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Rebuilding Angola –
Geotechnical Challenges
Dr Robert May
[email protected]
Australian Geomechanics Society,
Queensland
26th March 2015
Overview
• Angola – Quick Guide
• Geology & Climate
• Geotechnics
• Observations
• Explanations
• Foundation Engineering
• Challenges & Solutions
2
Angola
Quick Guide
Angola: Location & Size
4
Angola: Geographical Regions
Northern
Rainforest
Major highways
in poor condition
Coastal
Plain
Feb 2014
Transport malfunction
Interior
Plateau
Western
Highlands
5
Angola: Population
Dundo
Luanda
Angola
Malanje
Saurimo
• Population 19M?
Almost half under 15
• Acute shortage of
Benguela
skilled workforce
Luena
Huambo
6
Angola: Civil War & Aftermath
• Independence from
Portugal 1975
• Civil war 27 years
• Up to 1.5M dead &
4M internal refugees
7
Angola: Economic Wealth
• Very rapid economic
growth 2002-2010
• 2nd largest oil producer
in Africa. Oil exports to
China & India
• Diamond mining
• Other mineral reserves
(e.g. iron ore) not yet
developed
• Large hydro-electric &
farming potential not
yet developed
8
Social Housing
2008 onwards
9
Rebuilding Luanda
10
Geotechnical Background
Climate - Geology - Soils
Climate & Weathering
Combination of high temperature and
high rainfall produces moderate to strong
chemical weathering  Residual Soils
12
Angola Climate
Tropical
Under 1000m ASL
Rainfall >1500mm
Rains 8 months
Temperature 26°C
Brisbane:
Rainfall 830mm
Rains 8+ months
Temperature 26.5°C
Semi-Arid
Rainfall >500mm
Temperature 30°C
Cairns
Rainfall 1960mm
Rains 8+ months
Temperature 29°C
Semi-Desert
Rainfall 400mm
Rains 2 months
Temperature 26°C
Desert
Rainfall <250mm
Temperature 24°C
Longreach
Rainfall 390mm
Rains 4+ months
Temperature 31°C
Semi-Tropical
Over 1000m ASL
Rainfall 1200mm
Rains 7 months
Temperature 24°C
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When it rains....
Climates Change
Congo Rainforest
Kalahari Desert
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Angola Geology - Solid
Kwanza Basin
Tertiary marine
sediments
PreCambrian
Kalahari
Group
Continental
sediments
16
Angolan Soils
Observations
Angola Soils – Luanda Plateau
Luanda Plateau:
SW Coast
Sub-vertical excavations
Cracked & Settled Buildings
• Luanda hospital 3 years after
construction
• Hospital closed due to major
settlement damage
• Shallow foundations on
red sand
20
Erosion Gullies: Natural
• Erosion gully in Red Sand –
North-east Angola
• Note high back-cutting back
scarp (notch)
21
Erosion Gullies: Man-made
22
Angolan Soils
Explanations
Quelo Red Sand
●
Chemical weathering of rock:
wet & dry seasons + high temperature
●
Red colour – iron oxides
●
Appears to be weakly cemented:
disaggregates when saturated with water
●
Typically: 75% Sand
15% Silt
10% Clay
Quelo Red Sands
100%
0%
Clay
50%
50%
Silt
100%
0%
100%
50%
0%
Sand
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Residual Soils - Structure
Sub-vertical excavations
Slope Height 8m
Slope angle 70°
Angle of shearing resistance, f’ = 30°
Effective cohesion, c’ ≥ 12kPa
26
Soil-Water Characteristic Curves
Fredlund, Rehardjo & Fredlund, 2012,
“Unsaturated Soil Mechanics in
Engineering Practice”, pub Wiley
27
SWCC Sand
Moisture content ~ 5%
Soil suction, y = 25-50 kPa
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Sand Strength
 f  c' n  u w  tan f '
Fully saturated soil strength
 f  c' n  ua  tanf 'ua  uw  tanf b
Partially saturated
soil strength
fb = f’ at low suctions & reduces at high suctions
Estimation of strength:
Assume: c’ = 0
f’ = 30°
fb = 25°
(ua – uw)tanfb = 12 kPa
Notes
For engineering assessments:
1. Measure m/c
2. Measure SWCC
3. Ideally measure uw in situ
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Red Sands SPT Profiles
Luanda Plateau:
SPT profiles natural and wetted
North-East Angola:
SPT profiles natural and after rain
SPT N value
SPT N value
0
5
10
15
20
0
2
ZK42, after 30 min
rain
ZK42, after 7 hrs rain
4
Natural m/c
8
Wetted
Depth h / m
Depth h / m
6
ZK42', 24 hrs
recovery after rain
10
12
14
16
18
Liu Z-h et al (2010) Preliminary study of
physico-mechanical properties of Luanda
sand, Rock and Soil Mechanics 31, Aug.
