Abstract

The disturbance of Shanghai silty clay during earth pressure balance (EPB) tunnelling has been studied through field monitoring, field measurement and laboratory test. The soil disturbance during tunnelling consists of two parts: stress disturbance, which is the change of effective stress; and strain disturbance, which is caused by the soil movement. The definition of stress disturbance degree of Shanghai silty clay is given by the change in the in situ effective stress before and just after tunnelling at the same site. According to the changes in the static cone penetrometer resistance, the extent of stress disturbance in a transverse section is determined. The relationships between the mechanical properties and stress disturbance degree are also studied.

Author Keywords: Soil disturbance; Earth pressure balance (EPB) tunnelling; Stress disturbance; Stress disturbance degree; Mechanical property; Shanghai silty clay

Article Outline

1. Introduction

2. Engineering background

3. In situ instrumentation

4. Stress disturbance during tunnelling

4.1. Stress disturbance degree

4.2. Extent of stress disturbance

5. Variation in soil mechanical properties during tunnelling

5.1. Changes in undrained shear strength due to excess pore water pressure

5.2. Changes in undrained shear strength and deformation modulus due to stress disturbance

6. Conclusions

Acknowledgements

References

1. Introduction

With an increasing need for underground development in China, EPB tunnelling has become a major construction method, especially in urban areas. It is essential to protect the pre-existing structures and underground works from damage during tunnelling in urban areas.

Most researchers to date have focused on the assessment of ground surface settlement above the tunnel and its effect on the settlement, in particular the differential settlement, of adjacent buildings ([Mair et al., 1996]). It may be appropriate for buildings supported on shallow foundations to study the effect of the ground surface settlement above the tunnel on settlement of adjacent buildings. However, knowledge of only surface settlement may be inadequate for buildings that are supported on pile foundations ( [Chen et al., 1999]). For pile foundations, soil disturbance during tunnelling may play an important role in influencing the response of pile supporting structures ( [Mair and Taylor, 1998]).

To date, extensive research work on soil disturbance during EPB tunnelling appears to be sparse. Researches on soil disturbance due to sampling have published many results ([Ladd and Lambe, 1963, Nakase et al., 1985, Baligh et al., 1987, Clayton and Siddique, 1998, Shogaki and Kaneko, 1994 and Carrubba, 2000]). Soil disturbance during EPB tunnelling is similar to sampling disturbance ( [Xu and Sun, 1999]). Soil disturbance during EPB tunnelling is comprised of the change in effective stress (stress disturbance) and the soil movement (strain disturbance). [Rowe and Lee, 1993] have studied soil disturbance due to the change in pore water pressure and deformation induced by an earth balance shield tunnelling.

Recently, civil engineering constructions, especial in the underground works construction have been developing rapidly in China. Many cities in China are planning to begin the construction of metro projects. Shanghai Bund Sightseeing Tunnel has been constructed after and beneath the Shanghai Metro Tunnel-Line 2. As shown in Fig. 1, the Shanghai Bund Sightseeing Tunnel passes beneath the Bund scenic area and passes in the vicinity of the Shanghai Orient Pearl Television Tower, which is the highest tower in Asia. In order to evaluate the vulnerability of adjacent buildings and underground works, stress disturbance of Shanghai silty clay during EPB tunnelling of the Shanghai Bund Sightseeing Tunnel has been studied. The degree of stress disturbance and the induced changes in mechanical properties of Shanghai silty clay are discussed.

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Fig. 1. Plane figure of the Shanghai Bund Sightseeing Tunnel.

2. Engineering background

The construction of the Shanghai Bund Sightseeing Tunnel was started in March 1998 and finished in May 1999. The Shanghai Bund Sightseeing Tunnel trends from Chenyi's Square on the west near Nanjing West Road in Puxi beneath the Huangpu River to the Shanghai International Conference Centre in Pudong (Fig. 1). It was excavated by an EPB tunnel boring machine (TBM). The Shanghai Bund Sightseeing Tunnel consists of two straight lines at both ends and one curve line in the middle section, and its length is approximately 400 m. The longitudinal gradient of this tunnel is up to 4.8%. The thickness of the overburden soil in the middle of the Huangpu River is approximately 5.2–7.0 m.

