ATS11-03329
Building Damage Assessment Next to Karaj Subway Station
Eshagh Namazi1, Mohsen Hajihassani2, Mohammad khosrotash3, Hisham Mohammad4, Masoumeh Karimi Shahrbabaki5
1Research Assistant, Steel Technology Centre, Universiti Teknologi Malaysia. [email protected] 2Research Assistant, Department of Geotechnics and Transportation, Faculty of Civil Engineering, Universiti Teknologi Malaysia. [email protected] 3Managing Director, Tunnel Rod Construction Consulting Engineering Inc. Iran. [email protected] 4Senior Lecturer, Faculty of Civil Engineering, Universiti Teknologi Malaysia. [email protected] 5Department of Mining Engineering, Islamic Azad University, Tehran, Iran. [email protected]
ABSTRACT Tunnel excavation in the urban areas can induce ground movements, which distort and, in serve cases, damage overlying buildings and services. To predict damage in the buildings an analytical framework approach based on the concept of limiting tensile strain is used world-widely. In developing the approach clearly, there is the conspicuous shortage of well-documented case histories of measured building response to ground movement. In this paper, effects of construction of the Station E in Karaj Subway System on the adjacent 2-storey commercial buildings are presented. The Station was constructed by enlargement of the NATM tunnels. Damage to the building is assessed in two ways. First, the analytical assessment of building damage is made by calculating tensile strains due to settlement. Second, an external visual inspection was made of cracking or damage to verify the analytical prediction. The results provide an important frame of reference for interpreting the measured responses of the nearby buildings.
KEYWORDS Karaj Subway; Surface Settlement; Building Damage Assessment; Subway Station; Cracks.
1. INTRODUCTION Having 2.6 million inhabitants, Karaj is a large city located around 40 km to the west of Tehran, the capital of Iran. With C Kamal Shahr ever-increasing population and the number of vehicles, Karaj E N G is currently under development and construction of a new I subway system to address the transportation problems. Line 2 J B of Karaj Urban Railway (KUR), distancing for 24 km, is being K constructed between Kamal-Shahr, in north-western of Karaj, D F H
to Malard, placed in south of the city. The KUR includes a M
tunnel with 7.80 m height and 8.40m width at the depths of G
h O e
14-20 m below the ground level. The full length of the KUR is h P
e
m
shown in the simple map of Fig. 1. In all, there are 23 stations
S Q
t
C D B
r e
constructed in open cut and enlarged tunnels by NATM e R tunneling techniques. A t E F S One particular interest of KUR is a part next to station Station E E, where the station is bored next to businesses, Fig1. T A l 0 10 20m m Being confronted with increasing population and the e U Scale h d i numerous existing vehicles, Karaj is currently under
S V t r e
development and construction of a new subway system to e overcome the transportation problems. The aim of the study is t W to evaluate the ground movement influences on the existing X surrounding buildings (buildings A, B, C, D, E and F in Fig. Y 1). To minimize damage to adjacent buildings, it is essential to have a comprehensive documentation of the responses of these Malard buildings to station construction. This includes a complete record of ground conditions and the response during and after construction. This paper summarizes the conditions at the site Fig. 1. Line 2 of Karaj Urban Railway and the location of case study and presents the correlation among construction activities, buildings related to station E. [22] measured displacements, distortions and consequent damages to buildings. 2. GROUND CONDITIONS In general, 40 boreholes and 18 shafts were bored to obtain continuous samples for visual description [8]. The samples collected from boreholes were tested in the form of laboratory uniaxial compressive and direct shear tests. The in-situ plate loading test, in situ direct shear test and Standard Penetration Testing (SPT) also were performed to provide more information on the properties of the soil. Between Stations E and F, seven boreholes (BH201-BH207) with the distance of 100 m and one shaft (TP-239) were excavated. Borehole 201 is located about 100 m south-east of the reference point. Fig. 2 is a summary log of this borehole to show the associated sub-division of the soil in the site. Visual inspection reveals that the soil contains inorganic clay with clayey sand at the top, followed by clayey sand and seldom silty sand overlying clayey and silty gravel. Furthermore, Fig. 2 summarizes the properties of the mentioned soil layers obtained from geotechnical tests. The ground water table was not observed in any boreholes. 3 2 1
6 5 4
Fig. 3. Typical main stages for construction of station E. [22]
4 . BACKGROUND OF BUILDING DAMAGE ASSESSMENT (Tunnel Rod, 2005)
