9c- ground improvement techniques

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APPLICATION OF GEOTECHNICAL ENGINEERINGGEOTECHNICAL ENGINEERING

CE451A Dr. Rajesh Sathiyamoorthy, IIT Kanpur

GROUND IMPROVEMENT TECHNIQUES


Grouting


• Grouting technology has become a common ground improvement method used frequently for underground and foundation constructions.

• The process of grouting consists of filling pores or cavities in soil or rock with a liquid form material to decrease the permeability

Significance

Grouting


and improve the shear strength by increasing the cohesion when it is set.

• Injection of chemicals, lime, cement etc, into subsoils improve subsoil by formation of bonds between soil particles

Methods• Intrusion grouting• Permeation grouting• Compaction grouting • Jet grouting

This technique is applicable to fissure rock, gravely layer, sand or silty sand layer.

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• Grouting process is the injection of special liquid or slurry materials called grout into the ground for the purpose of improving the soil or rock.

Grouting

• There are two primary kinds of grout e.g. Cementitious grout and chemical grouts.

• Cementitious grouts are made of Portland cement that hydrates after injection, forming a solid mass.

• Chemical grouts includes a wide range of chemicals that solidify once they are injected into ground These includes silicates resins etc


they are injected into ground. These includes silicates, resins etc., Chemical grouts can be used where cement grouts are ineffective. Chemical grouts are expensive, toxic and corrosive.

• Cement base grout mixes are commonly used for gravely layers or fissure rock treatment. But the suspension grain size may be too big to penetrate sand or silty‐sand layers. In this case, chemical or organic grout mixes are also used. In recent years, the availability of ultrafine grout mixes has extended the performance of hydraulic base grout for soil treatment.

Criteria for design

Grouting

The grout volume to be injected depends on:• ground porosity, • geometry of the treated zone, • grout hole spacing, • stage length and• total depth to be treated


• total depth to be treated.

The groutability of soil with particulate grouting has been evaluated based on the Ng value:

• Ng is defined as Ng1 = (D15)Soil / (D65)Grout.• Grouting is considered feasible if Ng1 > 24 and not feasible if Ng1 < 11

• Another alternative is to use Ng2 = (D10)Soil / (D95)Grout.• Grouting is considered feasible if Ng2 > 11 and not feasible if Ng2 < 6

Methods of Grouting

Grouting

• Intrusion grouting / slurry grouting /High mobility grouting• Permeation grouting / chemical grouting / low viscosity grouting• Compaction grouting / displacement grouting / low mobility grouting• Jet grouting


For animation:http://www.haywardbaker.com/

Methods of Grouting

Grouting

Intrusion / slurry / cement / high mobility grouting:

• Intrusion grouting, is a grouting technique that fills pores in granular soil or voids in rock or soil, with flowable particulate grouts.

• Depending on the application, Portland cement or microfine


cement grout is injected under pressure at strategic locations either through single port or multiple port pipes.

• The grout particle size and soil/rock void size must be properly matched to permit the cement grout to enter the pores or voids.

• The grouted mass has an increased strength and stiffness, and reduced permeability. The technique has been used to reduce water flow through rock formations beneath dams and to cement granular soils to underpin foundations or provide excavation support.

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Methods of Grouting

Grouting

Permeation / chemical / low viscosity grouting:

• Permeation grouting is a grouting technique that transforms granular soils into sandstone‐like masses, by permeation with a low viscosity grout.

• The soils best suited for this technique are sands with low fines


content.

• Typically, a sleeve port pipe is first grouted into a predrilled hole. The grout is injected under pressure through the ports located along the length of the pipe. The grout permeates the soil and solidifies it into a sandstone‐like mass.

• A common application of permeation grouting is to provide both excavation support and underpinning when an excavation is planned immediately adjacent to an existing structure. It is well‐suited for tunneling applications.

Methods of Grouting

Grouting

Compaction / displacement / low mobility grouting:

• Compaction grouting is a grouting technique that displaces and densifies loose granular soils, reinforces fine grained soils and stabilizes subsurface voids or sinkholes, by the staged injection of low‐slump, low mobility aggregate grout.


• Typically, an injection pipe is first advanced to the maximum treatment depth. The low mobility grout is then injected as the pipe is slowly extracted in lifts, creating a column of overlapping grout bulbs. The expansion of the low mobility grout bulbs displaces surrounding soils.

• When performed in granular soil, compaction grouting increases the surrounding soils density, friction angle and stiffness.

• Compaction grouting for treatment beneath existing structures is often selected because the low mobility grout columns do not require structural connection to the foundations.

Methods of Grouting

Grouting

Jet grouting:

• Jet grouting is a grouting technique that creates in situ geometries of soilcrete (grouted soil), using a grouting monitor attached to the end of a drill stem.

