Question 3 (40 Marks)
It is proposed to construct a canal in the ground shown in Figure for ferry transport (the left bank is not shown). Your task is to assess the canal banks stability immediately after excavation of the canal without water (assume continuous pumping of water from canal bottom and no water is accumulating at the canal bottom immediately after excavation).
The analysis is to be undertaken with the computer package ‘‘SLOPEW”. Use Bishop method as your
analysis type.
45
40
35
30
25
20
15
10
15 25 35 45 55 65 75 85 95
• Initial ground level j? ?^ Initial wate« table
* s’
Giavellv sand

txcavation /A
1 inal ground profile /
—•
Silty «lay, firm

/
/ Silty day. stiff
/ Sikstone (very strong)

?»?»TM.I ? ?
The proposed canal is 70.0m wide at the existing ground surface, with the side slopes of 2H:1 V (horizontal: vertical). The top of the canal is at RL 35.0m for both banks. The deepest part of the canal is at RL 25.0m.
A summary of the soil profile, together with the soil properties is given in the following table (measured at borehole (BH01) located 95m back from top of each river bank). All soil layer interfaces can be assumed as horizontal. Normal operating water level of the canal is at RL =33m.
RL (m) (BH1) Soil description Bulk density kN’m3 Cn
kPa
C
kPa
Decrees
35 to 30 Gravelly SAND (fill) 18 6 35
30 to 24 Sihv CLAY firm (CIS) 160 40 6 30
24 to 20 Silty CLAY stiff (FBS) 178 70 12 32
<20 SILTSTONE (ven' stronc) Note: Undraincd analysis must be used because pore water pressure is unknown immediately after excavation. Please assume straight-line phreatic surface from borehole 01 to clay surface and then water table coincide with canal surface. No accumulation of water is expected on canal bottom due to pumping.