RECLAMATION AND SOIL IMPROVEMENT ON ULTRA-SOFT SOIL
By
Victor Choa
Nanyang Technological University, Singapore
ABSTRACT
Reclamation on ultra-soft soil is extremely difficult due to its low strength and high compressibility. Slow and steady application of load is required in order to maintain the stability of the foundation. The method of reclamation on ultra-soft soil is described in this paper. Large settlement usually occurs which does not follow Terzaghi’s small strain theory. The large settlement which occurs in the initial stage of deformation does not lead to a gain in effective stress. This paper presents a case study of a reclamation and improvement of an ultra-soft soil and the deformation characteristic of the ultra-soft soil after reclamation.
INTRODUCTION
Due to a shortage of land in coastal cities, reclamation works are being carried out to expand the land. Areas with favourable foundation for reclamation are becoming scarce .As such reclamation works have to be carried out on non- favourable foundation such as waste pond, slurry pond and recently formed estuary deposits which are still undergoing self-weight consolidation. Since such deposits are very soft with high compressibility and negligible shear strength, reclamation on them are extremely difficult. Special techniques are required for reclamation of this type of deposit. In addition to the difficulties in reclamation on such foundation, it also contributes large settlement upon application of the additional load. Large settlement usually occurs with minimum effective stress gain. This type of large strain deformation does not complyTerzaghi’s small strain theory. This paper describes a case study on reclamation and soil improvement on ultra-soft soil. Site investigation and characterization of the ultra-soft soil and the interim assessment of the improvement of the ultra-soft soil are also discussed.
SITE INVESTIGATION FOR ULTRA-SOFT SOIL
Since ultra-soft soil is too soft for conventionl me methods of site investigation, sampling was carried out with a twist sampler as shown in Figure 1. Field vane shear tests were carried with bigger dimension blades due to the low shear strength. Bulk density were measured with Gamma-Gamma Probe. Site investigations had to be carried out from the floating pontoon rather than jack-up barge.
CASE STUDY
In order to demonstrate the process of reclamation and soil improvement of ultra-soft soil, a case study of reclamation of a siltpond at Changi, Singapore is chosen. The location of the siltpond is shown in Figure 2.
Figure 1. Twist asmpler
Figure 2. Location of siltpond
Description of the Siltpond
During the reclamation works for Changi Airport, between 1975 and 1978 the borrowing of sand created a pit in the seabed to an elevation of about -22 m CD (Chart Datum). In 1986, a containment sand bund was constructed around this borrow pit. Silt and clay washings from sand quarrying activities inland was transported in pipelines with high water content and discharged into the bunded area. The pond was locally named as the siltpond. The siltpond was trapezoidal in shape and had an area of about 180 hectares. It measured about 2000 m in length and 750 m and 1050 m in width at the two ends.
The elevation of the top of the sediment varied from 0 to -5 m CD with an average at about -3 to-4 m CD. The top of the slurry was taken as the elevation at which the density was greater than 1.1 Mg/m3 . The elevation of the bottom of the slurry varied from 0 to -22 m CD with an average at – 12 to – 18 m CD. The thickness of sediment varied from 1 to about 20 m with an average of about 10 m to 15 m. Location of various types of soil investigation in the siltpond shown in Figures 3. Contour of depth to various density layers are shown in Figures 4. A typical density profile determined form Gamma-Gamma logging is shown in Figures 5. Details of the siltpond can be found in Bo et al., (1997)a, 1997b and 1998).
Figure 3. Location of site investigation in siltpond
Figure 4.Isopach map of zone B (Density 1.3 to 1.5)
Characterization of the Siltpond Slurry Prior to Reclamation
The siltpond material prior to reclamation consisted of low to high plasticity clay (Figure 6 ). The water content was as high as 250 percent with an average of 170 percent. The material was still undergoing self-weight consolidation. The density of the top part of silt was only slightly above that of salt water density. The mean grain size of silt (D50) was smaller than 3 microns and about 80 percent of the particles were smaller than 63 microns (Figure 8). The strength of the about 80 percent of the particles were smaller than 63 microns (Figure 8). The strength of the slurry was as low as 0.5 kPa at the soil surface and increasing with depth at only 0.2 kPa per meter. A summary of the properties of the siltpond material is shown on
Table 1. The properties versus depth are shown in Figure 7. It can be seen in the figures that the material had a very high moisture content and low bulk density. The preconsolidation pressures were much lower than the overburden pressure which confirmed that the material was under-consolidated and still undergoing self-weight consolidation. The liquidity index decreased with depth. Results of density logging also showed that the density increased with depth. This confirmed that consolidation commenced from the bottom rather than from the surface. Details of siltpond slurry were described in Bo et al. (2001).