• The jet grout monitor is advanced to the maximum treatment depth, at which time high velocity grout jets (and sometimes water and air) are


which time high velocity grout jets (and sometimes water and air) are initiated from ports in the side of the monitor. The jets erode and mix the in situ soil as the drill stem and jet grout monitor are rotated and raised.

• Depending on the application and soils to be treated, one of three variations is used: the single fluid system (slurry grout jet), the double fluid system (slurry grout jet surrounded by an air jet) and the triple fluid system (water jet surrounded by an air jet, with a lower grout jet).

• Jet grouting has been used to underpin existing foundations, construct excavation support walls, and construct slabs to seal the bottom of planned excavations.

Methods of Grouting

Grouting


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Methods of Grouting

Grouting


Reinforcement


• Reinforcement may be in the form of dense granular materials in the form of stone columns or horizontal or vertical strips and membrane.

• The reinforcement serve significantly to increase the capacity of soil to withstand tensile, shear and compressive loads and

Significance

Reinforcement


contribute towards improvement in stiffness and consequently load carrying capacity and suitability of soil mass.

This technique is applicable to sands as well as fine grained soils

Methods• Stone columns• Geosynthetic reinforcement• Steel reinforcement• Fiber reinforcement

Stone columns

• A method now being used to increase the bearing capacity of shallow foundations on soft clay layers or mixed deposits is the construction of stone columns.

• Stone column construction involves the partial replacement of unsuitable subsurface soils with a compacted vertical column of stone that usually completely penetrates the weak strata.

Reinforcement


weak strata.• It is generally considered to be part of dynamic replacement theme due to the method involved in construction.

• Stone columns are constructed by drilling holes through the weak layer(s) in the field at regular intervals. The holes are then backfilled with gravels and then compacted.

• After stone columns are constructed, 0.3m thick drainage blanket is placed over the ground surface and compacted before the foundation is constructed.

• The gravel used for the stone column has a size range of 6 to 40 mm.

Stone columns can be made using:

• Vibrofloatation technique• rotary drilling and then backfilling with stone

• drilling by continuous‐flight auger with a hollow stem and backfilling with stone

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Stone columnsSuitability/Benefits• Stone columns work more effectively when they are used to stabilize a large area where the undrained shear strength of the subsoil is in the range of 10 to 50 kPa. Subsoils weaker than that may not provide sufficient lateral support for the columns.

• For large‐site improvement, stone columns are most

Reinforcement


g peffective to a depth of 6 to 10 m but can go up to 20‐30m.

• To improve settlement by a factor of 1.5 to 3 on most projects at allowable loads.

• To improve stiffness of the ground• To reduce liquefaction potential• To accelerate the dissipation of excess pore water generated by loading pressures.

• As an alternative to more expensive piling systems ‐due to speed of installation and cheaper materials.

Stone columns

Reinforcement

Stone columns usually have diameters of 0.5 to 0.75 m and are spaced at about 1.5 to 3 m center to center.

• Undrained shear strength of the soil• In‐ situ lateral stress of the soil• Radial stress ‐ strain characteristics of the soil

• Friction angle and stress ‐ strain characteristics of the column material

Factors governing soil‐column behaviour


Common applicationsAirport taxiways and runways, chemical plants, storage tanks and silos, pipelines, bridge abutments and approaches, offshore bridge abutments, road and railway embankments, both land / offshore applications.

characteristics of the column material• Initial column dimensions• Construction methodology – disturbance• Treatment depth and termination

Stone columns

Stone Column Diameter, D:Installation of stone columns in soft cohesive soils is basically a self compensating process that is softer the soil, bigger is the diameter of the stone column formed. Due to lateral displacement of stones during vibrations/ramming, the completed diameter of the hole is always greater than the initial diameter of the probe or the casing depending upon the soil type its undrained shear strength gravel size characteristics of the vibrating probe/

Reinforcement

Basic design parameters


soil type, its undrained shear strength, gravel size, characteristics of the vibrating probe/ rammer used and the construction method.

Spacing, S:The design of stone columns should be site specific and no precise guidelines can be given on the maximum and the minimum column spacing. However, the column spacing may broadly range from 2 to 3 times stone column diameter depending upon the site conditions, loading pattern, column factors, the installation technique, settlement tolerances, etc.

Stone columns

Pattern:Stone columns should be installed preferably in an equilateral triangular pattern which gives the most dense packing although a square pattern may also be used.

Reinforcement



Equivalent diameter, De:The tributory area of the soil surrounding eachstone column forms regular hexagon around the column. It may be closely approximated by an equivalent circular area having the same total area.

De = 1.05 S (for an equilateral triangle pattern)De = 1.13 S (for a square pattern)Where S‐ spacing of the stone columns

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Stone columns

Reinforcement



Utilisation of Unit cell idealisation in the analysis:• An equivalent cylinder of composite ground with diameter De enclosing the tributary soil and one stone column is known as unit cell.

