Geotechnical Investigation of Collapsed Margalla Tower due to October 2005 Muzzafarabad Earthquake

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1 Geotechnical Invetigation of Collaped Margalla Tower due to October 2005 Muzzafarabad Earthquake Shama Shoukat Ali 1 Dr. K Mahmood 2 Dr. Ahraf Tanoli 3 Dr. Muhammad Atiq Tariq 4 1 Department of Civil Engineering, COMSATS Intitute of Information Technology Abbottabad, Pakitan 2 Department of Civil Engineering, COMSATS Intitute of Information Technology Abbottabad, Pakitan 3 Department of Civil Engineering, COMSATS Intitute of Information Technology Abbottabad, Pakitan 4 Department of Civil Engineering, COMSATS Intitute of Information Technology Abbottabad, Pakitan Abtract: Thi tudy evaluate the bearing capacity and ettlement of collaped Margalla Tower due to 2005 Muzzafarabad Earthquake. The geotechnical invetigation wa conducted at the ite to evaluate bearing capacity and ettlement analyi by field a well a lab tet. The oil ampling (diturbed and unditurbed ample) wa done by 21 m borehole at the ite. The grain ize ditribution and the electric reitivity tet reult howed that the oil beneath the tower wa mainly clayey and ilty oil. The hear wave velocity baed on SPT reult howed a range of 175 to 350m/. The bearing capacity calculated by uing lab a well a field tet reult howed a value of 2.64 and 4.06 tf at the raft foundation level. Similarly the ettlement evaluation from lab a well a field tet howed 3.80 and 50 mm repectively and wa in the permiible limit. The geotechnical invetigation reveal that the Margalla Tower wa afe againt bearing capacity and ettlement. Keyword: Geotechnical Invetigation; Margalla Tower; bearing capacity; ettlement Introduction In 1992, Capital Development Authority Pakitan planned three ite for multitory building in Sector F-10/3 Ilamabad. Among them Margalla Tower wa one of the 10 torie building with 60 luxuriou reidential apartment. The Muzafarabad Earthquake 7.6 detroyed forth block and a portion of fifth block of Margalla Tower Ilamabad. There were 250 caualtie including foreign national. The collaped tower i hown in Figure. 1. Preliminary tudie by CDA and Engineer in Chief (ENC) Branch Rawalpindi [1 2] reveal that geotechnical invetigation of thi tower had flaw in it i.e., there wa no conolidation tet wa performed although there wa clay below ground level and water table wa high. The unconfined 151

2 trength of ample from hallow and higher depth wa reported ame which i contradictory. Wherea, trength of clay hould improve with depth, The foundation i at 1.5 m while the weak upper trata containing debri, root and organic matter extend up to 2, 2.5 and 3.5 meter in three bore hole, Bearing capacity i 0.8 kg/q cm. Figure 1. The Collaped Margalla Tower Ilamabad Pakitan Foundation i to be placed between 2 to 3 m. The raft hould be deigned againt bearing capacity of 1.3 kg/q cm (vetting report of Margalla Tower by CES Pvt. Ltd.) [2]. Letter from reident to CDA and C.C.C. Aociate, reveal complaint of ubtandard material uage by conolidated Engineering Service. Therefore the Geotechnical invetigation of collaped Margalla Tower in F-10 Sector Ilamabad (33 42'1"N 73 0'33"E) [9] wa planned to invetigate bearing capacity and ettlement evaluation. Geotechnical Invetigation The detailed invetigation involved field and lab teting of the Margalla Tower. The field tet evolve tandard Penetration tet (SPT) and Electric Reitivity tet (ERT). The layout of SPT and ERT i hown in figure 2 given below. Wherea, the lab tet [5 6] were conducted both on 152

3 diturbed and unditurbed ample collected from borehole at different depth. Thee ample were then ued for moiture content [10], Atterberg limit[11], oil claification [12 13], unconfined compreion tet direct hear tet [15] and conolidation tet [14]. Figure 2 The Layout of Electric Reitivity tet at Margalla Tower Reult and Dicuion In the field work the tandard penetration (N) value were calculated from SPT at different depth up to 20.5 m ingle borehole. Thee N value were then corrected for N 60 a hown in Fig. 3. From the figure it i clear that the N 60 value are low up to 12.5 meter but after that there i increae in value tremendouly up to 25 m. Thi how that the upper trata i weak and lower i trong. 153

