CIVL7225 Dam and Embankment EngineeringAssessment 3: Internal Erosion Assignment Report

Dr Adnan Sufian

May 6, 2020

Key Dates:

Question 2 PSD Graph: 12pm Wednesday 13 May 2020Submission Date: 12pm Wednesday 20 May 2020

Weighting: 30%

Aims & Objectives

Internal erosion is a major contributor to failure of embankment dams. Failure can occur

within the embankment, its foundations or at the interface between the embankment and

its foundation. Design considerations for internal erosion requires the provision of effective

filters, in addition to assessing the susceptibility of available construction material and nat-

ural foundation material to internal erosion. In this assessment, you will explore laboratory

assessment of internal erosion, the design of an effective filter zone and assess the susceptibil-

ity to suffusion in a laboratory setting. This will require your knowledge of seepage through

embankments and its foundations covered in previous coursework and assessments.

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Requirements

You are required to work independently and prepare a report that addresses all the questions

set out below. All work towards any component of the report must be conducted solely by

you. The report must be submitted via Blackboard by 12pm Wednesday 20 May 2020,

along with the signed EAIT coversheet which can be accessed here.

Submit a PDF that contains either handwritten responses (ensuring that any handwritten

responses are neat and clearly legible) or a typed document that provides all of the steps

and assumptions made to attain the solution. The PDF file should have the following file

name convention:

{STUDENT_ID}_{FULL_NAME}_Assessment_03_CIVL7225.pdf

In addition, any external document that was required to obtain the solution, such as Excel

spreadsheet, MATLAB scripts/notebooks or Python scripts/notebooks, must also be sub-

mitted via Blackboard. Use an appropriate naming convention for these supplementary files.

For example:

{STUDENT_ID}_{FULL_NAME}_Assessment_03_CIVL7225_Q1_EXCEL.xlsx

Marking Criteria

The total available marks for this assessment will be 30. The marks available for individual

questions are set out below.

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Question 1 (12 marks)

Figure 1 shows the permeameter employed in the laboratory practical discussed in the course.

The apparatus consists of a head tank, where the water level in the head tank is located a

distance H above the base the permeameter cell. The cell has a total length L and a circular

cross-section area of A, where d is the diameter of the cell. The total flow rate measured at

the outlet is q. The permeameter cell comprises three piezometer ports, located at a distance

of he1, he2 and he3 from the base of the cell. The water in the piezometer tube rises to a

height of hp1, hp2 and hp3 above the piezometer port, as shown in Figure 1a.

The sample is prepared with a screen of thickness LS, which is overlain by the base soil

of thickness LB, and subsequently the filter soil of thickness LF , as shown in Figure 1b.

Above the filter is a column of water of thickness LW . When no erosion is initiated, the

configuration shown in Figure 1b is maintained. When erosion occurs, the formation of a

mixture zone of thickness LM is observed, as shown in Figure 1c.

Address each of the questions set out below. Show all working out clearly and logically. State

any assumptions made in attaining your solution and justify why they are appropriate.

(a) For a given H, discuss whether the flow rate, q, would increase or decrease if the thickness

of the base soil (LB) was increased or decreased. Assume that LS+LB+LF remains constant,

that is, the total thickness of the soil column remains the same, and that the screen thickness

remains constant.

(b) Under ideal conditions, the total head at the base of the permeameter cell is H. However,

head loss is inevitable through the pipes connecting the head tank and the permeameter cell.

Suppose that the total head at the base of the permeameter cell is αH, where α is the head

loss coefficient to account for losses in total head through the pipe. Derive an expression

for the head loss coefficient based on the sample configuration shown in Figure 1b. The

derived expression must be a function of the variables shown in Figure 1. Assume that the

lower-most piezometer is located within the base layer.

(c) Suppose that only the permeability of the base soil was known. Derive an expression

for the permeability of the mixture zone in Figure 1c as a function of the variables shown in

Figure 1 and the permeability of the base soil.