Damage to buildings due to excavation-induced ground movement depends on conditions of the building before excavation begins, the magnitude of the ground displacement cause by excavation, and the structural system of the effected Fig. 2. Summary log and geotechnical properties of the soil close to building [10]. Because the potential sources of damage are station E. [8] various, simple criteria which correlate the damage categories to a component of displacement are not universally applicable. The 3. SEQUENCE OF THE STATION earliest method based on empirical models used numerous case CONSTRUCTION studies to establish correlations between distortion parameters The station E was constructed as enlarged tunnels with and the corresponding damage limits (Skempton and approximately a length of 160 m and 16 m width. Fig. 1 MacDonald, 1956; Polshin and Tolkar, 1957 and Bjerrum, 1963). illustrates the position of the station from the buildings. All These methods usually deal with the risk of buildings owing to buildings are a two-storey, steel-framed building founded on a displacement under their own weight and not necessarily reinforced concrete raft. On the KUR, NATM technique has applicable to the movements of buildings due to adjacent been used for construction of station E. In this case, the large excavation. Building's criteria based on two most critical tunnel was excavated in several parts in order to help the components of displacement, called angular distortion and ground to stand until lining completion. The sequence deflection ratio, are shown in Table 1. Deflection ratio ∆/L, is the construction is presented in Fig. 3. The shape of the tunnel is relative deflection divided by the distance between the two different from conventional circular tunnels. As the figure reference points which relative deflection is the maximum presents, the process is started with removing and the displacement of the settlement profile of a structure relative to enlargement of sides’ drifts. Subsequently, the excavation and the straight line connecting two settlement reference points. compensation grouting of the piles are performed. Removing Angular distortion β is defined as the differential settlement and grouting of arch ribs is a next stage in the sequence of between two points divided by the distance between them, less construction followed by the excavation of the whole tunnel. the tilt. Finally, subsequent to the installation of waterproof Currently, the analytical method has been widely used in the membrane, the invert is closed by the concreting. Supposing engineering practice to predict building damage. This method the whole length of the station divided into three blocks (block was first proposed by Burland & Wroth (1974) and Burland et al. 1, 2 and 3 in Fig. 4), block 1 is excavated first. Prior to the (1977). They assumed the onset of crack is associated with construction termination of the block 1, the exaction of the average tensile strain within the building. To obtain the second block is started and so on. The first and the last maximum tensile strain in the buildings, they represented excavated ribs in block 1 were Rib 05 and Rib 10, whereas individual walls of the buildings as a linear-elastic deep beam there was Rib 31 and Rib 42 in blocks 2 and Rib 59 and Rib subjected to a point load at the centre. The authors used the 56 in block 3, respectively (Fig. 4). After process completion, structural engineering principles to derive the relationships the excavation height and width were around 15 m and 18.8 m between the deflection ratio and the limiting tensile strains in the at the depth of 8.4 m below the ground surface. beam. They consider the effects of different deformed shape called “hogging” for concave downward deflection profiles and “sagging” for concave upward deflection profiles. They had shown excavation-displacement can result in sagging, hogging or both in the buildings, whereas the hogging is far more damaging than sagging.
Table. 1. Empirical damage criteria caused by Building self- weight
Reference Building Severity Criterion Type
Skempton and Frame Onset of Β=1/300
McDonald building cracking (1956)
Polshin and Load Onset of 1/2000<∆/L<1/1400 Tolkar (1957) bearing cracking For L/H>5
brick wall 1/3300<∆/L<1/2500 For L/H<3
Bjerrum (1963) Frame Negligible β=1/500 Structure Structural β=1/150
Damage
Afterwards, Boscardin and Cording (1989) completed Burland and Worth (1974) model by including horizontal tensile strain using simple superposition to consider the role of horizontal displacement induced by adjacent excavation and tunneling. They defined categories of damage based on angular distortion and horizontal strain and compared the damage predicted by their suggested method with recorded cases of damage using Table 2. Recently, Mair et al. (1996), Burland (1995) and Finno et al. (2005) extended the previous studies by defining the new criteria for damage based on deflection ratio and horizontal strain. Even the approach presented in the previous section was originally developed for assessing the risk of subsidence damage for the London Underground Jubilee Line Extension project [5], it is now 5. FIELD INSTRUMENTATION widely used internationally with minor variation. But a Fig. 4 shows the excavation site along with monitoring problem is a limited amount of comprehensive case studies sections. There are six monitoring sections, which are shown were reported for urban tunneling project to verify the current with their position related to a local co-ordinate (named S-). method. Surface settlements in these sections were monitored using optical survey points established on the centre, left and right
sides of the tunnel axis with the distance of 5 m (Fig. 4b). The
monitoring points consisting of steel rods grouted into the ground about 100 cm below the ground surface to isolate the rods from asphalt movement.
Extensometer measurements also made across several spans within the tunnel in some sections (named C-). The spans were across the horizontal diameter (axis level, less often at the shoulder or knee level) and from the crown to the same points at the end of the horizontal spans. Only section S-3385, S-3400, S- 3440 and S-3460 are presented in this study to obtain the surface
settlement trough.
