Short-pile and long-pile cases were analysed to assess the effect of pile length with respect to the tunnel depth. Each case indicates considerably different tunnelling-induced behaviour on the pile, as described below.
• Short Pile. The tunnel axis is located below the tip of the existing pile (Lp/H<1).
Anlyses show significant pile settlement is induced, together with additional bending moments, lateral deformations and pile rotations. The pile head settlement exceeded the ground surface settlement for a pile located about half the tunnel depth away hori-zontally.
• Long Pile. The tunnel axis is located above the tip of the existing pile (Lp/H>1).
Analyses show the induced bending moments are significant in this case. The pile head settlements are less than the ground settlement and, therefore, the pile head settlement trough may be considered in the assessment of potential building damage due to tunnelling-induced ground movements.
The difference between the short- and long-pile behaviours may be attributed to the pile fixity and the slenderness effects with respect to the non-uniform ground movements with the depth induced by the tunnel excavation. Short piles have a tendency to move with the soil movements, whereas long piles resist soil movement. Soil can fail around the long pile, how-ever, due to the resistance; whereas generally, short piles will show elastic behaviour of the pile-soil interaction.
Based on the above observations, two sets of design charts were developed, one for long piles and one for short piles.
H Pile
d X
z
R Tunnel
Ground Level
SoilMovement
x
Lp
49
5.3 PARAMETRIC STUDY
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, εF
• Undrained soil shear strength, cu
• Depth of tunnel axis level, H
• Pile diameter, d
• Pile length, Lp.
The following observations were made:
1. Increasing tunnel radius R and ground loss ratio, εF, resulted in increases in the:
• Maximum bending moment Mmax
• Lateral pile deflection ρmax
• Compressive axial force (+Pmax)
• Tensile axial force (-Pmax)
• Pile head settlement vmax.
It is appropriate, therefore, to normalise the ground loss with the tunnel radius by introducing a factor “ground loss ratio, εF” where εF = R2ε0.
2. Increasing the strength of the ground, cu, resulted in increases in:
• Mmax, ρmax, +Pmax, and -Pmax because of an increase of the lateral soil pressure and skin friction
• Pile head settlement, vmax.
3. Increasing the pile diameter, d, tended to:
• Increase induced bending moment on pile, Mmax
• Decrease lateral deflection of the pile, ρmax (due to an increase of pile lateral rigidity)
• Increase +Pmax, and -Pmax
• Decrease vmax.
4. The effects of the depth of tunnel axis level, H, and the pile length, Lp, depend on the ratio Lp/H. The maximum pile responses could either increase or decrease with changing Lp/H depending on the other parameters.
5.4 DESIGN CHART CONCEPT
Based on the above parametric studies, it was found that within the range of parameters examined, the various maximum pile responses may be approximated as follows:
Lateral response:
Mmax = maximum induced bending moment
Mb = maximum induced bending moment on the pile for base case ρb = maximum lateral deflection of the pile for base case
ρmax = maximum induced lateral deflection +Pmax = maximum induced compressive axial force
+Pb = maximum positive axial force induced on the pile for base case -Pb = maximum negative axial force induced on the pile for base case -Pmax = maximum induced tensile axial force
vmax = maximum induced pile head settlement vb = pile head settlement derived for base case.
Based on the parametric study (by changing various factors), the following correction or influence factors were derived for the various parameters that affect the magnitude of the tunnelling-induced effects on piles:
• Undrained shear strength. Correction factors are kMcu, kρcu, k+Pcu, k-Pcu, and kvcu.
• Pile diameter. Correction factors are kMd, kρd, k+Pd, k-Pd, and kvd.
• Ratio of pile length to tunnel axis level. Correction factors are kMLp/H, kρLp/H, k+p Lp/H, k-pLp/H, and kvLp/H.
The base case, a single pile and a tunnel configuration as shown in Figure 5.1, was ana-lysed to develop the design charts. Details of the base case are as follows:
• The tunnel is excavated through homogeneous clay with the undrained shear strength of 60 kPa.
51
5.5 DESIGN CHARTS FOR SHORT PILES
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 normalised ground loss factor εF = R2ε0 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.
• Figure 5.2 shows the tunnelling-induced effects on short piles for the base case with the ground loss factor εFB = R2ε0 = 32 x 1% = 0.09.
• Figure 5.3 shows the variation of correction factors for the undrained shear strength of the soil varying from 10 kPa to 300 kPa.
• Figure 5.4 shows the variation of correction factors for the pile diameter varying from 0.25 m to 1.5 m (0.8 feet to 5 feet).
• Figure 5.5 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.
The steps involved in using the design charts are as follows:
STEP 1: Estimate the ground loss ratio, εF for a given problem using εF = R2ε0.
Estimate the factor, LR = εF/εFB , where εFB = 0.09.
STEP 2: Estimate the tunnelling-induced base behaviour from Figure 5.2 for the given horizontal distance, x, and multiply these values by the factor LR.
STEP 3: Estimate undrained soil shear strength, pile diameter, and pile length/tunnel depth ratio correction factors from Figures 5.3, 5.4 and 5.5 and multiply the values estimated in Step 2 by these correction factors (via Equations 5.1 to 5.5).