Performance of Encased Silica-Manganese Slag Stone Columns in Soft Marine Clay
Published at : 30 Oct 2019
Volume : IJtech
Vol 10, No 5 (2019)
DOI : https://doi.org/10.14716/ijtech.v10i5.2094
Prasad, S.S.G., Satyanarayana, P.V.V., 2019. Performance of Encased Silica-Manganese Slag Stone Columns in Soft Marine Clay. International Journal of Technology. Volume 10(5), pp. 887-896
| S. Siva Gowri Prasad | Department of Civil Engineering, GMR Institute of Technology, Andhra Pradesh 532127, India |
| P.V.V. Satyanarayana | Department of Civil Engineering, Andhra University, Visakha Patnam, Andhra Pradesh 532127, India |
Stone
columns are the most suitable and economical ground improvement technique for
soft soils. Stone columns accelerate the consolidation process, thereby
increase the stiffness of the soil. This increase may not be sufficient because
of the less lateral confinement, which leads to excessive bulging. The strength
of the composite soil can also be increased further by encasing the column with
geotextile. In this paper, model tests were conducted on end-bearing stone
columns with geotextile encasement and compared with the unreinforced (plain) stone
columns. The stone columns were prepared by placing the silica-manganese slag,
sand and were reinforced with geotextile with different encasement lengths of
D, 2D, 3D, and 4D (D is the stone column diameter; i.e., 5 cm). The tests
demonstrated that the engineering behavior of the soil was improved by
introducing the silica-manganese slag (when compared with conventional stone
columns) and also with encasement. Bulging can also be reduced by providing
encasement beyond the zone of bulging.
Bulging; Encasement; Geotextile; Marine clay; Silica-manganese slag; Stone column
Due to development of infrastructure in metropolitan cities, suitable
sites for construction have been reduced and caused a rise in land prices.
Because of this problem, industries are looking for cheaper land for
construction. As a result, some sites which were not used earlier due to low
strength are now being used for construction. When these soils are loaded, they
may experience failure due to excessive settlement. Greenwood (1970) was first
to propose load transfer theory, settlement prediction, and estimation of
ultimate bearing capacity. Hughes and Withers (1974) found that stone columns
fail under compressive loads in general shear, bulging, and sliding. The
load-carrying capacity of the columns is acquired via lateral confinement from
the surrounding soils (Greenwood, 1970). While the stone columns improve soft
soil, sufficient load-carrying capacity may not be achieved because of the less
lateral confinement. To overcome this situation, geosynthetic material can be
used for encasing stone columns. This is the most popularly used method.
Many researchers have used geosynthetic material as encasement for stone columns to improve soft soils. Murugesan and Rajagopal (2009; 2010), Gniel and Bouazza (2009), Samadhiya et al. (2009), and Hasan and Samadhiya (2016) studied the behavior of geosynthetic/geogrid-encased stone columns and found that the stiffness of soft soil can be improved by increasing the encasement length. Malarvizhi and Ilamparuthi (2004) reported that settlement can be reduced by providing the encasement by increasing the stiffness of the stone column. Murugesan and Rajagopal (2009) studied geosynthetic-encased stone column performance and found that the pressure settlement response showed linear behavior.
Malarvizhi and Ilamparuthi (2004; 2007) and Ali et al.
(2011) studied the effect of length to diameter ratio (L/D) and found that the
load-carrying capacity was increased by increasing the L/D ratio whereas the
influence is much less in floating columns (Malarvizhi & Ilamparuthi,
2004). The bearing capacity of composite soil increases with column length, but
the increase is not significant when the length exceeds beyond six times the
column diameter (Ali et al., 2011). Samadhiya et al. (2009), Murugesan and
Rajagopal (2010), Ali et al. (2011), and Hasan and Samadhiya (2016) conducted tests on stone columns
of different diameters and concluded that the stiffness of the soil increases
with a decrease in the diameter of the column. This is because of the higher
confining stresses mobilized on smaller diameter columns. Fattah et al. (2016)
studied the behavior of stone columns in embankments and concluded that the
Stress Concentration Ratio (SCR; the ratio of the stresses in the column to the
surrounding soil) increases gradually with increasing L/D ratio.
Dheerendra Babu et al. (2010) conducted experiments on
stone columns reinforced with vertical nails placed along the circumference and
found that the circumferential nails enhanced the stone column performance.
Furthermore, the behavior of composite ground was improved with the number of
nails. They also found that in order to enhance the stone column performance
significantly, the depth of embedment of nails required was 3D to 4D. Fattah & Majeed (2012a) studied the behavior
of capped stone columns encased with geogrid by the finite element method and
found that the capped stone column increased the bearing improvement ratio (q
treated/q untreated) and decreased the settlement for all L/D ratios. The
bearing improvement ratio also increased with the thickness of the cap, up to
0.4 times the footing diameter.
