Analytical study on the ground movement caused by pipe bursting

Oct 08, 2008

Trenchless pipe replacement technology by ‘EXPANDIT’ using hydraulic expansion was established in March, 2006 in Japan, and we received the construction technology certificate of JIWET (Japan Institute of Wastewater Engineering Technology).This trenchless technology of pipe replacement has much effective features. However, many sewer administrators would consider that pipe bursting might damage to adjacent pipes or structures, therefore this pipe replacement technology cannot be used widely for sewer pipes in Japan. In order to solve such problems, we performed experimental pipe bursting operation focused on the ground movement at 6 span’s model pipelines in 2005. Consequently, the relation between a distance from existing pipe to measuring point and the amount of ground movement at that point has been understood, and estimation charts of the ground movement were attained.In this paper, we describe the numerical study about the ground movement associated with the amount of radial expansion, while these data following advancing analysis of the data were analyzed in detail, estimation of the ground movement was available. One example of vertical ground movement by pipe bursting operation can be described.

It is said that more than 390,000 km long sewer pipeline has been constructed since 1945 in Japan. A design life of sewer pipelines is regulated as 50 years. Length of sewer pipelines which should be inspected and if necessary repaired is increasing every year. We expect large percentage of these pipeline length should be replaced in the big cities. Instead of pipe lining methods, the pipe bursting and replacement technology is needed because of big deformation of pipe and many other reasons. In many cities in Japan, construction works could not affect only the vehicular traffic but also the neighbour-shops. Therefore, the trenchless technology of pipe is necessary for the pipeline repairing and replacement. As for the repairing of pipes, the inside pipe lining technique, such as cured in place pipe (CIPP) is mainly adopted. However, the cured in place pipe techniques could not be available, for upsizing or size-for-size. Therefore the trenchless pipe replacement technology should be developed. In order to spread the technology, we have to know the movement of the subsurface around the pipe at the time of expansion. We have made study of ground movement during the hydraulic bursting equipment bursts pipes, and made the technology to be available in Japan. Although the result of field test was reported on the ISTT conference held at Brisbane 2006, we would like to describe the advanced analysis about the ground movement.
Overview of Pipe Replacement Technology
Pipe replacement is performed by a pipe bursting machine which has one hydraulic jack. The hydraulic system is called ‘Expandit’. Expandit can burst existing pipes by radial expansion, and new pipes could be installed on the same line. On the bursting process of old pipes, Expandit creates a cavity in the soil around the pipe and the new pipe is pulled into from the starting shaft. Radial expansion/reduction of the bursting machine are repeated successively and the new pipeline is constructed. An ‘Expandit’ is advanced by the pulling-chain to the arriving shaft, and a new pipe is pushed in the cavity by the hydraulic jack being set in the starting shaft. The shape of ‘Expandit’ is like a long and slender cone, the maximum diameter and the rear end diameter of ‘Expandit’ are larger a little than outer diameter of the newly installed pipe, and the front end diameter of ‘Expandit’ is a little smaller than the inner diameter of the existing pipe. Expandit enters into an existing pipe, and the machine is expanded and bursts old pipes. The current Expandit can accommodate to the pipes which have the inner diameter of 200 to 600. The inner diameter of new pipe can be up-sized about 150mm than one of the old pipe. Figure 1 is a picture of‘ Expandit’. Figure 2 shows the relation between Expandit and pipes.
Numerical Study Procedures
The final purpose of the numerical study is to develop estimation curves for ground movement and to ensure safety of adjacent utilities. Numerical study procedure is shown in Figure 3.
