bridge foundation design challenges associated with ... 2018.pdf · presented by: bridge foundation...

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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.


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


Figure 1: Failure at Thwake River bridge along Kathiani road in Machakos County, Kenya


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


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


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


Figure 2: Parts of a Fold


Figure 3: Types of Folds


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


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


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.


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

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