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International Symposium on Underground Excavation and Tunnelling2-4 February 2006, Bangkok, Thailand

Characteristics of Ground Movements from Shield Tunnelling ofthe First Bangkok Subway Line

Noppadol Phienwej 1, Attasit Sirivachiraporn 1, Sahapol Timpong 1, Sippavut Tavaranum 1,Suchatvee Suwansawat 2

1 School of Engineering and Technology, Asian Institute of Technology, Pathumthani, Thailand.2 Dept. of Civil Engineering, King Mongkut Institute of Technology-Lad Krabang, Bangkok, Thailand

ABSTRACT

The contracts for construction of the first Bangkok MRT Subway line called for the design to avoiddamages to existing buildings, thus instrumentation for monitoring of ground movements induced bythe excavation was an important component of the construction works. The ground and buildingmovement data associated with the EPBS bored tunneling are interpreted and the findings are presentedin the paper. Comparisons were made between the observed ground movements and predictions given

by well known empirical and analytical methods. The magnitude of building settlement was interpretedwith respect to building height (depth of piled foundation) and distance from tunnel.

1. BANGKOK MRT PROJECT

The Bangkok MRT Initial System Project (ISP), officially named Chaloem Ratchamongkhol Line, isthe first underground MRT of Bangkok. This project was constructed along traffic congested roads inthe inner city areas. The line which is 22 km long has been constructed as twin bored single-tracktunnels with 18 stations by the cut and cover method. Construction of the underground structures in the

project was implemented in two contracts, the South Contract and the North Contract, each with aboutequal scope of work.

Each of the tunnel tube is 6.3 m in outer diameter and lined with concrete segmental lining of 5.7m in inner diameter. The tunnels were bored by 8 EPB shields (6 Kawasaki machines and twoHerrenknecht machines). They were placed at depths between 8 and 25 m below the ground surface.Bangkok subsoil condition is relatively uniform through out the city area, of which the first 25-30 mthick soils comprise of a layer of soft marine clay (12-15 m thick) underlain by the first stiff clay layerand the medium dense sand layer. Although most of the tunnel alignment is placed in the stiff clay

layer, some shallow sections have roof in the overlying soft clay and some of the deep alignment wasexcavated in the underlying sand layer. Groundwater level in the sand layer was at 22-24 m depth below the surface. General layout of the twin tunnels is in side by side configuration (clear distance between tunnel walls mostly of 1.0-1.5 times the excavated tunnel diameter (5.0-9.0 m) and theminimum as close as 2.6 m). The vertical stacked configuration was adopted for the first 4.5 km of thealignment due to obstruction from foundation of viaducts over the roadway and water tunnels.

In many sections of the alignment, the tunnels were bored very close to the piled foundations of buildings, bridges and utilities lines. Thus control of ground movement and limiting damages toexisting nearby structures is the key for the success for the development of the project. In the designand construction planning, prediction of the ground movement characteristics, building conditionsurvey, and potential damages of buildings located within ground movement influence zone werecarried out prior to the start of the excavation work. A comprehensive instrumentation program formonitoring of ground movements and responses of concerned structures was carried out as an important

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part of the contract works. Over 5,000 instrumentations points were installed along the tunnel route.(1,276 surface settlement points, 39 settlement arrays, 36 subsurface instrument sets and 339 buildingswith settlement monitoring points)

The recorded instrumentation data served as a means to understand the ground movement behaviorinduced by the EPB tunneling in the stratified Bangkok soils and as indices for construction control andimplementation of remedial measured when needed. Although there have been previous studies on thecharacteristics of ground movements associated with shield tunneling in Bangkok soil (e.g. Phienwej,1998), the data from the MRT ISP were comprehensive and of the largest sized bored tunneling withEPB shields ever made in Bangkok. The findings on the characteristics of the ground movements and

building responses in the projects are reported herein.

2. GROUND MOVEMENTS

2.1 Surface settlement

Monitoring of ground surface settlements over the tunnel alignment at 50 m intervals showed that themaximum settlements were in the range of 5 mm to 120 mm, mostly between 20 mm and 60 mm (Fig.

1). The largest magnitudes were recorded in the early leg of tunneling of the North Contract and in asarea of the vertically-stacked tunneling in South Contract, in both of which the tubes were bored inlayers of sand with ground water.

