Ground Treatment

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GROUND TREATEMENT . Vibro/ Dynamic/ Impact Compaction are methods of densifying granular soils using a depth or surface vibrator/ impact. The effect is enhanced shear strength and stiffness. – Use in loose sands, wind blown sands, cavities, fill material, reclaimation. Vibro Replacement is the installation of stone or concrete columns in clayey or silty soils using a depth vibrator. The improved soil matrix has higher shear strength and stiffness. In addition, the stone columns may function as large diameter drains for rapid consolidation. (Form of piling typically achieving bearing capacities of 150kPa for SCC ). Deep Soil Mixing involves the mixing of soft soils with a binder, typically cement and/or lime, by mechanical means to modify its properties. This treatment results in higher shear strength and stiffness. These may be thought of a modified modulus columns. Compaction Grouting is a technique that creates grout bulbs by injecting a stiff grout (under 1 – 7MPa 1000psi, pressure) into the soil. This grout bulb displaces and compacts the in- situ soil. Low slump grout maintains a homogenous mass densifying the soil. Jet Grouting utilizes a fluid jet (air, water and/or grout) to erode and mix the in-situ soft or loose soils with grout slurry (up to 11,000psi pressures). The result is significantly increased shear strength and stiffness. In sandy soils, permeability can be reduced to control water flow.

Transcript of Ground Treatment

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GROUND TREATEMENT .

Vibro/ Dynamic/ Impact Compaction are methods of densifying granular soils using a depth or surface vibrator/ impact. The effect is enhanced shear strength and stiffness. –Use in loose sands, wind blown sands, cavities, fill material,reclaimation. Vibro Replacement is the installation of stone or concretecolumns in clayey or silty soils using a depth vibrator. Theimproved soil matrix has higher shear strength and stiffness.In addition, the stone columns may function as large diameter drains for rapid consolidation. (Form of piling typically achieving bearing capacities of 150kPa for SCC ). Deep Soil Mixing involves the mixing of soft soils with abinder, typically cement and/or lime, by mechanical means to modify its properties. This treatment results in higher shearstrength and stiffness. These may be thought of a modified modulus columns. Compaction Grouting is a technique that creates grout bulbsby injecting a stiff grout (under 1 – 7MPa 1000psi, pressure) into the soil. This grout bulb displaces and compacts the in-situ soil. Low slump grout maintains a homogenous mass densifying the soil. Jet Grouting utilizes a fluid jet (air, water and/or grout) toerode and mix the in-situ soft or loose soils with grout slurry (up to 11,000psi pressures). The result is significantly increased shear strength and stiffness. In sandy soils, permeability can be reduced to control water flow.

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1. Vibro – Dynamic Compaction The objective in Vibro Compaction (sand compaction)is to achieve densification of coarse grained soils with less than 10-15% silt using a ‘vibrofloat’

The effect of the process is based on the fact that particles of non-cohesive soil can be rearranged by means of vibration.

Effect of vibro compaction on soil density The angle of friction is increased resulting in much higher shear resistance. (see N spt - φ relationship charts).

The modulus of deformation (E) is also increased, resulting in reduced settlements (see settlements of foundations influence charts and equations). The Vibro Compaction process may be described as follows: 1. Penetration. Water jets are fully opened to allow speedy penetration to design depth. The vibrator penetrates the ground under its own self weight. 2. Compaction & Backfilling. Upon reaching design depth, the vibrator is held at that level to compact the soil. The vibrator is then lifted a specified distance and held at that position. This is carried out until the required zone of soil is treated. As the vibrator is being lifted stepwise, sand is filled into the developing crater.

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Dynamic/ Impact Compaction This is a widely used soil improvement technique that involves thedropping of large weights onto the soil, or the towing of "square" rollingcompactors. The success of dynamic compaction depends on soil type,moisture content, compaction energy, but the depth and extent of thesoil improvement is often difficult to predict.