4 Figure: 3 Variation of N 60 with depth The allowable bearing capacity baed on SPT reult wa the calculated by uing Mayerhof (N 55 & N 70 ) and Da method at different depth of borehole [3 4 5]. Thee reult are hown in figure 4a below. (a) (b) Figure. 4 Allowable Bearing Capacity baed on (a) field tet and (b) lab tet The q all from field tet 3.16 to 10 tf and from lab tet varie from 1.58 to 5.5 tf. Wherea, the recommended bearing capacity i 2.5 tf. Similarly, by lab teting the variation of coheion and friction angle along with the depth i hown in Fig. 5. The value alo how the above 10 meter oil ha low angle of internal friction a well a coheion which predict that an upper tratum i week which i alo predicted from the bearing capacity reult of field tet. 154

5 Figure 5: Variation of (a) coheion and, (b) friction angle with depth Thi allowable bearing capacity wa alo calculated baed on Da and Punmia method [6 7]. uing lab tet i plotted in figure 4b. From the figure 4 it i clearly hown that the allowable bearing capacity i low up to 10 m then it increae up to 25 m. The allowable bearing capacity i high from field tet a compare to lab tet. The Electric Reitivity tet (ERT) were conducted along and parallel to Block 4 of Margalla Tower a hown in figure 3 above. The ERT reult parallel to the collaped tower 4 i hown in figure 6 below. It i clear from the figure that approximately 60 % tratum below the tower i oft material. Zone of high aturation were alo marked. Trapped water (highly aturated zone) parallel to the underground water tank wa found. Thi might be due to eepage from the underground water tank. 155

6 Figure: 6 Electric Reitivity Tet (ERT) reult of Margalla Tower Soil Claification The oil claification according to Unified Soil Claification Sytem (USCS) [12] and American Aociation of Highway and Tranportation Official (AASTO) [13] baed on lab tet at different depth i given in Table.1 [10 11]. From the AASTO and USCS oil claification ytem i it i clear that the oil i ilty and clayey below ground level. Table. 1 Soil Claification by Uniform Soil Claification Sytem (USCS) Depth Platicity Index Liquid Limit USCS Soil type AASHTO Soil type CL-ML Silty Soil A-2-4 Silty or Clayey Soil CL - ML Silty Soil A-2-4 Silty or Clayey Soil CL Clayey Soil A-2-6 Silty or Clayey Soil CL-ML Silty Soil A-2-4 Silty or Clayey Soil CL Clayey Soil A-2-4 Silty or Clayey Soil CL Clayey Soil A-2-6 Silty or Clayey Soil The ettlement were calculated at three clay ample at repective depth uing odometer apparatu. The lab reult were then ued to plot graph between void ratio (e) and effective preure to evaluate pat maximum preure ( ) a hown in Fig

7 Figure 7: Void Ratio veru Effective Stre (at 8.5 m) For under conolidated clay i.e., the equation 1 i ued by [5]. Da CH σ c CcH σ ο +Δσ S c = Log + Log 1+e σ 1+e σ ο ο ο c (1) Similarly, for over conolidated clay i.e., the equation 2 i ued by Da S = c CH σ ο +Δσ Log 1+e σ ο ο (2) The Table 2 how the reult of ettlement of clay at three depth. The clay at 8.5 and 14 meter are under conolidated clay wherea the clay at 20.5 m i over conolidated clay. Thee ettlement were in the permiible limit (Da, 2007). It how that the tructure againt ettlement. Table 2: Settlement of Clay Sample (Da, 2007). Depth eₒ C c C σ c (kpa) σ (kpa) Δσ (kpa) OCR Settlement

8 Schmertmann in 1970 propoed method baed on SPT data to compute elatic ettlement [1].. To compute the ettlement the oil below ground level wa divided into five layer a hown in figure. Each layer had a contant value of train ( ε ) and oil modulu (E ) by uing relationhip influence of all layer. ΔH = C C Δq 1 2 E = 300 (N+6) (Bowle, table 5.5) [3]. Settlement wa calculated by umming the IzΔz E The following table 3 how the calculation of the IzΔz E at different depth. Table 3: Data for calculation of elatic ettlement Layer Elevation top Elevation bottom Δz I zp N 60 E (kpa) I Δz ΔH = C C Δq 1 2 IzΔz E ΔH = ΔH = ΔH = mm I Δz zp E z = E Similarly the ettlement againt 50, 75, 100, and 125 kpa wa calculated and hown in table 4. Table 4: Net bearing capacity and ettlement. 158