-180.0

-160.0

-140.0

-120.0

-100.0

-80.0

-60.0

-40.0

-20.0

0.0

10 .000 12.000 14.000 16 .000 18.000 20.000 22 .000 24.000 26 .000 28 .000 30.000

Station, Km

S e

t t l e m

e n t , m m

Side-by-Side TunnelsStacked Tunnels

Figure 1 Maximum surface settlement along the subway alignment

Figure 1a Relationship between maximum surface settlement and shield face pressure

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Studies were made to examine influencing factors controlling the magnitude of the groundmovement in EPB shield tunneling in the Bangkok MRT project (Suwansawat, 2002, Timpong, 2002,Tavaranun, 2003, etc). It was found that the most important factor seems to be the applied face pressurein the front chamber of the shield machine. The face pressure played an important role in maintainingstability of the excavation and minimizing ground movement. Figure 1a shows the plot between theobserved maximum surface settlement and the face pressure in the Project. The plot clearly shows theinfluence of the applied face pressure on the magnitude of ground settlement, in which larger settlementoccurred when lower face pressure was used in the shield excavation. In case of a very high face

pressure, heave occurred at the ground surface. On the other hand, when the shields were operated atvery low face pressures (


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Out of the 39 arrays of settlement trough monitoring, 19 arrays showed that the observed troughscould be reasonably fitted with the Gaussian curve (Peck, 1969) as shown in Figures 2 and 3. Thecalculated tunnel ground loss from the observed settlement troughs was mostly in the range of 0.5-2.5%. In difficult sections, the ground loss was up to 3.5%.

Settlement troughs of which shapes did not follow the Gaussian function were influenced byfoundation of adjacent foundations and flyovers. The existing piled foundations within the green-fieldtrough zone obstructed the ground movement and caused an unsymmetrical trough shape. The typicalshape in such a case is shown in Figure 4. Piled foundation acted as a barrier to confine the groundmovement, resulting in larger ground settlements in that side of the tunnel.

Zone 26Ratchada - Lat Phrao

Ground settlement array no. 26-AR-001

S o

f t c

l a y

S t i f f c

l a y

S a n

d

-20 m.

10.5 m.

NB SB

Figure 4. Effect of piled foundation on settlement trough

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00i/R

Z / 2 R

Single tunnelTwin Tunnel

Soft to Stiff clay

Rock, Hard Clays,Sand abov egroundwater lev el

Sand belowgroundwater lev el

Figure 5. Relation between Z/2R with i/R plotted in Peck(1969)s curves

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Maximum surface settlement occurred above the centerline of first tunnel after the shield passagethrough the section and normally the Gaussian shaped trough shape was observed. For the side-by-sidetwin tunnel case, once the excavation of the second tunnel was made at the section, the trough expandedand shifted toward the second tunnel. However the center of the trough was normally skewed towardthe first excavated tunnel.

The observed behavior agrees with that reported by New and Bowers (1993). In such a case, thesettlement trough of the twin tunnel is described by the shifted Gaussian function

2

max 2

( )exp

2 x a

i

=

where = surface settlement at transverse distance x from the centerline of twin tunnelsmax = maximum settlement (at x = 0)

x = horizontal distance from the centerline of twin tunnelsi = the distance from the centerline of twin tunnels to the point of inflectiona = shifted distance from twin tunnels centerline

For the Bangkok MRT, the offset parameter (a) observed on 24 arrays ranged from 0.03-0.58 ofthe distance between the two tunnel centerlines ( d ). The mean a/d is 0.16, The width of the settlementtrough observed after the excavation of the twin tunnels in the project was about 6 to 12 m for thevertically-stacked tunnels and 10 to 17 m for the side-by-side twin tunnels.

The settlement trough width data were analyzed according to Peck (1969)s method of prediction.The plot of i/R versus Z/2R for the single tunnel case is shown in Figure 5. The trough width parameter(i) was obtained by fitting the observed surface settlement data with the Gaussian function. As can beseen, most of the data points fall within the zone of sand below ground water level rather than soft tostiff clay zone. This implies that for tunneling in Bangkok subsoil with EPB shields, the settlementtrough width tends to be wider than the suggested value by Peck (1969)s empirical method.

0

5

10

15

20

25

30

35

40

0 5 10 15 20

Trough width parameter, i (m)

D e p

t h , Z

( m )

Single tunnel

Twin tunnel i = 0.3Z i = 0.4Z i = 0.5Zi = 0.6Z

Figure 6. Observed settlement trough width parameters after single tunnel compared withthe parameter recommended by O'Reilly and New (1982)

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A plot is made between relationship between the settlement trough width parameter ( i) and tunneldepth ( z), Figure 6. Most of the data points fall within the range of i = 0.5Z to 0.6Z for tunnels locatedin both the stiff clay and soft clay layers and i < 0.4Z for tunnels in sand layer. This is in agreementwith the observation of OReilly and New (1982).

