W16 - Groundwater Modification: Stabilizing the Foundations of

Public Works Projects Douglas Dycus, P.E.

Senior Engineer E Sciences, Inc.

April 2013

Causes of Unconsolidated Soils

• Water • Clays • Organics • Man-made • Karst

Soil Stabilization • Improvement of stability or bearing capacity of

soil by use of controlled compaction or by the addition of suitable admixtures or stabilizers.

Methods for Soil Improvement

• Deep Dynamic Compaction

• Drainage/Surcharge

• Electro-osmosis • Compaction

grouting • Blasting • Surface

Compaction

• Soil Cement • Lime

Admixtures • Flyash • Dewatering • Heating/Freezin

g • Vitrification

Ground Reinforcement Ground Improvement Ground Treatment Stone Columns Deep Dynamic Compaction Soil Cement Soil Nails Drainage/Surcharge Lime Admixtures Deep Soil Nailing Electro-osmosis Flyash Micro Piles (Mini-piles) Compaction grouting Dewatering Jet Grouting Blasting Heating/Freezing Ground Anchors Surface Compaction Vitrification Geosynthetics Fiber Reinforcement Lime Columns Vibro-Concrete Column Mechanically Stabilized Earth Biotechnical

Compaction

Mechanical Stabilization Process of improving the properties of soil by

changing its gradation.

Two or more natural soils are mixed to obtain a composite material.

Cement Stabilization Done by mixing soil and cement with water and compacting

the mix to attain a strong material.

Lime Stabilization Lime stabilization is done by adding lime (2%-10%) to soil.

Bituminous Stabilization Bituminous stabilization provide water proofing and binding.

Chemical Stabilization Stabilization by adding different chemicals.

Electrical Stabilization Done by a process known as electro-osmosis.

Stabilization by Grouting In this method grouting is done under pressure the stabilizers

with high viscosity are suitable only for soils with high permeability.

Stabilization by Geotextiles and Fabrics Geotextile which have very high tensile strength can be

used as reinforcement for strengthening soil.

Reinforced Earth Soil can be stabilized by introducing thin strips in to it .

Stabilization using Bio-Enzymes Bio-enzyme stabilization is a newer technique for

strengthening of sub grade soil. Terra Zyme is one of the largely used bioenzymes.

Vertical Drains • Act as free draining water channel. surrounded by a

thin filter jacket which prevents the surrounding soil from entering the core.

• A vertical sand drain accelerates the rate of consolidation.

• Installation of vertical sand drains is a convenient technique for stabilization of soft and compressible soil.

• There are two types of vertical drains - sand drains and sand wicks.

Vertical drains

Sand drains • Typically 200-500 mm in diameter • Formed by infilling sand in to a hole in the ground • Hole formed by driving, jetting or augering • Typical spacing 1.5 - 6.0

Sand wicks • Sand wicks are improved technique of sand drains • A small diameter hole is made by driving mandrel

or by boring • Then cylindrical bag with sand is lowered into this

• Excavation which has a blanket of filter material between 0.5m and 1.00 m thick against its upstream slope and at the bottom of system for collecting and eliminating water.

• Improves the stability of embankment by providing drainage and replacing weaker material with better material .

Stabilizing Trenches

Stabilizing Trench

Capillary Cut-Off

• In some cases capillary water accumulates and saturates the subsurface layers which results in failures.

• To arrest this capillary rise, capillary cut-off has to be provided.

• Capillary cutoff is of two types. • Permeable Capillary Cut-off

• Impermeable Capillary Cut-off

A layer of granular material is provided which has a thickness higher than the capillary rise so that

water cannot rise above the cut-off layer

Cross-Section of pavement showing permeable capillary cut-off

Permeable Capillary Cut-Off

An impermeable capillary cut-off is prepared by inserting bituminous layer in place

of permeable blanket.

Cross-Section of pavement showing impermeable capillary cut-off

Impermeable Capillary Cut-Off

Methods for Soil Improvement-Soil Nailing

Soil Nailing

• Earth retention structure that combines reinforcements and shortcrete to support excavations, hillside, embankment steeping, etc.

• The nails must have bending stress. The tension developed in nails provides resisting forces which stabilize the soil mass.

