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Chapter One
Problems of road construction in
weak soil and waterlogged areas.
By - Dumesa Gudissa (Assistant Professor)
August ,2021 G.C.

❖ take a minute to think about the question
1. What is the importance of quantifying the damage of
pavement layers due to problematic soils?
2. Which soils are categorized as a problematic soils and
remedial measurers should be take to use as subgrade
material?
3. Why do expansive soils expand? Which minerals are
responsible for expansiveness?
4. Is it possible to construct roads in water logging areas?
5. What are the remedial measures to should be taken for
roads in waterlogged areas ?
22/08/2021 compiled by Dumesa Gudissa 2

Introduction
expansive soil with crack

Introduction
Damage due to problematic subgrade condition

Introduction
❑The performance of any types of completed roads are
depend on the pavement structural design and subgrade
soil conditions.
❑ Subgrade soil that changes their volumetric and
stiffness properties leads to deformation and cracking of
road surface, frequently leading to premature failure.
❑ The most problematic subgrade soils including
expansive/swelling soil, highly compressible soft soils
and saturated soils.
compiled by Dumesa Gudissa 5
22/08/2021

Cont. . . .
❑In water stagnated areas where the water table is
fluctuating and vegetational growth possible swamp and
marsh deposits develop.
❑ soil formed under this environment is soft, high in
organic content.
❑Accumulation of partially or fully decomposed aquatic
plants in swamps or marsh is termed Muck or Peat.
❑Muck is spongy, light in weight, highly compressible,
and not suitable at all for construction purposes of any
kind.

Cont. . .
❑why expansive soil and collapsible soil are labelled as a
problematic soils?
❖ expansive or collapsible soil may cause high
differential movements in structures through excessive
heave or settlement.
❑ problematic soils(soft and unstable subgrade conditions)
may not identified during subsurface investigation and
can be detected during the construction phase, this may
led to claims and construction cost overruns.

Cont. . .
❑Expansive soil is generally defined as any soil or
rock material that has a potential to increase in
volume under increasing water content.
❑Collapsible soil, which are sometimes referred to as
metastable soil, are unsaturated soil that undergo a
large change in volume upon saturation.
❑Swelling and shrinkage of sub-grade soils are critical
factors contributing to increases in roughness and
degradation of serviceability of highway pavements.
compiled by Dumesa Gudissa 8
22/08/2021

Distribution of Expansive Soils
❑ The problems with expansive soils is widespread
throughout world except acratic.
❑The countries that are facing problems with
expansive soils are Australia, the US, Canada, China,
Israel , India , and everywhere.
❑ It was projected that by the year 2000, losses due
to expansive soil would exceed 4.5 billion dollars
annually(Chen,1988)

Expansive soil distribution in Ethiopia

Description of Expansive/swelling soil
❑Expansive or swelling soils are generally clayey soil
types that are susceptible to volume change due to
seasonal fluctuations in moisture content.
❑ A decrease in moisture will cause these soils to
shrink, while an increase in moisture will cause
expansion or swelling.
❑ The clay mineral that is responsible for
expansiveness belongs to the montmorillonite group.

Cont. . .
❑Expansive soils are residual soils which are the result of
weathering of the parent rock.
❑The swell - shrink potential of expansive soils is
determined by its initial water content; void ratio;
internal structure and vertical stresses, as well as the
type and amount of clay minerals in the soil.
❑Swelling pressures can cause heaving, or lifting of
structures whilst shrinkage can cause differential
settlement. Failure results when the volume changes are
unevenly distributed beneath the structures

Cont. . .
❑Swelling and shrinkage are not fully reversible
processes.
❑The process of shrinkage causes cracks, which on
re-wetting, do not close-up perfectly and hence
cause the soil to bulk-out slightly, and also allow
enhanced access to water for the swelling process.
❑If these soils were left untreated, longitudinal
cracks and significant surface roughness issues can
be expected on the finished pavement surface.

Characteristics of Expansive Soil
❑ Increases in volume with the addition of water.
❑ They shrink greatly on drying and develop cracks on
the surface.
❑ These soil possess a high plasticity index.
❑ their colour varies from dark grey to black.
. Shrinkage cracks are visible on the ground during dry seasons.

Clay minerals
• All clay minerals are made of two distinct building blocks:
tetrahedrons and octahedrons.
• The tetrahedron has oxygen atoms at the corners, and
there is a silicon in the center.
• Octahedron has six oxygen or hydroxyl atoms in the
corners, and an aluminum or magnesium ion at the
center.