2.2 Subsurface ground movement

Monitoring of subsurface ground movements was made by means of borehole extensometers andinclinometers installed at a number of sections along the tunnel alignment. The layouts of theinstruments in the two contracts are shown in Figure 7.

Extensometer

Tunnel

Soft Clay

Stiff Clay

Extensometer

Tunnel

Soft Clay

Stiff Clay3 - 6 m.

Extensometer installation inSouth contract

Extensometer installation inNorth contract

Inclinometer casing

Inclinometer casing

Figure 7. Subsurface Instrumentation for bored tunneling

0

-5

-1 0

-1 5

-2 0

-2 5

-3 0

-3 50 2 0-2 0-4 0

0 2 0-2 0-4 0

Sof t c lay

Stiff clay

Very s t i ff c lay

D e n s e s a n d- 2 5 m .

- 1 5 m .

St i ff clay- 2 0 m .

Soft clay

Ver y s t if f clay

S u b s u r fa c e s e t t le m e n tI n s tr u m e n t R E - 5 T- 7 ( B o n K a i - S ir ik i t)

D e p

t h ( m )

S u b s u r fa c e s e t tl e m e n t ( m m )

Figure 8. Subsurface settlement above tunnel centerline

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1) Subsurface settlement

For area above the tunnel centerline, the magnitude of subsurface settlement after the tunnel wasexcavated generally increased with depth toward the tunnel roof, as shown in Figure 8. For thesubsurface ground settlement to the side of the tunnel wall, it was generally observed that the largestmagnitude of the settlement usually occurred in the soft clay layer at location near the interface with theunderlying stiff clay (Figure 9). The settlement in the stiff clay along that vertical plane decreased withdepth as the point falls further from the movement cone. At some sections, a small heave was measured

below the tunnel level.

0

-5

-10

-15

-20

-25

-30

-35

Soft clay

Stiff clay

Dense sand

Subsurface settlementInstrument 23-IEX-001 (Thiam Ruam Mit - Pracharat)

020 -20 -40 -60

020 -20 -40 -60

-23 m.

-15 m.

Stiff clay-17 m.

Soft clay

Dense sand

3.42 m.

Subsurface settlement (mm)

D e p

t h ( m )

Measurement

Loganathan &Poulos (1998)

Figure 9. Subsurface settlement to the side of tunnel wall

Extenso me ter Monitoring R esultNo. 25 -IE-003 Sutthisan - Ra tchada

-50

-40

-30

-20

-10

0

10

20

30

0 20 40 60 80 100 120 140

Elapsed time (days)

S e t

t l e m e n

t ( m m

)

Shield passing-18.5 m.

25-ME -3/08 -2.02 m.

-15 m.

Soft clay

Stiff clay

25-ME -3/07 -5.15 m.

25-ME -3/06 -8.32 m.

25-ME-3/05 -11.45 m.

25-ME-3/04 -14.58 m.

25-ME-3/03 -17.71 m.

6.24 m.

Figure 10. Changing of subsurface settlement with time

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Long term ground settlement behavior for the EPB shield tunnel excavation was examined. Thesubsurface settlement data showed that after the shield passage, significant increase in the settlement inlong term could only be observed in the soft clay layer. This was probably attributed to the effect ofconsolidation of the soft clay brought about by the effect of the shield tunneling. In some case theamount of the increase in 3-4 months was almost 100 percent of the short term settlement (Figure 10).However, the long term increase in the settlement in the stiff clay layer near the tunnel roof wasgenerally not significant.

2) Lateral displacement

The lateral displacements of ground beside the tunnel were observed at 12 locations along thealignment. The instruments were placed at distance of 0.20-2.6 m from the tunnel wall. Most of themeasurements showed a similar pattern of the lateral ground movement which can be divided into twodifferent depth zones, i.e. above and below the level of the tunnel arch point (Figure 11).

Above the tunnel roof, the ground deformed laterally towards the tunnel with the maximumdisplacement occurring in the lower area of the soft clay layer. The inward movement within this depthzone continued to increase after the passage of the shield tail.

