foundations: soil improvement

8-1

8 TECHNIQUES FOR SOIL IMPROVEMENT

8.1 General aspects

Possible alternatives when poor conditions of the foundation soil are encountered:

- Bypass the poor soil using deep foundations.

- Remove the poor soil and replace it with engineered fill.

- Improve the soil properties in place (also referred to as soil stabilization).

Factors to be considered in choosing the proper solution:

- Soil characteristics: content of fine particles having silt- and clay-size; shear strength,

compressibility and hydraulic conductivity of the compressible soil.

- Thickness of the compressible layer.

- Characteristics of the structure to be constructed and acceptable settlements.

- Area and depth of the necessary treatment for soil improvement.

- Previous treatments carried out in the area.

- Availability of skills, equipment, materials.

- Economic aspects.

Primary objectives of soil improvement are:

- Clay: Increase bearing capacity or stability of excavations; Reduce foundation settlements.

- Sand: Reduce liquefaction potential; Increase bearing capacity; Reduce settlements.

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The liquefaction of saturated sand deposits is caused by the temporary loss of shear strength

of loose sand due to a rise of excess pore water pressure during cyclic loading, such as

seismic events.

Vibrations induce a rapid volume decrease of loose sands. If the sand is saturated, this

produces an increase of pore pressure in partially undrained conditions. If the increase of pore

pressure balances the total volumetric stress, the effective stress vanishes and the sand loses

its capacity to sustain shear stresses.

Grain size distributions of soil prone to liquefaction:

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Effects of liquefaction.

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Sand boils Quicksand (or running sand)

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Main mechanisms of soil improvement for cohesive soils:

1) Consolidation

- Preloading and drainage

2) Reinforcement

- Stone Columns

3) Reduction of water content (and consequent consolidation)

- Electro-osmosis

Main mechanisms of soil improvement for granular soils:

1) Vibration

- Dynamic compaction (impacts at ground surface)

- Vibro-compaction (deep vibration)

2) Vibration and displacement of backfill

- Vibro-replacement stone or concrete columns

3) Displacement of backfill material

- Compaction grouting

4) Binding of soil particles

- Permeation grouting

5) Mixing

- Soil-cement columns

- Jet grouting

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Main soil improvement methods

Method Cohesive

soils

Granular

soils

Preloading X -

Dynamic compaction - X

Vibro-compaction - X

Soil-cement columns (x) X

Stone columns X X

Compaction grouting (x) X

Permeation grouting - X

Jet grouting (x) X

Electro-osmosis X -

In general, the methods that aim at increasing the density of soil (e.g. vibration, compaction) and

those that aim at permeating it with grouting injections (e.g. permeation grouting) are not

effective in cohesive soils.

The methods that mix the soil with cement (e.g. soil-cement columns, jet-grouting) and

compaction grouting are suited for granular soils. They could be used also in cohesive soils

depending on their time dependent behaviour.

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8.2 Preloading

- Preloading is performed by placing a fill over the soft clay deposit.

- It improves the foundation soil for buildings, embankments, runways, bridge abutments.

- Type of preloads: earth fill, water tanks.

- Sand drains or prefabricated vertical drains (or wick drains) are used to reduce preloading time.

= time factor; = coefficient of consolidation; t = time; H = length of drainage paths

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Remarks.

- Wick drains: plastic core wrapped in geotextile (about 10-20 cm wide and 1-2 cm thick).

- Typical wick drain spacing is 1 to 2 m, depending on soil permeability and time available.

- Typical preloading period is 3 to 6 months, depending on soil permeability and degree of

consolidation to be achieved.

- Construction monitoring: settlement (settlement plates/probes); pore water pressure

(piezometerers); lateral movement (inclinometers)

Wick drains:

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’ Log scale

.

.

’ = pre-loading

. .

(1)

(2) (3)

(4)

(1) Initial in situ condition

(2) End of consolidation due to pre-loading

(3) Removal of preloading

(4) Construction of building

Pre-loading is most effective in normally consolidated or slightly over-consolidated clays.

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8.3 Dynamic compaction

- Dynamic Compaction involves repeated dropping of heavy weights onto the ground surface.

- Effective for sand, waste, and rubble fills.

- Pounders: concrete blocks, steel plates, or thick steel shells filled with concrete/sand.

- Typical weight of pounders: 6 to 30 tons, depending on the depth of soil to be improved.

- Typical drop heights: 10 to 30 m.

- Most effective for soil with less than 25% fines (i.e. silt- and clay-size particles; material

passing #200 sieve with 0.075 mm opening).

- Typical improvement depth D is 3 to 10 m. It can be estimated as follows:

√

where:

D = improvement depth in m

W = pounder weight in ton

H = drop height in m

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- The maximum velocity of the induced ground vibrations should be:

< 1 cm/sec to prevent cracks in walls of nearby buildings

< 5 cm/sec to prevent structural damages

- Construction monitoring: induced settlement; ground vibration; ground heave; pore water

pressure increase; verification testing (SPT, CPT).

