Stabilization of Soft Clay Using PVD’s by Combined Vacuum-Surcharge Pressures

STABILIZATION OF SOFT CLAY USING PVD’S BY COMBINED VACUUM-SURCHARGE PRESSURES

S. SakthirajaPostgraduate Student, Anna University Chennai, Chennai–600025, India. E-mail: sakthivivek@yahoo.co.in,K. IlamparuthiProfessor and Head, Anna University Chennai, Chennai–600025, India. E-mail: kanniilam@gmail.com

ABSTRACT: In this study an attempt is made to understand the effect of combined vacuum-surcharge pressure in the consolidation behaviour of soft clay improved with prefabricated vertical drains. Consolidation tests are conducted in conventional consolidation cell and on samples of 15.4 cm diameter and 6 cm thick. Two series of consolidation tests are performed on the soft clay samples, they are with and without prefabricated vertical drains (PVD’s). The tests are conducted at the consistency of 72%. For the tests with and without PVD’s, the coefficient of consolidation of vacuum and vacuum surcharge technique is higher than the conventional surcharge technique. The time taken for 90% consolidation for vacuum loading without PVD’s is reduced to about 0.65 times that of surcharge load. While using the PVD’s the time taken for 90% consolidation is reduced to 0.65 times, irrespective of the type of loading.

1. INTRODUCTION

Soft clays are deposits of recent origin and found in different parts of the world. They possess poor geotechnical properties such as high compressibility and very low bearing capacity. These deposits can be improved by various ground improve-ment techniques such as pre-loading sand compaction pile, deep cement mixing, stone column etc. These techniques are used in actual construction projects, and their improvement effects on soft ground are well established. Superior techniques are developed in this field. Vacuum preloading assisted with vertical drains is one such technique. The use of vacuum preloading, together with vertical drains for soft soil stabilization, can reduce construction and maintenance costs, while the increased soil strength will enhance the stability of the structure (Tang, 1995, Masse & Ihm, 1998). While combining the effect of vacuum with the surcharge, it is possible to support the rapid construction of the surcharge without posing stability problems to the soft soil (Hayashi et al. 2003, Kwong et al. 2006).

1.1 Vacuum Consolidation

Vacuum consolidation is a technique used to improve the strength of soft clayey soils. Kjellman (1952) first introduced the concept of using vacuum consolidation to improve the strength of soil. The vacuum consolidation method utilizes the atmospheric pressure to consolidate soft saturated sediments by similar principles as those used in surcharge preloading by vertical drains. The vacuum preload is

isotropic, independent of depth and leads to an immediate decrease of pore water pressure. Instead of increasing the effective stress in the soil mass by increasing the total stress by means of conventional mechanical surcharging, vacuum-assisted consolidation preloads the soil by reducing the pore pressure while maintaining a constant total stress. The change in soil pore water pressure produced from applied vacuum preloading induces discharge of pore water and consolidation thereby increase the shear strength of the soil. As an alternative to the conventional preloading, vacuum assisted consolidation can be used to consolidate soft alluvial soils, to improve bearing capacities prior to construction, and to reduce post construction settlements.

Hassan & Shang (2002) reported that vacuum consolidation can result in settlements nearly identical to those induced by a surcharge loading applied under odeometer conditions. Chai et al. (2005) suggested that vacuum consolidation is also influenced by the drainage boundary conditions and will normally result in less settlement than application of a surcharge load with the same magnitude as the vacuum pressure.

Even though the efficiency of vacuum consolidation technique has been demonstrated under different site conditions, there are still differing opinions as stated above regarding the important characteristics of vacuum consolidation. Hence in this project efforts are made to establish the effectiveness of vacuum consolidation on ground improvement and consolidation acceleration effect.

396

IGC 2009, Guntur, INDIA

Stabilization of Soft Clay Using PVD’s by Combined Vacuum-Surcharge Pressures

Tests are conducted to study the consolidation behaviour of soft clay under the combination of vacuum and surcharge consolidation assisted by prefabricated vertical drains.

2. METHODOLOGY

2.1 Materials Used

2.1.1 Soil

The soil samples were collected from Siruseri Sipcot site, and tested for basic properties. Fines in this soil are 90% and the balance 10% is sand. The LL and Ip values are 68% and 34% respectively. The soil is classified as CH.

2.1.2 Prefabricated Vertical Drains (PVD)

Prefabricated vertical drains selected for this study are currently used in practice where vacuum technique is adopted in soil stabilization. The type of drains used in this study is Mebra flodrain CX 1000.