For a detailed design of the Shanghai Bund Sightseeing Tunnel, a comprehensive geotechnical investigation was completed, which consisted of undisturbed sampling, cone penetration tests, in situ vane shear tests and pressuremeter tests. The soils in the upper 30–40 m are very soft silty clays deposited during the Holocene period (approx. 15 000 to 20 000 years ago) as shallow sea and Yangtze deltaic sediments. The underlying fine sand and silty clay strata were deposited in the Pleistocene period. According to the site investigation results, Shanghai soils can be differentiated into a number of well defined strata based on physical properties and soil types. The soil properties of Shanghai soil strata near instrumentation section 1 (Fig. 1) are summarized in Table 1. The Shanghai Bund Sightseeing Tunnel is situated within highly compressible and very soft silty clay deposits layer 4 and layer 5-1.

Table 1. Physico-mechanical properties of Shanghai soil

/ Means no data.

The Shanghai Bund Sightseeing Tunnel was constructed with a precast concrete lining, which is composed of six segmental concrete pieces connected by steel bolts in the circumferential and longitudinal directions. The outer diameter of the tunnel lining is 7.48 m, and the thickness of the lining is 0.36 m. The length of each lining section is 1.2 m.

The EPB machine is a second-hand apparatus from France, and re-equipped by the machine factory of Shanghai Tunnel Engineering Co. Ltd. The shield body is 7.65 m in outside diameter and 8.935 m in length. The corresponding physical gap (G_p), defined by [Lee et al., 1992], was 170 mm. Pressurized grout was injected into the tail void synchronized with the pushing action of the shield. Segmental concrete lining sections were installed at the back of the shield with the lining erector. The time required for each excavation cycle was on average approximately 1 h/m of tunnel advancement. Two 12-h shifts were adopted in the project. The normal tunnel advancement rate was approximately 10 m/day.

3. In situ instrumentation

In order to study the soil disturbance and ground responses in close proximity to the tunnel, an instrumentation array of section 1 was carefully designed and installed around the approaching tunnel. Items monitored include surface and subsurface ground movements, pore water pressure and earth pressure. The detailed layout of the instrumentation is described as follows.

•Surface and subsurface settlement markers were installed along the alignment every 2 m over the centreline and in several cross-sections (including sections 1, 2, 3 and 4 in Fig. 1) for measurement of the ground surface settlement ( Fig. 1). An auto-level was employed to monitor the surface movements. Initial readings were taken when the EPB shield was at a distance of at least five times the tunnel diameter before reaching the monitoring points.

•Three extensometer casings were installed around the tunnel as shown in Fig. 2. The interval of magnetic rings in each casing was 2 m. The movements at the top of each casing were also carefully monitored so that the magnetic ring readings can be calculated by combining these two sets of readings.

•The extensometer casings were also used as inclinometer casings. Thus, inclinometer readings in the directions along the centreline and perpendicular to the tunnel alignment were taken in the casings as shown in Fig. 2.

•Three vibrating-wire piezometers (w₁, w₂ and w₃) for pore water pressure measuring and three earth pressure cells (p₁, p₂ and p₃) for earth pressure measuring were installed in the vicinity of the tunnel (section 1 in Fig. 1) at the depth of 11 m as shown in Fig. 2.

•Three static cone penetration tests and six vane shear tests were performed around the tunnel before and after the passing of TBM.

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Fig. 2. Layout of instrumentation in section 1.

The instantaneous stress disturbance of Shanghai silty clay in close proximity to the tunnel during EPB tunnelling is mainly studied in this paper.

4. Stress disturbance during tunnelling

4.1. Stress disturbance degree

Stress disturbance consists of the changes in pore water pressures and total stresses. The changes in pore water pressures and total stresses are shown in Fig. 3 for the Shanghai silty clay. The buried depth of the monitoring points is approximately 11 m. Symbols p₁, p₂ and p₃, and w₁, w₂ and w₃, drawn in Fig. 2, are the locations where earth pressure and water pressure were measured. It is seen that the pore water pressures and total stresses increase with the decrease in distance between the monitoring points and the tunnel axial line in transverse section from Fig. 3. The pore water pressure and earth pressure increase before the shield cutter face reaches the measured section, and decrease after the shield cutter head passes through the measured section. The increases and decreases in earth pressure and water pressure synchronize. The maximum changes in earth pressure and water pressure occur at points of p₃ and w₃, and are 32 and 25 kPa, respectively.

