2011-09-09 tau tona trench replacement pipeline - design

TAU TONA TRENCH REPLACEMENT DETAILED DESIGN

DESIGN REPORT

September 2011

PREPARED BY:

AURECON AME

PO Box 74381

Lynnwood Ridge

0040

South Africa

Docex 264

CONTACT PERSON: P. GROBLER

Tel No: +27 12 427 3153

PREPARED FOR:

ANGLO GOLD ASHANTI

P.O. BOX 8044

Western Levels

2501

South Africa

CONTACT PERSON: MR L. SCHOEMAN

Tel No: +27 18 700 2680


Rev 2.0, September 2011

1

TABLE OF CONTENTS

1. INTRODUCTION ................................................................................................................. 1

2. RESPONSIBILITIES / INTERFACING ................................................................................ 1

3. SCOPE OF WORKS ........................................................................................................... 2

3.1 BATTERY LIMITS ............................................................................................................. 2

3.2 PIPELINES ..................................................................................................................... 2

3.2.1 Tau Tona Trench Replacement Pipeline ...............................................................................2

3.2.2 Refrigeration Plant Pipeline ...................................................................................................2

4. OVERVIEW OF EXISTING SYSTEM .................................................................................. 3

5. OVERVIEW OF PROPOSED SYSTEM .............................................................................. 5

5.1 DESIGN FLOWS .............................................................................................................. 5

5.1.1 Storm water ...........................................................................................................................6

5.1.1.1 Tau Tona Shaft and Red Soil Area....................................................................................6

5.1.1.2 Refrigeration Plant Area ....................................................................................................6

5.1.1.3 Clean water areas..............................................................................................................6

5.1.2 Process Water .......................................................................................................................7

5.1.3 Total Design Flow in Tau Tona Trench Replacement Pipeline .............................................7

5.2 PIPES ............................................................................................................................ 8

5.2.1 Selection of Pipe Material ......................................................................................................8

5.2.2 Pipe Jointing ..........................................................................................................................8

5.3 CONCRETE .................................................................................................................... 9

5.4 DESIGN OPTIONS ........................................................................................................... 9

5.4.1 Full Pressure System ............................................................................................................9

5.4.2 Full Gravity System – No Attenuation....................................................................................9

5.4.3 Storm water Attenuation ..................................................................................................... 10

5.4.4 Combined Pressure and Gravity System with Attenuation ................................................ 11

5.5 FINAL DETERMINATION OF PIPE DIAMETER .................................................................... 11

5.5.1 Upper Section ..................................................................................................................... 12

5.5.2 Traffic Circle ........................................................................................................................ 12

5.5.3 Lower Section ..................................................................................................................... 12

5.6 PIPE LONGSECTION...................................................................................................... 12

5.6.1 Route Description ............................................................................................................... 12

5.6.2 Inlet Works .......................................................................................................................... 14

5.6.3 Dissipation Structure .......................................................................................................... 14

5.6.4 Attenuation Basin................................................................................................................ 15

5.6.5 Drop Structure .................................................................................................................... 15

5.6.6 Parshall Flume .................................................................................................................... 15

5.6.7 Outlet Structures to Nursery Dams ..................................................................................... 16

5.6.7.1 Maintenance of the Nursery Dams ................................................................................. 16

5.6.8 Traffic Management ............................................................................................................ 16

6. CONCLUSIONS ................................................................................................................ 17

1. APPENDIX A .................................................................................................................... 18

1.1 HYDRAULIC DESIGN VARIABLES .................................................................................... 18

1.2 PIPE LENGTHS AND ELEVATIONS ................................................................................... 18

1.3 .PIPE DIAMETERS ......................................................................................................... 18

1.3.1 Pipe Roughness ................................................................................................................. 18

2. APPENDIX B: ................................................................................................................... 20



2

LIST OF TABLES

Table 1: Responsibilities List ...................................................................................................... 1

Table 2: Recorded Monthly Average Flows in the Tau Tona Trench........................................... 3

Table 3: Baseline Information – Tau Tona Trench ...................................................................... 4

Table 4: Summary of Chemical Analysis of Decanting Water ..................................................... 5

Table 5: Storage Volumes Required for Attenuation Structure. ................................................. 11

Table 6: Pipe Inside Diameter Calculations .............................................................................. 18

Table 7: Pipe Roughness ......................................................................................................... 19

Table 8: Hazen Williams Pipe Roughness ................................................................................ 19

LIST OF FIGURES

Figure 1: Example of traffic accommodation at turning circle. ................................................... 17



1

1. INTRODUCTION

Following on from the feasibility study undertaken by Aurecon in 2010, AngloGold Ashanti

Limited appointed Aurecon to carry out the detailed design of a new pipeline to transport the

water currently being released into the Tau Tona Trench. The pipeline will follow a similar route

to the existing trench, and will discharge into two small dams on the mine, referred to as the

Nursery Dams.

This report is compiled to inform on the following items:

• Calculation of the expected flow in the pipeline

• Discussion on the horizontal alignment of the pipeline

• Discussion on the vertical alignment of the pipeline

• Design of the inlet structure of the pipeline

• Design of the energy dissipation structures

• Design of the flow measurement structures

• Design of the outlet structure

The following information was made available to Aurecon and formed the basis of this report:

• Verbal confirmation of the water being pumped from the mine

• Survey information undertaken by Ingwe Topographic and Engineering Surveys of the route

of the pipeline

• Aerial photography of the route of the pipeline.

