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Pile-soil-tunnel interaction in some layered soil profiles

J. Zou, Y. K. Chow, G. R. Dasari, C. F. Leung, C. S. Ng Department of Civil Engineering, National University of Singapore, Singapore

engp1572; cvechow; cvegrd; cvelcf & eng90474@nus.edu.sg

Abstract: This paper presents the results of a study carried out to investigate the settlement induced in existing tunnel by piles during

loading in some layered soil profiles. The function of debonding is also investigated. The analysis indicates that while the debonded

 pile may be able to mitigate its effects on tunnel, the interaction behavior is strongly affected by tunnel depth to pile length ratio and

soil properties.

1 INTRODUCTION

The effect of tunneling on piled foundations is a subject of 

great interest. Mair & Taylor (1997) reviewed case histories, the

results of numerical analyses and model tests relating to this

  problem. A two-step procedure has also been developed for the

analysis of this problem: (i) using semi-analytical (Loganathan &

Poulos, 1998; Chen et al., 1999) or numerical solutions (Cheng

et al., 2002) to predict tunneling induced ground movements, and

(ii) analyzing the performance of pile subjected to such ground

movements (Chen et al., 1999; Cheng et al., 2002).

An interrelated problem, which has received less attention but

nevertheless is important, is that of piled foundations constructed

close to existing tunnels. Benton & Phillips (1991) analyzed the

stress changes and deformations of two existing tunnels beneath a

  building founded on bored piles. The method of analysis was based on two-dimensional finite element method, and the effects

on tunnel during both construction and loading of the piles were

considered. Calabrese & Monaco (2001) performed 2-D plane

strain analysis using FLAC and evaluated the effects induced in

the existing deep tunnels by driving foundation piles. The in-

crease in stresses in the lining was also estimated. Higgins et al. 

(2000) proposed a five-stage procedure based on a two-

dimensional model and analyzed the effects of installation of 

 piles close to existing tunnels. Responses of tunnel during both

  pile installation and loading of the piles were considered;

debonding of the piles was also taken into account. But, as only

two-dimensional analyses were found in the literature, it is ques-

tionable whether they can properly simulate a truly 3-D problem.

The pile-soil-tunnel interaction problem in an urban environ-

ment deserves more attention. However, such interaction is not

well understood. This is mainly due to its complexity in the di-

versity of pile construction methods, and more importantly the

  problem is truly 3-D. Typical pile and tunnel configuration is

shown in Fig. 1.

The objective of this paper is to study the effects of piles on

existing tunnel via 3-D finite element analysis. Considering large

diameter bored piles are usually used in supporting high-rise

  buildings in modern cities, the effects of loads acting on bored

 piles on nearby tunnel are investigated. Emphasis is placed on the

function of debonding /sleeving, which is usually taken as a

measure to mitigate the effects of pile foundations on the tunnel.

2 DEBONDING

Debonding is a commonly adopted procedure to reduce the in-

teraction between piles and tunnels. There are mainly two types

of debonding: single permanent casing debonding and double

  permanent casing debonding with bituminous membrane sheets

as debonding membrane. For piles with single permanent casing,

temporary casing is usually provided to prevent the debonding

membrane on the permanent casing from being damaged during

installation. There is no uniform practice concerning how the

debonding should be carried out, e.g. some piles are required to

  be debonded within a certain zone of influence, as is shown in

Fig. 2 (LTA, 1996), while others are proposed to be debonded till

above the invert of tunnels (Higgins et al., 2000).

When a pile is fully debonded, theoretically, the applied load

is transferred to the ground below the debonded zone. The actualinteraction between piles, which are debonded or not debonded,

and an existing tunnel is, however, not well understood.

Fig. 1. Tunnel adjacent to single pile.

3 METHOD OF MODELING

The simulations carried out in this paper were performed us-ing the Finite Element computer program ABAQUS (HKS,

2002). The drained condition was simulated, as long-term effects

L

 pile

S

D

tunnel

soil

d

Z

P

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were considered to be more significant. The soil was modeled as

a linear-elastic material with a Poisson’s ratio of 0.3. Concrete

material of grade 30 was assigned to properties of both pile and

tunnel lining to simplify the present analyses. The finite element

mesh was selected after consideration of mesh arrangement and

 boundaries. In ABAQUS, debonding of the pile was simulated by

creating contact between the surface of pile (Master surface) and

the surface of its surrounding soil (Slave surface) in the debondedzone, and assuming a small friction coefficient close to zero.