Soil Nail

• Tiebacks can be used in tension applications to anchor retaining walls.

• Helical tiebacks have shorter bond lengths than grouted ones so they can be used where space is limited.

Tension Anchor

Tiebacks can be used in tension applications to anchor shot-crete walls.

Tiebacks

Shotcrete Walls

Helical tiebacks were favored over grouted ones because they would not encroach beyond the property line.

Recesses were formed in the wall to allow the tiebacks to be stressed against bearing plates.

Stressing Tiebacks

Recesses were filled and the wall stuccoed..

The Finished Wall

Construction can proceed with the excavation and there is no need for backfill behind the wall.

Top Down Walls

The large 2-7/8” OD shafts can stand unsupported for the full depth of the trench.

Greater Span Without Buckling

Underpinning Underpinning is used when an existing structure has failed and

support must be restored. Underpinning brackets allow transferring of the structure load to the newly installed piles,

this helps to preserve the integrity of the structure.

• Passive Anchor • Small diameter tension element (not-stressed)

• Active Anchor • Small post tensioned element.

Definitions

NICHOLSON

Definitions Micropile • Small diameter drilled

and grouted pile. • Made with combinations

of pipe (casing) and treaded rods.

• Can be post grouted

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NICHOLSON

Grout under pressure

Excavation Support Wall Movements

What factors control wall movements?

• Wall Stiffness

• Ground Stiffness

• Depth of first level of brace/anchor

• Magnitude of preload

• Toe support

• Base Safety Factor

Stone Columns • Done to provide adequate support for relatively

light foundation. • The method consists of forming vertical holes in

ground which are filled with compacted crushed stone, gravel and sand or a mixture.

Methods for Soil Improvement – Jet Grouting

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Elephant and Compaction

Heavy Weight

Question?

The compaction result is not good.

Why?

Compaction and Objectives Compaction • Many types of earth construction, (dams, retaining walls,

highways, airport) require man-placed soil, or fill. To compact a soil, that is, to place it in a dense state.

• The dense state is achieved through the reduction of the air voids in the soil, with little or no reduction in the water content. This process must not be confused with consolidation, in which water is squeezed out under the action of a continuous static load.

Objectives • Decrease future settlements • Increase shear strength • Decrease permeability

Coarse-grained soils Fine-grained soils

Hand-operated vibration plates Motorized vibratory rollers Rubber-tired equipment Free-falling weight; dynamic compaction (low frequency vibration, 4~10 Hz)

Falling weight and hammers

Kneading compactors

Static loading and press

Hand-operated tampers

Sheepsfoot rollers

Rubber-tired rollers

Labo

rato

ry

Fiel

d

Vibration

Vibrating hammer (BS)

Kneading

General Compaction Methods

Field Compaction Equipment and Procedures

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Equipment

Smooth-wheel roller (drum)

• 100% coverage under the wheel

• Contact pressure up to 380 kPa

• Can be used on all soil types except for rocky soils.

• Compactive effort: static weight

• The most common use of large smooth wheel rollers is for proof-rolling subgrades and compacting asphalt pavement.

Equipment (Cont.)

Pneumatic (or rubber-tired) roller

• 80% coverage under the wheel

• Contact pressure up to 700 kPa

• Can be used for both granular and fine-grained soils.

• Compactive effort: static weight and kneading.

• Can be used for highway fills or earth dam construction.

• Compactive effort: static weight and kneading.

• Can be used for highway fills or earth dam construction.

Equipment (Cont.) • Has many round or rectangular shaped protrusions or

“feet” attached to a steel drum • 8% - 12% coverage • Contact pressure is from 1400 to 7000 kPa • It is best suited for clayed soils • Compactive effort: static

weight and kneading • It is best suited for

clayed soils • Compactive effort: static

weight and kneading

Sheepsfoot rollers

Equipment (Cont.) • About 40% coverage

• Contact pressure is from 1400 to 8400 kPa

• It is best for compacting fine-grained soils (silt and clay).

• Compactive effort: static weight and kneading.

Tamping foot roller

Equipment (Cont.) • 50% coverage

• Contact pressure is from 1400 to 6200 kPa

• It is ideally suited for compacting rocky soils, gravels, and sands. With high towing speed, the material is vibrated, crushed, and impacted.