• Cadastral Plan of the mine

• Chemical analysis of the decanting water

2. RESPONSIBILITIES / INTERFACING

Table 1: Responsibilities List

RESPONSIBLE ACTION

Aurecon Design of Tau Tona Trench Replacement Pipeline (TTTRP)

Aurecon Design of Pipeline from the Refrigeration Plant to the TTTRP

Aurecon Design drawings for Tender – TTTRP and Refrigeration Plant pipeline

Aurecon Design Report – TTTRP and Refrigeration Plant pipeline

Aurecon Specifications to SANS1200 and preliminary BoQ

AngloGold Ashanti Tender Documentation

AngloGold Ashanti Surface water improvements to separate clean and dirty water runoff entering the TTTRP

AngloGold Ashanti Backfilling of trench and surface water improvements to replace trench



2

3. SCOPE OF WORKS

This scope of works covered by this report is highlighted below.

3.1 BATTERY LIMITS

The inlet works to the new Tau Tona Trench Replacement Pipeline forms the upper battery limit

of this project. The existing structure onto which the inlet is being constructed, the upstream

channel and improvements to the drainage network upstream of the new pipeline are not

included in the scope of this project.

The outlet from the Refrigeration Plant forms the upper battery limit for the new pipeline from the

plant to the tie in with the new Tau Tona Trench Replacement Pipeline.

The outlets to the Nursery Dam form the lower battery limit.

3.2 PIPELINES

3.2.1 Tau Tona Trench Replacement Pipeline

The design of a new pipeline from the inlet structure to the Nursery Dams, including:

• The preliminary design of a new 710 mm external diameter HDPE pressure pipeline linking

the outlet structure to the stilling basin

• The preliminary design of a new 600 mm internal diameter HDPE gravity pipeline linking the

stilling basin to the Nursery Dams;

• Earthworks and bedding;

• A flume to measure flows;

• Pipe crossings under roads;

• Control structures to allow the flows to be diverted into either the upper or lower Nursery

Dams.

The water to be conveyed is process water from the Tau Tona shaft, and dirty water runoff from

the shaft area. The process water may become acidic.

3.2.2 Refrigeration Plant Pipeline

The design of a new process water pipeline linking the Refrigeration Plant to the new Tau Tona

Trench Replacement Pipeline, including:

• The preliminary design of a new minimum 200 mm internal diameter HDPE pipeline

• Earthworks and bedding;

• Pipe crossings under road;


4. OVERVIEW OF EXISTING SYSTEM

The Tau Tona Trench was constructed to transport water pumped from the adjacent shaft as

part of the mine dewatering, to the Nursery Dams. The water being pumped from the shaft is

classified as process water, and therefore requires treatment before it can be released to the

environment.

Pump flow rates from the Tau Tona Shaft vary between 220 ℓ/s and 440 ℓ/s. These are

discussed in more detail later in the report.

Also included in the trench is the storm water from certain areas of the development

surrounding the Shaft, road drainage, and runoff and process water from the refrigeration plant,

as described in detail later in this report.

The trench is culverted beneath the traffic circle, and passes through a gauging flume before

discharging into either the upper or lower Nursery Dams.

The average flows recorded at the flume and provided by AngloGold Ashanti are presented

below in Table 21.

Table 2: Recorded Monthly Average Flows in the Tau Tona Trench

Jan 09 Feb 09 Mar 09 Apr 09 May 09 Jun 09 Jul 09

Kℓ/Month 585 320 641 259 510 440 440 813 710 663 543 390 642 329

ℓ/s 226 247 197 170 274 210 248

Jan 10 Feb 10 Mar 10 Apr 10 May 10 Jun 10 Jul 10

Kℓ/Month 520 611 760 654 368 636 808 481 622 732 753418 691 201

ℓ/s 201 293 142 312 240 291 267

Aug 09 Sep 09 Oct 09 Nov 09 Dec 09 Avg. 2009

Kℓ/Month 654 131 567 365 609 237 380 704 724 052 585 320

ℓ/s 252 219 235 147 279 226

Aug 10 Sep 10 Oct 10 Nov 10 Dec 10 Avg. 2010

Kℓ/Month 682 883 578 050 689 436 667 641 636 380 648 344

ℓ/s 263 223 266 258 246 250

The recorded flows indicate that the average flow in the trench (recorded at the flume) for 2009

and 2010 were 226 ℓ/s and 250 ℓ/s respectively. This included rainfall runoff from a significant

area surrounding the shaft and trench.

Given the large surface water catchment currently draining to the trench, pumping in isolation is

therefore only expected to make up a portion of the recorded average flows mentioned above.

This suggests that there are either times when no process water is pumped into the trench, the

time-steps for recording flows are too course to properly capture rainfall events, or there are

potential inaccuracies in the readings.

1 Emailed by Luther Schoeman at AGA; 1 July 2011


In several months for example, average flows dropped below 220 ℓ/s (0.22m3/s), either due to

low rainfall, reduced pumping, or combination thereof.

The Nursery Dams form part of the mine’s dirty and process water containment system, which

includes the recently upgraded North Boundary Dam. It is understood that the Nursery Dams

are currently unlined, but that these will be lined in the future to prevent seepage losses into the

environment.

The baseline information is summarised in Table 3 below.