2.2 Experimental Procedure

2.2.1 Preparation of Soft Clay

Vacuum consolidation has been effectively practiced to improve the strength of very soft clays which is naturally at a consistency nearly equal to or greater than liquid limit. Therefore the tests were carried out at a water content of 72% which is 4% greater than the liquid limit. The soil collected from the pit was completely dried. For the fixed density and water content the weight of the dry soil to be taken is calculated. The required water content is added to it and soaked for 24 hours and then thoroughly mixed before placing it in the moulds. The soil sample is then filled in the moulds carefully by removing the air bubbles, so that the calculated density is achieved.

2.2.2 Experimental Facility

Two sizes of moulds were used to conduct the consolidation tests. One is the conventional consolidation mould and the other is a steel mould of size 15.4 cm diameter and 15 cm height. A central hole is made at the bottom of the mould as a provision for application of vacuum pressure to the sample. The hole is covered with fine metallic grid of opening size of 75µ as a filter to prevent any clay particles getting sucked out or clog the hole.

Prefabricated vertical drains are cut and scaled down for testing purposes. The size of the drains are cut such that the ‘n’ (n = rw /r where, rw is the equivalent radius of pvd and r is the radius of the mould) value is same for the both the sizes of moulds. In this study the tests are done for n = 2.7. For the test in consolidation ring the PVDs of 10 mm width is used and for tests in larger mould the width of the PVD is 37 mm. For tests with drains a single PVD is placed at the middle of the sample.

A facility is developed in this study to conduct consolidation test under vacuum pressure. The schematic diagram of vacuum preloading arrangement is shown in the Figure 1. The arrangement consists of a vacuum pump of capacity of 100 kPa, a vacuum regulator, a drainage chamber and a vacuum gauge. The required vacuum pressure is applied to the sample by operating the regulator and the vacuum gauge is used to measure and monitor the vacuum pressure applied on the sample. The water draining from the sample is collected in the chamber. For tests on consolidation cell a burette is used as the drainage chamber so as to measure the quantum of water being drained from the sample while consolidating the sample.

Fig. 1: Vacuum Preloading Arrangement

3. TESTS IN CONSOLIDATION CELL

In conventional consolidation cell tests are carried out for three magnitudes of pressures 30, 60 and 90 kPa independently. Two series of tests were carried out with and without PVDs by applying surcharge, vacuum and combined vacuum-surcharge pressure of same magnitudes as said above.

3.1 Surcharge Loading

For samples with and without drain, prepared in the consolidation cell tests were carried out according to the procedure of IS: 2720 Part XV. The consolidation cell was placed on the frame and tests were carried out for the required pressure. The dial readings are recorded continuously at predetermined time intervals as in regular consolidation test pressure.

3.2 Vacuum Loading

The Figure 2 shows the arrangement of vacuum loading for the test in consolidation ring. The test is conducted by adopting following procedure. Initially the vacuum pressure of required intensity is set by operating the vacuum pump

397

Vac

uum

R

egul

ator

Vac

uum

C

ham

ber

Vacuum Gauge

Vacuum Pump

Stabilization of Soft Clay Using PVD’s by Combined Vacuum-Surcharge Pressures

and by adjusting the regulator. Now the vacuum pump is turned off and the vacuum pressure line from the vacuum pump is connected to the drainage line of the clay sample in the consolidation ring. The dial gauge was placed in position to record the settlement of the clay sample. The surface of the consolidation ring is thoroughly sealed by means of grease to prevent the leakage. The vacuum pump is operated and the settlement was recorded continuously for pre-determined time intervals. The vacuum pressure of required intensity is maintained throughout the test by adjusting the knob of the regulator. The test is carried out till the dial gauge reading remains constant for a long time. Generally in consolidation ring tests were over within a period of 5hrs.

Fig. 2: Vacuum Loading Arrangement for Tests in Consolidation Cell

3.3 Combination of Vacuum-Surcharge Loading

All the steps for vacuum pressure application is done as explained previously and then the consolidation cell was placed on the conventional loading frame. Now the required surcharge and vacuum pressure was applied simultaneously and the dial readings were recorded in the similar fashion at predetermined time intervals. For the tests with Pvd instead of porous stone a sand layer is placed at the top of the consolidation ring.

4. TESTS ON THE LARGER SAMPLES

In current practice, to improve the soft clays the combination of vacuum-surcharge loading is practiced. Therefore tests on reasonably large size samples were planned. Tests in this series are conducted on samples of 15.4 cm dia and 6 cm thick. The samples are tested in a large scale consolidation loading frame, available in the laboratory. For the tests with and without Pvds, vacuum, surcharge and vacuum surcharge loading are applied to the sample for an equivalent pressure of 60 kPa.