Fig. 2. Zone of influence and length of debonding l (After LTA,

1996).

4 PILE-SOIL-TUNNEL INTERACTION IN SOME

LAYERED SOIL PROFILES

Settlement of a tunnel due to a loaded pile is governed by

many factors, such as load on the pile, soil properties, ratio of 

tunnel depth to pile length, and distance from pile to tunnel. If a

 pile is debonded, settlement of tunnel will inevitably be affected.

From preliminary studies by the authors, it has been identified

that soil properties and ratio of tunnel depth to pile length are

significant factors affecting the effect of debonding. In the fol-

lowing simulations, single pile-soil-tunnel interaction in some re-

alistic layered soil profiles is investigated. The effect of debond-ing is also studied.

4.1  Typical soil profiles

There are four typical soil profiles adopted in the analyses.

Soil profile 1 at the site is constituted by Fill and Fluvial layers

down to about 12m depth, below this level are thick upper Old

Alluvium and lower Old Alluvium layers. In soil profile 2 there is

approximately 6m of made ground above 10m thick Marine clay.

Beneath Marine clay are a layer of sandy silty clay and a very

thick layer of Bouldery clay. While, profile 3 and profile 4 are

mainly made up of Kallang Formation, Old Alluvium and Kal-

lang Formation, Jurong Formation, respectively. The properties

of different soil layers, which are used in subsequent FEM analy-ses, are summarized in Table 1.

Table 1. Soil properties of four typical soil profiles.

4.2 Geometries and working load 

The pile and tunnel geometries used in this study in different

soil profiles are listed in Table 2. They are chosen based on sim-

 plification of realistic pile-tunnel interaction problems as follows:

(i) piles within the influence zone of tunnel are fully debonded

(refer to Fig 2); (ii) length of pile is determined based on the full

material strength with a factor of safety of 2.5. The working load

of 13250kN, determined from material capacity of pile, is applied

in all the cases.

Table 2. Geometries for pile and tunnel in FEM modeling (refer 

to Figs 1 & 2).

Soil Profile1 Thickness(m) γt (kN/m3) K 0 

Fill 2 19 0.5

Fluvial 10 18 0.5

Old Alluvium (upper) 8 - -

Old Alluvium (lower) 100 - -

Ф' Cu (kPa) Es (MPa)

Fill 30 - 3

Fluvial 30 - 20

Old Alluvium (upper) - 150 75

Old Alluvium (lower) - 240 120

Soil Profile2 Thickness(m) Cu (kPa) Es (MPa)

Fill 6 123 62

Marine clay 10 22 11

Sandy silty clay 2 47 24

Bouldery clay 82 477 239

Soil Profile3 Thickness(m) Cu (kPa) Es (MPa)

Fill 8 50 10

Kallang Formation 10 25 5

Old Alluvium 102 250 100

Soil Profile4 Thickness(m) γt (kN/m3) K 0 

Fill 2 18 0.5

Kallang Formation 4 - -

Jurong Formation 94 - -

Ф' Cu (kPa) Es (MPa)

Fill 30 - 8

Kallang Formation - 20 6

Jurong Formation - 600 300

L (m) d (m) l (m) D (m)

Profile1 70 1.5 14 6

Profile2 42 1.5 18 6

Profile3 70 1.5 12 6

Profile4 24 1.5 6 6

t (m) S (m) Z (m)

Profile1 0.25 12 20

Profile2 0.25 12 24

Profile3 0.25 12 18

Profile4 0.25 12 12

6

Z

6 + R  Z - R  34 - (Z - R)

2nd1st 3rd Reserves

l

45°

Zone of influence

40 + R 

Railway safety zone

( unit: m )

Pile

Tunnel

R 

d

L

P

l

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4.3  Selection of pile length

Practically, a normally designed pile and a pile designed for 

debonding under the same working load may not have the same

length, as contribution from top soil layers is not considered in

the latter. Therefore, tunnel settlements induced by a pile with

  both a normally designed length (case 1) and a debonding de-

signed length (case 2) in soil profiles 1 and 2 are examined be-fore the study of debonding effect is carried out. In both cases,

 piles are not debonded, the results of which are illustrated in Ta-

 ble 3, where 1t W  and 2t W  are settlements of tunnel in case 1 and

case 2, respectively.

Table 3. Tunnel settlement by a pile with both normally designed

and debonding designed pile length.