Field Test and Measurements [1]
The pipelines for the field test were constructed and pipe burst and replacement was done by two Expadits. One is EXP-250, another is EXP-400. The outline of the field test is shown in table 1. The amounts of radial expansion are 73mm and 115mm. The ground cover depth of pipelines for the field testing are provided, 1.5m and 2.0m. Measurement of the ground displacement was performed in the upper part of an existing pipe, and the both horizontal direction of right-and-left sides. The ground movement of an upper part and horizontal both sides of the existing pipe were measured at 5 points from 200mm to 1000mm in each section. Measurement of the vertical movement was done by the settlement and upheaval plate at the time of pipe bursting. The inclinometer insertion pipes were installed for measurement of the horizontal displacement, and the horizontal ground movement was measured by the inclinometer before and after passage of Expandit.
Span
No		 Existing
Pipe		 Installed
Pipe		 Type of
Expandit		 Cover
Depth		 Radial
Expansion
1 350 CP 400 CP EXP-400 2.0 m 115 mm
2 350 CP 400 CP EXP-400 2.0 m 115 mm
3 250 CP 250 CP EXP-250 1.5 m 73 mm
4 250 CP 250 CP EXP-250 1.5 m 73 mm
5 250 CP 350 VP EXP-250 1.5 m 73 mm
6 350 CP 400 CP EXP-250 1.5 m 115 mm
Table 1 Outline of Field Testing
Sampling and Laboratory Testing of Sub-surface Soil
Field Test was carried out in Miyazaki prefecture which located in the southernmost end Kyushu area in Japan. The site is a fairly flat area, and the field was reclaimed on the farm ten years ago. 6 lines of trench were excavated about 3.0 m deep, and about 12 m long by open-cut method, and the 6 spans of pipeline and 9 shafts were constructed. The diameter of each shaft is 1.5m. The soil conditions are indicated in table-2 obtained by the field sampling and laboratory test. The pipes were covered by sand which was compressed sufficiently and upper portion of pipes was backfilled with original soil compressed sufficiently. Sampling, laboratory testing, and a standard penetration test were held at two points, original ground and a backfilled ground. A standard penetration test is an examination which evaluates the quality of the ground by N-value is available for decision of Young’s Modulus E of soil. Young’s Modulus E (kN/cm2) could be calculated by Equation 1.
E = 2,800N           (1)
Where, N is N-value obtained by standard penetration test on the field.
E is Young’s Modulus of the materials.
Figure 5 shows the cross section of trenches around the pipeline and the sampling location.
Soil Number of N-value γ (kN/m3) C (kPa) φ (deg)
Original 3 1.688 E11.7 36.4
Back-fill 4 1.801 11.2 33.0
Table 2 Soil Condition from Sampling and Laboratory Test
Simulation for Sub-surface Soil Condition
Soil constants γ (Unit Weight), ν (Poison Ratio), C (Cohesion), φ (Internal Friction Angle) for FEM analysis are on the Table 3. These constants γ, ν, C and φ were used for FEM analysis. Young’s modulus E of soils were predicted from N-value by the Equation 1.
  N- Γ
(kN/m3)		 E
(kN/m3)		 v Φ
(°)		 C
(kN/m2)
Initial After
Original 1~10 15 2800N 0.33 0.45 20 20
Back-fill 1 18 2800 0.33 0.45 20 10
5 18 14000 0.33 0.45 20 10
10 18 28000 0.33 0.45 30 10
Sandfill 5 19 14000 0.33 0.45 30 10
Table 3 Soil Constants for Simulation
After field testing of pipe replacement, in order to evaluate the soil constants, at the first step of the numerical study, the theoretical maximum horizontal and vertical displacement of span 1 to 6 calculated by finite elements method was compared with the actual displacement measured on the field test. According to this simulation, calculated displacement of upper and side direction showed that an actual measurement and a calculated displacement suited well, when analyzing as elastic modulus E=2,800×N.
Comparison of an FEM analysis and a field measurement is shown in Figure 7. The soil conditions which were adopted for every section by the simulation horizontal direction and vertical direction were set up. On the simulation, the calculated displacement of horizontal movement was compared to the measured amount of horizontal movement at the section 1. A comparison result of 40cm distance in section-1 is shown in Figure 8.