The surcharge consolidation tests were carried out with and without drains for a pressure of 60 kPa. The sample was filled in the mould gently and slight tamping is given to

remove air voids. Since the test is conducted at moisture content higher than the liquid limit, there is no difficulty in preparation of sample of required size. Sand layer of 0.5 cm thickness was placed both top and bottom of the clay sample for effective drainage purpose. A layer of geotextile is placed at the interference of clay and sand layer in order to avoid intermixing of both the layers and also to prevent the penetration of fine particles (i.e., clay) into the sand layer particularly in the case of vacuum load. For the test with drain, the sample is prepared as said above and the drain is inserted at the centre of the bed as it is been done at the field. The soil bed thus prepared in the mould is placed in the loading frame and a load of 60 kPa is applied. The dial readings are recorded continuously for the first one hour and then readings were taken for every one hour.

Vacuum and Vacuum-surcharge loading tests were also conducted for the pressure of 60 kPa. Required vacuum pressure is set in the vacuum pump by adjusting the regulator and vacuum pressure line is connected to the drainage line of the sample. Now the vacuum pressures of 30 kPa and surcharge load of 30 kpa are applied simultaneously. Dial reading are recorded as indicates above. After the test the sample is kept in oven for the measurement of final water content.

5. RESULTS AND DISCUSSIONS

The coefficient of consolidation for conventional surcharge loading, vacuum loading and vacuum-surcharge loading are obtained by Terzaghi’s one dimensional consolidation equation. The co-efficient of radial consolidation for the tests on clay with PVD is obtained by Barron’s equation. The coefficient of consolidation, time taken for 90% consolidation, settlement corresponding to 90 % consolidation and total settlement of the clay sample are determined. The laboratory tests results are presented in Table 1 and Table 2.

Table 1: Results of Oedometer Consolidation Tests without PVD

Type of loading

Pressure (kPa)

t90

(min)Cv × 10–3

cm2/min

Settlement (mm)

90% Total

Surcharge30 49 14.11 1.65 1.8860 64 9.20 4.2 4.6090 100 6.12 3.6 4.40

Vacuum30 36 18 2.05 2.4760 46 12.8 4.14 4.5390 64 9.51 4.11 4.90

Vacuum 60 58 12.6 3.2 3.68

398

Vacuum regulator

Vacuum pump

Motor

Drainage chamber

Dial gauge

Consolidation cellVacuum gauge

Stabilization of Soft Clay Using PVD’s by Combined Vacuum-Surcharge Pressures

Surcharge 90 64 10.8 4.8 5.72

Table 2 Results of Oedometer Consolidation Tests with PVDType of loading

Pressure kPa

t90

minCvr

cm2/minCvr/Cv Settlement

(mm)90% Total

Surcharge30 38 0.113 8 1.8 2.0260 50 0.094 9.34 3.8 4.2190 70 0.061 10.05 3.5 4.17

Vacuum30 27 0.159 8.833 1.9 2.2060 36 0.120 9.375 2.48 3.0290 46 0.094 10.30 3.7 4.2

Vacuum Surcharge

60 42 0.102 8.095 3.0 3.6590 46 0.094 8.70 4.2 4.59

The validity of Terzaghi’s one dimensional equation and Barron’s radial consolidation equation for the vertical and radial consolidation process by vacuum technique is revisited by means of establishing the relation between degree of consolidation and time factor for both vertical and radial drainage conditions. The comparison of experimental results with the theorotical curves for a single pressure is presented in Figures 3 and 4 for vertical and radial consolidation respectively. The behaviour is similar for all the pressures.

Fig. 3: Variation of U vs Tv for the Pressure of 60 kPa

Fig. 4: Variation of Ur vs Tvr for the Pressure of 60 kPa

For the tests without Pvd’s, the experimental results compare fairly well with the theoretical results for the entire range of consolidation period irrespective of the loading conditions. From the Figure 3 it can be said that the response of vacuum loading compares well than the surcharge loading. For the tests with Pvd the experimental results show some variation from the theorotical results for all the loading conditions. But the deviation is found to be less for vacuum loading technique compared to surcharge loading conditions. However the degree of consolidation values achieved from the tests are higher than the theorotical values.

From the test results of larger samples (Table 3), it is observed that the time taken for 90% consolidation for vacuum loading is just higher than the value of vacuum-surcharge loading and it is of about 1.6 times less than that of surcharge loading. The total settlement achieved is 15% high for the combined vacuum-surcharge loading than other two methods of loading. This observation is similar to that of results of tests on smaller size samples.

