Chapter 14

Soil Stabilization

14.1. Introduction
Sometimes the strength of the soil is not adequate for it to carry the loads we intend to impose. We
have several choices, we can:
 Replace the soil with stronger soil;
 Overlay the soil with stronger material to reduce the loading on the soil; or
 Increase the strength of the soil (ground modification/soil stabilization)

14.2. Ground Modification or Soil Stabilization

When the strength of the soil is not adequate to carry the loads it intends to impose, one of the
techniques to address this concern is ground modification or soil stabilization. Soil stabilization increases
soil strength by mixing a stabilizing agent or additive with the soil. Stabilization:
 Allows the use of otherwise unsuitable natural soils
 Eliminates the cost of excavating and removing the unsuitable soil and replacing it with suitable
material.

14.3. Common methods of stabilization

1) mechanical methods
2) hydraulic methods
3) reinforcement methods, and
4) physiochemical

Mechanical Hydraulics Reinforcement Physiochemical

 Compaction  Drainage  Confinement  Admixtures
 Deep compaction  Preloading  Inclusions  Freezing
 Vibroflotation  Electroosmosis  Minipiles  Grouting
 Soil nailing  Heating
 Stone columns

 Mechanical methods:
 Compaction (see Chapter 12)
 Deep compaction (sometimes called Dynamic compaction)
- Involves dropping a heavy weight from a crane onto the ground surface to achieve soil
densification. Typically, drop weights of 9 metric ton – 36 metric ton are used with a drop
height of 15–30 m to produce soil densification to a depth of about 9 m. The horizontal spacing
of drop points usually ranges from 2–8 m.
 Vibratory compaction (vibroflotation and vibrocompaction)
- Involves inserting a vibratory probe into the cohesionless soils. After the probe is jetted
and/or vibrated to the required depth, the vibrator is turned on and the device is slowly
withdrawn while the soil is kept saturated. Clean, granular material is added from the surface
as the soil around the probe densifies and subsides. The process is repeated in a pattern such
that a column of densified soil is created under each footing or other load.
 Hydraulics
 Drainage
- Involves forcing the soil grains of saturated cohesive soil closer together by draining the soil’s
void spaces with water.

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 Surcharging
- Involves placing additional weight on the soil surface. This is a very-long-term process
(months to years) unless natural soil drainage can be increased. Sand columns consisting of
vertical drilled holes filled with sand have often been used for this purpose. A newer
technique that provides faster drainage at lower cost involves forcing wicks, or plastic drain
tubes, into the soil at intervals of a few feet.
 Electro-osmosis
- Employs electrical current to speed up the drainage of cohesive soils. The external
electromotive force applied across a solid liquid interface causes the movable diffuse double
layer to displaced tangentially with respect to the fixed layer. As the surface of fine grained
soil particles causes negative charge, the positive ions in solution are attracted towards the
soil particles and concentrate near the surfaces. Upon application of the electro motive force
between two electrodes in a soil medium the positive ions adjacent to the soil particles and
the water molecules attached to the ions are attracted to the cathode and are repelled by the
anode. The free water in the interior of the void spaces is carried along to the cathode. By
making the cathode a well, water can be collected in the well and then pumped out.
 Reinforcement
 Confinement
- Cellular confinement systems (CCS), also known as geocells are widely used in construction
for erosion control, soil stabilization on flat ground and steep slopes, channel protection, and
structural reinforcement for load support and earth retention. Typical cellular confinement
systems are geosynthetics made with ultrasonically welded high-density polyethylene
(HDPE) strips or novel polymeric alloy (NPA) and expanded on-site to form a honeycomb-like
structure and filled with sand, soil, rock, gravel or concrete. A Cellular Confinement System
when infilled with compacted soil creates a new composite entity that possesses enhanced
mechanical and geotechnical properties. When the soil contained within a CCS is subjected
to pressure, as in the case of a load support application, it causes lateral stresses on perimeter
cell walls. The 3D zone of confinement reduces the lateral movement of soil particles while
vertical loading on the contained infill results in high lateral stress and resistance on the cell-
soil interface.
 Inclusions
This category of soil improvement may also be known as "in-situ densification" because it results
in densing the natural soil existing in the construction site. Stone columns and sand compaction
piles are two of the common techniques used in this way.

o Stone columns
- Stone columns are constructed by drilling holes that extend through clay to firmer soil.
Then the hole is filled with compacted gravel. They can be installed as either independent
columns or as continuous walls or panels of columns.