Table 3: Baseline Information – Tau Tona Trench

Existing connection to Nursery Dams from Tau Tona Shaft and Refrigeration Plant

Type of trenches Unlined earth

Conveyance method Gravity

Source of Water

• Tau Tona Shaft – pumped to trench.

• Surface water runoff – conveyed in pipes and ditches from

the Tau Tona Shaft and Refrigeration Plant areas. Overland

flow for other areas.

• Refrigeration Plant – combined in surface water system

(unlined open ditch).

Types of Water

• Tau Tona Shaft – process water from below ground.

• Tau Tona Shaft area – both clean and dirty surface water

runoff.

• Red Soil Area (recently remediated) and residual parts of

catchment - clean surface water runoff.

• Refrigeration Plant –process water and clean surface water

runoff.

Destination Nursery Dams (linked to North Boundary Dam)

Flow capacity

• Tau Tona Shaft – 220 ℓ/s to 440 ℓ/s of process water

• Surface water runoff – not calculated.

• Refrigeration Plant – 75 ℓ/s max of process water

Length from inlet to lower Nursery

Dam outlet 2000 m (approx. length)

The decision by AngloGold Ashanti to replace the Tau Tona Trench with a pipeline was

motivated by the need to remove the potential hazard of an open trench. It is deep and runs

immediately adjacent to the access road to the shaft, with little to prevent access into the trench.

The flow rate and velocity of the water in the trench are also high, thereby contributing towards

the hazardous classification of the trench.

The trench is also unlined, and no barrier exists between the process water and the

environment. The installation of a pipeline to convey the process water and replace the trench

will be a significant environmental improvement on the current situation.


The quality of the water being pumped out of the mine is show in Table 4 below. This data was

made available to Aurecon by AngloGold Ashanti.

Table 4: Summary of Chemical Analysis of Decanting Water

Constituent Min Ave Max

pH 6.5 7.4 8.1

EC (mS/m) 122.0 256.7 307.0

TDS (mg/l) 891.0 1868.9 2478.0

SS (mg/l) 10.0 35.2 127.0

CN_R (mg/l) 0.0 0.1 0.1

Cl (mg/l) 116.0 339.0 503.0

NO3 (mg/l) 1.7 11.5 24.0

SO4 (mg/l) 401.0 756.3 1042.0

Fe (mg/l) 0.0 0.0 0.1

Mn (mg/l) 0.1 0.3 0.8

Ca (mg/l) 91.0 229.8 302.0

Mg (mg/l) 24.0 40.9 49.0

Na (mg/l) 96.0 232.1 299.0

K (mg/l) 6.0 15.0 34.0

TALK (mg/l) 10.0 38.6 63.0

CaCO3_TH (mg/l) 352.1 743.8 941.0

It is evident from the data in Table 4 that the Total Dissolved Solids (TDS) are elevated, and that

the dominating ions are Sulphate (SO4), Chloride (Cl) Calcium (Ca), Magnesium (Mg) and

Sodium (Na). The quality of the water does not exhibit the quality typically seen from Mine

Drainage Water. It is, however, not know whether the quality of the water may deteriorate with

time, and the corrosive potential of the water therefore needs to be taken into account in the

specification of the construction materials.

5. OVERVIEW OF PROPOSED SYSTEM

5.1 DESIGN FLOWS

As summarised in Table 3, the existing Tau Tona Trench currently receives the following flows:

• Process water being pumped from the mine shaft;

• Storm water from the developed area adjacent to the shaft;

• Storm water from the access road to the shaft, traffic circle and the adjacent open veld;

• Process water gravity fed from the Refrigeration Plant;


• Storm water from the area surrounding the Refrigeration Plant.

5.1.1 Storm water

5.1.1.1 Tau Tona Shaft and Red Soil Area

The Tau Tona Trench currently discharges into the Nursery Dams/North Boundary Dam

containment area, and conveys process water, and clean and contaminated surface water

runoff.

The storm water catchment surrounding the Tau Tona mine shaft and open veld (Red Soil Area)

was reviewed at a strategic level as part of the West Wits Mining Area Infrastructure Plan2. The

resulting report showed that if un-remediated, the area surrounding the inlet to the existing

trench and adjacent Red Soil Area may generate runoff that would be termed dirty water, under

Regulation 704 of the National Water Act3. Runoff from these areas would therefore need to be

directed to the North Boundary Dam rather than directly to the environment.

The report highlighted areas for rehabilitation and possible mitigation measures to remove

contaminants from the runoff. A subsequent report4 considered the clean and dirty water

separation for the built up area at the shaft in greater detail and expanded upon improvements

to the drainage in that area.

Following review of these aforementioned reports, the separation of runoff streams was

discussed with AngloGold Ashanti’s engineering and environmental representatives5, and it was

confirmed that the Red Soil Area had been remediated and was no longer classified as a dirty

water runoff area. It was also confirmed that the drainage improvements proposed at the shaft

area would all be implemented, which would result in a residual dirty water runoff area of 3.2

Ha.

Runoff from this area will need to be accommodated in the new pipeline for events up to the

critical 1 in 50 year storm. The corresponding critical surface water runoff rate to be

accommodated in the pipeline, as specified in the Etek Report, is 1100 ℓ/s 4.

Based on the information provided, the remaining areas will be reclassified as clean water

runoff areas, which do not require treatment. Storm water from these areas will no longer be

directed to the Nursery Dams, and will be collected in a separate drainage system. Flows can

consequently be released directly into the environment.