0

-5

-10

-15

-20

-25

0-2-4 2 4 6Lateral Deformation (mm)

D e p

t h ( m )

Made ground

Soft clay

Stiff clay

Clayey sand

Very stiff clay

Zone 1 Zone 2 Zone 3

C.L. = 18.8 m2.35 m from the inclinometer

Avg. face pressure = 100 kPaAvg. grouting pressure = 2.5 bar

A. 2D before the shield faceB. 1D before the shield faceC. at the shield faceD. 0.5D behide the shield face

E. at the shield tailF. 2D behide the shield faceG. 3D behide the shield faceH. 4D behide the shield face

Inclinometer No. IN-T7-04(Phetchaburi - Sukhumvit stations

Figure 11. Lateral displacements (After Suwansawat, 2002)

In the zone below the tunnel roof level, two types of lateral ground displacement were observed.Generally, inward movement toward the tunnel was observed as can be seen in Figure 12. Themagnitude was smaller than those occurring above the tunnel roof. However, at few sections wherehigh face pressure was used in the shield operation, the ground in this depth zone experienced outwardmovements during the passage of the shield as shown in Fig 11. The movement ceased after the passageof the shield tail. This clearly indicates the effect of the applied face pressure in the EPB shield

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tunneling in Bangkok soils as reported by Suwansawat (2002) who presented a correlation between theoutward lateral ground movement and the EPB face pressure observed for the project.

Figure 12. Comparison between lateral displacements with predictions

Studies were made to investigate applicable methods for prediction of ground movements inducedfrom shield tunneling in Bangkok soils. Both the empirical approach (e.g. Verruijt and Booker, 1996;Loganathan and Poulos, 1998; etc.) and the numerical approach (e.g. FEM PLAXIS, FDA FLAC, etc.)were investigated (Tavaranum, 2003). On the empirical approach, it was found that the method ofLoganathan and Poulos (1998) method generally can give reasonable fits to subsurface settlement

pattern with field data for both single and twin tunnels cases (8 out of 12 sections). The gap parameter,g, from back calculation was in the range of 20-60 mm (Timpong, 2002). Reasonable prediction wasalso offered by the Verruijt and Booker (1996)s method.

However, both of the empirical methods could not offer reasonable prediction of lateral groundmovement from the EPB shield tunneling in case of excavation with a high face pressure where

outward movement of ground from shield face occurred (6 out of 12 sections).

3. BUILDING RESPONSE

Along the alignment, there were 339 buildings and structures located in the zone of influence. Thesestructures were subjected to a detailed building condition survey. Instruments were installed to monitorany movements and cracks induced during and after the passages of the shields.

With the existence of the soft clay layer in the entire Bangkok area, buildings and other structuresconstructed in Bangkok need to be founded on piles. Different length and size of piles have been useddepending on the size of the building and the period of their construction. Generally, the depths of piletips of the buildings can be conceived as shown in Figure 13. Five categories of buildings based on thedepth of foundation piles were made in the project for consideration of potential damage from shieldtunneling in the project.

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- Type 1: Old buildings with expected pile length not exceeding 6 m.- Type 2: Buildings and shophouses, 2 to 3 storeys with expected pile lengths of

between 6 m and 14 m.- Type 3: Buildings , 4 to 10 storeys with expected pile lengths of between 15 m and

22 m- Type 4: Modern buildings exceeding 10 storeys or other structures with pile lengths

in excess of 22 m.

Made ground

Soft clay

1st Stiff clay

1st Sand deposit

0.00 m.-1.5 m.

-15 m.

-22 m.

-40 m.

14 to 30 m.

2 to 3 storeybuilding &

shophouseOld buildingand house

4 to 10 storeybuilding

> 10 storeybuilding

Pile length > 22 m

15 to 22 m

6 to 14 m

< 6 m

Figure 13. Expected pile length of buildings in Bangkok

The monitoring data of all the instrumented buildings (mostly reinforced concrete frame types)were evaluated and the broad picture of the building response to the tunnel excavation could bedepicted by the summary plots of building settlements versus number of storey of the building anddistance from the tunnel excavation (Figure 14 and 15).

It clearly shows that buildings on deep piles were less influenced by the tunnel excavation (Types4 Buildings: settlements < 10 mm). For smaller buildings (Types 2&3), the settlement could be larger(up to 30 mm). For buildings on short piles (Type 1), settlement could be excessive. Fig 14 shows thatsignificant building settlement (> 20 mm) only recorded for buildings located within distance of 30 mfrom the tunnel centerline.