During the late 1940’s in South Africa, a young civil engineer by the

name of Aubrey Berrangé, was watching a fleet of equipment build an

embankment on a remote road building project. The soil was being

placed in thin layers and compacted with multiple passes of a vibratory

roller. As he watched, he was struck by the fact that, of all the

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processes happening on the site, the compaction of the newly placed

soil was both the most critical to the success of the project and at the

same time, the least efficient. He began to wonder what it would take to

enable soil to be compacted in much thicker layers. Little did he know

at the time that his thinking was about to lead him into a study that

would last a lifetime and lead to the only real revolutionary advance in

soil compaction since the invention of the steam roller

Between the years of 1949 and 1953, Aubrey Berrangé worked on the

development of a “square wheel” roller. The principle was to provide a

large compactive force on a large contact area similar to a “stamper” or

a “rammer” but on a continuous basis. There were many development

challenges as the forces were enormous and it wasn’t as simple as one

might imagine at first. Nonetheless he persevered and through the

development of a series of trial machines, he was able to produce a

machine that effectively compacted soil to far greater depths than was

possible using the conventional vibratory equipment. Despite the

effectiveness of the “square rollers” the idea seemed to be a little

“ahead of it’s time” since the real need for deep compaction was not

clearly apparent to most people in the industry.

Some 20 years later, during the 1970’s, conditions for the use of “deep

compaction equipment” became more favourable. Roads were having

to withstand the strain of carrying larger truck loads, airports were

having to cater for much larger aircraft [747’s etc] and many places

around the world were looking at reclaiming land from the sea. With a

renewed interest in the requirement for deep compaction becoming

apparent, Aubrey Berrangé dug his old machine out of mothballs and

went in search of industrial development partners. What followed was a

twenty year development story that was filled with much intrigue and

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interesting developments. Many collaborations were undertaken

including a lot of work done under the auspices of the South African

Council for Scientific and Industrial Research [CSIR]. During this twenty

year development period, the impact compactor became widely

accepted in Southern Africa and Australia. As the design was refined, a

twin mass [split-mass] configuration was adopted. This proved to

overcome some of the constraints of the single mass machines.

Different shapes of masses were also developed within the twin mass

range of equipment to better suit different applications.

One recent development was the introduction of Continuous Impact

Response [CIR] measurement to enable the effective control and

“certification” of the compaction performed by Landpac Impact

Compactors. This system is now being used as a quality control tool

and further development is taking place to enhance the capabilities of

this new technology

High energy impact compaction (HEIC) involves the transfer of

compaction energy into the soil by means of the lifting and falling

motion of non-circular rotating masses. The rotation of such masses to

their highest point results in an effective potential energy build-up.

Further rotation of these masses results in the conversion of this

potential energy into a falling kinetic energy, which is transferred to the

soil upon the impact of the lowest point of the masses with the surface

of the soil. The amount of energy transferred, in the form of compactive

effort, is closely related to the amount of potential energy generated in

the lifting process.

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The high energy impact compaction equipment is towed along the

ground by a tractor at a relatively high speed of 10-13 km/h. The non-

circular masses are thus caused to rotate and generate a series of high

impact and high amplitude blows that are delivered to the surface of the

soil at a relatively low frequency of 90 to 130 blows per minute. The

energy per blow varies between 10kJ and 25kJ depending on the type

of compactor being utilised. Because the energy is transferred in the

form of a “dynamic load” it is possible to generate very high compaction

forces when utilising high energy impact compaction equipment. The

main features of this high energy impact compaction process include

the following:

Compaction Loads

The high energy and dynamic compaction action of the Landpac HEIC

equipment leads to typical compaction loads of between 1200kN and

2500kN being generated depending upon the type and condition of the

material being compacted.

Moisture Content

The high energy of the Landpac HEIC equipment leads to the ability to

compact material to a higher maximum dry density than is achievable

with conventional roller type of compaction equipment.

This high energy also allows for the compaction of material over a wider

range of moisture conditions particularly dry of optimum moisture

content.