5.1.1.2 Refrigeration Plant Area

The refrigeration plant area is considered to be a clean water runoff area, which does not

require treatment. It can consequently be released directly into the environment.

The installation of a separate pipeline to separate process and surface water runoff from the

refrigeration plant will mean that storm water from this area will no longer be directed to the

Nursery Dams, and will be collected in a separate drainage system.

5.1.1.3 Clean water areas

As the existing drainage system of the local area has functioned on the basis that all storm

water drains to the Tau Tona Trench, and on into the Nursery and North Boundary Dam, the

2West Wits Mining Area Infrastructure Plan, Volume 3: Document Number: 105225-W-03; Aurecon; 10 December 2009

3 Regulation 704 of the National Water Act, Act 36 of 1998

4 Conceptual Design Report: Surface Water Investigation Project TauTona Mine Rev 1; E-Tek Consulting; May 2011

5 Minutes of meeting (402688 - Tau Tona Trench Replacement meeting); Aurecon; 22 July 2011


proposed trench replacement pipeline will allow for clean and dirty water runoff streams to be

separated, in line with Regulation 704 3.

It is a requirement that subsequent to the separation of process as well as clean annual dirty

storm water and the Tau Tona trench is filled in, provision is made for the area to be free

draining. It is recommended that a study is carried out to identify the storm water inflow points,

means of conveyance, and discharge points. This should be carried out prior to backfilling the

trench.

5.1.2 Process Water

The Tau Tona Shaft is dewatered by two pump sets, each consisting of two pumps. Each pump

set has an individual delivery line to surface.

Each pump within the set can deliver between 220 ℓ/s and 225 ℓ/s6. The pumps are operated

such that only one pump per pump set will be operational at any given time. This will mean that

with only one pump set running, approximately 220 ℓ/s will be pumped out of the mine.

The maximum pumping rate with two pump sets running will be approximately 440 ℓ/s. This

value was confirmed to Aurecon on 4 August 2010 as a maximum capacity and was agreed as

part of the feasibility study7.

On average the Tau Tona Shaft pumps approximately 22 Mℓ of water per day, equivalent to 255

ℓ/s. This is in excess of what a single pump set can deliver (220 ℓ/s).

To achieve this, both pump sets are therefore run concurrently for approximately 5 hours per

day, at a total combined flow of approximately 440 ℓ/s. For the remainder of the time, only a

single pump set is required to achieve the remainder of the daily flow.

In addition to process water from the shaft, the refrigeration plant also produces process water

which currently drains into the Tau Tona Trench. Based on information provided by AngloGold

Ashanti8, the peak flow from the refrigeration plant occurs when the associated storage dam is

emptied. AngloGold Ashanti requested that an allowance of 75 ℓ/s is provided in the new

pipeline to accommodate flows from this plant.

The design of the process water pipeline from the refrigeration plant is based on this maximum

design flow rate.

5.1.3 Total Design Flow in Tau Tona Trench Replacement Pipeline

The maximum design flow in the pipeline will be 1 540 ℓ/s.

The use of attenuation may be used to reduce the peak flow rate in a 1 in 50 year storm event,

and this is considered further in Section 5.4.3.

No additional flow allowance has been made for the refrigeration plant in the overall peak

design flow, as the process water storage facility at the plant will not be drained during storm

events. It will only be drained during dry weather and will consequently be able to utilise the

available storm water capacity in the pipeline.

6 Email from Werner Grobbelaar (AGA), 3 Aug 2011

7 Conceptual Design and Feasibility Report, Tau Tona Trench Replacement Conceptual Design; Aurecon; 25 Aug 2010

8 Email from Luther Schoeman (AGA), 4 July 2011


5.2 PIPES

5.2.1 Selection of Pipe Material

The pH of the process water currently being pumped out of the mine is almost neutral. The pH

can, however, drop lower in the future and the process water will become acidic and corrosive.

The material of the new pipeline should thus be able to accommodate water with a low pH.

During the feasibility study, several pipe materials were considered including concrete, steel and

High Density Polyethylene (HDPE).

Acidic water will dissolve the aggregates of a concrete pipeline thereby reducing the pressure

rating of the pipeline to the point where the pipeline would collapse. Concrete pipelines could

therefore be lined on the inside with a lining that can withstand acidic and corrosive water.

Steel pipes also have to be lined on the inside with either an epoxy or rubber. They should also

be coated on the outside and require the installation of proper cathodic protection measures.

Steel pipes could also be affected by the growth of sulphate reducing bacteria, due to the high

concentrations of sulphate in the water.

An existing set of 760 mm nominal diameter unlined steel pipes is available at the AngloGold

Ashanti Vaal River Operation. This was considered as an option, however, the remaining design

life of the pipes was unknown and they would have needed to be sandblasted and lined/coated.

The cost of refurbishment of these pipes and the additional protection measures required, as

mentioned above, resulted in this option not being considered further.

An alternative to steel or concrete pipelines is HDPE, which can withstand water with low pH-

values without requiring a lining.

It was concluded that HDPE would be used as the pipe material for the project.

