Abstract : Marine clay is one of the problematic soil which cause excess in settlement and having low in bearing pressure. For improving the bearing pressure and reducing the settlement, deep mixing technique is carried out by using different additives. The paper mainly deals with the Deep Cement Mixed column technique as a ground improvement method. The main objective of the project to determine the load – settlement characteristics of marine clay using deep mixed with cement. In deep cement mixing, the amount of cement which is mixed with marine clay to improve the strength characteristics. Plate load tests are conducted to investigate the performance on deep mixed columns which affects the marine clay. Effect of curing on cement is determined. Also different pattern (square and triangular) columns are installed for the determination of which pattern will give good results in bearing pressure. The parameter varying for deep mixing are curing time(1 day, 3 days, 5 days), Water to binder ratio(0.45,0.55,0.65) and different pattern(square and triangular). High bearing pressure is obtained at a water to cement ratio of 0.45.
Keywords : Marine clay, deep mixed column, Plate load test
Introduction
Marine clay is one of the problematic soil which cause excess in settlement and having low in bearing capacity. Inorder to improving the bearing capacity there are various ground improvement methods. The deep mixing is applied for the improvement in stability, settlement reduction, seepage reduction. There are mainly two types of mixing wet and dry mixing. Wet mixing mainly includes binder with water slurry. Dry mixing includes the powdered form of binder. Dry method employs mechanical mixing which consist of vertical shaft. Deep mixed columns can be provided by single and compounded in order to load transferring and settlement analysis. Typically triangular and square pattern are used to improvement in bearing capacity and reducing settlement.
Deep mixing method of ground improvement is environmentally friendly and its application is deals with deals with on weak soil. Significant improvement in the physical and mechanical properties of soft soils has been achieved through by mixing of soil with cement, lime, fly ash and other hydraulic binders to produce a soil- binder column. The resulting soil material posses higher strength, lower compressibility and lower hydraulic conductivity
Objectives of the paper
- To determine the load settlement characteristics of marine clay load settlement by marine clay.
- To determine load settlement characteristics of marine clay after deep mixing by cement
- To determine the effect of curing period after deep mixing by plate load test
- To determine load settlement characteristics of marine clay after deep mixing
- By using different pattern (square and triangular )
Materials used
Marine clay used in this work is collected at a depth of 7m below the ground level at Kundaloor near Vytila, Ernakulum.
The physical properties of marine clay is summarized is below. The liquid limit is about 74% and the specific gravity is 2.38.
Fig 1. Marine clay
Table 1 Properties of marine clay
| Description | Value |
| Specific gravity | 2.38 |
| Organic content(%) | 7 |
| Liquid limit(%) | 74 |
| Plastic limit(%) | 34 |
| Shrinkage limit(%) | 18 |
| Plasticity index | 40 |
| Maximum dry density(kN/m3) | 13.7 |
| Optimum moisture content (%) | 32 |
| Clay (%) | 36 |
| Silt(%) | 46 |
| Sand (%) | 18 |
| Type of soil | CH |
Experimental Programme
The experiments were carried out in a square test tank of 0.5m side and 0.5m height made of galvanized iron sheets for plate load tests. The initial tests for the assessment of improvement in load carrying capacity through densification were conducted by filling the clay in a tank at maximum dry density and optimum water content. Marine clay bed prepared by using the maximum dry density and optimum moisture content from the proctor compaction test. The clay was mixed an optimum moisture content of 32%. Clay filled in the tank with 3 layers of equal weight and being compaction should be done.
Fig. 2. Footing Placed Over the Leveled Surface
A 10 mm thick steel square footing plate of size 100 mm x 100 mm was used. Figure 3.2 shows the footing placed over the leveled clay bed. The load on the footing was applied through a mechanical jack.
Fig. 3. Loading by Mechanical Jack on Footing
The same procedure was adopted during each plate-load test for grouted case. The load test was conducted in this grouted bed after the days of curing. Vertical shaft was installed between footings and proving ring provided. A augerized PVC pipe of 38mm diameter and 200mm length is used for the mixing the grout. The pipe is connected with a DC motor with a DC adapter having a capacity of 30 rpm. The system is connected to the tank by a horizontal frame. Binder is mixed by using the augerized PVC pipe attached to the tank. After mixing is done the entire PVC pipe system is withdrawn. Plate was placed above the column to determine the load carrying capacity.
Fig. 4. Auger
Fig. 5. Augerized PVC pipe
Applied load was monitored using proving ring of capacity 25 kN and 25 mm capacity dial gauges were used to settlement monitoring. Load was applied using a hand operated mechanical jack system. A series of load deformation tests as per IS codes provisions (IS 1882: 1982) of plate load test. Tests were continued until failure of the footings. Load and settlement corresponding to each load increment was noted carefully.
Inorder to apply grouting pressure a grouting chamber is used. Grouting chamber consist of a inlet, outlet, agitator, an air pump and a grouting nozzle. Grout is poured into the chamber through the inlet and when switched ON the motor agitates the grout, which rotates the blades attached to the shaft. After the required pressure attained the outlet valve is released. Cement grout was pumped under a constant pressure of 1 kg/cm2 (100 kPa).
Fig. 6. Grout chamber
Fig. 7. Grout Pouring Through Inlet
Applied load was monitored using proving ring of capacity 25 kN and 25 mm capacity dial gauges were used to settlement monitoring. Load was applied using a hand operated mechanical jack system. A series of load deformation tests as per IS codes provisions (IS 1882: 1982) of plate load test. Tests were continued until failure of the footings. Load and settlement corresponding to each load increment was noted carefully.