• For purpose of settlement and stability analysis, the composite ground representing an infinitely wide loaded area may be modelled as a unit cell comprising the stone column and the surrounding tributory soil. The behaviour of group columns beneath a uniformly loaded area can be simplified to a single column installed at the centre of a cylinder of clay representing the column’s zone of influence.

Stone columns

Area replacement ratio: asTo quantify the amount of soil replaced by the stone, the term replacement ratio, as, is used. It is defined as the fraction of soil tributary to the stone column replaced by the stone. i.e

as = As/A = As/(As+Ag) Where: As = area of the stone column, Ag = area of ground surrounding the column, and A = total area within the unit cell

Reinforcement



total area within the unit cell.

The area replacement ratio, as, can be expressed in terms of the diameter and spacing of the stone columns as follows:

as = C1(D/S)2

Where, D = diameter of the compacted stone column, S = center‐to‐center spacing of the stone columns, C1 = a constant dependent upon the pattern of stone columns used;

For a square pattern C1 = π / 4 and for an equilateral triangular pattern π/(2√3)

For an equilateral triangular pattern of stone columns the area replacement ratio is then

Stone columns

Stress Concentration Factor, n

• Stress concentration occurs on the stone column because it is considerably stiffer than the surrounding soil. From equilibrium considerations, the stress in the stiffer stone columns should be greater than the stress in the surrounding soil.

• The stress concentration factor n due to externally applied load σ is defined as the ratio of

Reinforcement



• The stress concentration factor, n, due to externally applied load σ is defined as the ratio ofaverage effective stress in the stone column, σs to, the average effective stress in the soil, σc within the unit cell.

n = σs / σc

Improvement in the soil owing to the stone column

Stone columns

Stress Concentration Factor, n

• The value of n generally lie between 2.5 and 5 at the ground surface.

• The stress concentration factor increases with time of consolidation and decreases along the length of the stone column Higher n value at ground surface may result if load is

Reinforcement



the length of the stone column. Higher n value at ground surface may result if load is applied to the composite ground through a rigid foundation as compared to the flexible foundation.

• The stress concentration factor may be predicted using elastic theory as a function of the modular ratio of the stone and the clay assuming equal vertical displacements. However, as the modular ratio can vary within wide limits, it should be selected from the equation.

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Stone columns

• Failure mechanism of a single stone column loaded over its areasignificantly depends upon the length of the column.

• For columns having length greater than its critical length (that is about 4 times the column diameter) and irrespective whether it is end bearing or floating, it fails by bulging (see Fig. a).

Reinforcement

Failure Mechanism

a


g g y g g ( g )

• However, column shorter than the critical length are likely to fail in general shear if it is end bearing on a rigid base (see Fig. b) and in end bearing if it is a floating column fail by punching (Fig. c.)

b c

Stone columns

In practice, a stone column is usually loaded over an area greater than its own in which case it experiences significantly less bulging leading to greater ultimate load capacity and reduced settlements since the load is carried by both the stone column and the surrounding soil

Reinforcement

Failure Mechanism


stone column and the surrounding soil.

A group of stone columns in a soft soil undergoes a combined bulging and local bearing, type failure (Fig. a). Short stone column groups can fail in end bearing (Fig. b) or perhaps undergo a bearing capacity failure of individual stone columns similar to the failure mode of short, single stone columns. In the case of embankment over the weak foundation, the soil beneath and to the sides of the foundation move laterally (Fig. c) or can fail by circular failure surface.

ab

c

Stone columns

Load capacity of the treated ground may be obtained by summing up the contribution of each of the following components for wide spread loads, such as tankages and embankments:

• Capacity of the stone column resulting from the resistance offered by the surrounding soil against its lateral deformation (bulging) under axial load,

• Capacity of the stone column resulting from increase in resistance offered by the surrounding soil due to surcharge over it and

Reinforcement

Estimation of Load Capacity of a Stone Column


surrounding soil due to surcharge over it, and• Bearing support provided by the intervening soil between the columns.

Capacity based on Bulging of column:Considering that the foundation soil is at failure when stressed horizontally due to bulging of stone column, the limiting (yield) axial stress in the column is given by the sum of the following:

Various Ground Improvement Techniques


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INTRODUCTIONBenefits (1 ‐Main benefit/ 2 – Secondary benefit)

Technique Higher BearingCapacity

Less/ moreEvensettlement

FasterSettlement

Ground watercontrol

ReducedLiquefactionpotential

IncreasedErosionresistance

Improvedslope / facestability

Vibro‐replacement 1 1 2 1 2

Dynamic Compaction

1 1 1 2

Pre compression 1 1

Vertical Drainage 1 1 1 2

Reinforcing soil / 2 1 2 1g /soil nailing

Structural fill 2 2 2

Lime/cement admixture

1 1 2 1 2 2

Permeation grouting

2 1 1 2 1

Grouting 2 2 2

Ground freezing 1 1

Slurry cut off trench & pumping

1 2 2


9c- ground improvement techniques

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