HDPE is available in both solid walled (pressure) and structural walled (gravity or very low

pressure) pipes. Given the positive gradient between the shaft and the nursery dams, three

options were available:

• Running the pipeline under pressure, using a solid walled pressure pipe. A minimum

SDR17, PE100, class 10 pipe would be required for this application, dictated largely by the

potential for unacceptable deflection in lower rated/thin walled HDPE pressure pipes.

• Utilising the positive gradient to convey water along the new PE100 HDPE structural walled

gravity pipeline. These pipes were cheaper than HDPE pressure pipes. A minimum ring

stiffness of 8kN/m2 would be required for this application to prevent unacceptable levels of

deflection along the pipe.

• Running part of the pipe under pressure, and part under gravity, using a combination of the

abovementioned pipes.

These are discussed further in section 5.4.

5.2.2 Pipe Jointing

The pipe material will be either solid or structural walled PE100 HDPE. It will be installed in 12

m lengths to minimise the number of joints. Connections between the individual lengths of pipe

will be done either through butt-welding (solid walled) or electro-fusion welding (structural walled

pipes), which is robust and produces a watertight joint which effectively creates a continuous

length of pipe.


The use of flexible HDPE pipes with butt and electro-fusion welded joints comply with design

guidance for gravity pipework in dolomitic areas9.

5.3 CONCRETE

As mentioned, the pH of the process water that is currently being pumped out of the mine is

almost neutral. This may nonetheless change and the process water could become acidic and

corrosive. The likelihood of this occurring is however unknown.

On this basis, the investment in lining of the concrete structures was reviewed and the decision

taken by AngloGold Ashanti10 not to line the structures at this stage, based on current water

quality.

The corrosiveness of the water will consequently be monitored by AngloGold Ashanti, along

with the condition of the concrete structures. Should the water quality deteriorate, AngloGold

Ashanti will undertake to install protective coatings on the structures, if necessary.

All concrete will use dolomitic aggregate to reduce the impact of any corrosive water onthe

integrity of the concrete, although this will not obviate the need to line the structures should the

corrosiveness of the water increase.

Wherever practical, HDPE manholes have been used to replace the need for concrete

structures.

5.4 DESIGN OPTIONS

5.4.1 Full Pressure System

Given the topography, the pipeline could be operated as a pressure system, whereby the static

head in the upper section is used to drive water through the lower section.

The static head available on the system is approximately 37 m to 39 m, depending on where the

pressure line is terminated.

Considering the peak design event, an 800 mm external diameter (705.9 mm internal diameter)

SDR17 HDPE pipe would be required.

The outlet flow controls and flow measuring structures would need to be designed to

accommodate the full design flow of 1 540 ℓ/s.

This option was rejected due to the high costs of large diameter HDPE pressure pipes.

5.4.2 Full Gravity System – No Attenuation

Given the topography, the pipeline could also be operated as a gravity system. The steep upper

sections could utilise the gradients to reduce pipe diameters, however as the gradients flatten

out a short distance upstream of the traffic circle, the pipe diameters need to be increased to

continue to provide the design capacity.

To dissipate the excess residual energy in the flows, running in excess of 6m/s at the change in

gradients, a stilling basin is required to protect the downstream section of pipeline and prevent

unstable flow conditions and hydraulic jumps from forming where the transition between highly

supercritical and subcritical flows could occur.

9 Appropriate Development of Infrastructure on Dolomite: Manual for Consultants; Department Of Public Works;

September 2010 10

Minutes of design review meeting (402688 - Tau Tona Trench Replacement meeting Rev A); Aurecon; 7 Dec 2010


Using a gravity system has the advantage of free flowing conditions under atmospheric

pressure. This allows for structural walled HDPE pipes to be used, with a significant cost benefit

when compared to solid walled pressure pipes.

Initial calculations indicated that a 600 mm internal diameter structural walled pipe would be

sufficient to convey design flows in the upper section. Diameters for the lower section increased

to 900 mm.

Along the 600 mm internal diameter upper section, velocities as high as 7.5 m/s could occur in

the steepest sections, and with the pipe running very nearly at full capacity. Due to the

extremely high velocities and supercritical flow characteristics, any minor disruptions to the flow

or increases in roughness could result in the pipe running full, with the associated drop in

capacity below the design threshold (typically when flows exceed 94% of the barrel area,

capacity begins to reduce). The energy grade line was also calculated to be between 1.8 m and

2.7 m above the pipe soffit, which also lies above normal ground levels. Any disruptions to the

flow could consequently result in overflows from manholes, and place additional strain on the

pipe and joints.

This led to the consideration of converting the upper section to a pressure line.

5.4.3 Storm water Attenuation

The design flow in the upper section of the pipeline will be 1 540 ℓ/s, comprising of 1 100 ℓ/s of

dirty water runoff, and 440 ℓ/s of process water pumped from the shaft.

The upper section uses the steep gradient to accommodate the flows in a smaller diameter

pipe, when compared to the lower section. Insufficient space and several constraints created by

existing infrastructure also limit the possibilities for attenuation closer to the inlet.

A short distance upstream of the traffic circle, the topography flattens out, therefore requiring

the pipe diameter to be increased to accommodate the flows.

Under normal dry weather conditions, flows in the pipeline will vary between 220 ℓ/s and 440 ℓ/s.

The capital investment to increase the downstream pipe diameters, flow control and measuring

structures to accommodate the 1 in 50 year design storm was therefore questioned. An

attenuation basin was investigated as an option to reduce the design flows for the lower flatter

section of the pipeline.

