Influence of Tunneling on Adjacent Existing Pile Foundation

Influence of Tunneling on Adjacent Existing Pile Foundation

Mr. K. Raja

Assistant Professor, Department of Civil Engineering,

1. College of Engineering and Technology, Karur, India

   Dr. K. Premalatha

   Professor, Department of Civil Engineering,

   College of Engineering, Guindy, Anna University Chennai, India

   Mr. S. Hariswaran

   Assistant Professor, Department of Civil Engineering,

   Agni College of Engineering and Technology, Chennai, India

   Abstract One of the important issues of tunneling in urban areas is the assessment of the impact on adjacent buildings of tunnel construction. Many high rise buildings are supported by pile foundations in major cities. Tunneling activities carried out adjacent to these pile foundations invariably cause ground movements, which in turn will impose axial and lateral forces on the pile foundations. The study focuses on the estimation of ground movements induced by tunneling (both vertical displacements and horizontal displacements) and the imposition of these soil movements on the pile and computation of the consequent pile responses. Tunneling induced behavior of piles was estimated using PLAXIS software. A parametric study was carried out for short pile in sand. Based on the parametric study, design charts were developed for short piles to estimate the tunneling induced behavior of pile easily.

   Keywords Tunneling, Free field ground movements, Settlement, Short pile

   1. INTRODUCTION

      In recent years, there has been an increase in infrastructure development in highly populated or urban area. There transportation plays a vital role to facilitate the people to move from one place to other. In order to meet the demand for transportation in urban area several projects have been undertaken including road, rail transport on flyovers and underground tunnels. Apart from transportation numerous utility lines are also used below the ground level.

      Excavations below ground level causes relaxation of insitu stress i.e. ground loss. Tunneling is also one form of excavations which cause ground losses. In the highly populated or urban areas the underground excavation for tunneling is carried adjacent to the high rise buildings. Therefore in urban areas, the performance of nearby piled foundations was affected by tunneling activities. Necessary steps are required to assess the responses of pile foundation due to tunneling at various depths and to prevent the pile from deformation and settlement.In many large heavily populated cities, tunnel excavation is very close to the

      adjacent buildings. This induces vertical and lateral soil movement on piles, which in turn induce axial responses such as axial force and settlement. It also induces lateral responses such as pile bending moment and deflection.

      This present study focuses on the estimation of ground movements induced by tunneling and the imposition of these soil movements on the pile and computation of the consequent pile responses.

   2. METHOD OF ANALYSIS

      The fast growing population and the scarcity of land in the urban areas needs diversification in traffic. By providing subways and tunnels in these urban areas, technically the traffic can be made feasible and viable. While constructing the tunnels and subways in the urban areas, it may happen to cross near the high rise buildings. When these tunnels are to be executed adjacent or below the foundations of the adjacent buildings, the relaxation in stress causes ground loss, which in turn affects the behavior of pile foundation .Necessary steps are required to assess the responses of pile foundation due to tunneling at various depths and to prevent the pile from deformation and settlement. Many solutions are available to predict the ground movements they are analytical, empirical and numerical solutions. Software tools are also used to predict these ground movements. Some of them are PLAXIS and Xdisp. Ground movement profile of a tunnel depends on the ground loss. Despite other factors, the major factor that influences the ground loss is the installation technique. Experiences in tunneling technology advocate to do the tunnel work with minimum ground loss. The reported ground loss is 1-3%. The present analysis is carried out in two stages, they are

      STAGE 1: Prediction of Free Field Ground Movements

      STAGE 2: Response of Pile Foundation for Ground Movement

   3. PREDICTION OF FREE FIELD GROUND MOVEMENTS

      The major ground movements discussed here are surface settlement, subsurface settlement and lateral displacement. These ground movements can be estimated using various methods such as empirical method, analytical method and numerical method. For the present analysis the input parameters reported in literature for Heathrow Express Trial Tunnel in clay soil, central London was used. The ground movements estimated using different methods were compared with the observed ground movements.

      Fig-1 Surface settlement

      Fig-2 Sub Surface settlement

      Fig-3 Lateral Displacement

      3.1 PREDICTION OF GROUND MOVEMENTS FOR DIFFERENT CASES

      It was decided to predict the ground movements for different conditions, by varying the following parameters and the soil type. The soil types considered are cohesive and cohesionless deposits of 35m thickness. Cohesive soil of three different consistencies and cohesionless soil of three different relative densities are considered. The tunnel parameters considered are placing depth and tunnel diameter. The different soil conditions and the input parameters are summarized in the table-1.

