PEATY CLAY IMPROVEMENT WITH 
PREFABRICATED VERTICAL DRAINS 
Thesis submitted in partial fulfillment of the requirements for the 
Degree of Master of Science. 
----''tS^flS^P^": :cr.. 
immsm OF w i o M T u m ifik»} 
Rasiah Kugan 
Department of Civil Engineering 
University of Moratuwa 
Sri Lanka 
January 2004 
79576 
urv) Thesis coll 
11516 
University of Moratuwa 
79576 
DECLARATION 
The work included in this thesis in part or whole, has not been submitted for any other 
academic qualification at any institution. 
e candidate 
Certified 
Signature of the Supervisor 
ABSTRACT 
Peaty clay obtained from the Colombo-Katunayake Expressway route was tested in the 
laboratory for its consolidation characteristics. The main objective was to quantify the 
improvement in peaty clay by the use of pre-fabricated vertical drains (PVD). 
Two large-scale model set-ups were erected to monitor the consolidation process of peaty 
clay, one without the use of PVD, and the other with the PVD installed. Both tests were 
conducted by depositing remoulded peaty clay in cylindrical barrels, and load was 
applied by several increments. Settlements and pore-water pressure were monitored over 
a long period. Axi-symmetric conditions were simulated by these tests. Based on the test 
results, the performance of PVD in accelerating the consolidation process is quantified. 
The model test results are back-analysed using the finite difference method, in order to 
give an indication of an appropriate numerical modeling process which can be used to 
model other instances of similar process. 
In addition, the effect of sample size and the duration of the load increment period on 
consolidation in peaty clay is investigated. Tests were carried out in three different sizes 
of specimen dimensions. Values of c v were calculated by several methods found in the 
literature, after carrying out tests using different load increment periods. Load increment 
periods for different sample thicknesses were calculated according to Terzaghi theory, for 
time simulation tests. The time necessary to obtain a specified degree of consolidation 
was calculated by an appropriate equation. The values of c v obtained by these methods 
are of the same order for the time simulation tests. The secondary compression index of 
the peaty clay is found to be very high. 
The improvement of shear strength in peaty clay due to consolidation is also investigated. 
Shear strength of peaty clay was measured at the end of laboratory tests and the results 
were compared with the strength of sample taken prior to consolidation. Soil strength 
after the treatment was evaluated by undrained triaxial tests, vane shear tests and 
consolidated drained triaxial tests. Soil strength before the treatment was evaluated only 
by vane shear tests as sample preparation with the soft soil was not possible. Gain in 
shear strength in peaty clay due to consolidation is quantified. 
I 
ACKNOWLEDGMENT 
E v e n a s m a l l r e s e a r c h o w e s i t s e x i s t e n c e t o m a n y m o r e p e o p l e t h a n t h e o n e w h o s e n a m e a p p e a r s as 
r e s e a r c h e r . A m o n g t h o s e w h o d e s e r v e c r e d i t f o r t h e a s s i s t a n c e e x t e n d e d d u r i n g t h e r e s e a r c h p r o g r a m m e , 
D r . U . G . A . P u s w e w a l a , w h o p r o v e d a n e x c e l l e n t s u p e r v i s o r t o w o r k w i t h , h a s c o n t r i b u t e d a g r e a t d e a l t o t h i s 
r e s e a r c h . H i s g u i d a n c e , s u p p o r t a n d s t i m u l a t i n g c r i t i c i s m s m a d e t h e r e s e a r c h a c o m p l e t e s u c c e s s . T h e 
s u p p o r t o f r e s e a r c h c o - o r d i n a t o r D r . S . A . S . K u l a t h i l a k e i s h i g h l y a p p r e c i a t e d . I w i s h t o a c k n o w l e d g e a n d 
e x p r e s s m y g r a t i t u d e t o D r . S . A . S . K u l a t h i l a k e , w h o f o u n d t i m e t o s u p e r v i s e m e t h r o u g h o u t t h e s t u d y p e r i o d . 
H i s d e v o t i o n i n t h e s u b j e c t o f s o f t g r o u n d i m p r o v e m e n t i s a l s o a c k n o w l e d g e d . I e x p r e s s m y s i n c e r e t h a n k s 
t o D r . T . A . P e i r i s f o r g i v i n g n e c e s s a r y s u p p o r t a n d g u i d a n c e . M y g r a t i t u d e i s e x t e n d e d t o P r o f e s s o r 
B . L . T e n n e k o o n f r o m w h o m I g a r n e r e d i n s i g h t s a n d s u g g e s t i o n s d u r i n g t h e p r o g r e s s r e v i e w s a n d i n s o m e 
s p e c i f i c a r e a s . A s p e c i a l t h a n k s i s e x t e n d e d t o D r . S . T h i l a k a s i r i f o r h i s v a l u a b l e i n s t r u c t i o n s a n d s u p p o r t . I 
w o u l d h a v e n o t g o t t h e c h a n c e o f d o i n g t h i s r e s e a r c h s t u d y i f n o t f o r t h e S c h o l a r s h i p g r a n t e d b y A s i a n 
D e v e l o p m e n t B a n k a n d t h e M i n i s t r y o f S c i e n c e a n d T e c h n o l o g y , S r i L a n k a , a n d t h e i r c o n s i d e r a t i o n i s v e r y 
m u c h a p p r e c i a t e d . 