The attenuation volume is influenced by the outflow rates from the basin, and the pumping rate

of process water from the shaft, which will make up the baseflow in the pipeline. For

approximately 19 hours of the day, a single pump is operated to dewater at a rate of 220 ℓ/s,

and this is the most probable baseflow rate should a storm occur. A nil baseflow rate and

maximum baseflow rate of 440 ℓ/s were also considered.

These scenarios with various outflow rates are presented in Table 5.


Table 5: Storage Volumes Required for Attenuation Structure.

Storage Volume Required (m3)

Outlet Discharge (m3/s)

Base Flow = 0 m3/s

Base Flow = 0.22 m3/s

Base Flow = 0.44 m3/s

0.66 440 849 1509

0.70 382 762 1358

0.75 315 662 1194

0.80 254 570 1049

0.85 199 487 920

0.90 150 410 805

5.4.4 Combined Pressure and Gravity System with Attenuation

Given the topography, the upper section of pipeline could be operated as a pressure system,

whereby the static head in the upper section is used to drive high flows through this length of

the pipeline. The lower section would then run under gravity with larger diameter pipes.

The static head available on the upper part of the system is approximately 28 m, measured to

the stilling basin.

Considering the peak design event, a 710 mm external diameter (626.5 mm internal diameter)

SDR17 HDPE pipe would be required.

With the steep gradient and high flows and velocities experienced in the upper section, this

length of pipe will be installed as a butt welded HDPE pressure pipe. This is to control pressure

spikes caused by potentially unstable hydraulic conditions, and to ensure that water can be

retained within the pipeline for high flows and velocities up to the 1 in 50 year storm design

event.

To reduce these pipe diameters and the size of the flow measuring a control structures, an

attenuation basin to regulate peak flows was introduced immediately downstream of the stilling

basin. Flows will be limited to 660 ℓ/s. Final pipe diameters are discussed in the next section.

The lower section of pipe downstream of the stilling basin could continue to operate as a gravity

line, utilising structural walled HDPE pipe.

This first section of gravity pipe is also used as a flow restriction to choke downstream flows and

force excess water to overflow into the attenuation basin designed to accommodate the excess

runoff volume in a 1 in 50 year 24 hour storm event.

5.5 FINAL DETERMINATION OF PIPE DIAMETER

The Colebrook White and Manning equations were used to estimate a minimum inner diameter

of the new pipeline.

The pipeline can be broken into three sections:

• The steep slope from the inlet to upstream of the traffic circle (terminating at the stilling

basin), where gradients range between 3% and 5%;


• the crossing beneath the traffic circle (including the attenuation basin and drop structure),

and;

• the lower section from the traffic circle to the Nursery Dams, where gradients are shallower

and range between 0.67% and 1%.

5.5.1 Upper Section

In the upper section, the steep slope results in velocities between 6.0 m/s and 7.5 m/s under

peak flow conditions. Flow is supercritical and fast flowing, and a minimum internal diameter of

600 mm is required to convey water under gravity.

Under pressure, a 626.5 mm internal diameter pressure pipe was confirmed to be the minimum

standard SDR17 HDPE pipe diameter acceptable to convey water through this section.

5.5.2 Traffic Circle

Due to the raised construction of the traffic circle, the original pipe crossing proposed under the

road required deep excavation, with associated high costs and construction hazards. To

mitigate this, the gradient of the pipeline was reduced beneath the circle to minimise

construction depths. This requires minor ground raising upstream of the traffic circle to achieve

the minimum cover over the pipeline. A drop structure downstream of the crossing was required

to accommodate the change in elevation between the pipe crossing under the circle and the

downstream pipeline.

To minimise the gradient various pipe diameters were considered. It was concluded that a

600 mm diameter pipe laid at a shallow gradient would reduce the depth of excavation to

between 2.3 m and 2.5 m, whilst ensuring sufficient cover and capacity (660 ℓ/s) in the pipe.

5.5.3 Lower Section

Downstream of the drop structure at the traffic circle, velocities are significantly reduced to

between 2.4 m/s and 3.2 m/s. As with the upper section however, flows remain supercritical.

Pipe gradients for the lower section must remain steeper than 0.67% in order to maintain the

capacity of the pipeline.

5.6 PIPE LONGSECTION

5.6.1 Route Description

The pipeline will connect to the existing outfall channel at the shaft. (See Photograph 1 and

Photograph 2)


Photograph 1: Channel at Shaft

Photograph 2: Connection point into existing channel

The existing Tau Tona trench will be replaced by a HDPE pipeline. The pipeline will typically run

with a 10m centreline-offset from the edge of the existing trench, although its route is dictated in

places by existing infrastructure constraints.

From the new inlet works the pipeline will run parallel to the existing trench, underneath the

conveyor belt, through the existing traffic circle and down to the Nursery Dams. The new

pipeline will cross beneath several existing pipelines en route. A layout of the horizontal

alignment of the pipe route is given in drawing 346-M11315 in Appendix B. The route from the

inlet structure is shown in Photograph 3.

As shown in the photograph, large quantities of building rubble and redundant infrastructure are

present in the vicinity of the inlet structure, and for the length of new pipeline down towards the

crossing beneath the conveyor belt. It is recommended that this area is fully cleared, remediated

and levelled prior to the installation of the pipeline.