Fig. 8. Failed Surface of grouted clay bed
Fig. 9. Deep cement mixed column after plate load tes
Results and discussions
The visual properties of marine clay are listed below.
Table 2. Visual properties
| Physical properties | Characteristics | |
| Color | Gray in color | |
| Texture | Fine grained | |
| Odour | Decaying vegetation | |
| Plasticity | Highly plastic | |
The bearing pressure – normalized settlement graphs were plotted from the plate load tests.
Fig. 10. Bearing pressure – Normalized settlement of marine clay
The plate load test was conducted by using deep cement mixing by using a special grouting chamber. The grout was cured for 1 day in a 38mm diameter pipe at 200mm length pipe which is placed on the tank. Different water – cement ratio is used as 0.45, 0.55, 0.65 at 1 day curing. The graph plotted between bearing pressure – settlement is shown in the figure. From the graph bearing pressure is obtained as at 0.65 water cement ratio 100 kPa at a settlement of 10.5%, at 0.55 W/C ratio the bearing pressure is 167 kPa and corresponding settlement is obtained as 10% and for 0.45 W/C ratio, the bearing pressure is obtained as 188.8kPa at a normalized settlement of 11%. The maximum bearing pressure is obtained as at 0.45 water cement ratio.
Fig. 11.Bearing pressure-settlement ratio of clay deep mixed with cement at 1 day of curing
The graph is plotted between bearing pressure-settlement is shown is figure. The water cement ratio is used as 0.45, 0.55, and 0.65 at a 3 days of curing. The bearing pressure is obtained as 140 kPa at a settlement of 4% at a water cement ratio of 0.45. As water content increase to 0.55, the bearing pressure is obtained as 179.4 kPa which have a corresponding settlement of 10.5%. The bearing pressure is obtained as 147 kPa for 0.65 water cement ratio. As water content increases the bearing pressure decreases from 205.9kPa to 147kPa. The percentage decrease for increasing water content is obtained as 28.6%
Fig. 12. Bearing pressure-settlement ratio of clay deep mixed with cement at 3 day of curing
The effect of square pattern is determined by using four deep mixed cement columns at equal spacing of 40 mm provided below the plate of 100mm square size. The test conducted at different days of curing from 1 to 5 days. The bearing pressure is obtained as for 1 day of curing 167.6 kPa at a normalized settlement of 10%. By increasing period of curing from 1 to 3days, the bearing pressure is obtained as 80 kPa at a normalized settlement of 7%. For 5 days of curing, the bearing pressure is obtained as 217.7kPa at a normalized settlement of 8.5%. The percentage increase in bearing pressure is obtained as from 1 to 5 days of curing is 30%. By comparing it with triangular pattern, the square pattern is more effective. The percentage increase in triangular pattern is 23% and for square is 30%. By comparing these two type of patter, square pattern have better performance than that of triangular pattern of same area.
Fig. 13. Bearing pressure-settlement curve of cemented marine clay by square pattern at different curing period
Plate load test conducted after deep mixing by using different pattern such as triangular and square. The graph refers the triangular pattern at a water cement ratio of 0.45. For determining the effect of triangular pattern 3 number of deep cemented column are provided. And different days of curing are conducted as 1, 3 and 5 days. At 1 day of curing, the maximum bearing pressure is obtained as 147.1 kPa at a normalized settlement of 10% and for 3 days of curing, the bearing pressure is obtained as 160kPa at a normalized settlement of 5.5%. For 5days of curing the bearing pressure is obtained as 180 kPa at a normalized settlement of 7%. As curing period increases the bearing pressure increases and settlement reduces. The percentage increase in bearing pressure is 22.36% from 1day to 5 days of curing.
Fig. 14. Bearing pressure-settlement curve of cemented marine clay by triangular pattern at different curing period
The below graph shows the comparison of bearing pressure cemented marine clay at a normalized settlement of 5%.
Fig. 15. Comparison of bearing pressure at a normalized settlement of 5%
Conclusions
Based on index properties of marine clay the soil is classified as soft. And the IS classification of marine clay is obtained as CH, that is high clay content. From the result specific gravity is obtained as 2.38 and this low value is due to organic content in marine clay.
The following conclusions are drawn from plate load tests:
- Installation of deep cemented technique on marine clay improves the load carrying capacity.
- As water content increases from 0.45 to 0.65 bearing pressure decreases as 132.9 kPa to 58.84 kPa at a normalized settlement of 5%
- By increasing curing days from 1 to 3 days the bearing pressure increased as 132kPa to 148kPa at a water cement ratio of 0.45 for a normalized settlement of 5%. The percentage increase in bearing pressure is obtained as 13%.
- At a constant water cement ratio of 0.55, the bearing pressure is obtained as 130 kPa corresponding settlement 5% at 3 days of curing. For 1 day curing with deep mixed cement, the bearing pressure is obtained as 102.97 kPa and at a normalized settlement of 5%
- At a water cement ratio of 0.65, the bearing pressure is obtained as 58.84 kPa at a 56normalized settlement of 5% at a curing period of 1 day curing. At 3 days of curing, the bearing pressure is obtained as 94.144 kPa at a normalized settlement of 5%.
- The bearing pressure is obtained as for 1 day of curing 102.9 kPa at a normalized settlement of 5%. By increasing from 1 to 3days of curing, the bearing pressure is obtained as 160.6 kPa at a normalized settlement of 5%. For 5 days of curing, the bearing pressure is obtained as 164.7kPa at a normalized settlement of 5%. The percentage increase in bearing pressure is obtained as from 1 to 5 days of curing is 60%.
- By comparing it with triangular pattern, the square pattern is more effective. By comparing these two type of pattern square pattern have better performance than that of triangular pattern of same area.
Reference
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