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(d) Derive an expression for the total head at the interface of the mixture zone and base

soil shown in Figure 1c. The derived expression must be a function of the variables shown in

Figure 1. Show that the proportion of head loss in the mixture zone as a proportion of the

total head loss in the sample approaches a value of 1 when the base in completely eroded

into the filter layer.

(e) Sample experiment data from the laboratory practical is shown in Table 1. Note that

the units of all entries is mm (millimetres), with the exception of the flow rate q which is in

units of mm/s (millimetres per second). The permeameter cell length is L = 300, while the

diameter of the cell is d = 75. The piezometer ports are located at a distance of he1 = 80,

he2 = 140 and he3 = 200 from the base of the cell. The screen layer thickness is LS = 40.

Using this sample data, calculate the hydraulic gradient in the base soil and mixture zone.

Calculate the pressure head that would be observed in the lower-most piezometer (hp1) when

H = 1300 and a mixture zone is formed.

Table 1: Sample Data from Laboratory Practical

H LB LM LF q hp1 hp2 hp3

700 80 0 80 1.2× 102 230 160 100

1300 70 20 70 5.6× 102 ? 160 100

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𝐻

𝐿

𝑄

ℎ ℎ ℎ

ℎ

ℎ

ℎ

𝐴 = 𝜋 4⁄ 𝑑

(a)

𝐿𝐿

𝐿𝐿

Screen, 𝑘

Base, 𝑘

Filter, 𝑘

(b)

𝐿𝐿

𝐿𝐿

𝐿

Filter, 𝑘

Screen, 𝑘

Base, 𝑘

Mixture, 𝑘

(c)

Figure 1: (a) Permeameter apparatus used to investigate base filter compatibility; (b) Sampleconfiguration when no erosion has occurred; (c) Sample configuration when erosion hasoccurred resulting in a mixture of the base and filter.

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Question 2 (8 marks)

Figure 2 shows a zoned embankment dam with a central core. The embankment is overlying

an impervious bedrock. A filter-drain has been proposed downstream of the core, as shown

in Figure 2.

The parameters required to generate the particle size distribution are provided in Table 2,

where individual values have been assigned to each student. Use the attached Excel file,

q2 Generate PSD.xlsx, to generate the particle size distribution for the core using the pa-

rameters provided in Table 2. It is a mandatory requirement to email a.sufian@uq.edu.au

a graph of the particle size distribution generated from this Excel file by 12pm Wednesday

13 May 2020. The attached Excel file also contains the particle size distribution of the

shell.

(a) Using the Recommended Filter Design Method (Lecture 09), determine the range of

allowable particle size distributions for the filter zone of this zoned embankment dam. Clearly

show all steps taken to achieve this solution and state any assumptions required. A particle

size distribution graph of the filter zone must be provided with the solution.

CORE SHELLSHELL

FILTER DRAIN

80m

90m

75°75°

𝑤

Figure 2: Zoned embankment dam with a central core and filter drain system.

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(b) The permeability of the core is k = 2 × 10−6m/s. The permeability of the filter zone

material can be estimated from the Kozeny-Carman equation:

k =1

k0T 2S20

· γwµw

· e3

1 + e(1)

where k0 = 2.5 is the pore shape factor, T =√

2 is the tortuosity factor and S0 is the specific

surface area (ratio of surface area to volume). Consider the approximation S0 = 6/Drep,

which can be obtained by considering the surface area to volume ratio of a sphere with

diameter Drep. The representative particle size Drep is usually obtained by considering the

entire distribution, but in this assessment it can be approximated by DF15 of the filter zone

particle size distribution. γw = 9.8kN/m3 is the unit weight of water and µw = 0.001Pa·s is

the dynamic viscosity. Filter zones typically consist of well compacted materials, so the void

ratio can be assumed to be e = 0.56 which is the random close packing limit of spheres.

The discharge capacity of the filter drain is given by:

q =khw

l(2)

where k is the permeability of the filter zone material, h is the vertical height that the

phreatic surface intersects the filter drain, w is the perpendicular width of the filter drain

and l is the free seepage length on the downstream face of the core (Lecture 07, Slide 56).

Specify the minimum thickness, w, of the filter drain and discuss the implications of your

results.