Table. 2. Category of damage (modified after Burland et al. 1977).
Degree of Crack width (C.W) Ease of repair severity
Negligible (0) <1mm -
Very slight(1) 1mm Other monitoring methods such as monitoring the displacements of buildings and floor across existing cracks or other discontinuity were made during construction. The widths of cracks were measured using a matt steel ruler or gauge. Fig. 4a demonstrates the position of old and new cracks which are developed in response to ground movements induced by station construction. During construction, the monitoring team obtained the location of these cracks from simple visual inspection and then measured the cracks and joints frequently. Rib 01 Rib 05 Rib 10 S-3+350 B l C-3+350 o c k 1 Fig. 4.Location Fig. ofInstrumentation(a) Planeand Section,(b) S-3+385 Rib 26 C2 Rib 31 (b) (a) S-3+400 C16 C21 C-3+400 B l o c C19 C20 k 2 C26 S-3+425 Rib 42 C-3+425 S-3+440 C3 [8] B l o C-3+450 c k 3 C8 S-3+460 Rib 56 C9 Rib 59 C14 Rib 64 6. OBSERVED SURFACE DISPLACEMENT The Gaussian error function of the settlement profile suggested by Peck (1969) is chosen to obtain a graphical The vertical transverse displacements at the location “best fit” to the observed movements in the sections. of monitoring points in the sections are shown in Fig 5. Based on the suggested displacement, the ground The displacements are given after completing movement under the buildings was represented. These construction of the station when the movements had ground movement models close to the buildings are reached to the steady state's conditions. The figure shown in Fig. 6. The shapes of settlement profiles show illustrates the transverse displacement against the that all the buildings experienced hogging mode of distance from tunnel centre line for four sections. The deformation. The boundaries between the two zones of displacement in section S-3400 is the highest one with a hogging and sagging were defined by the inflection maximum around 75 mm. The surface settlement trough points of the settlement profiles obtained in a 10 m in sections S-3385 and S-3460 are identical excluding distance from the tunnel centre pint. Given the data from the monitoring point S1. The maximum displacement in Canadian Geotechnical Society (1992), the cracks are these sections was 30 mm, while on the contrary, in onset at lower angular distortions in the region of section S-3440, that shows the shallowest surface hogging than sagging. Fig. 6 clearly shows the settlement trough, was just 11 mm. The volume loss difference in the settlement trough under the existing incorporated with maximum surface settlement and soil buildings. The deflection ratio from these settlements properties were obtained in the site. Volume loss is can be obtained from the relative deflection dividing by defined as the volume of ground loss as a proportion of the length. In the absence of other information, the the final tunnel volume [9]. At the site, the volume horizontal displacements adjacent to the excavation losses in sections S-3385, S-3400, S-3440 and S-3460 were assumed equal to vertical displacement [16]. The were estimated 0.4%, 0.9%, 0.1% and 4%, respectively. horizontal strain, εh on the surface in every distance These volume losses in comparing to the previous case from tunnel centre line can be obtained by studies reported by Lake et al. (1992) in the greenfield differentiating horizontal displacement [17]. From these conditions with the same excavation geometry and analyses, each building was evaluated and categorized geological conditions, are lower. It is important to note for potential damage using the method suggested by that the stiffness of the buildings decrease the Burland (1995). Table 3 summarizes the assessed displacement and volume loss induced by excavation displacement's parameters and predicted damage of the [20]. buildings. Refer to table 2 as one basis for judging damage severity. S-3385 0 S-3400 -10 S-3440 -20 S-3460 -30 -40 -50 -60 -70 -80 (mm) Settlement Surface -5 -3 -1 1 3 5 7 9 11 Transverse Dispalacement from Tunnel Axis (m) Fig. 5. Measured ground settlement, [12] Distance from Tunnel Centre line (m) 0 0 5 10 15 20 25 30 35 40 -5 -10 -15 A -20 S-3385 Surface Settlement (mm) Settlement Surface -25 S-3460 -30 -35 Distance from Tunnel Centre line (m) 5 -5 0 5 10 15 20 25 30 35 40 45 50 -15 -25 -35 B, C, D -45 -55 (mm) Settlement Surface -65 S-3400 -75 Distance from Tunnel Centre line (m) 0 0 5 10 15 20 25 30 35 40 -2 -4 -6 E, F -8 -10 Surface SettlementSurface (mm) S-3440 -12 Fig. 6. Profile of measurements and predicted ground settlement adjacent to the buildings in the site Table. 