T h e a s s i s t a n c e r e c e i v e d f r o m M r . K . R . P i t i p a n a a r a c h c h i , t e c h n i c a l o f f i c e r , M r . D . G . S . V i t h a n a g e , t e c h n i c a l 
o f f i c e r a n d M r . D . B a n d u l a s e n a , l a b a s s i s t a n t , o f S o i l M e c h a n i c s L a b o r a t o r y o f t h e U n i v e r s i t y o f M o r a t u w a , 
d u r i n g t h e l a b o r a t o r y - t e s t i n g p r o g r a m m e i s a c k n o w l e d g e d . I w o u l d l i k e t o a c k n o w l e d g e t h e a s s i s t a n c e 
e x t e n d e d b y M r . G u n a s e r k a r a , t e c h n i c a l o f f i c e r , C i v i l E n g i n e e r i n g W o r k s h o p f o r f a b r i c a t i n g e x p e r i m e n t a l 
m o d e l s s u c c e s s f u l l y . 
M r . G . V . S . K . K u m a r a s r i , C h i e f E n g i n e e r , K e a n g - N a m C o n s t r u c t i o n s , C o l o m b o - K a t u n a y a k a H i g h w a y 
P r o j e c t n e e d s t o b e t h a n k e d f o r a l l o w i n g m e t o c o l l e c t s o m e i n f o r m a t i o n o n t h e i n s t a l l a t i o n o f v e r t i c a l 
d r a i n s a t C o l o m b o - K a t u n a y a k e E x p r e s s w a y P r o j e c t o f t h e R o a d D e v e l o p m e n t A u t h o r i t y , a n d p r o v i d i n g 
n e c e s s a r y m a t e r i a l s t o m a k e t h e l a r g e - s c a l e m o d e l . 
M y s i n c e r e t h a n k s a r e a l s o f o r w a r d e d t o a l l t h e g e o t e c h n i c a l r e s e a r c h s t u d e n t s f o r t h e i r s u p p o r t t h r o u g h o u t 
t h e r e s e a r c h p e r i o d . I e x t e n d m y t h a n k s s p e c i a l l y t o u n d e r g r a d u a t e s t u d e n t s w h o s u p p o r t e d m e d u r i n g t h e 
t e s t i n g p r o g r a m m e . 
Rasiah Kugan 
01-01-2004 
II 
f 
CONTENTS Page 
ABSTRACT i 
ACKNOWLEDGEMENT ii 
CONTENTS iii 
List of Figures vii 
List of Tables x 
CHAPTERS 
1 Introduction 
1.1 Problems due to peaty clay in Sri Lanka 2 
1.2 Improvement of peat 2 
1.2.1 Methods applicable to cohesive soils 3 
1.2.2 Ground treatment in peaty soils 3 
1.2.2.1 Excavation and replacement by stable fill material 3 
1.2.2.2 Displacement methods 4 
1.2.2.3 Pre-loading 4 
1.2.2.4 Preloading with Vertical Drains 4 
1.2.2.5 Placement of compacted fill on peat 8 
1.2.2.6 Dynamic compaction 8 
1.2.2.7 Vacuum consolidation 9 
1.2.2.8 Deep mixing method 10 
1.3 Consolidation testing of peaty clay 10 
1.4 Objectives and Scope of the current research 11 
1.5 Arrangement of the thesis 12 
2 Literature review 
2.1 How to approach a project involving possible site-improvement with 
vertical drains 14 
2.2 Prefabricated Vertical Drains (PVD) 17 
2.2.1 General 17 
2.2.2 Definition of PVD 18 
III 
V 
2.2.3 History 18 
2.2.4 Types, material and installation procedure for vertical drains 20 
2.2.5 Installation 21 
2.2.6 Factors affecting the efficiency of PVD 26 
2.2.6.1 Design Factors 27 
2.2.6.1.1 Drain Spacing 27 
2.2.6.1.2 Equivalent Diameter 28 
2.2.6.2 Well resistance 29 
2.2.6.3 Smear Zone 30 
2.2.6.4 Degree of consolidation 31 
2.3 Preloading 34 
2.4 Settlement 35 
2.5 Numerical prediction of model behavior 38 
2.5.1 Modeling the pore pressure response 38 
2.6 Properties and behaviour 39 
2.6.1 Classification of peat and other organic soils 39 
2.6.2 Amorphous granular peat 41 
2.6.3 Mires and Peats 41 
2.6.4 Physical and chemical characteristics 42 
2.6.4.1 Water content 42 
2.6.4.2 Permeability 43 
2.6.4.3 Void ratio 43 
2.6.4.4 Specific Gravity 44 
2.6 Overview of some methods proposed to find c v. 44 
3 Methodology 
3.1 Geological condition in the borrow area 54 
3.2 Sample collection 54 
3.2.1 Taking of undisturbed sample 54 
3.2.2 Taking disturbed sample 56 
3.3 Sample preparation 59 
IV 
3.4 Testing programme 60 
3.4.1 Testing programme for investigating the influence of the prefabricated 
vertical drains on the consolidation process in peaty clay 60 
3.4.2 Consolidation tests on treated large-scale sample without PVD 65 