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Table 2: Student Data for Question 2

Student ID Name 𝒂 𝒃 𝒅𝒎𝒂𝒙 𝒇𝒄

46365967 Ricardo Antonio Angola Escalona 1.1 10 0.5 40

45565274 Yvan Manuel Arosquipa Nina 1 8 0.7 42

45537486 Diana Lizet Villena Bejarano 0.8 11 0.6 34

45218141 Dipanshu Dhiman 0.9 20 0.5 40

45215458 Jingying Dong 0.6 13 1 46

46206613 Samuel Alexander William Holden 1.1 6 1 34

45964439 Haoyi Hu 1.4 20 0.6 32

44310860 Yin Fung Lee 1.1 13 0.8 46

45865264 Haolin Liao 1 14 0.8 40

45651636 Boliang Liu 1.1 14 0.7 34

45124970 Chang Liu 1.3 6 0.5 50

45843208 Yujie Liu 1.3 11 0.7 32

44087117 Zhengtao Lu 1.3 9 0.9 40

46030340 Charity Joseph Manoppo 0.7 11 0.9 32

44935218 Mirabbas Parmoon 1.4 18 0.7 48

45419283 Jawad Qassem 1.3 13 0.7 42

45348927 Amrit Ranabhat 1.2 17 0.8 48

40566052 Christopher Paul Sampford 1.3 18 0.9 38

44783503 Jia Song 1 14 0.7 48

43714050 Ruolin Wang 0.6 7 0.6 42

45430756 Tengzhou Wang 0.7 18 0.8 48

45347881 Zhidiao Weng 1 14 0.6 46

45695694 Jin Wu 1.1 7 0.9 48

43852176 Ziqi Xue 1.4 17 1 40

45995152 Qifan Yang 1.5 7 0.6 48

44160953 Pok Man Yau 0.5 16 1 36

46176903 Jiapeng Zhang 1.5 5 1 46

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Question 3 (10 marks)

The research group at UQ is planning an extensive laboratory investigation of suffusion.

However, there are currently no material available which are susceptible to suffusion, and

hence, the samples must be artificially manufactured by mixing available soils. Soil samples

labelled s1 to s8 in Figure 3 will be considered for the mixing process. Numerical data for the

particle size distributions is shown in Table 3, where particle size is indicated in millimetres

(mm) and the percentage passing (%) is shown for each soil sample.

By mixing the available soil samples shown in Figure 3, create six experimental test samples

that each satisfy one of the following criteria:

• Split Distribution - stable: 3 ≤ (DC15/D

F85)max ≤ 4

• Split Distribution - borderline: 4 ≤ (DC15/D

F85)max ≤ 5

• Split Distribution - unstable: 5 ≤ (DC15/D

F85)max ≤ 6

• Entire Distribution - stable: 1.3 ≤ (H/F )min ≤ 1.6

• Entire Distribution - borderline: 1.0 ≤ (H/F )min ≤ 1.3

• Entire Distribution - unstable: 0.7 ≤ (H/F )min ≤ 1.0

For each identified experimental sample, state the two soil samples considered for mixing,

along with the proportion of each soil sample in the mixture. In addition, provide a graph

of the particle size distribution for each identified experimental sample showing the critical

condition. This graph show include the distribution of both soils, along with the distribution

of the mixture. Show all working out and any assumptions required to arrive at the solution.

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Figure 3: Particle size distribution of available soil samples.

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Table 3: Particle Size Distribution Data

Size (mm) s1 s2 s3 s4 s5 s6 s7 s8

4.750 100

2.360 82

1.700 69

1.400 100

1.210 99 56

1.000 77 100

0.850 54 100 99 100

0.710 22 99 100 86 98

0.600 8 98 94 100 51 96 37

0.500 2 78 17 93

0.425 0 46 97 5 88 99 28

0.355 21 0 70 96

0.300 9 57 73 41 89 21

0.250 4 20 80

0.212 0 23 8 72 16

0.150 15 1 2 40 11

0.106 0 0 9 5

0.075 2 3 3

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