This article clarifies your queries about soil stabilization and its’ types. The main focus of this article is a specific technique for slope stabilization, i.e., soil nailing.
We shall go through the
introduction of soil nailing, its uses, advantages and disadvantages, design
criteria, construction methodology, and performance checks. Let us dive into
it.
What Is Soil Stabilization?
Soil stabilization is the
act of altering a soil’s physical properties to provide long-term, permanent
gains in strength. A soil is stabilized by enhancing its shear strength and
overall bearing capacity.
These are the three
primary methods for soil stability. The first is mechanical, the second is
compaction, and the third is chemical.
What Is Soil Nailing?
Soil nailing is a method
of ground stabilization. It is a construction technique used to remediate unstable
natural soil slopes or unstable man-made (fill) slopes.
Soil slopes are
stabilized by driving passive bars (nails) into predrilled and grouted holes.
This method is capable of withstanding tensile stresses, shearing forces, and
bending moments.
Source: designingbuildings.co.uk
Additionally, soil
nailing increases soil stability. It stabilizes slopes, excavations, retaining
walls, and other similar structures. The main deciding factor for soil nailing
is the characteristics of the ground, the applicability of other systems such
as ground anchors, geosynthetic materials, and so on, and the cost. However, it
should be established that the slope under consideration is above the
groundwater table.
Soil nail walls are
generally constructed from top to down. It is called soil nailing because it’s
like having a nail being hammered into the soil, where the nails are the steel
bars. This technique is generally used to bring soil stability where landslides
might be a problem.
The first use of the soil
nailing technique dates back to 1972 in Versailles, France. The technique
originated from a tunnel construction method called the New Austrian Tunnelling
Method (NATM), which uses reinforcing bars drilled into the soil and covered
with shotcrete.
Application Of Soil Nailing
- · Stabilization of Natural Slopes
- · Stabilization of Embankment Slopes
- · Stabilization of Cuts
- · Retaining Walls
- · Tunnels
- · Excavation Shoring
- · Bridge Abutments
Soil nailing for retaining wall Source: geostabilization.com
Advantages of Soil Nailing
· It can be used on new construction, temporary
structures, remodelling process, and to repair the existing wall system with
comparative ease.
- · Soil nailing helps limit the slopes’ boundaries.
Hence more space is available.
- · It needs minimum slope preparation for nailing
operation.
- · No restriction on wall height.
- · The construction time is lesser as compared to the
traditional slope stabilization method.
- · The nails are placed at intervals; hence no need
to put shoring throughout.
- · The cost is generally lesser than constructing a
retaining wall instead.
- · It may prevent slope erosion with applied with
mesh and shotcrete.
- · Less noise, fewer traffic obstructions, and less
impact on surrounding areas. Thus environment-friendly.
- · This technique is flexible, relatively quick, and
easy to install.
- · Time-saving, cost-saving, requires less labour.
Limitations of Soil Nailing
- · A high-water table is not a desirable condition
for soil nailing.
- · Soil nailing is not a good choice for non-cohesive
soils, fine-grained soils, highly corrosive soils, and loose granular soils.
- · The soil nail density increases with a decrease in
the soil’s shear strength. This implies that low-strength soils need more nails
for stabilization.
- · The use of nails is restricted for permanent
long-term applications in sensitive and expansive soil.
- · Metal nails will corrode.
- · Soils with high plasticity have a higher liquid
limit and hence lesser undrained shear strength, which can lead to long-term
deformations.
- · The soil might be overexposed prior to the
installation of the nail.
- · Sand and gravel might not be compatible with soil
techniques.
- · Specialists may be required to design and install
the nails.
Types Of Soil Nailing
There are four types of
soil nails based on the method of construction.
1. Grouted Nails
As the name suggests,
these nails are grouted. The holes for this particular nailing method are bored
into the walls or slope face.
After this, nails are
hammered into place inside the pre-drilled holes. Later, the hole is stuffed
with grouting materials having a water-cement ratio between 0.45-0.6.
In conclusion, the diameter
of the nail’s ranges from 100 to 200 mm, and the distance between each nail is
typically 1.5 m.