3.4.3 Testing programme for investigating the improvement of shear strength 
in peaty clay due to consolidation 65 
3.4.4 Testing programme for investigating the influence of sample thickness 
in consolidation 68 
3.4.5 Testing programme for investigating the moisture content variation in 
the sample 68 
4 Results 
4.1 Experimental results and analysis of improvement with PVD 69 
4.2 Experimental results and analysis of improvement in shear strength 69 
4.2.1 Consolidation test results 69 
4.2.2 Undrained unconsolidated triaxial test results 75 
4.2.3 Field vane shear test 77 
4.2.4 Consolidated drained test 79 
4.2.5 Moisture content variation with depth for treated sample without PVD 81 
4.2.6 Effect of friction on the process of consolidation of large-scale model 81 
5 Back analysis of test results 
5.1 Back analysis of test data using Terzaghi model by finite different 
method 82 
5.2 Development of the finite difference numerical schemes 82 
5.2.1 Numerical solution for one-dimensional consolidation 82 
5.2.2 Numerical solution for radial drainage 84 
5.2.3 Modelling of the vertical drainage 86 
5.2.4 Modelling of the radial drainage 86 
5.2.5 Modelling of the settlement behaviour 87 
5.3 Modelling of the large-scale test using the finite difference method (FDM) 88 
V 
4 
7 
5.4 Analysis of results 89 
6 Application of various methods to analyse test results for 
consolidation characteristics 
6.1 Experimental results 94 
6.2 Analysis of results 94 
6.3 Application of the methods to test data 97 
7 Analysis of long-term consolidation test result 
7.1 Analysis of long-term consolidation test results and calculation of 
secondary compression index 100 
7.2 Improvements in compression index and compression ratio 100 
7.3 Improvements in the coefficient of volume compressibility 101 
7.4 Secondary compression 102 
8 Conclusions and Recommendations 
8.1 Conclusions 105 
8.2 Recommendations 106 
9 References 107 
Appendix A A - l - A - 9 
Appendix B B - l - B - 9 
Appendix C C-l -C-6 
VI 
List of Figures 
Figure 1.1 General arrangement of preloading with vertical drains and field monitoring 
arrangements 
Figure 1.2 Vacuum consolidation arrangement 
Figure 2.1 Flow chart illustrating the foundation design process (Hartikainen, 1983) 
Figure 2.2 Prefabricated vertical drain 
Table 2.1 Types of vertical drains, common installation methods and typical geometric 
characteristics (after Jamiolkowski, Lancellotta and Wolski, 1983a) 
Table 2.2 Dimension and materials of some prefabricated drains 
Figure 2.3 Typical mandrels for band-shaped drains (Sectional plan views) 
Figure 2.4 Typical shoes 
Figure 2.5 Illustration of PVD installation procedure 
Figure 2. 6 Equivalent diameters for square and triangular grid pattern 
Figure 2.7 Equivalent diameter 
Figure 2.8 Approximation of disturbed zone around the mandrel (Rixner et al., 1986) 
Figure 2.9 Schematic diagram of PVD with drain resistance and soil disturbance 
(Rixner et al., 1986) 
Figure 2.10 Consolidation due to vertical and radial drainage (Rixner et al., 1986) 
Figure 2.11 Graph of influence factors for calculation of immediate settlement 
Figure 2.12 Logarithm-of-time method for determination of c v 
Figure 2.13 Square-root-of-time method for determination of c v 
Figure 2.14 Maximum slope method for determination of c v 
Figure 2.15 The log8 - log t method for determination of c v (point A corresponds to U = 
88.3%) 
Figure 2.16 Asaoka's(1978) method for determination of c v 
Figure 2.17 (a) Typical experimental 5-log t plot 
(b) A5/Alog t - log t plot 
Figure 2.18 t/ 8 - t plot for determination of c v 
VII 
Figure 3.1 Location map of the sample collection area 
Figure 3.2 Removal of top vegetated layer 
Figure 3.3 Pushing the barrel vertically into the peat layer 
Figure 3.4 pushing the barrel into the peat layer by using backhoe 
Figure 3.5 Taking the barrel out from the soil by holding the bottom of the barrel with 
backhoe. 