Reclamation of Siltpond
First Phsae Sand Spreading
The material at the surface of the siltpond was extremely soft and virtually no shear strength. Direct hydraulic filling or dumping of sand on this ultra-soft foundation was not possible. A sand spreading method was therefore proposed. In this method sand was pumped through pipelines with high water content and deposited in a loose form with low density. The spreader used in the sand spreading work is shown in Figure 9. The photographic features of siltpond prior toreclamation is shown in Figure 10.
Figure 5. Typical density profile
Figure 6.Siltpond material on classification chart
Figure 7.Geotechnical parameters VS.depth prior to sand spreading (a) bulk density (b) liquidity index (c) water content &Atterberg limit (d) initial void ratio (e) compression index (f) coefficient of consolidation (g) preconsolidation pressure (h) overconsolidattion ratio (i) shear strength from field vane test (i) sensitivity from field vane test
Figure 8. Grain size distribution curve ofsiltpond materials.
Table 1. Soil properties of siltpond clay prior to reclamation and after sand spreading
Properties Prior to Reclamation
Range of Values After sand spreading
Range of Values
Bulk Density (Mg/m3) 1.25-1.6 1.3-1.9
Water Content (%) 75-170 40-150
Liquid Limit (%) 60-115 50-105
Plastic Limit (%) 22-45 18-30
Initial Void Ratio 2-4.5 2-3.5
Specific Gravity 2.66-2.7 2.5-2.7
Compression Index 0.5-1.7 0.6-1.4
Overconsolidation Ratio 0.2-1.2 0.2-1.0
∆Cu/∆p’ 0.078 0.094
The sequence of sand spreading in the first phase in the silt pond consisted of evenly spreading sand in 20 cm lifts until the total sand thickness deposited was 2 m. A certain rest period was required after each 20 cm lift to ensure stability of the foundation. A hydrographic survey was carried out after each lift to verify the thickness of the deposited sand. Hydrographic survey profiles after each phase of sand spreading are shown in Figure 11. The sand spreading in this phase was carried out to an elevation of about-1 to 0 m CD. Details of the method of reclamation on siltpond was extensively described in Bo et at., 1998.
Figure 9. Sand spreader used in siltpond reclamation
Figure 10.Siltpond prior to reclamation
Figure 11.Hydrographic survey profile after each phase of spreading.
Figure 12.Comparison of density profile after first phase of sand spreading.
Site Investigation During and After First Phase of Sand Spreading
In order to determine the build up of sand on top of the slurry and to investigate the improvement of the slurry, interim boreholes, field vane shear tests and density loggings were carried out. The build up of sand were found to be up 5.2 meters. The increase in density at the bottom part of the slurry but not at the top of the soil were noted at several places. Figure 12 shows a comparison between typical density logging results before and after the sand spreading. It can be seen that the bottom portion of the slurry has reached the soil stage while the top portion was still in the slurry stage. Characteristics of the material inside the siltpond after sand spreading to about 0m CD are shown in Figures 13 (a) to (j)
It can be seen in the figures that the bulk densities of soil increased to about 1.3 to 1.5 Mg/m3.However from the liquidity index the moisture content in the top 10 meters was still above the liquid limit. The void ratio reduced to about 3 to 4 within the top 10 metes and to about 2 or slightly greater at depths below 10 meters. The preconsolidation pressures of most cases were still below the original effective overburden pressure (excluding the additional load from the new sand fill). This indicated that the soil was under consolidated. The overconsolidation ratio of the material was below unity. The undrained shear strength measured by field vane tests were still below 5 kPa with an average of 2 kPa up to 7 meters depth and 5 to 7 kPa below 7 meters depth. The summary of soil properties in the siltpond after the first phase of sandspreading is shown in Table 1. It can be seen that only a slight improvement of the parameters is evident.