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8.4 Vibro-compaction (or vibroflotation)

- Vibro-Compaction increases the density of very loose granular soil by inserting a vibrating

probe into it. It represents an artificial liquefaction of soil.

- The probe spacing ranges from 2 to 5m.

- The treatment is suitable for sand with less than 15% fines (silt- and clay-size particles).

- Vibrator is a torpedo shaped horizontally vibrating probe, 3 to 5 m long, and weighs about 2

tons. The probe penetrates to the design depth under its own weight assisted by water jetting

- The action of vibrator and water jetting reduces the intergranular forces (liquefaction)

allowing the soil to become denser.

- The vibrator starts at the bottom of the hole and raised to treat the next interval; the procedure

is repeated as backfill sand is added.

- If backfill is not added, craters with diameters of 3 to 4 m can form around the vibrator.

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To be effective, the probe should vibrate at a frequency close to the resonant frequency of

the soil into which it penetrates. Hertwig (1933) and Lorenz (1934) studied the effect of

vibration on soils using a vibrator placed at the ground surface.

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8.5 Vibro-replacement stone and concrete columns

- This technology is an extension of the previous one and could be adopted, by a certain extent,

also for silt and clay.

- The probe penetrates to design depth. Then, gravel/crushed rock (or concrete) is placed in the

hole as the probe is withdrawn in vertical increments of 0.5 to 1.5 m.

- A column is formed with gravel or concrete laterally compacted against the surrounding soil.

- Construction Monitoring: settlement and ground heave; amount of stone backfill used;

verification testing (SPT, CPT)

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Stone columns can be also constructed using an auger to drilling the borehole

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8.6 Soil-cement columns (or grout-mix columns, or deep mixing)

- Soil-cement columns are constructed mixing in-situ soil with cementitious materials using

mixing shafts consisting of auger cutting heads, auger flights, or mixing paddles.

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- When the grout-mix column is completed, a steel beam can be inserted into it, thus obtaining a

foundation pile.

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8.7 Soil grouting

- Soil grouting is carried out with medium-low pressure of the grouting fluid. This includes:

Compaction grouting; Permeation grouting; Claquage (or hydrofracturing).

- Jet grouting requires much higher pressures with respect to the previous ones.

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Compaction grouting

- Compaction grouting is used to compact loose granular soil or to underpin existing

foundations or to produce controlled heave of structures.

- It involves injection of viscous grout (soil-cement mixture) which does not enter the soil pores

but remain in a homogeneous mass. As a results a series of bulbs is formed around the

injection pipe.

- Compaction grouting is not typically used for excavation support. However, compensation

grouting, a variant of this technique, is increasingly used to control the settlements induced by

tunnelling.

- Grout material may consist of fine sand mixed with 12% cement and water to produce stiff,

mortar-like mixture.

- Grout pipe is installed to maximum treatment depth and grout is injected as the pipe is

withdrawn incrementally, forming a column of interconnected grout bulbs.

- Construction monitoring: injected grout volume; pressure loss; ground surface heave;

verification testing pre- and post-grouting (SPT/CPT)

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Compensation grouting

By means of compensation grouting it is possible to compensate settlements caused by the

excavation of shallow tunnels which could damage existing buildings.

Grouting occurs through steel sleeved pipes who are installed in horizontal holes starting from a

shaft. By this way, a net of grouting points is created between the foundations and the tunnel.

Grouting of every point can be precisely controlled in order to compensate settlement with the

injection of cement grouts. Building movements are continuously controlled by means of a net

of displacement measuring devices.

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Permeation grouting

- The scope of this technique is to increase the shear resistance of granular soil (providing a

cohesion to them), and to improve their stiffness, by filling their voids with grout without

changing their original structure and volume. It is the oldest grouting technique: the first

documented applications date back to the early 1800’s.

- Permeation grouting is frequently adopted for the temporary support of shallow and deep

excavations in granular (pervious) soils and for underpinning existing foundations.

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- Grouts of fine and ultra-fine cement, with particle size ranging from 1 to 10 microns that can

penetrate fine sands, are used in most cases.

- Chemical solutions (which are appreciably less viscous than cement grouts) can be employed

for increasing the mechanical characteristics of fine soils or to produce impervious

underground barriers in order to control the water flow towards excavations and subterranean

structures.

- Cement based suspension grouts are only suitable for the grouting of soils with a coefficient of

hydraulic conductivity k in excess of 0.08 cm/s.

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Microfine cement based grouts are typically suitable to permeate soils with k greater than

0.001 cm/s, provided the silt content is less than 12%.

Both cement grouts produce hydrofracturing when k is one order of magnitude lower.

Solution grouts can typically be used for permeation grouting as long as k is greater than .0005

cm/s. The layer thickness and the silt percentage also play an important role in the injectability

of the soils and the permeability limits are therefore only indicative.

- Permeation can be achieved in a single pass, while withdrawing the probe from the borehole,

or in multiple passes (through pre-installed tubes-a-manchette) alternating between solution

grout (typically sodium silicate solutions) and cement based grout or suspension grout.