20
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Settlement
Oedometer tests:
Dry and flooded
Plate bearing tests:
Dry and flooded
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African Lateritic Red Sand
0.800
Natural unsaturated:
Moisture content 1.5%
Void Ratio, e
0.700
0.600
Sample A natural
Sample B natural
0.500
Soaked:
Moisture content 12.6%
0.400
0.300
10
100
1000
Vertical Effective Stress (kPa)
10000
African Lateritic Red Sand
0.800
Sample soaked at vertical
effective stress = 400 kPa
Void Ratio, e
0.700
0.600
Sample A natural
Sample B natural
Sample C natural
0.500
Sample D natural
Sample soaked at vertical
effective stress = 200 kPa
0.400
0.300
10
100
1000
Vertical Effective Stress (kPa)
10000
Luanda Quelo Red Sand
0.800
Collapse settlement
Dev = 8 to 11% of
specimen height for
uncompacted sand
Void Ratio, e
0.700
0.600
Sample A natural
Sample B natural
Sample C natural
Sample D natural
Compacted unsat:
m/c 1.9%
0.500
Sample A compacted
Sample B compacted
Sample C compacted
0.400
Soaked:
m/c 12.1%
0.300
10
100
1000
Vertical Effective Stress (kPa)
10000
Erosion
Erosion in Luena – Angola (Google Earth)
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Foundation Engineering
on Red Sands
Foundation Solutions
●
5 – 18 storey Buildings on Red Sands
●
Shallow foundations (A)
●
Piled foundations (B)
B
A
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Red Sand Shallow Foundation
1. Excavate and re-compact red sand
1 to 2m depth
Red Sand Shallow Foundation
2. Construct foundation & services
Red Sand Shallow Foundation
3. Load foundation
Apron 1 to
2m width
5 to 13
Stories
Red Sand – Shallow Foundations
Red Sand – Shallow Foundation
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Foundation Construction
Red Sand Shallow Foundation
3. Load foundation
5 to 13
Stories
Red Sand Shallow Foundation
4. Rain and/or pipe leakage
Red Sand Shallow Foundation
5. Collapse settlement
Red Sand Foundations - Aprons
Poor compaction &
no edge drainage 
Collapse settlement
under apron
Red Sand Foundations - Aprons
Zhang et al 2015, “Experimental study on water
sensitivity of the Red Sand foundation in Angola”,
Eng. Geol. For Soc. & Territory, Vol 6, p 229-235
Red Sand Foundations - Aprons
1d
81d
34d
11d
15d
1 day observation by author, other observations Zhang et al
Are aprons effective?