Parametric Study
Pipe diameter, radial expansion, soil cover depth, and soil condition represented N-value should be considered for the parametric study. As can be seen in Table 4, there are several factors that must be carefully considered to study the relation between ground movement and distance from pipe. We made N-value obtained by the standard penetration test the representative parameter for the subsurface soil. N-value for the parametric study were ranged from 1 to 10. There are two cases for the backfilled portion of trench, same N-value or not same. The cover depth on the installed pipe shall be considered to be ranged from minimum 1.0m to maximum 4.0m. According to usual open cut construction for sewage pipeline, almost all the cover depth of pipeline is in this range. Our pipe bursting technology can be applied to existing pipe diameter 200 mm to 600 mm, and also to newly installed pipe in diameter 200mm to 600 mm. However, at the field test we have used pipes in diameter 250 mm and 350 mm. Generally, one or two size-up can be accomplished by our pipe bursting and replacement technology. The influence of pipe diameter should be considered for the parametric study. The amount of radial expansions was necessary for up sizing of the existing pipe. The maximum radial expansion is 600mm inner diameter.
Factors Amount
to be counted		 Field Testing Our Technology
Soil Condition N-Value 3 to 5 1 to 10
Locating of Pipe Cover Depth 1.5m and 2.0m more than 1.0m
Pipe Diameter Existing Pipe
Inner Diameter		 250mm and
350mm		 From 200mm to 600mm
Original soil movement Radial
Expansion		 115 mm and 73 mm Maximum 115 mm
Points of Soil
Movement Around Pipe		 Distance from
Existing pipe		 20 cm, 40 cm,
60 cm, 80 cm, 100 cm		 Minimum 2D (D is a
Radius of old pipe)
Table 4 Factors and Amount of Parameters
Difference of Soil Condition
Figure 9 is the result of parametric study under the various soil conditions. It is shown that Ο marks are meaning the case of original ground N=1, Δ marks for N=10, Θ marks for N-4 that is basic case. Other factors on these cases are existing pipe diameter 250mm, cover depth h=1.5m, radial expansion is 73 mm. The horizontal direction under the ground is the same level of an existing pipe, and the horizontal displacement is expressed with the ground movement at the points which is away the distance from an existing pipe. The vertical direction of the existing pipe is upper of the existing pipe, and the distance from an existing pipe shows distance between the point and the crown of the pipe.
Figure 10 is the alternative case of radial expansion 115mm. Even if the soil conditions are changed N-value as 1 to 10, it is seen that the difference is not so big. The case where the backfill ground is set with the same N-value as one of the original ground is also compared. If the backfill ground is set by N-value = 1, in the case of N-value of original ground =1, the horizontal displacement becomes bigger and ground movement of upper portion will decrease. On the contrary, if a original ground is set to N-value= 10, the horizontal movement becomes small and the upper movement becomes large. If N-value of backfill ground is set to 5, the displacement of horizontal direction seems to be small where N value of the surrounding original ground is higher.
Difference of Cover Depth
According to the parameter study about influence of pipe cover depth, horizontal displacement of the sub-surface have little influence of cover depth. On the other hand, the displacement of vertical direction when the cover depth is 1.0m becomes 20% larger than when the cover depth is 1.5m.
Difference of Pipe Diameter and Radial Expansion
The ground movement on the parametric study about the size of existing pipe diameter are shown on the figure 11. On the field testing the EXP-250 and EXP-400 were used, and the number following Exp- indicates the ability of the machine. That is, in case of new 250mm concrete pipe, EXP-250 is available normally. By our trenchless pipe replacement technology, 200mm to 600mm diameter pipes can be burst and installed. According to the theoretical consideration, the larger diameter pipe makes larger ground movement. Even if it is the same radial expansion, it is thought that expansion area around pipe, in case of bigger diameter pipe, is bigger. The ground movement becomes large as compared with the case of smaller diameter pipe. If an existing pipe diameter is 350 mm, radial expansion 115mm, ground movement at the point of 80 cm away from the pipe side is horizontally 10 mm and vertically 9 mm. The existing pipe diameter becomes 600 cm, in the case that other conditions are same, ground movement becomes horizontal 14.1 mm and vertical 15.1 mm. Multiple ratios are horizontal 1.41 and vertical 1.75.