 Minipiles (sometimes called Micropiles)

- Micropiles are defined as small-diameter (typically less than 300 mm), drilled and grouted
replacement piles that are typically reinforced. A micropile is constructed by drilling a borehole,
placing reinforcement, and grouting the hole.
 Soil nailing
- The process starts on drilling the soil and locating the nail and steel bar target points. Once
drilling is completed the nails must be inserted on holes. Then, the grout is applied into the
soil to develop a structural member. Then reinforcement is placed on a soil surface to protect
the soil.

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  Physiochemical
 Admixture
- Soil stabilization method is widely used to improve soil strength and decrease its
compressibility through bonding the soil particles together. Additives or grout are mixed with
soil to bring about the stabilizing action required.

 Lime stabilization:
- Lime often is used with soils that have a high clay content. Quicklime (calcium oxide) or
hydrated lime (calcium hydroxide) reacts chemically with the clay causing small
particles to combine forming larger particles. It also reacts with available silica and
alumina in the soil to cement soil components into a hardened matrix. The construction
process for lime stabilization is:

1) Loosen and pulverize the soil to be stabilized

2) Spread the lime on the soil to be stabilized
3) Mix the lime with the soil
4) Spread the soil/lime mixture to the desired thickness
5) Wet the soil/lime mixture
6) Compact the wet soil/lime mixture
7) Cure the soil/lime mixture for three to seven days

 Cement stabilization
- Cement is the oldest binding agent since the invention of soil stabilization technology in
1960’s. In this technique, cement is mixed with water and soils by special equipment in
site. Physical and chemical reactions within cement and soil are happened. Setting of
cement will enclose soil as glue, but it will not change the structure of soil. The soil is
hardened as cemented soil.

Two types of cement and soil mixtures:

1. soil-cement and
- Soil-cement contains sufficient cement to form a hardened mixture. It is
compacted to a high density and cures into a hardened slab-like structural
material. Soil-cement should be kept moist and cured for at least seven to eight
days to achieve desired strength.
2. cement-modified
 Fly-Ash stabilization
- Stabilization of soils with coal fly ash is an increasingly popular alternative nowadays.
Fly ash is a product of coal fired electric power generation facilities; it has little
cementitious properties compared to lime and cement. Most of the fly ashes belong to
secondary binders; these binders cannot produce the desired effect on their own.
Therefore, the use of fly ash to stabilize clay must usually be in concert with lime or
cement.
 Asphalt stabilization
- Asphalt can be added to granular soils, such as sand or gravel, to produce a more
durable, stable soil. The construction process for asphalt-stabilized soil is:

1) Loosen and pulverize the soil to stabilized

2) Apply the required amount of asphalt
3) Mix the asphalt with the soil
4) Spread the asphalt–soil mixture to the desired thickness
5) Compact the asphalt–soil mixture
6) Finish the surface of the asphalt–soil mixture

