BRACED EXCAVATION IN SOFT CLAY A CASE STUDY

BRACED EXCAVATION IN SOFT CLAY A CASE STUDY OF KOLKATA METRO CONSTRUCTION BY CUT AND COVER METHOD N. N. Som Consulting Geotechnical Engineer Former Professor of Civil Engineering Jadavpur University, Kolkata 1

MAP OF WEST BENGAL SHOWING LOCATION OF CALCUTTA METROPOLITAN DISTRICT 2

CALCUTTA METROPOLITAN DISTRICT 3

RAPID TRANSIT SYSTEM FOR KOLKATA METRO - FIRST PHASE OF CONSTRUCTION BY CUT AND COVER METHOD 4

CALCUTTA METRO ALIGNMENT 5

GEOLOGICAL FEATURES OF CALCUTTA SOIL 6

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Description of Strata 0 DEPTH (M) 4 I. Light brown/brownish grey silty clay/clayey silt with occasional lenses of fine sand Soil Properties (average) N blows/ 30 cm 5– 8 II. Grey/dark grey silty clay, clayey silt with semi-decomposed timber pieces 2– 4 III Bluish grey silty clay with calcareous modules 10 W (%) LL (%) 30 54 50 30 PL (%) Cu (t/m 2) mv m 2/t 24 4. 0 0. 002 55 28 2. 5 0. 007 60 25 6. 0 0. 003 25 10. 0 14 20 25 30 40 IV. Brown/yellowish brown sandy silt/silty fine sand with occasional lenses of brown and grey silty clay V. Mottled brown/grey silty clay with laminations often with rusty brown spots VI Brown /light brown silty fine to medium sand 20 20 – 30 40 --- 30 65 0. 002 / = 350 SUBSOIL OF KOLKATA : NORMAL CALCUTTA DEPOSIT 9

0 I. Light grey/brownish grey silty clay/clayey silt DEPTH (M) 3 IIA. Grey / light grey fine to medium sand N = 15 – 30 blows / 30 cm 25 V. Mottled brown /grey silty clay with VI. 30 laminations often with rusty brown silt SUBSOIL OF KOLKATA : RIVER CHANNEL DEPOSIT 10

LENGTH OF FIRST LINE OF KOLKATA METRO – 16. 633 Km Elevated – 1. 228 Km. Surface – 0. 270 Km. Underground – 15. 135 Km Ø Sheet Piles : 0. 356 Km. Ø H-Pile with Timber Lagging : 0. 684 Km. Ø Diaphragm Wall : 12. 687 Km. Ø Shield Tunnel : 1. 090 Km. 11

OPEN CUT IN MAIDAN AREA 12

0 (m) Dessiccated Brownish grey silty clay Cu = 4 t/m 2 STRUT 1 - 3. 8 2 Grey / dark grey silty clay with decomposed wood 3 Cu = 2. 5 t/m 2 4 - 13. 2 Bluish grey silty clay with kankar - 13. 5 - 1. 5 - 2. 0 - 6. 5 - 9. 0 - 9. 5 - 11. 0 - 11. 5 Cu = 6 t/m 2 - 17. 2 Yellowish Brown sandy silt with clay N = 21 - 37 TYPICAL EXCAVATION PROFILE OF CUT-AND-COVER CONSTRUCTION WITH DIAPHRAGM WALL AND STRUTS 13

+ 4. 69 m + 1. 80 - 0. 12 - 2. 00 F I L L Brownish grey clayey silt Grey silty clay / clayey silt Silt with Decayed Wood Tunnels - 8. 00 - 13. 00 - 19. 50 Bluish grey silty clay g = 1. 9 t/m 3 Cu = 3 t/m 2 = 1. 9 t/m 3 LL = 58 W = 50 PL = 32 Cu = 2 t/m 2 g = 1. 9 t/m 3 Cu = 3 t/m 2 LL = 1. 9 t/m 3 PL = 3. 6 t/m 2 Brown silty fine sand SHIELD TUNNELLING BETWEEN BELGACHIA AND SHYAMBAZAR 14

BRACED CUT WITH DIAPHRAGM WALL Diaphragm Wall Excavation & Strutting Box Construction Backfilling 15

C u 4 N c + γ 3 D 2 + γ 4 D f + å C u (H + D 2 + D f ) /D 1 F = å γ (H + D 2 + Dγ ) STABILITY AGAINST BOTTOM HEAVE IN STRATIFIED SOIL 16