Span Existing pipe Installed pipe Radial Expansion EXPANDIT
1 350 CP 400 CP 115 EXP-400
2 350 VP 400 CP 115 EXP-400
3 250 CP 250 CP 73 EXP-250
4 250 CP 250 CP 73 EXP-250
5 250 VP 250 VP 73 EXP-250
6 250 VP 400 CP 115 EXP-400
Table 5 Radial Expansion and Expandit used at Field Test
It is theoretically significant that the bigger radial expansion, the bigger ground movement. At the field testing, the amounts of radial expansion are 115 mm and 73 mm. According to numerical study, ground movements change following to the radial expansion. Table 5 shows amounts of radial expansions gotten on the combination of pipes and Expandit. Amount of radial expansion is calculated by inner diameter of existing pipe and size of Expandit diameter. Ground movements should be increasing depend on the amount of radial expansions.
Parametric Study Results
The parametric study indicates that the maximum sub-surface displacement is 20 mm at the points of 80 cm away from side-end and crown of the existing pipe. On the calculations, the other conditions of pipe bursting are in the following ranges.
N-value of backfill ground is 1 to 10.
N-value of original ground is 1 to 10.
Diameter of existing pipes is 200 mm to 600 mm.
Maximum radial expansion is 115 mm.
Minimum cover depth is 1.0 m.
In addition, the amount of ground displacement on each factor is as follows. Based on the measurement on the backfill ground, N-value 3 to 5, sub-surface displacements were calculated. N-value on the backfill ground is usually 1 to 10 because the pipelines were constructed by open cut method. As the result, the bigger N value the bigger the ground movement. However, there is no big difference between N-value 5 and 10. To a contrary, in the case that N-value is small, the amount of movement becomes small.
On the field testing, the ground movement in the case of pipe diameters 250 and 350 were measured. Our pipe replacing technology can be applied currently to pipe diameter 200 to 600 mm. The maximum pipe diameter was checked by calculation. Consequently, when the 250 mm diameter increases to 600 mm, the amount of ground movement becomes 1.6 to 2.0 times for horizontal direction and 1.4 to 1.7 times for vertical direction. On this case, pipe diameter ratio 600/250=2.4. In the case of increasing diameter from 350 to 600, the diameter ratio 600/350 = 1.7, the ground movement becomes 1.2 to 1.6 times. The ratio of ground displacement is about 80% of the ratio of pipe diameter.
On the field testing, in the cases of 115mm and 73mm radial expansions, the amount of ground movement was measured. Based on the calculation, ground movement changes by the radial expansion ratio.
According to the cover depth, 1.5 m and 2.0 depth were carried out on the field testing. The cases of 1.0 m and 4.0 m were calculated. Consequently, the amount of movement of a horizontal direction does not change, and then the amount of ground displacement could be increasing 20% when 1.5m cover depth becomes 1.0m.
The cases where the ground displacement becomes biggest in the scope of this technology are the following.
N-value of original and backfill ground is 10.
Existing pipe diameter is 600 mm.
Radial Expansion is 115 mm.
Cover depth is 1.0m.
On the above described condition, the ground displacement should not over 20mm at the point of 80 cm away from existing pipe. Figure 11 is an example of these calculated results, that is, the parametric study on radial expansions.
Estimation of Sub-surface Displacement
Horizontal movement of sub-surface can be estimated according to the ground movement estimation chart, Figure 13. In the figure, Ratio of Distance/Pipe Radius is the ratio of two distances, one is the distance between the point which is to be known about the ground movement and pipe side end and another is inner radius of the existing pipe. Range of N-value of the sub-surface is in the range of 3 to 5, such as backfilled sub-surface for the open cut trench.