Figure 13. Geotechnical Parameters vs. Depth after first phase of sand spreading (a) bulk density (b) liquidity index (c) water content &Atterberg limits (d) void ratio after sand spreading (e) compression index
(f)coefficient of consolidation (g)
RECLAMATION AND SOIL IMPROVEMENT ON ULTRA-SOFT SOIL
By
Victor Choa
Nanyang Technological University, Singapore
ABSTRACT
Reclamation on ultra-soft soil is extremely difficult due to its low strength and high compressibility. Slow and steady application of load is required in order to maintain the stability of the foundation. The method of reclamation on ultra-soft soil is described in this paper. Large settlement usually occurs which does not follow Terzaghi’s small strain theory. The large settlement which occurs in the initial stage of deformation does not lead to a gain in effective stress. This paper presents a case study of a reclamation and improvement of an ultra-soft soil and the deformation characteristic of the ultra-soft soil after reclamation.
INTRODUCTION
Due to a shortage of land in coastal cities, reclamation works are being carried out to expand the land. Areas with favourable foundation for reclamation are becoming scarce .As such reclamation works have to be carried out on non- favourable foundation such as waste pond, slurry pond and recently formed estuary deposits which are still undergoing self-weight consolidation. Since such deposits are very soft with high compressibility and negligible shear strength, reclamation on them are extremely difficult. Special techniques are required for reclamation of this type of deposit. In addition to the difficulties in reclamation on such foundation, it also contributes large settlement upon application of the additional load. Large settlement usually occurs with minimum effective stress gain. This type of large strain deformation does not complyTerzaghi’s small strain theory. This paper describes a case study on reclamation and soil improvement on ultra-soft soil. Site investigation and characterization of the ultra-soft soil and the interim assessment of the improvement of the ultra-soft soil are also discussed.
SITE INVESTIGATION FOR ULTRA-SOFT SOIL
Since ultra-soft soil is too soft for conventionl me methods of site investigation, sampling was carried out with a twist sampler as shown in Figure 1. Field vane shear tests were carried with bigger dimension blades due to the low shear strength. Bulk density were measured with Gamma-Gamma Probe. Site investigations had to be carried out from the floating pontoon rather than jack-up barge.
CASE STUDY
In order to demonstrate the process of reclamation and soil improvement of ultra-soft soil, a case study of reclamation of a siltpond at Changi, Singapore is chosen. The location of the siltpond is shown in Figure 2.
Figure 1. Twist asmpler
Figure 2. Location of siltpond
Description of the Siltpond
During the reclamation works for Changi Airport, between 1975 and 1978 the borrowing of sand created a pit in the seabed to an elevation of about -22 m CD (Chart Datum). In 1986, a containment sand bund was constructed around this borrow pit. Silt and clay washings from sand quarrying activities inland was transported in pipelines with high water content and discharged into the bunded area. The pond was locally named as the siltpond. The siltpond was trapezoidal in shape and had an area of about 180 hectares. It measured about 2000 m in length and 750 m and 1050 m in width at the two ends.
The elevation of the top of the sediment varied from 0 to -5 m CD with an average at about -3 to-4 m CD. The top of the slurry was taken as the elevation at which the density was greater than 1.1 Mg/m3 . The elevation of the bottom of the slurry varied from 0 to -22 m CD with an average at – 12 to – 18 m CD. The thickness of sediment varied from 1 to about 20 m with an average of about 10 m to 15 m. Location of various types of soil investigation in the siltpond shown in Figures 3. Contour of depth to various density layers are shown in Figures 4. A typical density profile determined form Gamma-Gamma logging is shown in Figures 5. Details of the siltpond can be found in Bo et al., (1997)a, 1997b and 1998).
Figure 3. Location of site investigation in siltpond
Figure 4.Isopach map of zone B (Density 1.3 to 1.5)
Characterization of the Siltpond Slurry Prior to Reclamation
The siltpond material prior to reclamation consisted of low to high plasticity clay (Figure 6 ). The water content was as high as 250 percent with an average of 170 percent. The material was still undergoing self-weight consolidation. The density of the top part of silt was only slightly above that of salt water density. The mean grain size of silt (D50) was smaller than 3 microns and about 80 percent of the particles were smaller than 63 microns (Figure 8). The strength of the about 80 percent of the particles were smaller than 63 microns (Figure 8). The strength of the slurry was as low as 0.5 kPa at the soil surface and increasing with depth at only 0.2 kPa per meter. A summary of the properties of the siltpond material is shown on
Table 1. The properties versus depth are shown in Figure 7. It can be seen in the figures that the material had a very high moisture content and low bulk density. The preconsolidation pressures were much lower than the overburden pressure which confirmed that the material was under-consolidated and still undergoing self-weight consolidation. The liquidity index decreased with depth. Results of density logging also showed that the density increased with depth. This confirmed that consolidation commenced from the bottom rather than from the surface. Details of siltpond slurry were described in Bo et al. (2001).