● Not if poorly
constructed or
poorly maintained
● May provide some
protection for
foundation against
transient seepage
from surface (long
duration ponding
unlikely)
● BUT – infiltration
from surface may
not pose biggest
threat
Typical Sewer Construction
No rocker pipe
No joint gasket
Collapse settlement
potential on wetting
Sewers – Collapse settlement of fill
Water Supply Pipelines
Galvanised steel water supply
pipelines:
● Buried pipeline life depends
on various factors including:
● Thickness & quality of
galvanising
● Making good of joints
after welding
● Aggressivity of partially
saturated soil/water
environment
● Leakage may not be noticed
for some time
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Red Sand Shallow Foundation
Interim conclusions:
●
Foundations are vulnerable to
collapse settlement
●
Biggest risk is from rising
groundwater levels
●
Sources of water:
●
Rainfall – relatively low risk in semiarid areas
●
Leaking pipes (sewers & water
supply) - relatively high risk
●
Irrigation – significant risk
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Red Sand – Screw Piles
● Location:
● NE Angola
● Tropical climate with 4
month dry season
● Soils:
● 40m depth Red Sand
● Water table 30m bgl
● Foundation Solution:
● Floating screw piles
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Red Sand – Screw Piles
Pile Size
L x Dia (m)
18.0 x 0.5
13.0 x 0.4
10.0 x 0.4
12.0 x 0.5
13.2 x 0.5
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Red Sand – Screw Piles
Dundo Pile Load Test Data - All Piles
Piles A, B & C
fs = 74 kPa
qb = 9.2 MPa
Vertical Load (kN)
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0
20
Vertcal Settlement (mm)
40
60
80
100
Piles F&G
Very soft shaft &
toe response
Piles D & E
fs = 16 kPa
qb = 2.1 MPa
Test No A: 18m x 0.5m pile
Test No B: 18m x 0.5m pile
Test No C: 18m x 0.5m pile
120
Test No D: 13m x 0.4m pile
Test No E: 13m x 0.4m pile
140
Test No F: 10m x 0.4m pile
Test No G: 12m x 0.5m pile
160
180
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Rainfall
(mm)
Red Sand – Screw Piles
Date
Red Sand – Screw Piles
Observations:
● Piles built, cured & tested in dry
have normal cfa capacity
● Piles lose substantial capacity if soil
is wetted after construction
● Piles have least capacity if tested in
wet conditions
r = K v
Explanations:
● K in dry soil ~ 0.8 to 0.9
● K if soil wets ~ 0.2 to 0.3
Also
● C in dry soil ~ 12 kPa
● C in wet soil ~ 0 (recoverable)
● Nq dry & wet ~ 30
Consequences:
● Pile capacity in ground which has
been wetted is ~ 60% initial capacity
● Pile capacity in wet soil is < 50%
initial capacity & stiffness v low
58
Red Sand – Screw Piles
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Conclusions
Tropical Red Sands
Fundamentals:
● Residual soils
● Open structure with large voids
● Typically collapsible if loaded
and wetted
● Partial saturation
● Strength, stiffness & in situ
stress related to soil suction
● Soil suction can be assessed
from SWCC & m/c
● Erosion
● Loss of strength when wetted
leads to high erosion potential
Foundations:
● Strips/rafts & compacted pad + apron
● Foundation stresses deeper
collapsible soil
● May suffer large collapse
settlement if deeper sand wetted
● Apron may provide some
protection but not a full solution
● Floating screw piles
● Liable to suffer large reduction in
capacity and stiffness if ground is
wetted after pile construction
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Residual Soils Queensland
Residual Soils:
● Quite common in Queensland
(and across other parts of
Australia)
● Deep weathering occurred in
several episodes through the
Tertiary & Quaternary in Qld
● Residual soils may be buried
under more recent alluvial soils
Engineering:
● Collapse settlement (and shrinkswell) may affect spread
foundations & pile capacities
● Partial saturation has strong
effects on observed soil strength
& stiffness
● Residual soils are prone to
erosion especially when
disturbed.
Ref: TMR Technical Note WQ32
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International Applied Geotechnics
• Liquid Limit Testing
• Critical Steak Framework
Questions?
[email protected]
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