2. Driven Nails
The driven soil nailing
is the process of temporarily stabilizing soil slopes. It is quite a bit
quicker than other methods. As it is a temporary method, neither the nails nor
the reinforcement steel will be protected from corrosion.
The driven soil nailing
method involves driving the nails into the slope face while the excavation is
taking place. The diameter of the nail ranges from approximately 19mm to 25mm
and is considered on the smaller side. Also, the distance between nails is
between one and one and a half meters.
3. Self-Drilling Nails
In the method of
self-driven soil nailing, hollow bars are the tool of choice. The surface of
the slope has holes bored into it to accommodate bars. The drilling operation
is simultaneously accompanied by the injection of grout.
It is more efficient than
drilling and grouting before nailing. In conclusion, the self-drilling soil
nailing method offers more corrosion resistance to nails than the driven nail
method.
4. Jet-Grouted Nails
The Jet-grouted soil
nails use jets to erode the soil to create holes in the surface of the slope.
After that, concrete is used for grouting the space between the steel bars inserted
in this hole. The jet-grouted soil nailing offers excellent protection against
corrosion for steel bars (nails).
Design Considerations of Soil Nailing
There are eight points to consider when deciding the feasibility of the nailing technique. Let us discuss all of them.
Strength Limit
The first criterion for
designing the soil nails is their failure limit. The strength limit defines the
limit state of potential failure or collapse.
Service Limit
The second criterion for
designing the soil nails is their service limit. The service limit defines the
limit state of deformation, which renders the nail unfit for supporting the
slope.
Height & Length
The height and length of
the slope affect the design. The slope height affects the strength and service
limits, while the length affects the cost. Also, higher heights need special
installation arrangements like a winch, crane, etc.
Vertical & Horizontal Spacing of Nails
The vertical and
horizontal spacing of the nails depends upon the strength and service criteria
as well as the soil condition. The spacing between the nail decreases with the
decrease in the cohesion of the soils. Hence an increase in cost due to the
increase in the number of nails for a given height and length of the slope.
Ground Properties
Soil conditions like
density, texture, cohesion, water table, etc., affect the technical and
economic feasibility of the soil nailing technique. It affects the nail
spacing.
Nail Diameter and Length
After considering all the
factors mentioned above, a preliminary assessment of the maximum nail force and
length is done.
The length, diameter, and
spacing of the nails are the typical factors that govern external and internal
wall stability. During the design process, these factors can be altered to fulfil
all of the exterior and internal stability requirements.
Drainage & Frost Penetration
The drainage and frost
penetration detailing are incorporated into the design in the final stage.
These parameters are equally important for the nails to serve for the intended
duration.
External Loads due to Wind & Hydrostatic Pressure
Lastly, nails are checked
against external loads like wind and hydrostatic pressure. If the design can
potentially fail due to the external loads, it is revised accordingly.
Construction Methodology
The construction involves
many phases, from excavation to grouting. RJ Byrne and other contributors
developed the “Soil Manual for Design & Construction Monitoring of Soil
Nail Wall” [5]in 1998. We shall discuss key points from the manual to explain
the construction method of soil nail installation.
Excavation
The excavation of
vertical cuts and slopes is carried out up to a height that can support itself
for 24-48 hours. Ensure the drainage allows surface water to drain out without
eroding the soil. The surface should be smooth to minimize the shotcrete
quantity.
Drilling Nail Holes
Nails can be installed by directly pushing them into the soil. Alternatively, holes can be drilled using a drill bit machine, rope core drill, air leg rock drill, or horizontal drill machine, and then nails can be inserted in holes.
In the second method,
centralizers are used to correctly place the nails into the holes, and then the
holes are grouted.
Construction of Temporary Shotcrete Facing
A wire mesh is welded to the nails to support the upcoming shotcrete. A 3-4-inch-thick layer of shotcrete is sprayed with a shotcrete gun. The concreting can be done by any of the two methods- dry mix and wet mix.
In the dry mix method,
the cement-aggregate mix is mixed with water at the nozzle. In the wet mix
method, the cement, aggregate, and water are blended and conveyed by a
hydraulic pump.
Construction of Permanent Facing
When the bottom of the excavation is reached, a permanent facing arrangement is done. It can be either concrete or shotcrete. A toe drain is connected to the bottom of the slope.
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