The measured differential settlements of the buildings were calculated and expressed in term ofsettlement ratio or tilt (differential settlement/distance between the two reference points)., The observedtilts of these buildings were mostly smaller than 1:1000 (0.001) which was the alert level set forth forthe control of work in the Bangkok MRT project. According to Burland (1995) a (masonry) buildingexperiencing a maximum tilt of 1:500 (0.002) and a settlement of less than 10 mm has negligible risk ofany damage. In fact, very few cases of damages of the existing buildings due to shield tunnelingoccurred in the project. These were limited to old 1-3 storey buildings founded on short piles (probablywith tips within soft clay layers). All of the tall buildings were not damaged by the tunnel excavationeven though the tunnel was excavated right next to the piles. A study is currently made to evaluate theinfluence of the shield tunneling on the load carrying capacity and integrity of the piles.

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Figure 14. Relation between building settlement and distance of building from tunnel

Building settlements vs distance from tunnel

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80

distance from tunnel (m.)

b u

i l d i n g s e

t t l e m e n

t ( m m

)

Buildings on foundation Type 1

Type 2 Type 3 Type 4

Figure 15. Relation between building settlement and number of storey of building

Building settlement vs No. of storey

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60

No. of storey

b u

i l d i n g s e

t t l e m e n

t ( m m

)

4. CONCLUSIONS

- Maximum ground surface settlement above the tunnels ranged from 5 mm to 120 mm, mostly 20-60 mm. The corresponding volume loss in the tunnel excavation was 0.4-2.50%. Tunneling inwater bearing sand layer tended to give the largest ground movement.

- Most of the surface settlement trough could be fitted by Gaussian function. The trough width parameter ( i) is approximately between 0.4Z and 0.6Z for tunnels in clays and less than 0.4Z fortunnels in sands.

- The long-term settlements in soft clay layer above the tunnel could be significant in some sections but not for the stiff clay layer.

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- Outward lateral ground movement from the tunnel excavation occurred for depths below the tunnelroof in sections where high face pressure was used in EPB excavation. Above the tunnel roof,inward lateral displacement occurred in all cases.

- In general, the observed subsurface settlements and lateral displacements could be reasonably fittedwith the prediction by the analytical methods of Verruijt and Booker (1996) and Loganathan andPoulos (1998). However, the prediction was poor for lateral ground movements in case wherelateral outward ground movement occurred.

- Settlement of buildings showed correlation with the height of building (depth of foundation piles).

REFERENCES

Burland, J.B., 1995. Assessment of risk of damage to buildings due to tunnelling and excavation, ProcInt. Conf. Earthquake Geot. Eng., IS-Tokyo 95.

Loganathan N. and Poulos H.G., 1998. Analytical prediction for tunneling-induced ground movementin clays, J. Geotech. Engrg ASCE, 124, no. 9, pp.846-856.

Mair R.J., Gunn M.J. and OReilly M.P., 1981, Ground movement around Shallow Tunnels in SoftClay, Proceedings of The 10 th Conference on Soil Mechanics and Foundation Engineering,

Stockholm, Vol.1, pp. 323-328 New, B.M. and Bowers, K.H., 1993. Ground Movement model validation at the Heathrow ExpressTrial Tunnel. Tunnelling 94, IMM, London, pp. 301-329.

OReilly, M.P. and New B.M., 1982. Settlements above Tunnels in United Kingdom-Their Magnitudeand Prediction, Tunnelling82, pp 173-181.

Peck, R.B., 1969. Deep Excavation in Soft Ground, Proc. 7th International Conference on SoilMechanics and Foundation Engineering, State of the Art Volume, pp 225-290.

Phienwej, N. 1998. Ground movements associated with soft ground tunnelling in Bangkok. Proc.World Tunnel Congress 98: Tunnels and Metropolises, Sao Paulo, Brazil.

Suwansawat, S., 2002. Earth Pressure Balance Shield Tunneling in Bangkok: Ground Response andPrediction of Surface Settlements using Artificial Neural Networks. Doctoral Dissertation, MIT,USA.

Timpong, S., 2002. Analysis of Ground Movement in Bangkok MRT Blue Line Project. M.S. Thesis,AIT, Bangkok.

Tavaranum, S., 2003. Analysis of Subsurface Ground Movement Induced by EPB Shield Tunnelling inBangkok MRT Blue Line Project. M.S. Thesis, AIT, Bangkok.

Verruijt, A. and Booker, J.R., 1996, Surface settlements due to deformation tunnel in an Elastic Half Plane, Geotechnique, Vol. 46, No. 4, pp. 753-756.

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