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  Freezing
- Soil freezing involves lowering the temperature of the soil until the moisture in the pore
spaces freezes. Freezing of pore water acts as a cementing agent between the soil particles
causing significant increase in shear strength and permeability. Unlike soil heating, soil
freezing may be applicable to a wide range of soil types, grain sizes and ground conditions.
Fundamentally, the only requirement is that the ground has sufficient soil moisture (pore
water). The process typically involves installing double walled pipes in the soil. A coolant is
circulated through a closed circuit. A refrigeration plant is used to maintain the coolant’s
temperature.
 Grouting
- Jet grouting proves its effectiveness across wide range of soils. It is an erosion-based system.
Granular soils are considered the most erodible and plastic clays the least. The technique
hydraulically mixes soil with grout to create in situ geometries of soilcrete. Hydraulic Rotary
drill is used to reach the design depth and at that point grout and sometimes water and air are
pumped to the drill rig. This create a cementitious soil matrix called soilcrete.
 Heating
- This technique can be effectively used when a large and inexpensive heat source is located
near the site. Heating is applied to the soil by burning liquid or gas fuels in boreholes or
injection of hot air into 0.15 to 0.2 m diameter boreholes that can produce 1.3 to 2.5 m diameter
stabilized zone after continuous treatment for about 10 days.

Effect of Temperature Increase on the Properties of Clayey Soil

Temperature Effect
1000 C Can cause drying and significant increase in clay strength
5000 C Can cause permanent changes in the structure of clays,
hence decreasing its plasticity.
1000 C Can cause fusion of clay particles into a solid substance.

14.4. Types and Uses of Soil Stabilizing Equipment

Rotary mixers are used to pulverize the soil, mix the desired additive, and spread the resulting
mixture. It consists of a carrier whose large engine drives a rotor that rotates in a mixing chamber. The rotor
cuts, pulverizes, and mixes the soil with the stabilizing agent.

14.5. Production Estimation

The productivity of a rotary mixer can be determined on an area basis or a volume basis.

productivityarea = SWE Where:

S = speed of the machine
W = cutting width
productivityvolume = SWDE D = cutting depth
E = operational efficiency

14.6. Cost and Time Analysis

To estimate the direct cost of stabilizing the soil with a rotary mixer, first estimate the hourly ownership
and operating. These costs can be used in conjunction with the productivity to estimate the unit cost for using
the rotary mixer to stabilize the soil. Multiplying the unit cost by the total quantity of work to be performed
yields an estimate of the total cost for the task. Dividing the total volume of work to be performed by the
productivity yields an estimate of the time required to complete the task.

Problem Set for Chapter 14:

1. A regional port authority has awarded a contractor a contract for the construction of a large paved area
that will be used for storage of shipping containers. The project design calls for the construction of 250
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 mm of soil-cement covered with 150 mm of asphalt. The contract specifies that the cement content of the
soil-cement is to be 8% of the soil dry unit weight. The granular fill material that is to be used for the soil-
cement weighs 1 423.86 kg per loose cubic meter at a moisture content of 3%. How much cement should
the contractor add to each loose cubic meter of fill material to achieve the content specified in the
contract?
2. The contractor in Prob. 1 has decided to use a rotary mixer to mix the cement with the soil. The cutting
width of the machine is 2.4 m, and the cutting depth is 254 mm. The average travel speed of the machine
while stabilizing the soil will be 9.10 meter per minute. The contractor plans an operational efficiency of
50 minutes per hour.
a) What is the estimated productivity of the rotary mixer in square meter per hour?
b) What is the estimated productivity of the rotary mixer in bank cubic meter per hour?
3. The contractor of Problem 1 has decided to use a water truck and a compactor to support the rotary
mixer in stabilizing the soil. Overall productivity for the set of equipment is governed by the productivity
of the rotary mixer. Hourly ownership and operating costs for the rotary mixer are estimated to be ₱9625
per hour, for the water truck are estimated to be ₱5500 per hour, and for the compactor are estimated to
be ₱7150 per hour. The operator for the rotary mixer earns ₱3410 per hour, the operator for the
compactor earns ₱3190 per hour, and the driver for the water truck earns ₱2915 per hour.
a) What is the estimated unit cost in peso per bank cubic meter for stabilizing the soil?
b) How many days do you estimate it will take the contractor to complete the soil stabilization? The area
to be stabilized measures 45.7 m by 106.7 m.

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