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Fig. 3. Stress change during shield tunnelling.

The stress disturbance degree (SDD) is defined as:

(1)

σ′d=σ′0−uw

(2a)

σ′0=γshs−γwhw

(2b)

4.2. Extent of stress disturbance

Change in peak undrained shear strength occurs due to the stress disturbance in tunnelling ([Baligh et al., 1987 and Xu and Sun, 1999]). The extent of the stress disturbance during tunnelling can be determined by the changes in the peak undrained shear strength of soils before and just after tunnelling. The undrained shear strength can be measured using the field static cone penetration tests (SCP). Thus, the extent of stress disturbance during tunnelling can be determined from the results of SCP tests before and just after tunnelling.

The typical results of static cone-penetration tests are shown in Fig. 4. The locations of the SCP tests are shown in Fig. 2 (i.e. AA, BB and CC line). It is seen from Fig. 4 that the static point resistance decreases during tunnelling. In Fig. 4, p_sd is the static point resistance after tunnelling, and p_s0 is the static point resistance before tunnelling. According to the measured results shown in Fig. 4, the changes of static point resistance occurs at the depth of 2 m in AA line, at 4.3 m in BB line and at 6.5 m in CC line in section 1. At the depth of 13 m in line CC, the measured static point resistance just after tunnelling is nearly equal to that before tunnelling and keeps constant. The boundary between disturbed and undisturbed areas is the outline through those points, where no change in the measured static point resistance occurs. The stress disturbance range is depicted in Fig. 5.

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Fig. 4. Results of static cone-penetration tests in situ.

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Fig. 5. Extent of stress disturbance during tunnelling obtained from the results of the static cone-penetration tests and the vane shear tests.

The results of vane shear tests measured before and just after tunnelling are shown in Table 2. The testing points are marked in Fig. 2 (e.g. A-1, A-2, etc.). It can be seen that there are large decreases in the peak unconfined compression strength (q_u0) before and just after tunnelling, while the residual unconfined compression strength measured before tunnelling (q_ur0) is nearly equal to that measured just after tunnelling (q_urd). The change in the peak unconfined compression strength at point C-1 during tunnelling is very little, which is consistent with the fact that point C-1 is at the edge of the stress disturbed region determined by the results of SCP tests. The largest change in the measured unconfined compression strength occurs at point A-2, located in the centre of the stress disturbance region. The ratio of (q_u0−q_ud)/q_u0 is shown in Fig. 5 at the related locations, here q_u0 and q_ud are the peak unconfined compression strength before and just after tunnelling. The larger is the stress disturbance degree (SDD) in the measured positions where the larger change occurs in unconfined compression strength, and where the larger is the (q_u0−q_ud)/q_u0. The ratio of (q_u0−q_ud)/q_u0 gives an expression to the stress disturbance degree during tunnelling.

Table 2. Results of vane shear tests before and just after tunnelling

5. Variation in soil mechanical properties during tunnelling

In order to study the influence of the stress disturbance on strength and modulus of deformation during tunnelling, many soil samples of Shanghai silty clay were collected from the layer 4 (Table 1) using thin-wall samplers. The length of the sampler was 50 cm, the wall thickness was 1.5 mm and the inside diameter was 60 mm.

The test programs were designed as follows (Fig. 6). K₀-consolidation tests were performed to eliminate the effects of stress disturbance. The undrained shear strengths and the initial modulus of deformation were obtained from triaxial compression tests on the K₀-consolidated samples.

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Fig. 6. Laboratory test programs to study stress disturbance during tunnelling.

5.1. Changes in undrained shear strength due to excess pore water pressure

It is assumed that the parameters of shear strength expressed by effective stress are constant and are independent of excess pore water pressure. According to the Mohr–Coulomb failure criterion, the relationship of two effective principal stresses is written as:

(3)

(4)

σ3f″=σ3f′−uw

(5)

(6)

If the clay microstructure will be damaged and cohesion will be reduced during tunnelling, the cohesion of the disturbed soil with damaged microstructure is nearly equal to the effective residual cohesion (c_r′). Hence Eq. (6) can be written as follows:

(7)

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Fig. 7. Undained shear strength vs. excess pore pressure measured in situ by the vane shear tests.