It can be seen that the pile length which is normally designed

in case 1, as expected, is shorter than that designed for debonding

in case 2. In profile 1, the pile length only increased by 4m if 

debonding-design is used, while there is an even smaller increase

in profile 2. This is because the top soil layers are much weaker 

than lower soil layers in the adopted profiles, and hence the con-

tribution of top soil layers is small. Therefore, although tunnel

settlement is reduced when a debonding designed pile length is

used, the reduction is rather small, as it is only 0.9% and 1.5% in

 profiles 1 and 2, respectively.

Theoretically, normally designed pile length should be used to

study the effects of a normal pile. However, it is shown by theabove analyses that if debonding designed pile length is adopted,

it will only reduce tunnel settlement by a small amount. There-

fore, to simply the process of modeling, debonding designed pile

length will be used in the subsequent analysis comparing the in-

fluence of a normal pile and a debonded pile.

5 RESULTS AND DISCUSSIONS

5.1 Settlement of tunnel

Typical variations of settlement along the longitudinal and

transverse directions of tunnel in profile 1, when the pile is

debonded and without debonding, are illustrated in Figs 3 and 4,

respectively. It is clear that starting from the cross-section of tun-

nel which is closest to the pile (where x coordinate is zero), set-

tlement of tunnel decreases along the longitudinal direction, since

the influence of pile reduces with the increase of distance, i.e. the

distance between pile and any cross-section of tunnel. As far as

the transverse section of tunnel with the smallest distance from

the pile is concerned, larger settlement of tunnel occurs in the

side closer to the pile; and the maximum value occurs when θ is

about thirty degrees (refer to Fig 4). Similar trends of variation of 

tunnel settlement can be observed from other soil profiles, but

debonding may not always reduce settlement of tunnel as it does

in profile 1. Detailed comparison is given in the next section.

5.2 Comparison of debonding effects

The maximum settlements of pile and tunnel in different soil

  profiles are summarized in Table 4. Z/L is the ratio of tunnel

depth to pile length, and l is the length of debonding determined

following the code of LTA (refer to Fig 2). t W ∆ , is the differ-

ence of tunnel settlement caused by a normal pile and a debonded

 pile )( tntd  W W  − divided by normal pile induced tunnel settle-

ment tnW  (refer to Eq. 1):

%100×

−

=∆tn

tntd t 

W 

W W 

W  (1)

It is useful to provide a means of describing the effects of 

debonding, e.g. a negative t W ∆ means reduction occurs in the

settlement of tunnel if the pile is debonded; while a positive

t W ∆ implies an increase in tunnel settlement due to debonding.

As expected, the settlement of a debonded pile  pd W  is larger 

than that of a pile without debonding  pnW  in all the profiles.

Fig. 3. Settlement of tunnel along longitudinal direction.

Fig. 4. Settlement of tunnel in transversal section closest to pile

(θ is counter clockwise).

As far as settlement of tunnel is concerned, two types of re-

sults can be observed: (i) settlement of tunnel induced by a

debonded pile is smaller than that induced by a normal pile,

i.e. t W ∆ is negative, as is shown in profiles 1 and 3; (ii) settle-

ment of tunnel by a debonded pile is unchanged or even larger than that by a normal pile, i.e. t W ∆ is greater than or equal to

zero, as is in profiles 2 and 4.

Case 1 Case 2Pf.

L1 (m) Wt1 (mm) L2 (m) Wt2 (mm)(Wt2-Wt1)/Wt1

1 66 1.904 70 1.886 -0.9%

2 40 1.133 42 1.116 -1.5%

0 10 20 30 40 50

2.0

1.6

1.2

0.8

0.4

0.0

X (m)

   W   t

   (  m  m   )

Debonded

Normal

Tunnel in profile 1

longitudinal plot

0x

0 90 180 270 360

2.0

1.6

1.2

0.8

0.4

0.0

(o

)

   W   t

   (  m  m   )

Debonded

Normal

Tunnel in profile 1

transversal plot

0

z

y

θ 

θ

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Table 4. Settlement of tunnel and pile during single pile-tunnel

interaction

As is well known, total load on the pile is supported by shaft

friction and base resistance. In a pile-soil-tunnel interaction prob-

lem, as a pile is loaded, the tunnel will be affected by the part of 

the load taken by pile shaft (shaft load), as well as the part of the

load taken by base (base load). If total load is not changed, the

 behavior of tunnel will depend on (i) the soil profile; (ii) the ratio

of tunnel depth to pile length Z/L, which has been mentioned be-

fore; and (iii) the variation of proportions of load taken by shaft

and base, which is induced by changes of working conditions of 

the pile, e.g. from a normal pile to a debonded pile.