Figure 3.6 Removal of undisturbed sample with barrel 
Figure 3.7 Properly sealed barrels after the sample collection 
Figure 3.8 Sample preparation 
Figure 3.9 Schematic diagram of model test apparatus (arrangement shown without 
PVD) 
Figure 3.10 Arrangement of excess pore pressure measuring system and settlement 
measuring system (arrangement shown with PVD) 
Figure 3.11 Outer casing and inner casing arrangement of filter tip 
Figure 3.12 Positioning the filter tip arrangement 
Figure 3.13 Closed hydraulic piezometer arrangement with null indicator 
Figure 3.14 Photograph of the model test arrangement 
Figure 3.15 PVD installation 
Figure 31.6 Field vane shear test arrangement 
Figure 3.17 Test in progress with the medium scale model 
Figure 3.18 Consolidation test using the small scale model (oedometer test) 
Figure 3.19 Cutting sample using guitar string 
Figure 3.20 Sample pieces 
Figure 4.1 Test result for loading without PVD 
Figure 4.2 Test result for loading with PVD installed at center of the remoulded peat 
slurry prior to loading 
Figure 4.3 Comparison of test results without PVD and with PVD 
Figure 4.4 Consolidation test result of top layer 
Figure 4.5 Consolidation test result of middle layer 
Figure 4.6 Consolidation test result of bottom layer 
Figure 4.7 Unconsolidated undrain Traxial test results for top treated layer
 > v 
VIII \ 
Figure 4.8 Unconsolidated undrain Traxial test results for middle treated layer 
Figure 4.9 Unconsolidated undrain traxial test results for bottom treated layer 
Figure 4.10 Field vane shear test result at location 1 
Figure 4.11 Field vane shear test result at location 2 
Figure 4.12 Field vane shear test result at location 3 
Figure 4.13 Field vane shear test result before treatment 
Figure 4.14 Consolidated drained traxial test results for top treated layer 
Figure 4.15 Consolidated drained traxial test results for middle treated layer 
Figure 4.16 Consolidated drain traxial test results for bottom treated layer 
Figure 4.16 The variation of moisture content with depth 
Figure 5.3 Idealized section used for FDM 
Figure 5.1 Illustration of FDM meshes 
Figure 5.2 Illustration of FDM mesh 
Figure 5.4 Excess pore water pressure mesh developed for the analysis 
Figure 5.5 Comparison of observed and estimated excess pore water pressure (constant 
c v values) 
Figure 5.6 Comparison of observed and estimated excess pore water pressure 
dissipation (variable c v value) 
Figure 5.7 Variation of c v value with days 
Figure 5.8 Comparison of observed settlements with predictions c v 1 lm2/yr 
Figure 5.9 Comparison of observed settlements with predictions c v 5.5m2/yr 
Figure 5.10 Comparison of observed settlements with predictions by varying the c v 
value with time 
Figure 5.11 Comparison of observed settlements with predictions by varying the c v and 
Ch values with time 
Figure 5.12 Variation of Ch value with time 
Figure 6.1 Consolidation test results 
Figure 6.2. A possible deviation from Asaoka's construction (Orleach, 1983) 
Figure 6.3 Summary of c v values by different methods 
Figure 7.1 e vs log (ff) plot for remoulded peaty clay (long term test results) 
IX 
Figure7.2 
Figure 7.3 
Figure 7.4 
Effect of preloading on m v for remoulded peaty clay 
Test results from long term oedometer test for each load incremental 
c a vs log (a) plot for remoulded peaty clay 
List of Tables 
Table 2.1 Types of vertical drains, common installation methods and typical geometric 
characteristics (after Jamiolkowski, Lancellotta and Wolski, 1983a) 
Table 2.2 Dimension and materials of some prefabricated drains 
Table 2.3 Classification of peat ( The Canadian system after Radforth 1969) 
Table 4.1 Unconsolidated undrained traxial test results 
Table 4.2 Field vane shear test results 
Table 4.3 Consolidated drain triaxial test results 
Table 6.1 Comparison of results obtained by using various methods 
Table 7.1 Values of Void ratio 
Table 7.2 Values of m v 
Table 7.3 Classification of soil based on secondary compressibility (Mesri, 1973) 
Table 7.4 Values of c a 
Table 7.5 Comparison of results obtained from samples of different size 
X