3. Summary of displacement parameters and damage category estimation ⃰ ** Buildings L×H (m) Δ/L(%) εh(%) εbr(%) εdr(%)** Damage Severity A 7×6 0.014 0.03 - 0.03 Negligible B, C, D 35×6 0.026 0.21 0.25 - Moderate E, F 10×6 0.020 0.08 0.1 - Slight *length (L) and height (H) of buildings **Resultant bending tensile strain ***Resultant diagonal tensile strain (in every case, only resultant bending or diagonal strain which is critical is given in the table) In this study, it is illustrated that when the buildings are occurred in the hogging zone the limiting value of 7. CRACK OBSERVATIONS deflection ratio and horizontal strain were less than the limiting values proposed by Burland (1995). It can be The visible cracks on the interior and exterior walls the reason of under predicting of the damage in some of in the existing buildings were surveyed by consulting these buildings. groups during and after construction of the stations. In Fig. 4a, just cracks possessing the width of greater than 1-2 mm were represented. Only a few cracks were opened up more than 15 mm. These cracks were References observed in buildings D and E extended in the whole height of the interior wall from floor to roof (C8, C19 [1] Bjerrum L. (1963). Allowable settlement of structures. Proc. European Conf. on Soil Mech. and Found. Engr., Wiesbaden, and C26 in Fig. 4a). According to these cracks, the Germany, No. II, pp. 135–137. observed damage in these buildings is characterized as “severe” and “very severe” (table 2). In building B, the [2] Boscardin, M. D. and Cording, J. C. (1989). Building response to excavation-induced settlement. ASCE Journal of Geotechnical damage was mainly consisted of hairline cracks in the Engineering, 115, No. 1, 1–21. walls except crack C21 with the width of 5 mm and the [3] Burland J. B. (1995). Assessment of risk of damage to buildings length of more than 1 m. Moreover, in this building a due to tunnelling and excavation. Invited Special Lecture. 1st Int. Conf. crack of around 8 mm wide is reported in the floor from on Earthquake Geotech. Engineering, Tokyo, pp. 1189–1201. which the building can be classified as suffering [4] Burland J. B., Broms B. B., and de Mello V. F. B. (1977). “Slight” damage. The damages recorded within Behaviour of foundations and structures. State-of-the-Art Report. buildings A and E were incorporated with cracks C2 and Proc. 9th Int. Conf. on Soil Mech. and Found. Engr., Tokyo, Japan, C14 less than 5 mm. The existing cracks within building pp. 495–546. A and F did not exceed more than 5 mm. The recorded [5] Burland, J.B., Standing, J.R., Jardine, F.M. (2001) .Building damages within these buildings were “very slight” and response to tunnelling: case studies from construction of the Jubilee “slight." Line Extension, London, CIRIA and Thomas Telford, ISBN: 9780727730176. 8. CONCLUSION [6] Burland, J. B. and Wroth, C. P. (1974). Settlement of buildings and associated damage. Settlement of Structures, Proc. Conf. Organized by From the results of these case studies, a number of Geotechnical Engineering Society. Pentech Press, London, pp. 611– conclusions can be drawn. In general, the damage to the 645. buildings consisted mainly of hair cracks except a few [7] Canadian Foundation Engineering Manual. (1992). Canadian cracks with the width more than 1-2 mm. These few Geotechnical society. 3rd Edition, BiTech Publishers, Vancouver, BC. cracks have an important role in categorizing the [8] Darya Khak Pay Consulting Engineering Company. (2005). damage based on the observation. The method proposed Geotechnical reports of Karaj urban railway organization. In Persian. by Burland and Wroth (1974) underestimated the [9] Dimmock, P. S., Mair, R. J. (2007). Estimating volume loss for damage in most of the cases except building B and F. open-face tunnels in London clay. 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(1982). Settlements above tunnels in the united kingdom- their magnitude and prediction. The institution of mining and metallurgy, London. Pages: 55-64. [18] Peck, R. B. (1969). Deep excavations and tunnelling in soft ground. proc.7th int. Conference on soil mechanics and foundation engineering. Pages 225-290. [19] Polshin D. E. and Tokar R. A. (1957). Maximum allowable non- uniform settlement of structures. Proc. 4th Int. Conf. on Soil Mech. and Found. Engr., London, England, pp. 402–405. [20] Potts, D. M., & Addenbrooke, T. I. (1997). A structure's influence on tunnelling-induced ground movements. Proc. Instn. Civ. Engrs. Geotech. Engineering, 125, 109-125. [21] Skempton A. W. and MacDonald D. H. (1956). The allowable settlement of buildings. Proc. Inst of Civ. Engrs., Part III, 5, pp. 727– 784. [22] Tunnel Rod Construction Consulting Engineering Inc. (2008). Monitoring reports of line 2 of Karaj subway tunnels. In Persian.