5.2. Changes in undrained shear strength and deformation modulus due to stress disturbance

In order to study the effect of stress disturbance on mechanical properties of Shanghai silty clay, the stress state in situ was simulated by K₀-consolidation tests in the laboratory. If the vertical effective stress in situ is σ_v0′ and the vertical effective stress in the K₀-consolidation test is σ_vd′, the stress disturbance degree (SDD) is calculated from Eq. (1).

The relationships between stress disturbance degree (SDD) and the initial tangent modulus of deformation, the undrained shear strength and the axial strain at the peak deviator stresses of Shanghai silty clay are shown in Fig. 8, Fig. 9 and Fig. 10, respectively. From Fig. 8, it is seen that the initial tangent modulus of deformation decreases with increasing the stress disturbance degree (SDD) and a power function relationship exists between the stress disturbance degree (SDD) and initial tangent modulus, i.e.

(8)

(9)

(10)

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Fig. 8. Measured relationships between stress disturbance degree (SDD) and initial tangent modulus of deformation in the laboratory, the solid dots are experimental data and the solid line is the regressing line.

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Fig. 9. Measured relationship between the stress disturbance degree (SDD) and undrained shear strength in the laboratory, the solid line is the regressing line.

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Fig. 10. Measured relationship between the stress disturbance degree (SDD) and axial strain at peak deviator in the laboratory, the solid dots are experimental data and the solid line is the regressing line.

The changes in unconfined (undrained) shear strength, initial tangent modulus and axial strain at peak deviator stress are induced by the changes in the effective stress, which occurs in the EPB tunnelling. The changes in effective stress are due to the soil disturbance in tunnelling. The soil disturbance in tunnelling is the basic reason for the changes in soil mechanic properties. Controls in tunnelling process and compensation grouting are the main means to limit the changes in soil mechanical properties. Controls in tunnelling processes can well diminish the soil disturbance induced by tunnelling. Controls in tunnelling processes mainly consist of the controls in soil pressure at face, controls in driving speed and rate of discharge and controls in shield pose. Compensation grouting will improve the disturbed soils and diminish the strain disturbance and can therefore effectively limit the changes in soil mechanic properties.

6. Conclusions

The definition of the stress disturbance degree (SDD) during EPB tunnelling is presented based on the variation in the in situ effective stresses before and after tunnelling. In the Shanghai Bund Sightseeing Tunnel, the stress disturbance degree (SDD) spans from 0.197 to 0.274 at points located side distance from 3R/2 to 7R/2 (R is the tunnel radius), respectively, from the tunnel centre, when the maximum change in pore water pressure occurs. The static cone penetrometer resistance before and after tunnelling decreases with the stress disturbance degree. According to the depths of changes in the static cone penetrometer resistance measured around the tunnel, the extent of stress disturbance in close proximity to the Shanghai Bund Sightseeing Tunnel was obtained.

Stress disturbance during EPB tunnelling causes variation in the mechanical properties of soils around the tunnel. According to experimental results, the changes in the undrained shear strength, initial tangent modulus of deformation and axial strain at peak deviator stress occur to disturbed soils.

1The undrained shear strength decreases with the increase in pore water pressure. The proposed method to predict the effect of excess pore water pressure on the undrained shear strength is examined by the measurement of the vane shear test in situ.

2The initial tangent modulus of deformation, the undrained shear strength and the axial strain at the peak deviator stress vary with the stress disturbance degree. For Shanghai silty clay, power functions exist between the stress disturbance degree (SDD) and the initial tangent modulus of deformation, the undrained shear strength and hyperbola relation exists between the stress disturbance degree (SDD) and the axial strain at peak deviator stress.

The proposed stress disturbance degree (SDD) is capable of evaluating the variations in soil mechanical behaviour during EPB tunnelling, and is a potential index to access the influence of the EPB tunnelling on the closely existing adjacent structure.

Controls in tunnelling process and compensation grouting can effectively limit the changes in unconfined (undrained) shear strength, initial tangent modulus and axial strain at peak deviator stress due to EPB tunnelling.

Acknowledgements

The National Nature Fund of China (Grant No. 59716038 and No. 40201024) and the Chinese Post-doctor Science Fund are acknowledged for their financial support. The authors also wish to express their gratitude to Prof. Hajime Matsuoka of Nagoya Institute of Technology, for his kind help and valuable input to this paper.

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