Vertical movement of sub-surface can be estimated according to the ground movement estimation chart, Figure 14. Ratio D/R is the inner radius of the existing pipe to distance between the point and the pipe crown.
From the results obtained by measuring of the vertical and horizontal movement, the following simulation was performed. Ground conditions range N-value from 1 to 10. The original ground condition for the field trial was classified as N-value 4 or 5. By the open cut method the ground condition surrounding existing pipes should be sand. Installed pipe diameter ranges from 200mm to 600mm. The maximum expansion is 115mm, because if the 600mm pipeline is replaced to the same size of concrete pipe, the expansion should be 115mm. The cover depth ranges from 1.0m to 4.0m.
Application to Projects
We have obtained two measured data for the estimation curves after the described field test in Miyazaki Pref. One example could be as following. The drainage under railways was replaced by ‘Expandit System’. As the cross section is shown in Figure 16, replaced pipe is 15.6m long. The original pipe inner diameter was 460mm and installed pipe inner diameter was 600mm. In Figure 15, p-1 and p-4 are the subsurface points, and p-2 and p-3 are the rail elevation points.
Although the rail level would rise about 20mm according to our estimation prior to pipe bursting, the rail elevation did not rise at all. The displacement of p-1 that is 0.91 m away from the top of pipe was 21mm, and the displacement of p-4 that is 1.97 m away was 2mm. It can be said that these subsurface displacements are very similar to estimation.
From the estimation curves and the measured data of project, Table 6 was obtained. At the P-1 point, sub-surface heaving at 91cm upper from the pipe crown was maximum 21mm. According to the estimation curves, ground movement is 38mm.
point Distance
from Pipe		 Radial
Expansion		 radius D/R Sub-surface
movement		 Estimation
P-1 91cm 150mm 23cm 4.0 21mm 38mm
P-4 197cm 150mm 23cm 8.6 2mm Out of Range
Table 6 Comparisons of Measured Data and Estimation
Conclusions
Conclusions are as follows.
Numerical study presented in this paper shows a good capability to estimate ground displacement by pipe bursting technology. The followings are main findings from the field trials and the numerical study.
  • The field trial has provided valuable information on the movement of the ground during pipe bursting operations.
  • Estimation Charts of ground movement could be used for estimating the ground movement of some project on safety side.
  • In order to make the charts certificated, it is better to get more data of ground movement.
It should be noted that this paper refers to the results of one type of soil condition and it is evident from the previous studies that the type of soil will result in different ground displacement characteristics. It is important to test many different types of soil, at least sandy or clayey, in order to establish general safe proximity guides. Due to the requirement for over-burst, i.e. expansion of a void that has a greater diameter than the new pipeline being installed, the magnitude of the typical outward pattern of displacements caused by pipe bursting in the longer term is much smaller than the temporary displacements that occur while the process were being carried out. However, ‘Up-sizing’ operations become as common practice, in which the new pipeline has a significantly greater diameter than the existing pipeline that is being replaced. The ground movements that occur during construction will be larger still and generally provide the worst case for design. The pattern of movement is complicated by the possibility of residual ground settlements if the works carried out in loose granular soils or in soft cohesive deposits in which positive pore water pressures are generated. Therefore significant geotechnical knowledge is required in planning the safety operation of pipe bursting. The authors are currently using the information collected from the field trial to develop numerical and analytical estimation techniques to allow wider application of the information. The information appears to suggest that the current safe proximity charts for pipe bursting operations.
Acknowledgements
The authors would like to acknowledge to many people offered the excellent supports throughout this field testing and numerical study. Particular thanks to the organization and technical members of SPRSE (The Society of Pipe Replacement System by Expandit).
References
[1] Hirai M., Yamaoka R., Cho M., Sato T., The Field Test of pipe bursting and replacement system by the ‘Expandit’ and the ground movement, International No Dig Conference, Brisbane 2006.


1 University of kyushu, Japan
2 Obayashi-Road Corp., Tokyo, Japan

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