1. 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

2. MAP OF WEST BENGAL SHOWING LOCATION OF CALCUTTA METROPOLITAN DISTRICT

3. CALCUTTA METROPOLITAN DISTRICT

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

5. CALCUTTA METRO ALIGNMENT

6. GEOLOGICAL FEATURES
OF CALCUTTA SOIL

9. SUBSOIL OF KOLKATA : NORMAL CALCUTTA DEPOSIT
Soil Properties (average)
Description of Strata
N blows/30 cm
W LL PL Cu mv (%) (%) (%) (t/m2) m2/t
Light brown/brownish grey silty clay/clayey silt with occasional lenses of fine sand
5 – 8 4
II. Grey/dark grey silty clay, clayey silt with semi-decomposed timber pieces
2 – 4 14
III Bluish grey silty clay with calcareous modules
DEPTH (M) 10 20
IV. Brown/yellowish brown sandy silt/silty fine sand with occasional lenses of brown and grey silty clay 20
--- 25
V. Mottled brown/grey silty clay with laminations often with rusty brown spots
20 – 30 30
VI Brown /light brown silty fine to medium sand 40
/ = 350 40
SUBSOIL OF KOLKATA : NORMAL CALCUTTA DEPOSIT

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

11. LENGTH OF FIRST LINE OF KOLKATA METRO – 16. 633 Km
Elevated – Km.
Surface – Km.
Underground – Km
Sheet Piles : Km.
H-Pile with Timber Lagging : Km.
Diaphragm Wall : Km.
Shield Tunnel : Km.

12. OPEN CUT IN MAIDAN AREA

13. Dessiccated Brownish grey silty clay
0 (m)
STRUT 1
Dessiccated Brownish grey silty clay
- 1.5
- 2.0
Cu = 4 t/m 2
- 6.0
Grey / dark grey silty clay with decomposed wood
- 6.5 3
- 9.0
- 9.5
Cu = 2.5 t/m2
- 13.2 4
- 11.0
- 11.5
- 13.5
Bluish grey silty clay with kankar
Cu = 6 t/m2
- 17.2
Yellowish Brown sandy silt with clay N =
TYPICAL EXCAVATION PROFILE OF CUT-AND-COVER CONSTRUCTION WITH DIAPHRAGM WALL AND STRUTS

14. SHIELD TUNNELLING BETWEEN BELGACHIA AND SHYAMBAZAR
F I L L
+ 1.80
= 1.9 t/m Cu = 3 t/m2
Brownish grey clayey silt
- 0.12
Grey silty clay / clayey silt
 = 1.9 t/m3
- 2.00
Silt with Decayed Wood
LL = PL = Cu = 2 t/m2
W = 50
T u n n e l s
- 8.00
= 1.9 t/m Cu = 3 t/m2
Bluish grey silty clay
LL = 1.9 t/m PL = 3.6 t/m2
Brown silty fine sand
SHIELD TUNNELLING BETWEEN BELGACHIA AND SHYAMBAZAR

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

16. STABILITY AGAINST BOTTOM HEAVE IN STRATIFIED SOILC N +
γ3D + γ D + å C (H + D + D ) /D F = u 4 c 2 4 f u 2 f 1 å γ (H + D + D ) 2 γ
STABILITY AGAINST BOTTOM HEAVE IN STRATIFIED SOIL

17. GUIDE WALL FOR DIAPHRAGM WALL TRENCH

18. REINFORCEMENT CAGE OF DIAPHRAGM WALL

19. DESIGN OF BRACED CUTS

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

21. GROUND SETTLEMENT FOR EXCAVATION BELOW 1ST STRUT

22. WALL DEFLECTION IN SEQUENTIAL EXCAVATION AND STRUTTING (CONCEPTUAL)

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

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

25. INSTRUMENTATION

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

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

28. LOCATION OF EARTH PRESSURE CELLS IN DIAPHRAGM WALL

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

30. GROUND SETTLEMENT MEASURING POINTS
Concrete Cover
G.L
Pit x600
600
50 mm Lean Concrete
Settlement Stud
GROUND SETTLEMENT MEASURING POINTS

31. PORE PRESSURE DISTRIBUTION AS MEASURED BY VIBRATING WIRE PIEZOMETERS

33. EARTH PRESSURE ON DIAPHRAGM WALL BEFORE COMMENCEMENT OF EXCAVATION

34. VALUES OF IN – SITU KO
Depth Location Location Location Average (m)
* *
1.0
0.70
0.50
* Low values due to inadequate seating

35. DIAPHRAGM WALL DEFLECTION AT DIRRERENT STAGES OF EXCAVATION

36. MEASURED STRESSES ON DIAPHRAGM WALL AT DIFFERENT STAGES OF EXCAVATION

37. SUMMARY OF MEASURED EARTH PRESSURE AT DIFFERENT STAGES OF EXCAVATION

38. COMPARISTON OF EARTH PRESSURE BY PECK’S METHOD AND OBSERVED DATA
Depth Method Maximum earth m Depth at which pressure linear variation changes to constant pressure m (t/m2) (m)
Peck Observation
Peck Observation
Peck Observation

39. FORCES ON DIAPHRAGM WALL

40. FORCES ON DIAPHRAGM WALL

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

42. CONSTRUCTION CONTROL

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

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

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

46. 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

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

48. MAXIMUM SETTLEMENT OF BUILDINGS

49. Time of Construction

50. Building Settlement at Different Stages of Excavation

51. TIME EFFECT

52. SETTLEMENT OF ADJACENT BUILDINGS : EFFECT OF TIME OF CONSTRUCTION

53. SETTLEMENT OF PREMISES NO. 161, C. R. AVENUE : EFFECT OF TIME