Photograph 3: View of route of the pipeline from the inlet

It is also understood that the conveyor belt will be demolished and this area rehabilitated. The

timeframe relative to the installation of the new pipeline is however unconfirmed. Should the

conveyor belt be decommissioned after the construction of the pipeline, then traffic

management should be implemented to prevent heavy plant from driving over the pipeline. An

allowance will be made in the design to accommodate a heavy vehicle crossing at the existing

access road to the conveyor belt.

5.6.2 Inlet Works

The most practical approach to the construction of theapproach to the construction of thenew

inlet works would be to construct a small concrete side channel out of the existing inlet works

and then to link the pipe to the side channel. To minimise disruption to the existing structure

and to the pipework currently supported on and around the structure, the existing opening in the

structure will be used for the new inlet works. The proposed concrete inlet works will be

constructed at 90 degrees to the existing structure. Water will enter the new inlet works and

then be turned through a long radius bend to run parallel to the existing trench again.

The gradient and size of the initial section of the pipeline has been designed to maintain

subcritical flow until the water has been turned to run parallel with the existing trench. After the

bend, the gradient increases to follow the slope of the ground, flow becomes supercritical within

the pipeline.

The new inlet works will be constructed as shown in drawing No. 346-M11317 of

Appendix B.

5.6.3 Dissipation Structure

The steep gradient for the upper section of the pipeline results in high velocities and

supercritical flow, with Froude Numbers in excess of 3.7 in places.

To manage the energy present in the water at the transition between the steep upper section

and the approach to the crossing beneath the traffic circle, an impact stilling basin will be

installed.

The stilling basin will reduce the residual energy and prevent the formation of uncontrolled

hydraulic jumps in the pipeline at the transition in gradient.


It also forms the downstream limit of the pressure system.

5.6.4 Attenuation Basin

The design flow in the upper section of the pipeline will be 1 540 ℓ/s. Downstream of the

dissipation structure; this will be reduced to 660 ℓ/s.

The basin will be able to attenuate flows up to the 1 in 50 year storm event, with an 800mm

freeboard. Under normal flow conditions, water will remain in the pipeline and not enter the

pond. Only when the water level reaches the soffit of the downstream pipe will water begin to

spill into the attenuation basin.

The basin will be lined with a HDPE liner.

5.6.5 Drop Structure

The existing traffic circle has been raised above the surrounding ground. To reduce the depth of

crossing beneath the raised traffic circle, the pipe will be installed at a shallow gradient, with

minor ground raising upstream of the circle to maintain the minimum depth of cover required

above the pipe.

The pipe will emerge aboveground on the downstream side of the circle and a drop structure

will be installed to return the flows to the downstream pipeline. The downstream pipeline will be

buried with a minimum cover of 900mm below the existing ground level downstream of the

circle, which is notably lower than the raised traffic circle and the level of the associated pipe

crossing beneath the circle.

The drop structure will include a steep chute and basin, installed with baffle blocks to force a

hydraulic jump. The jump will dissipate energy transferred to the flows by the rapid drop in level.

5.6.6 Parshall Flume

An impact stilling basin will be used to dissipate the energy in the system before the water flows

into the measuring flume. The energy dissipation structure will form part of the upstream

channel works associated with stilling and straightening the flow before it enters the flume.

The peak design flow for the flume is 515 ℓ/s, assuming all surface water is removed from the

pipeline.

A 1 oot Parshall flume (based on the width measured at the narrowest point), is able to

accommodate a maximum flow of only 457 ℓ/s, and is consequently too small for measuring

flows in the new pipeline11.

A 2 foot Parshall flume can measure a minimum flow of 12.1 ℓ/s and a maximum flow of 937 ℓ/s,

which will be sufficient for the flows expected in the Tau Tona Trench.

Measurement accuracies of 2-5%12 can be achieved for flow measurement using a flume,

although this is influenced by several factors, including installation tolerances. This is

considered sufficient for the mine’s flow measuring requirements.

A prefabricated mould will be used to ensure that the flume section complies with the design

dimensions, as suitable tolerances may not be achieved using concrete alone.

Upstream flow depths were considered using available formulae and doing backwater analysis

up the approach channel. Although turbulence can be expected at the outfall of the pipe through

the stilling structure, approximate levels expected at the outfall could be estimated. Flow depths

11

Handbook of Hydraulic Engineering (Table 169, p 506), Armando Lencastre, 1987 12

BS 3680-4C:1981 Liquid Flow in Open Channels, Part 4: Weirs and Flumes


in the order of 0.5 m for maximum flow conditions resulted in a channel approach length of 10

m, excluding the transition length. This is 20 x the flow depth and considered adequate despite

the anticipated initial turbulence. The performance of the stilling structure and approach channel

should be monitored on commissioning to ensure that the flow lines stabilise sufficiently prior to

entering the flume, to ensure suitable measuring accuracy. If necessary, flow guides can be

fitted retrospectively to stabilise the flow.

The flume and associated stilling structure and channel will be covered with removable concrete

slabs to mitigate the risk of accidental entry into the channel.

See drawing No. 346-M11338 of Appendix B for the design of the Parshall flume.

5.6.7 Outlet Structures to Nursery Dams

A requirement of the design was to allow for flows to be diverted into either of the Nursery

Dams. This is to facilitate draining and maintenance on a dam, whilst flows are continued to the

other.

