bridge foundation design challenges associated with ... 2018.pdf · presented by: bridge foundation...
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Presented by:
Bridge Foundation Design Challenges Associated
with Geological Structures in Kenya
I.K. Mugamangi
International Transport and Road Research Conference (iTRARR 2018)
Introduction
Bridges are constructed as linkages for sustainable provision
of good road network through different terrain and landscape.
Ground suitability check is done through investigations
preceding designs production for bridge construction
However occasional design changes occur due to unforeseen
features not suitable for sustainable designs
This results in efforts to identify existence of features like
geological structures and determine impacts on bridge
construction.
Significance of identifying geological structural controls on
bridge foundation designs is critical in provision of sustainable
road infrastructure in Kenya.
International Transport and Road Research Conference (iTRARR 2018)
Site Investigations
Collecting subsurface information focuses on intended
foundation bearing points
Ground profile across the bridge then would determine whether
shallow or deep foundation design should be adopted
Changes in ground state or water cause as observed in figure 1
result in failure and revision of bridge design
Table 1 gives summary of geological structures accounting for
ground profile variations resulting in need for design changes
International Transport and Road Research Conference (iTRARR 2018)
Figure 1: Failure at Thwake River bridge along Kathiani road in Machakos County, Kenya
International Transport and Road Research Conference (iTRARR 2018)
Geographical
Region in
Kenya
Structure Observations
Depth to Rock (m bgl) Posible
Geological
structure
Right
Abutment Left Abutment
Eastern Thwake Bridge
Abutment
undermined as
river changes
course
0.00 12.70 Fold
Eastern Muti Bridge
Foundation
levels differ
significantly
10.50 2.50 Fold
Rift Nginyang
Bridge
Abrupt change
in ground
profile
material
along bridge
alignment
- -
Fault at pier
position with
no rock deep
to
investigated
depths of
30.00m bgl
Nairobi
Globe Round
About Bridge Foundation
levels differ
significantly
Pier 1 -
5.00m
P4, P6, P9 –
15.00m Unconformity
Museum Hill
Interchange
Chiromo Side
– 11-24m bgl
Museum Side -
2.5m Unconformity
Table 1: Summary of depths to foundations at various sample locations in Kenya
International Transport and Road Research Conference (iTRARR 2018)
Features developed when rocks are subjected to stress that result in deformation and failure.
The type of structure depends on the nature of prevailing stress at the time of formation.
In Kenya, the stresses attributed to formation of varied land forms are related to compression, tensional and shear movements.
The resulting structures vary in category and are grouped as primary, secondary and compound forms.
Major secondary and compound structures of significance to bridge foundation design are folds and faults.
Geological Structures in Kenya
International Transport and Road Research Conference (iTRARR 2018)
Result from compressional stresses acting over
considerable time with rock deforming in a ductile
manner.
Broadly classified as anticlines and synclines
based on the orientation of the different parts of the
structure (figure 2).
Symmetry and orientation of major fold structures
is summarized in figure 3.
Fold structures
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Figure 2: Parts of a Fold
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Figure 3: Types of Folds
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Represent plane surfaces of
displacement discontinuity in brittle
rocks of the ground.
Classified using movement patterns of
two displacement blocks into normal,
reverse, strike-slip and oblique (figure
4)
Faults
Presented by:
Figure 4: Fault classification
International Transport and Road Research Conference (iTRARR 2018)
The geological character of a site determines placement
of bridge abutments and piers on sound, strong and stable
rock foundation.
Primary layering structures influence depths of bridge
foundations due to intermittent variations in physical
character of each subsequent layer.
Sites that experience occurence of recumbent and
overturned folds face critical foundation depths
variations.
Common bridge design challenges involve ground profile
variation between sound and weak horizons
Discussions
International Transport and Road Research Conference (iTRARR 2018)
Faulted areas have severe effects on horizontal distribution
of bridge spans to avoid weak fault thrust zones.
Where pier location coincides with position of fault plane,
span width has to be reviewed to better foundation layers.
Depths factor to stable ground in faulted areas is undefined
hence the need for lateral shift in foundation position.
When fault or fold structures occur at a bridge site, designs
should consider potential impacts and have mitigations defined
Discussions cont.
International Transport and Road Research Conference (iTRARR 2018)
Detailed site investigations for bridge designs should
cover foundation loading points and surrounding area.
In the event of occurrence of poor ground condition
near the bridge foundation elements, the designs
should be flexible to accommodate potential cases of
undermining the foundations.
Whenever geological structures are encountered at a
proposed bridge site, the foundation designs should
propose action plans to mitigate potential impacts the
structures would have on the bridge foundation.
Conclusions and Recommendations
END