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Depth of Influence

The high compaction loads that are generated by the Landpac HEIC

equipment lead to high surface contact pressure on the soil. This

coupled to the relatively large contact area over which the compaction

energy transfer takes place, leads to a vastly increased depth of

influence of the compaction. Ground improvement is typically measured

to effective depths of 2m-3m with depths of up to 5m being recorded in

some applications.

Soil Compressibility

The shape of the non-circular masses allows for the high energy

parcels to be transferred in the form of a “rolling impact”. This means

that the load duration of the Landpac HEIC process is relatively long

[typically 0.12s]. This extended load transfer duration in turn leads to a

softer soil response to the load and hence an enhanced soil

compressibility is achievable.

Compaction Productivity

The relatively high operating speed and depth of influence of the

Landpac HEIC process leads to very high productivity of compaction.

The HEIC process can typically cover 15,000m² per hour per surface

coverage. The productivity of the Landpac HEIC process can be

between 2 and 5 times higher than that of conventional shallow

compaction equipment when performing fill works and many times

more productive than that when it comes to the improvement of in-situ

materials.

Landpac high energy impact compaction equipment thus provides a

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process that allows for a wide range of applications from fill works

compaction through to deep in-situ ground improvement. In all of the

appropriate applications of this equipment it is possible to ensure

project cost savings whilst at the same time enhancing the quality

assurance of the works relative to the “in service” performance of the

materials that have been treated using this equipment.

A solid steel tamping weight is dropped onto the ground from asufficiently great height to achieve the necessary compaction. Theresulting high energy impact transmits shock waves through the treatedground. This reduces voids between soil particles resulting in enforcedsettlement, thus reducing long term settlement. The treatment is carried out in a series of impact positions on variousgrid patterns which are arranged so that compacted zones beneath theweights overlap to ensure the whole area has been treated. As a result,foundation loads can be supported without shear failure or excessivesettlement. This technique is used in conjunction with good control andinsitu testing procedures.

This treatment technique is not as widely used as stone columns due tothree main factors:

The range of soil types suitable for improvement is less wide thanthat for stone columns e.g. soft clays and silts or sites with a highwater table are unsuitable

Dynamic compaction sets up much higher levels of vibrations than stone column installation. Typically dynamic compaction can be

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carried out within 15m from existing services and 30m of existingbuildings

L Lower cost per square metre than stone columns

Sites where dynamic compaction would have a cost advantage over stone columns include:

Large greenfield/brownfield sites Sites where there are a significant amount of underground

obstructions. These would require extensive preparatory earthworksby the Main Contractor to prepare the site for the stone columns or piles

Summary: Vibro Compaction Applicable soil(s) • Coarse grained soils with

silt/clay content less than 10-15%

Principle • Densification by vibration

Effect(s) • Increased shear strength • Increased stiffness • Reduced liquefaction potential

Common applications • Buildings • Chemical plants • Storage tanks & silos • Pipelines • Wharf structures • Embankments • Roads

Maximum depth • 60 m vibro piling technique • • (2-4m rolling dynamic

compaction)

Land / offshore application

• Both

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2. Vibro Replacement Vibro Replacement is a technique of constructing stone/ concrete columns through fill material and weak soils to improve their load bearing and settlement characteristics. Unlike clean granular soils, finegrained soils (such as clays and silts) do not densify effectively undervibrations. Hence, the necessity to form stone columns to reinforce andimprove fill materials, weak cohesive and mixed soils.

Principle of Vibro Replacement The stone columns and intervening soil form and integrated foundation support system having low compressibility and improved load bearing capacity. In cohesive soils, excess pore water pressure is readily dissipated by the stone columns and for this reason, reduced settlements occur at a faster rate than is normally the case with cohesive soils.

