quantity surveying purposes (Bauldry et al., 2008). Interpretation of
the graphical printouts obtained in this way is very important but requires
experience. Deficiencies of the column installation, particularly necking of
the column diameter, can however only be detected when real-time recordings of the parameters are made available.
Where the stone backfill quantity cannot be measured directly versus
depth and recorded by the monitoring device, a realistic estimate of the
total volume of stone needs to be carried out. It should be correlated with
the total length of stone columns executed during reasonable time intervals and calibrated with the delivered stone quantity. Material wastage and
excessive heave developing at the working platform during column installation have to be considered.
Where heterogeneous site conditions prevail and compactable sand layers alternate with cohesive soil layers, improvement can be achieved by a
access to a working recipient. Thorough compaction of this layer is recommended before concreting work commences. Measurements and the above
parametric study reveal that this layer helps to concentrate foundation
loads into the stone columns. It helps to equalize the bearing stresses acting
on the footing. However, the load distribution layer itself does not increase
the improvement factor of the stone column-reinforced strata any further.
Footing and slab stiffness help to evenly distribute foundation loads on
the stone columns leading finally also to evenly distributed settlements.
stone columns should always be arranged directly below footings and foundation slabs. Only when other criteria dictate—bearing capacity, slip failure, or earthquake mitigation—do they also need to be placed outside.
For the prevention of liquefaction, it is also recommended to extend the
stone columns outside the structure over a width which is equal to at least
two-thirds of the soil improvement treatment depth.
4.4 QUALITY CONTROL AND TESTING
In addition to the quality control measures that were discussed in Section 3.4
for the vibro compaction method, concerning assurance of the geometrical
layout of the vibro stone column pattern and their treatment depth, verification of the column diameter d is of prime importance. This geometrical
relation is represented by the area replacement factor a c , which significantly
influences design and performance of the vibro stone column foundation.
Modern data acquisition systems for vibro replacement collect, store,
and display all relevant data of a stone column including column identification number and, as a function of real time, depth, AC current or hydraulic
pressure of the vibrator motor, stone volume (measured directly or deduced
from measured stone weight), air pressure, and installation duration. All
data are generally collected numerically and can be stored or used for graphical displays to support construction and supervision procedures or to serve
for quantity surveying purposes (Bauldry et al., 2008). Interpretation of
the graphical printouts obtained in this way is very important but requires
experience. Deficiencies of the column installation, particularly necking of
the column diameter, can however only be detected when real-time recordings of the parameters are made available.
Where the stone backfill quantity cannot be measured directly versus
depth and recorded by the monitoring device, a realistic estimate of the
total volume of stone needs to be carried out. It should be correlated with
the total length of stone columns executed during reasonable time intervals and calibrated with the delivered stone quantity. Material wastage and
excessive heave developing at the working platform during column installation have to be considered.
Where heterogeneous site conditions prevail and compactable sand layers alternate with cohesive soil layers, improvement can be achieved by a