SEV362 Geotechnical Engineering
Assessment 1: Problem Solving Task
Total marks: 20%
Due: 8:00 pm AEST on Monday 7 August 2023
Part A: Analysis of 2D seepage under a dam
A dam, shown in Figure 1, retains 10 m of water. A sheet pile wall (cutoff curtain) on the
upstream side, which is used to reduce seepage under a concrete dam, penetrates 7 m into a
20.3 m thick silty sand stratum. Below the silty sand is a thick deposit of practically impervious
clay. The average hydraulic conductivity of the silty sand is 2.0 x 10-4
cm/s. Assume that the
silty sand is homogeneous and isotropic. Gs = 2.7
Compulsory questions:
(a) How do you propose to estimate the seepage loss through the silty sand layer Determine
the flowrate.
(b) Calculate and draw the porewater pressure distribution along the base of the dam.
(d) Determine the uplift force.
(e) Determine the maximum hydraulic gradient.
(f) Will piping occur if the void ratio of the silty sand is 0.8
(g) What is the effect of reducing the depth of penetration of the sheet pile wall
Figure 1
Extended questions:
(a) How do you estimate the seepage loss if the silty sand was anisotropic Assume kH =
2.0 x 10-4
cm/s and kV = 1.0 x 10-4
cm/s. Estimate the new flowrate.
(b) Compare the flowrate obtained above with that of a typical concrete dam. Comment.
(c) What are the factors that can increase the seepage loss overtime
(d) What are the other remedial measures available to reduce seepage and the risk of
piping
Part B: Analysis of consolidation data
A sample of dark grey silty clay obtained from borehole #bh2001-12 (E321858.2
N5811549.08), at depth of from 5.3 m to 5.8 m, in Burnley tunnel site investigation. A
specimen from this sample has been subjected to a consolidation test using the procedure
AS1289.6.6.1-1998. The depth to the ground water level was 2.8 m.
The results of the test, starting at 13 kPa (seating pressure), involving four pressure increments
that produced a final pressure on the sample of 213 kPa, are given below.
Sample diameter (mm) Initial
height
(mm)
Initial
Pressure
(kPa)
Particle
Specific
Gravity
Initial
sample
mass (g)
Oven dry
mass (g)
60 20 13 2.62 96.28 65.1
Final
Pressure
(kPa)
27 53 106 213
Time
(minutes)
Height
change
(mm)
Time
(minutes)
Height
change
(mm)
Time
(minutes)
Height
change
(mm)
Time
(minutes)
Height
change
(mm)
0 0.000 0 0.000 0 0 0 0
0.1 0.064 0.1 0.046 0.1 0.068 0.1 0.106
0.25 0.080 0.25 0.062 0.25 0.09 0.25 0.136
0.5 0.100 0.5 0.074 0.5 0.112 0.5 0.206
1 0.126 1 0.094 1 0.14 1 0.29
2.25 0.150 2.25 0.122 2.25 0.188 2.25 0.362
4 0.178 4 0.156 4 0.234 4 0.442
6.25 0.206 6.25 0.182 6.25 0.28 6.25 0.51
9 0.230 9 0.202 9 0.318 9 0.538
16 0.262 16 0.228 16 0.382 16 0.642
30 0.292 30 0.270 30 0.45 30 0.812
60 0.314 60 0.300 60 0.52 60 1.002
120 0.332 120 0.330 120 0.58 120 1.166
300 0.354 180 0.348 180 0.612 180 1.248
1440 0.386 240 0.358 240 0.638 240 1.316
2805 0.398 300 0.366 360 0.664 360 1.382
420 0.380 1230 0.736 1215 1.53
540 0.390 1440 0.742 1695 1.59
1425 0.412 2655 1.646
1605 0.414 4185 1.7
6975 1.764
Compulsory questions: Determine the followings.
1. Coefficient of consolidation .
Determine the coefficient of consolidation using both Taylor and Casagrande methods
Quote your values in m2
/year.
2. Coefficient of Volume Change mv.
For each of the four pressure increments compute the coefficient of volume change (kPa-1
) and
coefficient of permeability (m/sec).
3. Void ratio – Log (pressure) plot
Calculate the void ratio at the start of the first test, and at the completion of primary
consolidation for each load increment, then plot these void ratios against the logarithm (base
10) of the applied pressure. From this plot estimate
i. Compression Index Cc
ii. Recompression Index Cr
iii. Preconsolidation pressure
iv. Over Consolidation Ratio of the in-situ soil.
Extended questions:
A site containing a similar dark grey silty clay is being investigated for development. It consists
of a 5 m thick sand layer which overlays the silty clay layer (4 m thick). The bottom of the clay
is marked by an impermeable rock. If a proposed structure on the site is expected to apply a
surcharge of 30 kPa, what would be the ultimate primary consolidation settlement
Comment on your answer and discuss geotechnical solutions available.
Report Format
Report should not exceed 12 A4 pages including cover page, figures and appendices. Show
specimen calculations where necessary.