Table 3: Results of Consolidation Tests on Larger Samples (15.4 cm dia 10 cm thick)

Type of loading

Pressure (kPa)

t90

(min)Cv × 10–3

cm2/min

Settlement (mm)

90% TotalSurcharge 60 1936 3.23 10.80 11.37Vacuum 60 1024 6.11 11.17 11.30Vacuum surcharge 60 1089 5.67 11.80 13.10

From the results presented in Table 4, it can be seen that the coefficient of radial consolidation is more or less same for the vacuum and vacuum surcharge loading and it is about two times that of surcharge loading. Also the time taken for 90% consolidation is for the vacuum and vacuum surcharge loading is of about two times lesser than that of surcharge loading. Hence one can say that the effect of Pvd is better for all types of loading.

Table 4: Results of Consolidation Tests on Larger Samples (15.4 cm dia 10 cm thick) with PVD

Type of loading

Pressure (kPa)

t90

minCvr

cm2/minCvr/ Cv

Settlement (mm)

90% Total

Surcharge with pvd 60 1332 0.024 7.43 13.8 14.05

Vacuum with pvd 60 702 0.046 7.52 13.7 14.2

Vacuum surcharge with pvd

60 729 0.044 7.76 14 14.5

399

Time Factor

Time Factor

Deg

ree

of c

onso

lidat

ion

Deg

ree

of c

onso

lidat

ion

Stabilization of Soft Clay Using PVD’s by Combined Vacuum-Surcharge Pressures

From the results it is observed that the combined vacuum surcharge technique has resulted in settlements higher than the vacuum technique. Since in vacuum technique the consolidation behaviour is three dimensional there will be certainly some inward displacement of the soil particles, which in turn results in reduction of settlement in vertical direction. In the combined vacuum-surcharge technique this attribute of vacuum pressure is arrested by means of the surcharge pressure upto a certain extent and hence results in better settlements than the vacuum technique.

6. CONCLUSIONS

From the test results following conclusions are drawn.

The relation between time and consolidation for vacuum and vacuum surcharge technique in preloading of soft clay follows classical theories of Terazaghi’s one dimensional consolidation and Barron’s equal strain for vertical and radial consolidation respectively.

For the tests with and without Pvd conducted on consolidation cell the coefficient of consolidation of vacuum and vacuum surcharge technique is higher than the conventional surcharge technique. The difference in coefficient of consolidation values for vacuum and vacuum-surcharge techniques is marginal. But the settlement achieved is higher for the vacuum-surcharge loading.

For larger size samples tested in this study (i.e. 15.4 cm dia and 6 cm thick) the time taken for 90% consolidation for vacuum loading is just higher than the value of vacuum-surcharge loading and it is about 0.65 times that of surcharge loading. The total settlement achieved is 15% high for the combined vacuum-surcharge loading than other two methods of loading. This observation is similar to that of results of

tests on smaller size samples. Irrespective of the loading condition the Cvr/Cv ratio is increasing with the intensity of pressure. The ratio is high for the vacuum loading than the surcharge and combined vacuum surcharge loading. While using the Pvds the time taken for 90% consolidation is reduced by 0.65 times irrespective of the type of loading.

REFERENCES

Chai, J.C., Carter, J.P and Hayashi, S. (2005). “Ground Deformation Induced by Vacuum Consolidation”, Jl. of Geotechnical and Geoenvironmental Engineering, Vol. 131, No. 12, pp. 1552–1561.

Hayashi, H., Nishikawa J., Nishimoto, S. and Sawai, K. (2003). “Performance of Vacuum Consolidation and Prefabricated Vertical Drain in Peat Ground”, Proceedings of 37th annual meeting, The Japanese Geotechnical Society, pp. 2–9 (in Japanese).

Kjellmen, W. (1952). “Consolidation of Clayey Soils by Atmospheric Pressure”, Proceedings of the Conference of Soil stabilization, Boston, Massachusetts Institute of Technology, pp. 258–263.

Kwong, K.L., Lui, V. and Jacky, N.G. “Reclamation Ground Settlement monitoring by using GPS and other technologies” at Shenzhen Airport.

Masse, F. and IHM, C.W, “Successful application of Menard Vacuum Consolidation method to Nakdong River soft clay in Kimhae”, http://www.dgi-menard.com/MV-kimhae.

Mohamedelhassan, E. and Shang, J.Q. (2002). “Vacuum and Surcharge Combined one Dimensional Consolidation of Clay Soils”, Canadian Geotechnical Journal, Vol. 39, No. 6, pp. 1126–1138.

400