If the pile toe is relatively far from the tunnel centre, as is in

 profiles 1 and 3 with a small Z/L ratio of 0.3, the tunnel will be

more affected by shaft load, i.e. main proportion of tunnel settle-

ment is induced by shaft friction. In such circumstance, if the pile

is debonded, shaft friction decreases and it will take a smaller 

 proportion in the total load. As a result, settlement of tunnel re-

duced by 4% and 3% in profile 1 and 3, respectively ( t W ∆ is

negative).

On the contrary, when the pile toe is closer to tunnel centre, as

is in profile 2 with a large Z/L ratio of 0.6, base load becomes

more dominant and its influence on the tunnel settlement may be

more significant. In such a situation, if the pile is debonded, base

load will increase its proportion in total load with the decrease of shaft friction. Therefore in this case, settlement of tunnel in-

creases by 2% ( t W ∆ is positive).

The soil profile plays an important role in the effect of 

debonding in pile-soil-tunnel interaction. In any case for the soil

 profiles studied, the effect of debonding is small, as t W ∆ is no

more than 5%.

6 CONCLUSIONS

  Numerical modeling via 3-D finite element analysis enables

the estimation of the settlement of tunnel due to loads acting on

nearby piles. In this paper, single pile-soil-tunnel interaction in

some realistic layered soil profiles, was studied; the effect of 

debonding was also investigated.

The results of the analysis indicate that behavior of debonding

is greatly influenced by Z/L ratio, and soil profile and properties.

While the results show some trend, the number of cases studied is

too few to generalize the explicit impact of each of these factors.

For the cases considered, the effect of debonding is small.This paper is a preliminary attempt to study only one aspect of 

the pile-soil-tunnel interaction problem, namely the induced set-

tlement of tunnel. It is noted that soil is a nonlinear material; the

assumption of linear elastic soil behavior has a tendency to over-

estimate the interaction effects. These results presented will,

however, need to be verified by well-controlled laboratory centri-

fuge experiments and field studies. Another aspect of the interac-

tion behavior, namely the effect on stresses in the tunnel lining

will also need to be investigated.

REFERENCES

ABAQUS, 2002. ABAQUS User’s and Theory Manuals, Version

6.3. Rhode Island: Hibbitt, Karlsson & Sorensen, Inc.

Benton, L. J. & Phillips, A., 1991. The behavior of two tunnels

 beneath a building on piled foundations. Proc. 10th

European

Conference on Soil Mechanics and Foundation Engineering:

665-668. Florence.

Calabrese, M. & Monaco, P. 2001. Analysis of stresses induced

in an old deep tunnel by pile driving from the surface. FLAC 

and Numerical Modeling in Geomechanics: 199-204. France.

Chen, L. T., Poulos, H. G. & Loganathan, N. 1999. Pile Re-

sponses Caused by Tunneling.   Journal of Geotechnical and 

Geoenvironmental Engineering, ASCE Vol. 125 (3): 207-215.

Cheng, C. Y., Dasari, G. R., Leung, C. F. & Chow, Y. K. 2002.

A novel FE technique to predict tunneling induced ground

movement in clays. The 15th KKCNN Symposium on Civil

 Engineering: 43-48. Singapore.

Higgins, K.G., John, H. D. St., Chudleigh, I. L. J. & Potts, D. M.

2000. An example of a pile-tunnel interaction problem. Geo-

technical Aspects of Underground Construction in Soft 

Ground : 99-103. Rotterdam.

Loganathan, N. & Poulos, H. G. 1998. Analytical prediction for 

tunneling-induced ground movements in clays.   Journal of 

Geotechnical and Geoenvironmental Engineering, ASCE

Vol. 124 (9): 846 -856.

Land Transport Authority, 1996. Code of practice for railway

 protection. Singapore.

Mair R. J. & Taylor R. N. 1997. Bored tunneling in the urban en-

vironment. Theme lecture to the 14th

international conference

on soil mechanics and geotechnical engineering: 2353-2385.

Hamberg.

(1) normal (2) debonded profile

W pn (mm) Wtn (mm) W pd (mm) Wtd (mm)

1 11.01 1.886 11.88 1.816

2 9.54 1.116 10.74 1.138

3 13.27 2.071 13.74 2.012

4 6.89 1.054 6.95 1.054

l (m) Z/L ∆Wt

1 14 0.3 -4%

2 18 0.6 2%

3 12 0.3 -3%

4 6 0.5 0%