Samadhiya et al. (2009) and Hasan and Samadhiya (2016) studied
the lateral reinforcement of geogrid strips by varying the vertical spacing and
concluded that the load intensity was increased by decreasing the spacing. The
strength of granular pile was increased by increasing the length of
reinforcement to a depth of three times the diameter and no further increment
was observed. Basu et al. (2016) worked with fiber-reinforced stone columns and
found that the diameter of bulging can be decreased by increase the length and
the fiber content. The depth of maximum bulging from the surface also
decreased, but the total length of bulging was increased. Prasad and
Satyanarayana (2016) studied the behavior of geotextile-reinforced stone
columns by placing the reinforcement laterally at different spacings and found
that the load-carrying capacity increased with the decrease in spacing.
Ambily and Gandhi (2004) carried out experimental
studies by loading stone columns on their area alone and found that the failure
occurred in the form of bulging of the stone column at a depth of about 0.5D to
1.0D below the surface. When the load was applied to the tank wall, the
load/settlement behavior was linear and the failure did not take place. Fattah & Majeed (2012b) studied the
geogrid-encased floating stone columns and found that the maximum lateral
displacement occurred at an effective encasement length ratio (length of
geogrid encasement along the stone column/total stone column length) of 0.6. Gniel & Bouazza
(2009) carried out experiments on geogrid-encased stone columns and found that
maximum bulging occurred at a depth of 2D. This could be reduced by providing
encasement beyond the zone of bulging. Damoerin et al. (2015) carried out a
series of tests to increase the shear strength of the soil by improving the
cement column and found that this increased the shear strength of the soil.
Fattah and Majeed (2009) studied the behavior of encased floating stone columns
and found that the bearing improvement ratio increased by increasing the area
replacement ratio for both ordinary and encased stone columns.
Fattah et al. (2010) carried out tests on stone
columns by varying the SCR and found that the stiffness was increased with an
increase in stiffness of the treated soil. Malarvizhi and Ilamparuthi (2007),
and Murugesan and Rajagopal (2010) studied the stone column behavior and
concluded that the SCR increased by inclusion of the encasing material and also
with the stiffness of the encasing material.
Ambily and Gandhi (2007) carried out tests on stone
columns by varying the spacing between the columns and shear strength of the
soil for both single and group columns. They found that the stiffness
improvement factor (the ratio of stiffness of treated soil to untreated soil)
depends on the angle of internal friction between the stones and the spacing
between the columns, independent of the shear strength of the soil. They also
found that the settlement increased and the load-carrying capacity decreased
with an increase in spacing up to an L/D ratio of 3 (beyond this, the change
was negligible). For stone columns reinforced to L = 2D the improvement ratio
was very high and the settlement reduction ratio was very low (Fattah et al.,
2016).
From the literature review, it is clear that many
researchers have studied soft soil improvement by using different types of
stone aggregates. However, there has been limited research on the replacement
of stone aggregates with other materials. In this study an alternate material
(Silica-Manganese slag) was used as the column material and the sand was
replaced within the voids between the aggregates. This column was further
encased with geotextile material with different encasement lengths, and the
bulging and load versus settlement behavior was studied.
Experimental studies were conducted on stone columns
by replacing the column material with silica-manganese slag, and the columns were
reinforced with geotextile for various encasement lengths. The following
conclusions were made: (1) Silica-manganese slag is a potential alternative for
improvement of soft soil as it has a better load-carrying capacities than
conventional stone columns (about 9%) because of the superior properties of the
slag over the stone chips; (2) The load-carrying capacity of the stone column
was increased by introducing the encasement, due to mobilization of the hoop
stresses which resist bulging. These hoop stresses help to transfer the load to
the bottom of the stone column, and thus the bearing capacity increases. The
load-carrying capacity also increased by increasing the encasement length of
the column; (3) Bulging was reduced by providing reinforcement to the columns.
The maximum bulging occurred at the center of the column for both the plain and
fully-reinforced columns. For the other reinforcement lengths, bulging was
found just below the encasement depth. This indicates that the reinforcement
transfers the bulging to the greater depths; (4) The bulging can be decreased
by providing reinforcement beyond the zone of bulging (i.e., beyond an
encasement length of 2D) due to the increase in confinement on the stone column
where the bulging is occurring.
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| R2-CVE-2094-20190622203756.jpg | Figure 1(c) |
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| R2-CVE-2094-20190622203933.jpg | Figure 6 |
| R2-CVE-2094-20190622203956.PNG | Figure 7 |
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