GUIDE WALL FOR DIAPHRAGM WALL TRENCH 17

REINFORCEMENT CAGE OF DIAPHRAGM WALL 18

DESIGN OF BRACED CUTS 19

Adjacent building movement Buckled struts Wall position (a) Movement of (b) wall Water pressure (c) Bottom heave (d) Sand and/or gravel bottom BRACED CUT : MODES OF FAILURE (WINTERKORN & FANG 1972) 20

GROUND SETTLEMENT FOR EXCAVATION BELOW 1 ST 21 STRUT

WALL DEFLECTION IN SEQUENTIAL EXCAVATION AND STRUTTING (CONCEPTUAL) 22

APPARENT EARTH PRESSURE DIAGRAM IN BRACED CUT (PECK 1969) 23

GROUND SETTLEMENT IN BRACED CUT WITH SHEET PILE 24 SUPPORT (PECK 1969)

INSTRUMENTATION 25

OBJECTIVE To Study Earth Pressure on Diaphragm Wall Movement Strut Load Ground Settlement 26

CALCUTTA METRO CUT : LOCATION OF INSTRUMENTED TEST SECTIONS BETWEEN GIRISH 27 PARK AND M. G. ROAD STATION

LOCATION OF EARTH PRESSURE CELLS IN DIAPHRAGM WALL 28

FLUID PRESSURE MEASURED BY EARTH PRESSURE CELLS IN 29 DIAPHRAGM WALL TRENCH BEFORE CONCRETING

Concrete Cover G. L 600 Pit 600 x 600 Settlement Stud 50 mm Lean Concrete GROUND SETTLEMENT MEASURING POINTS 30

PORE PRESSURE DISTRIBUTION AS MEASURED BY VIBRATING 31 WIRE PIEZOMETERS

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EARTH PRESSURE ON DIAPHRAGM WALL BEFORE COMMENCEMENT OF EXCAVATION 33

VALUES OF IN – SITU KO Depth (m) 2 Location 1 1. 65 Location 2 Location 3 0. 77* 0. 55* 4 1. 70 0. 85 0. 80 6 0. 95 0. 85 0. 73 8 10 0. 83 0. 70 1. 16 0. 84 0. 70 0. 65 12 0. 67 0. 90 0. 60 13. 5 16 0. 50 0. 37 0. 80 0. 48 0. 70 0. 30 17. 5 0. 44 0. 65 0. 45 Average 1. 0 0. 70 0. 50 * Low values due to inadequate seating 34

DIAPHRAGM WALL DEFLECTION AT DIRRERENT STAGES OF EXCAVATION 35

MEASURED STRESSES ON DIAPHRAGM WALL AT DIFFERENT STAGES OF EXCAVATION 36

SUMMARY OF MEASURED EARTH PRESSURE AT DIFFERENT STAGES OF EXCAVATION 37

COMPARISTON OF EARTH PRESSURE BY PECK’S METHOD AND OBSERVED DATA Depth Method m Maximum earth pressure m (t/m 2) Depth at which linear variation changes to constant pressure (m) 10. 0 Peck Observation 10. 8 12. 5 0. 80 0. 55 2. 5 6. 0 12. 0 Peck Observation 12. 0 17. 0 1. 00 0. 50 3. 0 7. 2 13. 6 Peck Observation 15. 0 18. 0 1. 00 0. 65 4. 4 8. 2 38

FORCES ON DIAPHRAGM WALL 39

FORCES ON DIAPHRAGM WALL 40

DISTANCE FROM EXCAVATION MAX. DEPTH OF EXCAVATION VARIATION OF GROUND SETTLEMENT AT DIFFERENCE TEST SECTIONS 41

CONSTRUCTION CONTROL 42

CONTROL PARAMETERS Effect on Adjoining Buildings Settlement at Different Stages of Excavation Angular Distortion of Buildings Placement of Strut Time Effect 43

FIELD MEASUREMENTS Observations Settlement of Plinth / floor Tilt Cracks in Walls / Floors 44

EFFECT ON ADJOINING BUILDINGS Observations Settlement of Plinth / floor Tilt Cracks in Walls / Floors 45

GROUP A BUILDINGS : 60 Buildings Monitored Small and Minor Damages in the form of * Cracks & Minor Tilts * No Structural Distress * No Detailed Observation done except monitoring cracks & tilts 46

GROUP B BUILDINGS 28 Buildings Required Detailed Study Moderate to severe damages Temporary Protective Measures Final Rectification after Construction 47