Cont. . .
❑Several tetrahedrons joined together form a tetrahedral
sheet.
❑Different combinations of tetrahedral and octahedral
sheets form different clay minerals:
➢1:1 Clay Mineral (e.g., kaolinite, halloysite):
hexagonal hole

Cont. . .
➢2:1 Clay Mineral (e.g., montmorillonite, illite)

kaolinite
➢ Halloysite - kaolinite family;
hydrated and tubular structure.

Cont. . .
❑Montmorillonite- also called smectite; expands
on contact with water.
➢Highly reactive (expansive ) clay.
➢ Bentonite - montmorillonite family

Cont. . .
Illite

Identification clay minerals
❑ Scanning Electron Microscope
❑ X-ray diffraction (XRD)- to identify the molecular
structure and minerals present.
❑ Differential Thermal Analysis (DTA) - to identify the
minerals present
plate-like structure

Cont. . .
❑Casagrande’s Plasticity Chart

Damage to Structures from Expansive Soil
❑All pavement structures experience various levels of
damages during their lifetime. For structures to be
economical especially those made of concrete, a
certain degree of cracking is inevitable.
❑Swelling pressures can cause heaving, or lifting of
structures whilst shrinkage can cause differential
settlement. Failure results when the volume changes
are unevenly distributed beneath the pavement
foundation.

Cont. . .
❑Often, damage from expansive soils can be seen
within the first few months or years after a
pavement is constructed.
❑ As water from irrigation or rainfall migrates
underneath the pavement foundation, the soil around
the edge of the pavement foundation expands,
pushing up on the edges of the pavement.
❑ This condition, called edge-lift, can cause cracking
of surface course of pavement.

Cont. . .
❑Over a period of years, as the moisture further
migrates underneath the center of the pavement
, center-lift can occur, causing additional damage
to the pavement.

Road construction in water logged areas
❑When water from any source find no path to escape or
drain out and create a hazardous situation is known as
water logging.
❑As a result of migration of water by capillary action
from the high-water table/high flood level, the subgrade
soil gets further saturated and this leads to a gradual loss
in its bearing capacity.
❑It is important to ensure the drainage of storm water
and lowering of subsoil water so that subgrade remains
dry.

Cont. . .
❑Absence drainage system, improper maintenance of
drainage facilities may cause waterlogging during rain
season.
❑ Many roads in Ethiopia have deteriorated in areas with
high water tables.

Cont. . .
❑The rise in water table causes a capillary rise of water into
the sub-grade.
❑With this upward flow of water, the injurious salt such as
sulphates, chlorides and carbonates of sodium, calcium and
magnesium which are present in the water also rises
towards the surface.
❑These salts increase in volume when they crystallize under
a suitable condition.
❑The repeated volume changes of salts due to alternate
hydration and dehydration leads to breaking of pavement.

Solution of Road Construction in Water Logged Area
❑This problem can be resolved by taking variously
measured.
❑ One of the most effective measures is to depress the
water table (lowering water table)
❑This can be achieved by a system of a sub-surface drain.
❑The other alternative is to keep the road embankment
sufficiently high.
❑The bottom of the pavement should be kept at least 1.5
m above the highest water table.

Cont. . .
❑ Provision of capillary cut-off placed 0.15 m above the
ground level can effectively arrest the rise of water.
❑ Capillary cut-off may be provided in the form of a sand
blanket, bituminous stabilized soil.
❑Provision of suitable type and grade of Geotextile over
the sub-grade also effectively serves as a cut-off for the
capillary rise of water and salt migration.
❑The pavement should also be designed adequately for
adverse conditions.

Cont. . .
✓Lowering water table to protected damage of
pavement layers.

Cont. . .
Seepage control

Cont. .
Capillary cut-off : (1) with Granular capillary cut-off
(2) Impermeable capillary cut-off

Cont. . .
❑ Provision of capillary cut-off placed 0.15 m above the
ground level can effectively arrest the rise of water.
❑ Capillary cut-off may be provided in the form of a sand
blanket, bituminous stabilized soil.
❑Provision of suitable type and grade of Geotextile over
the sub-grade also effectively serves as a cut-off for the
capillary rise of water and salt migration.
❑The pavement should also be designed adequately for
adverse conditions.

Next . . .
Chapter 2
Methods of strengthening weak
pavement subgrade soil

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