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There are different types of installation methods which can be broadly classified in the following manner: • Wet top feed method • Dry bottom feed method • Offshore bottom feed method The installation process may be described as follows: . 1. Penetration The depth vibrator is used to penetrate the ground to be treated by a combination of vibrations, self weight, push down force (the Vibrocat method) or water jetting (wet method). Depth sensors allow the operator to know when design depth has been reached. 2. Stone feeding & Compaction Stone is feed into the hole either from the surface via a wheel loader (topfeed method) or through a stone tube to the tip of the vibrator (bottom feed method). This single charge of stone is then compacted by the vibrator. The stone is forced downward and outward leaving a short length of compacted stone, with a diameter larger than the diameter of the hole created by the vibrator. Once this stage is completed, another charge of stone is introduced and compacted. This process is repeated upwards till the desired length is achieved.

The VCCs technique is most appropriate when there is a limitedthickness of particularly soft/weak soils. It is applicable for soil stratawith a shear strength of greater than 15kPa, up to around 60kPa, under specific conditions and can be utilised for soils with shear strengths inthe range of 8kPa to 15kPa - providing these are generally not thickerthan 1.0m. Ideally the soil to be penetrated should be of constant strength orincreasing strength with depth, to eliminate the risk of ‘necking’.

UUse of VCC Light to medium loaded structures such as industrial units,offices and low rise housing. They are principally designed to takevertical axial loads and can only accommodate low horizontal loads or tensile forces.

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To support foundation loads and slabs, but slabs must besufficiently reinforced to prevent ‘punching’ of the concrete columnsthrough the slab.

Summary: Vibro Replacement Principle • Reinforcement

• Drainage

Applicable soil(s) • Mixed deposits of clay, silt and sand

• Soft and ultra soft silts (slimes) • Soft and ultra soft clays • Garbage fills

Effect(s) • Increased shear strength • Increased stiffness • Reduced liquefaction potential

Common applications • Airport taxiways and runways • Chemical plants • Storage tanks & silos • Pipelines • Bridge abutments and approaches • Offshore bridge abutments • Road and railway embankments

Maximum depth • 20-40 m

Land / offshore application • Both

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Deep Soil Mixing Dry mixing is a highly effective ground treatment system used to

improve the load performance of soft clays, peat and other weak soils.

By varying the proportion of lime, cement and admixtures, a range of

strength gains can be achieved. The greatest improvements can be

achieved in inorganic soils with low moisture content. Extremely good

results can also be achieved in sensitive clays. This constructs a

modified modulus soil column, similar to pile element.

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The mixing rig and shuttle

The Deep Mixing Method can be performed in most soft soils. Mixing in

the soil is performed through:

• Rotating a mixing tool, drilling the tool into the soil

• Reverse drilling rotation, extracting it again at the same time as the

dry binder is blown out and mixed into the soil.

Through the rotating movement, the soil is mixed with the binder and an

immediate reaction starts. The improved soil acquires the share of a

column. Soil mix column diameters of 500mm to 1000mm and lengths

of up to 25m can be constructed to a controlled height and depth. The

columns can also be interlocked to provide cellular structure of in-situ

wall or the entire mass cab be stabilized.

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Summary: Dry Deep Soil Mixing Principle • Chemical stabilization

Applicable soil(s) • Clayey silt • Clays • Marine clay • Sensitive clay • Mud • Peat

Effect(s) • Increased shear strength • Increased stiffness

Common applications • Deep excavation • Road and railway embankment • Quay walls

Maximum depth • 25 m

Land / offshore application

• Land

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Compaction Grouting Compaction grouting is a technique in which a stiff to plastic grout is

injected into the soil under pressure. It expands in the soil as a

relatively homogenous mass and at the same time forming almost ball-

shaped grout bulbs. The soil surrounding the grouted area is displaced

and at the same time compacted. Compared to other grouting

techniques, the grout material neither penetrates into the pores of the

in-situ soil (as is the case with classical injection) nor are local cracks

formed (as is the case with the Soilfrac® technique). During the

compaction grouting process, the following are monitored:

Pressure

• Grout quantity

• Ground surface deformation

• Heave of structures

Depending on the design requirements, the compaction grouting

process will be terminated either when reaching a maximum pressure,

a maximum grout volume, when achieving the desired uplift of the

structure of in case of grout material flowing out on the site surface.