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- During the first pass (solution grouting), grout will predominantly permeate the most pervious

layers in the pressurized grout zones. In addition, once the first pass is finished, the solution

sags under gravity and migrates in the most pervious soils. Becoming a gel, it reduces their

permeability.

The solution grout does not appreciably improve the mechanical characteristics of the

permeated soil. Hence a second pass of suspension grouting (cement) is necessary to this

purpose.

Due to the grouting pressure, the suspension could flow within pervious layers reaching zones

far beyond the one that was intended to be treated. The previous reduction of the permeability

tends to limit, by some extent, this negative affect.

By alternating solution grouts with suspension grouts, the suspension grouts will hydrofracture

through the treated zone close to the tube-a-manchette, thus extending the diameter of the

grouted zone.

A proper treatment could require 3-4 grout passes. As a result, grout cylinders with a

predictable size are formed around the injection pipe.

Their elastic modulus and shear strength parameters are higher, and the permeability is lower,

than those of the original granular soil.

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The grout spread (or radius of the treated soil) can be estimated with the equation of Cambefort-

Naudts

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Claquage (or hydrofracturing)

- Hydrofracturing uses a relatively high grouting pressures and aims at fracturing the ground,

thus creating lenses of grout mainly along planes normal to the injection pipe.

- Claquage may develop as a consequence of permeation grouting when the grout pressure

overcomes the in situ vertical stress.

- It could be used to lift the foundations of buildings that underwent settlements. However it can

produce severe damages because the inherent difficulties in controlling the produced heave. Its

use is not frequent in engineering practice.

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Jet grouting

- Jet grouting is an in-situ injection technique employing specialized equipment that includes

grout pumps, grout mixer, drill rig, drill rods and injection monitor with horizontal radial

nozzles delivering high velocity fluids to erode, mix, and stabilize in-situ soils using an

engineered grout slurry.

- The treatment is effective in granular soils, while it is not appropriate for clays.

- The pressure of the fluids can reach 500 bars.

8-40

The jet grouting technique can use different kind of fluids:

- Single fluid jet grouting: neat cement grout, is injected at high

velocity through horizontal radial nozzles to directly erode and

mix with the in-situ soil.

- Double fluid jet grouting: cement grout is injected through

radial nozzles and is assisted by a second fluid, typically air,

delivered through a coaxial nozzles, to directly erode and

mix with the in-situ soil.

- Triple fluid jet grouting: water is injected through radial

nozzles and is assisted by a second fluid, typically air

delivered through a coaxial nozzles, to erode the in-situ soil,

while a separate nozzle placed lower on the monitor delivers

a third fluid, typically neat cement grout, at lower velocity

to simultaneously fill the soil zone eroded by the cutting

fluids (air and water).

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- The Monitor (adjusted for single, double, and triple systems) is a drill pipe attached to the end

of a drilling string and designed to deliver one to three fluids of the jet grouting process

through one or more injection points (nozzles).

- Jet grouted columns could form panels (vertical structures), slabs (horizontal structures) or

canopies (sub horizontal structures within ⁄ from the horizontal plane).

(plane view of a

jet grouting slab)

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- Jet grouting parameters are: pressure of the fluid(s) within the jet grouting string; flow rate of

the fluid(s); grout composition; rotational speed of the jet grouting string; rate of withdrawal

or insertion of the jet grouting string. Their choice depends on the characteristics of soil.

- In layered cohesive-granular soils, the diameter of jet grouted columns tends to be quite

variable with depth.

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- Particular care must be adopted when using jet grouting for underpinning. In fact, if the high

pressure injection reaches the foundations it could cause large heave, severe damages and even

collapse of the building.

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8.8 Electro-osmosis

Electro-osmosis is a method sometime used for the stabilization of silts, silty clays and soft

sensitive clays. It is also used for the remediation of contaminated soils.

It can be described as follows.

When an electric potential gradient is applied across a mass of fine grained soil, the cations

(positive ions) belonging to the double layer of water surrounding its particles are attracted to

the cathode while anions (negative ions) are attracted to the anode. As ions migrate, they drag

water molecules with them.

Because there are more cations than anions in a soil containing negatively charged particles

(such as clay), there is a net flow of water towards the cathode.

This flow, called electro-osmosis, induces a progressive decrease in water content and a

corresponding progressive consolidation of the clay starting at the cathode. As a consequence,

the soil undrained strength tends to increase.

If the double layer is much smaller than the characteristic length dimension of the channels

between the particles, electro-osmotic velocities are independent of conduit size. This makes the

treatment applicable also to clays and silts where standard drainage methods would not be

affective because of their low permeability.

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Drawbacks of this method are its cost and the time required for completing the treatment.

The electro-osmotic flow rate is primarily a function of the applied voltage. It could be increased

by injecting saline solutions at the anode, however it is typically in the range of few millimeters

per second.

foundations: soil improvement

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