• Compactive effort: static weight and vibration.

Mesh (or grid pattern) roller

Equipment (Cont.) • Vertical vibrator attached to smooth wheel rollers

• The best explanation of why roller vibration causes densification of granular soils is that particle rearrangement occurs due to cyclic deformation of the soil produced by the oscillations of the roller

• Compactive effort: static weight and vibration

• Suitable for granular soils Vibrating drum on smooth-wheel roller

Equipment-Summary 56

Variables-Vibratory Compaction

Characteristics of the compactor:

(1) Mass, size (2) Operating frequency and

frequency range Characteristics of the soil:

(1) Initial density (2) Grain size and shape (3) Water content (4) Towing speed

Construction procedures: (1) Number of passes of the roller (2) Lift thickness (3) Frequency of operation vibrator (4) Towing speed

There are many variables which control the vibratory compaction or densification of soils.

Dynamic Compaction

• This involves in increasing the density of soil near the surface by tamping.

• Density improvement up to 10m is feasible.

• This method consists of dropping heavy mass of 8 to 40 tonnes known as pounder on the surface from a height 5 to 30m

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Dynamic Compaction Dynamic compaction was first used in Germany in the mid-1930’s.

The depth of influence D, in meters, of soil undergoing compaction is conservatively given by D ≈ ½ (Wh)1/2

W = mass of falling weight in metric tons

h = drop height in meters

Dynamic Compaction Equipment

Vibro Compaction

• For loose sand deposits, the density index can be increased by vibro compaction.

• This process employs a depth vibrator suspended from crane.

• Compaction of sand can be achieved up to distance of 2.5m from axis of vibrator.

• Compaction can be carried out to significant depths up to 12m.

Vibro Compaction

Vibroflotation

Vibroflotation is a technique for in situ densification of thick layers of loose granular soil deposits. It was developed in Germany in the 1930s.

Vibroflotation Procedures

What is Benefit of Pressure Grouting?

• How much pressure? – 300 to 600 kPa

• How long? – < 1 minute

NICHOLSON

Grout under pressure

Duplex Drilling With Air

Duplex Drilling with Water

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Hollow Bar Drilling with Grout

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Seepage Stress Important to Stabilize Hole

Chemical Grouting

• Same Principles as Pressure Grouting but changing the product from slurry grout to polyurethane.

• Use of either single part or two part polyurethane depending on the situation.

• Benefit: – Quicker & Cleaner

– Less down time/MOT

New York, July 18, 2007

• An underground steam line ruptured, blasting a hole in a Manhattan street and releasing large quantities of asbestos into the air along with the escaping steam.

• Companies like ConEd in New York need to have a regular schedule of replacement of parts of the system that weaken with age.

A New York City policeman wears a mask as he walks past the scene of the steam pipe explosion.

Collapsed Sewer Line Erodes a Sinkhole in Tucson, Arizona

Old sewers need to be replaced before they rupture or collapse.

St. Louis, MO – 2007

A 100-year-old large brick sewer line in downtown St. Louis collapsed causing a very large hole in a downtown street.

Many old cities like St. Louis have old masonry sewers or pipes made

of wood – these have limited serviceable life.

Sinkhole collapse in Nixa, MO.

This is a danger wherever streets or buildings are built on Karst limestone bedrock.

Very Large Sinkhole

This large sinkhole destroyed homes and streets. Broken water or sewer lines can create collapses much like this.

Taum Sauk Reservoir Water (1.5 bil. gal.) stored in the upper reservoir was released in peak usage periods to produce extra hydroelectric power.

December 14, 2005 • There was a breach in the upper reservoir to the

Taum Sauk Hydroelectric plant in Southern Missouri early this morning. A 20 foot wall of water came rushing down into the Black River like the water of a gigantic bathtub being drained.

• Negligence in maintenance and repair and refusal of management to heed warnings seem to be responsible for the catastrophe.

Before the breach

After the breach

Remains of Home of Johnson Shut-ins Park Superintendent

20-ft. Wall of Water Scoured the Land

Conclusions

While constructing public works facilities, different ground conditions are encountered.

Considering all factors a suitable ground improvement technique has to be done. Ground improvement techniques have been extensively

used by developed countries.

Questions