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The compaction grouting method may be used for the improvement of

non-cohesive soil, especially in cases where soils of loose to medium

density are encountered.

This method is also used in fine-grained soils in order to install

elements of higher strength and bearing capacity in soils of low bearing

capacity. This improves the load bearing behaviour of such soils. When

using this technique in saturated clayey soil, a temporary increase of

pore water pressure can be observed. (Due to the fact that fine-grained

soils cannot be compacted from the soil mechanics point of view- by

applying the same technique- strictly speaking, “consolidation grouting”

is carried out.)

The degree of compaction can be controlled by the following

parameters:

• Evaluation of the automatically recorded process parameters by

means of software

• Deformation measurements at ground surface or structures

• Soundings (CPT, SPT) before and after the compaction grouting

process

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Summary: Compaction Grouting Principle • Reinforcement

• Compaction

Applicable soil(s) • Loose to medium dense sands • Fine-grained soils

Effect(s) • Increased shear strength • Increased stiffness

Common applications • Ground improvement • Foundation rehabilitation • Cavity grouting

Maximum depth • 40 m

Land / offshore application

• Land

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Jet Grouting The jet grouting process is known as Soilcrete ® is a recognized method for cement soil stabilization. With the aid of high pressure cutting jets of either water or cement suspension, the soil around the borehole is eroded. The process uses nozzle velocities greater than or equals to 100 m/s and also has the possibility of air-shrouding. The eroded soil is rearranged and mixed with the cement suspension. The soil-cement mix is partly flushed out to the top of the borehole through the annular space between the jet grouting rods and the borehole. Different geometrical configurations of Soilcrete elements can be produced. The erosion distance of the jet varies according to the soil type to be treated, the kind of Soilcrete process and the jetting fluid being used.

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Basic forms of Solicrete® elements There are two broad categories of main Soilcrete applications- stabilization and sealing. Underpinning work for adjacent construction pits is one of the main applications of soil stabilization, followed by foundation modifications and restoration. The techniques opens news ways for tunnel construction in loose soil. Excavation pits constructed with waterproof Soilcrete elements in the walls, combined with low permeability slabs enables the execution of deep excavations without large-scale ground water lowering. Environmentally safe binder materials are used for Soilcrete. Drilling rigs manufactured by Keller have masts ranging from 2m to 35m to enable work with low headroom as well as optimal work rate in open spaces. The process may be summarized as follows. 1. Drilling Drill rods equipped with jet nozzle holder and drill bit are used to drill the jet grouting hole down to the required depth. Normally the jet grout mixture is used as drill flushing to stabilize the borehole during the drilling operation. In masonry and concrete, special drilling bits are used.

2. Jetting The dissolution of the grain texture with a powerfulfluid jet starts at the lower end of the Soilcrete element. The excess water-soil-cement mixture is removed to the surface through the annular space between drill rod and borehole wall. The pre-selected production parameters are constantly monitored.

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3. Grouting For all Soilcrete variations, cement suspension is injected under pressure simultaneously with the erosion of the soil. The turbulences caused by the jetting technique results in the uniform mixing of the grout with the soil within the treatment zone. 4. Extension Soilcrete elements of each type may be constructed fresh-on-fresh as well as fresh-against-firm and combined in a variety of ways. The working sequence follows the technical requirements and the conditions of the structure to be treated.

Summary: Soilcrete(®- Keller GE) Jet Grouting Principle • Chemical stabilization

• Reinforcement

Applicable soil(s) • Clay • Silt • Sand

Effect(s) • Increased shear strength • Increased stiffness

Common applications • Underpinning & foundation rehabilitation

• Tunnel protection • Horizontal grouting • Shaft support • Earth pressure relief • Watertight sealing slab • Joint sealing between pile • Vault slabs • Dam sealing • Panel walls

Maximum depth • 35 m

Land / offshore application

• Land