9 Δq (kpa) ΔH (mm) The graph between net bearing capacity and ettlement i hown in figure 8. The foundation of the Margalla Tower wa kept at 1.5 m below ground level. The bearing preure on the foundation wa KPa. Wherea the net bearing preure under the footing wa 56.8 KPa (unit weight of oil i KN/m3). The net bearing preure (60 KPa) againt 50 mm ettlement wa more than the applied net bearing preure, i.e., 56.8 KPa under the foundation. Hence the Margalla tower wa afe againt ettlement. Figure 8: Variation of Settlement with increae in Net Bearing Stre. Shear velocity wa calculated from oberved N value baed on the relationhip of JRA 1980, Lee, 1990 and Imai et al., The variation of hear wave velocity and hear modulu i given in Fig. 9. The Shear wave velocity calculated by three relation ha average value of 337, 350 and 315 m/ec. Thee value fall in range of 175 to 350 m/ec. According to building code of Pakitan the oil type i S D which repreent a Stiff Soil Profile [8]. From thee hear wave velocitie the hear modulu wa then calculated for known denitie. The average value by JRA, Lee and Imai et al are 2348, 2629 and 2049 MPa repectively a hown in figure 9 below. 159

10 Figure 9: Variation of Shear Velocity and Shear Modulu with depth. Concluion The initial geotechnical invetigation reveal that there were alternate layer of clay and ilty clay with ome gravel. It wa confirmed by AASHTO and USCS SOIL Soil claification ytem. The oil claification wa alo confirmed with ER reult. The upper trata up to 10 m i weak that i why raft foundation wa uggeted. The average bearing capacity i from the field tet i KN/m 2 and from the lab tet i KN/m 2. The ERT reult how the eepage from the under ground water tank which would have caue reduction in effective tre of the oil. A a reult failure would occur. The Schmertmann elatic train method and conolidation tet reult howed 43 mm and 15.5 mm repectively. Thi i in permiible limit for raft foundation. The Margalla Tower wa thu afe with repect to ettlement analyi. The Shear wave velocity calculated by three relation ha average value of 337, 350 and 315 m/ec. Thee value fall in range of 175 to 350 m/ec. According to building code of Pakitan the oil type i S D which repreent a Stiff Soil Profile [8]. In hort the geotechnical invetigation reveal that Margalla Tower wa afe againt bearing capacity and ettlement. The oil tratum below Margalla Tower i firm and tiff non problematic oil. The failure wa due to tructural flaw. Reference 160

11 [1] Capital Development Authority, Geo-technical Report of the Site Contruction of Flat in F-10/3, Ilamabad. [2] Conolidated Engineering Service (CES) private Ltd, Vetting Soil Invetigation Report of Margalla Tower, F-10/3, Ilamabad. [3] Joeph E. Bowle, Foundation Analyi and Deign publihed by Mc Graw- Hill Companie, USA. Chapter three pp [4] Holtz, R. D. and Kovac, W. D., An Introduction to Geotechnical Engineering. Printed by Prentice Hall, New Jerey, USA. Chapter 8, pp: [5] Braja M. Da, Principle of Foundation Engineering, Sixth Edition. Publihed by Thomon Limited, Canada. Chapter 6, pp: [6] Braja M. Da, Principle of Geotechnical Engineering, Fifith Edition. Publihed by Thomon Limited, Canada. Chapter 4, pp: and Chapter 10, pp: [7] B.C. Punmia, Soil Mechanic and Foundation. LAXMI Publication, New Delhi. Chapter Twenty Five, pp [8] Building Code of Pakitan (BCP), Seimic Proviion for Building Code of Pakitan Minitry of Houing and Work Government of Pakitan Ilamabad. [9] Geological Survey of Pakitan (GSP), Tectonic Map of Pakitan, Minitry of Petroleum and Natural Reource Ilamabad. [10] ASTM D Standard Tet Method for Laboratory Determination of Water (Moiture) Content of Soil and Rock Ma. [11] ASTM D Standard Tet Method for Liquid Limit, Platic Limit and Platicity Index of Soil. [12] ASTM D Standard Tet Method for Claification of Soil for Engineering Purpoe Unified Soil Claification Sytem (USCS). [13] ASTM D Standard Practice for Claification of Soil and Soil-Aggregate Mixture for Highway Contruction Purpoe. American Aociation of tate Highway and Tranportation Official (AASHTO). [14] ASTM D Standard Tet Method One Dimenional Conolidation Propertie of Soil Uing Incremental Loading. [15] ASTM D Standard Tet Method for Direct Shear Tet of Soil Under Conolidated Drained Condition. 161