Reclamation of Siltpond
First Phsae Sand Spreading
The material at the surface of the siltpond was extremely soft and virtually no shear strength. Direct hydraulic filling or dumping of sand on this ultra-soft foundation was not possible. A sand spreading method was therefore proposed. In this method sand was pumped through pipelines with high water content and deposited in a loose form with low density. The spreader used in the sand spreading work is shown in Figure 9. The photographic features of siltpond prior toreclamation is shown in Figure 10.
Figure 5. Typical density profile
Figure 6.Siltpond material on classification chart
Figure 7.Geotechnical parameters VS.depth prior to sand spreading (a) bulk density (b) liquidity index (c) water content &Atterberg limit (d) initial void ratio (e) compression index (f) coefficient of consolidation (g) preconsolidation pressure (h) overconsolidattion ratio (i) shear strength from field vane test (i) sensitivity from field vane test
Figure 8. Grain size distribution curve ofsiltpond materials.
Table 1. Soil properties of siltpond clay prior to reclamation and after sand spreading
Properties Prior to Reclamation
Range of Values After sand spreading
Range of Values
Bulk Density (Mg/m3) 1.25-1.6 1.3-1.9
Water Content (%) 75-170 40-150
Liquid Limit (%) 60-115 50-105
Plastic Limit (%) 22-45 18-30
Initial Void Ratio 2-4.5 2-3.5
Specific Gravity 2.66-2.7 2.5-2.7
Compression Index 0.5-1.7 0.6-1.4
Overconsolidation Ratio 0.2-1.2 0.2-1.0
∆Cu/∆p’ 0.078 0.094
The sequence of sand spreading in the first phase in the silt pond consisted of evenly spreading sand in 20 cm lifts until the total sand thickness deposited was 2 m. A certain rest period was required after each 20 cm lift to ensure stability of the foundation. A hydrographic survey was carried out after each lift to verify the thickness of the deposited sand. Hydrographic survey profiles after each phase of sand spreading are shown in Figure 11. The sand spreading in this phase was carried out to an elevation of about-1 to 0 m CD. Details of the method of reclamation on siltpond was extensively described in Bo et at., 1998.
Figure 9. Sand spreader used in siltpond reclamation
Figure 10.Siltpond prior to reclamation
Figure 11.Hydrographic survey profile after each phase of spreading.
Figure 12.Comparison of density profile after first phase of sand spreading.
Site Investigation During and After First Phase of Sand Spreading
In order to determine the build up of sand on top of the slurry and to investigate the improvement of the slurry, interim boreholes, field vane shear tests and density loggings were carried out. The build up of sand were found to be up 5.2 meters. The increase in density at the bottom part of the slurry but not at the top of the soil were noted at several places. Figure 12 shows a comparison between typical density logging results before and after the sand spreading. It can be seen that the bottom portion of the slurry has reached the soil stage while the top portion was still in the slurry stage. Characteristics of the material inside the siltpond after sand spreading to about 0m CD are shown in Figures 13 (a) to (j)
It can be seen in the figures that the bulk densities of soil increased to about 1.3 to 1.5 Mg/m3.However from the liquidity index the moisture content in the top 10 meters was still above the liquid limit. The void ratio reduced to about 3 to 4 within the top 10 metes and to about 2 or slightly greater at depths below 10 meters. The preconsolidation pressures of most cases were still below the original effective overburden pressure (excluding the additional load from the new sand fill). This indicated that the soil was under consolidated. The overconsolidation ratio of the material was below unity. The undrained shear strength measured by field vane tests were still below 5 kPa with an average of 2 kPa up to 7 meters depth and 5 to 7 kPa below 7 meters depth. The summary of soil properties in the siltpond after the first phase of sandspreading is shown in Table 1. It can be seen that only a slight improvement of the parameters is evident.
Figure 13. Geotechnical Parameters vs. Depth after first phase of sand spreading (a) bulk density (b) liquidity index (c) water content &Atterberg limits (d) void ratio after sand spreading (e) compression index
(f)coefficient of consolidation (g)
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