      Table-1. Soil condition and soil parameters

      Soil Type	Conditi on	Deformati on Modulus of soil, Es (MPa)	unit weight of soil, d (kN/m3)	Poisso ns ratio,	Shear strength Paramete r, Cu (kPa) or
      Cohesiv e Soil	Soft	3.5	15	0.4	50
      	Medium	6.0	17	0.45	100
      	Hard	14.5	19	0.5	200
      Cohesio nless Soil	Loose	28	16	0.3	300
      	Medium	35	18	0.4	350
      	Dense	70	20	0.45	400

      The variation of maximum ground movements with respect to deformation modulus and tunnel dimensions are represented in charts. Charts shown in figure 4and 5 represents the maximum vertical settlement and maximum horizontal displacement for cohesionless soil. The chart shown in figure 6 and 7 represents the maximum vertical settlement and maximum horizontal displacement for cohesive soil.

      Fig-4 Maximum vertical settlement cohesionless soil

      Fig-5 Maximum horizontal settlement cohesionless soil

      Fig-6 Maximum vertical settlement cohesive soil

      Fig-7 Maximum Lateral displacement of cohesive soil

      These charts can be used for evaluation maximum ground movements for other conditions.

      The general observations made from the table and figures are

      1. Irrespective of the type of deposits, increase in the tunnel diameter increased the ground movements.

      2. Irrespective of the type of deposits, increase in placing depth of tunnel decreased th maximum ground movements.

      3. The ground movement for dense sand deposit is higher than loose and medium sand deposits.

      The maximum ground movements observed for the different conditions of the analysis are also complied as charts and are shown in figure 4, 5, 6 and 7.

   4. VALIDATION OF PLAXIS ANALYSIS

      A case study reported in the literature was selected to validate the Plaxis modeling and analysis. The present case study briefs the construction of a tunnel for the Angel Underground Station in London. The tunnel was driven between pile foundations supporting a seven-story building with a two- story basement. The tunnel axis line is 5.7 m from the centerline of the nearest piles. The tunnel was excavated using hand tools in two stages, the first a pilot tunnel of 4.5 m diameter and the second an enlargement of 8.25 m diameter. Measured ground loss ratio was 1.5% for the pilot tunnel and 0.5% for the tunnel enlargement (Mair 1993). The piles were driven through 28 m of London clay to the underlying Woolwich and Reading beds. Ground investigation data showed that the average undrained shear strength of London clay increased linearly from 50 kPa at the top to 220 kPa at the bottom. Inclinometers were installed at various locations within the ground to measure the lateral soil movements and within some of the piles to measure lateral pile deflections. Measured results were presented for a pile that had a diameter of 1.2 m and was located 5.7 m away from the tunnel axis line. At the pile location, the depth h of the tunnel axis level was approximately 15 m. The pile was treated as a single pile and the analysis was done. The undrained shear strength and deformation modulus used for the present analysis are cu

      = 135 kPa, soil Youngs modulus Es = 54 MPa. The results are comparable and hence the analysis is valid. However the measured values are comparatively lower than the values that are predicted from software.

      Fig-8 Lateral Pile deflection for case history studied

      From figure 8 it is observed that the agreement between the computed and the measured profile is good. But the predicted maximum lateral deflection is higher than the measured maximum deflection.

   5. RESPONSE OF PILE FOUNDATION FOR GROUND MOVEMENT

      Response of pile foundation towards these predicted ground movement was determined by using software PLAXIS.

      1. The Pile response determined using PLAXIS was compared with the case study reported in literature to validate the results of PLAXIS analysis.

      2. Parametric studies were carried out to investigate the influences of various parameters on the pile responses. In these studies, the following parameters were varied:

         • Tunnel radius, R

         • Ground loss ratio,

         • Angle of internal friction,

         • Placing depth of tunnel, H

         • Pile diameter, d

         • Pile length, LP.

      A base case was considered for the analysis. The input parameters assumed for the analysis of base case are

      • The tunnel is excavated through homogeneous sand with the friction angle 30o.

      • Tunnel outer diameter, OD, is 6 m.

      • Tunnel depth to centerline, H, is 20 m.

      • Pile diameter, d, is 0.5 m.

      • Pile length, Lp, is 15 m for short pile and Lp, is 25m for long pile.

      • Youngs modulus of the pile is 30 GPa.

      • Ground losses are 1 & 5 percent.

      Table-2 Different Parameters for analysis

      Description	Parameters varied
      Angle of internal friction,	300
      	350
      	400
      Pile diameter, d in m	0.25m
      	0.50m
      	0.75m
      	1.00m
      	1.25m
      Short Pile (Lp/H<1)	0.50
      	0.75
      	1.00

      A parametric study was carried out by changing the base case parameters. The parameters varied with respect to base case for the analysis are shown in table-2 above. Based on the parametric studies, it was found that within the range of parameters examined, the various maximum pile responses can be approximated as follows:

      Lateral response:

      d

      Mmax = Mb. kM . kM . kMLp/H . (1) max = b. k. kd. kLp/H . (2)

      Axial response:

      d

      Pmax = Pb. kP . kP . kPLp/H . (3)

      • Angle of internal friction. Correction factors are kM k kP and kv

        , ,

        d d, d, d

      • Pile diameter. Correction factors are kM , k kP and kv

      • Ratio of pile length to tunnel axis level. Correction factors are kMLp/H, kLp/H, kPLp/H, and kvLp/H.