Enlarged HDPE manhole chambers installed with sluice gates are proposed, which will allow for

the outlet to each dam to be closed off, diverting all flows into a single dam. The chambers have

been fitted with overflow pipes to allow water to bypass the sluices into the dams should the

sluices become unexpectedly blocked.

The pipe outlets have been designed to outfall above the surveyed normal water levels in the

dams. Under normal operating conditions, the outlets will have free discharge conditions.

The levels in the Nursery Dams are currently known to fluctuate during rainfall events, due to

the catchment area currently draining to the Tau Tona Trench. Part removal of the storm water

element from the pipeline is expected to reduce the fluctuation of water levels in the dam.

5.6.7.1 Maintenance of the Nursery Dams

The Nursery Dams are linked by a shallow spillway, and under normal operating conditions the

inflows of water will be balanced between the two dams. Each dam is able to overflow into the

recently refurbished North Boundary Dam. Should work be required on either of the Nursery

Dams, then the interlinking spillway will need to be closed temporarily, ensuring however that

the dam receiving flows is still able to overflow directly to the North Boundary Dam. Under no

circumstances should the dam being filled by the new pipeline be isolated from one of the

spillways.

This will also facilitate the future lining of these dams.

5.6.8 Traffic Management

The crossing of the traffic circle will require that the circle is partially closed, and that special

traffic control measures will have to be implemented by the contractor. The use of the existing

Tau Tona Trench culvert beneath the circle was discounted as its condition is unknown, and

flows from the shaft need to be maintained throughout the construction process. Figure 1 shows

an example of the traffic accommodation at a circle.

AngloGold Ashanti will carry out the Traffic Management Plan for the crossing, and it is

consequently outside of the scope of this report.


Figure 1: Example of traffic accommodation at turning circle.

6. CONCLUSIONS

It is recommended that the existing Tau Tona Trench is to be replaced by a 710 mm nominal

diameter HDPE pipeline for the upper section.

Downstream of the stilling basin, the flow will continue under gravity to Nursery Dams. Structural

walled HDPE pipe with a minimum ring stiffness of 8 kN/m2 is recommended.

The overall pipeline will have an approximate length of 2 000 m.

An attenuation pond will be used to relieve surges in flow due to large storm events, limiting flow

to the downstream gravity section of the pipeline to 660 ℓ/s. This attenuation pond will be lined

with a HDPE liner and design to store the 1 in 50 year storm to meet legislative requirements for

dirty water.

The pipeline will be joined using either butt fusion or electro-fusion welding, depending on the

type of pipe used. This will create a continuous watertight pipeline.

A Parshall flume near the outlet will be used to measure flows.


1. APPENDIX A

1.1 HYDRAULIC DESIGN VARIABLES

The following information was used in the hydraulic calculations.

1.2 PIPE LENGTHS AND ELEVATIONS

The lengths and elevations for the different pipes sections were measured off the topographical

survey.

1.3 PIPE DIAMETERS

For structural walled HDPE gravity pipes, the pipe inside diameter is taken as the nominal

diameter. For HDPE pressure pipes the pipe inside diameter was calculated by subtracting the

wall thickness of the pipe from the pipe outside diameter. The calculations for the different pipe

diameters are shown in Table 6.

Table 6: Pipe Inside Diameter Calculations

Nominal

Diameter

(mm)

Pipe Material

Allowable

Pressure

(bar)

Wall

Thickness

(mm)

Inside Diameter

(mm)

225 PE80 SDR21 HDPE 6 10.7 203.6

710 PE100 SDR17 HDPE 10 41.8 626.5

800 PE100 SDR17 HDPE 10 47.1 705.9

900 PE100 structural walled

HDPE, 8kN/m2 ring stiffness 1 N/A 900

600 PE100 structural walled

HDPE, 8kN/m2 ring stiffness 1 N/A 600

1.3.1 Pipe Roughness

The Darcy-Weisbach and Manning’s roughness coefficients were used in the calculations for

the frictional head loss of a pipe for gravity flow. The roughness values for the HDPE pipes in

the calculations were taken from Hydraulics in Civil and Environmental Engineering, Fourth

Edition (Andrew Chadwick, John Morfett and Martin Borthwick) Table 4.2, and manufacturers’

technical specifications. The values used are shown in Table 8. A range of values were tested,

and the lower values are shown in brackets.


Table 7: Pipe Roughness

Pipe Material Darcy-Weisbach Roughness

Coefficient (mm)

Manning’s Roughness

HDPE - solid 0.03 (0.06) 0.009 (0.01)

HDPE – structural walled with

smooth inner walls 0.03 (0.06) 0.009 (0.01)

The pressure calculations considered both Darcy-Weisbach and Hazen Williams

methodologies. Hazen-Williams C-values were based on the Institution of Water Engineers

(now The Chartered Institution of Water and Environmental Management) 1969 "Manual of

British water engineering practice" (Fourth edition, volume II). The values used are shown in

Table 8. A range of values were tested, and the lower values are shown in brackets.

Table 8: Hazen Williams Pipe Roughness

Pipe Material Hazen Williams Roughness

Coefficient (C)

HDPE - solid 152 (145)

HDPE – structural walled with smooth inner

walls 150 (145)

Both sets of methods provided similar results.


2. APPENDIX B:

DOCUMENT TITLE NUMBER

Longsection 346-M11315

Inlet works 346-M11317

Parshall Flume 346-M11338

2011-09-09 tau tona trench replacement pipeline - design

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