        The design chart was developed for the analysis of Short Pile responses in sandy soil. Compilation of the observed results was done and presented in the form of design charts.

        5.1 DESIGN CHART FOR SHORT PILES (Lp/H<1)

        The base case, a single pile and a tunnel configuration as shown in Figure 9, was analyzed to develop the design charts. Details of the base case are as follows:

      • The tunnel is excavated through homogeneous sand with the friction angle 30o.

      • Tunnel outer diameter, OD, is 6 m. Radius R=3m.

      • Tunnel depth to centreline, H, is 20 m.

      • Pile diameter, d, is 0.5 m.

      • Pile length, Lp, is 15 m

      • Youngs modulus of the pile is 30 GPa.

      • Ground losses are 1 & 5 percent.

      The maximum pile responses for the short-pile case were established for the base case. Based on the observations made from the parametric study, it was decided to adopt normalized ground loss factor F = R20 to produce the design charts for the base case. Correction factors will be assessed based on the differences between the parameters for a specific project and those for the base case.

      vmax = vb. kv. kvd. kvLp/H . (4)

      Where:

      Mmax = maximum induced bending moment

      Mb = maximum induced bending moment on the pile for base case

      b = maximum lateral deflection of the pile for base case max = maximum induced lateral deflection

      Pmax = maximum induced axial force

      Pb = maximum axial force induced on the pile for base case vmax = maximum induced pile head settlement

      vb = pile head settlement derived for base case.

      Based on the parametric study, the following correction factors were derived for the various parameters that affect the magnitude of the tunneling-induced effects on piles:

      Fig-9 Short Pile (Lp/H<1)

      Figure 10(a) to 10(d) shows the tunneling-induced effects on short piles for the base case with the ground loss factor FB = R20 = 32 x 1% = 0.09 and FB = R20 = 52 x 1% = 0.45

      Figure 11(a) to 11(d) shows the variation of correction factors for the angle of internal friction of the soil varying from 300 to 400.

      Figure 12(a) to 12(d) shows the variation of correction factors for the pile diameter varying from 0.25 m to 1.25 m.

      Figure 13(a) to 13(d) shows the variation of the correction factors for the pile length/tunnel depth ratio varying from 0.5 to 1.0. This ratio has the most influence on the response of the pile.

      Fig-10(a) Maximum bending moment

      Fig-10(b) Maximum Axial Force

      Fig-10(c) Maximum Pile head settlement

      Fig-10(d)Maximum Lateral Deflection

      Fig-11(a) Max. B.M. Correction factor for()

      Fig-11(b) Max. Lateral deflection Correction factor for()

      Fig-11(c) Max. Pile head Settlement Correction factor for()

      Fig-11(d) Max. Axial force Correction factor for()

      Fig-12(a) Max. B.M. Correction factor for(d)

      Fig-12(b)Max. Lateral deflection Correction factor for(d)

      Fig-12(c) Max. Pile head Settlement Correction factor for(d)/p>

      Fig-12(d) Max. Axial force Correction factor for(d)

      Fig-13(a) Max. B.M. Correction factor for (Lp/H)

      Fig-13(b) Max. Lateral deflection Correction factor for(Lp/H)

      Fig-13(c) Max. Pile head Settlement Correction factor for(Lp/H)

      Fig-13(d) Max. Axial force Correction factor for(Lp/H)

   6. OBSERVATIONS

      The observations made from the parametric studies in short pile is

      Short Pile

      1. Increase in the friction angle increases all the tunneling induced behavior such as bending moment, lateral displacement and axial force but pile head settlement reduces

      2. Increase in the pile diameter increases all the tunneling induced behavior

      3. Increase in the Pile length/ tunnel depth ratio causes decrease in pile head settlement.

   7. CONCLUSIONS

Here the ground movements were evaluated using different method and these ground movements were compared with the case studies reported in the literature. On comparing the ground movements estimated using different methods and with the measured results from the case studies PLAXIS 2D software is found to give upper bound estimate of ground movements than the other methods. Ground movements for different input parameter of soil and tunnel dimensions are also obtained using Plaxis software. Design charts for maximum ground movements were developed and are presented.

PLAXIS software is used to predict the responses of pile foundation for these ground movements. A case study was considered to validate the analysis of pile responses using PLAXIS. Parametric studies were carried out by changing the parameters of the base case. Design charts were developed for the tunneling induced behavior of short pile. Also the design charts for the correction factors were developed based on the parametric study for short pile. The design charts presented here can be used by the designers to predict the tunneling induced effects on short piles sand effectively.

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