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Soft Ground
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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
41
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
45
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
49
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
58
Dynamic Compaction Dynamic compaction was first used in Germany in the mid-1930s.
The depth of influence D, in meters, of soil undergoing compaction is conservatively given by D (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
67
Hollow Bar Drilling with Grout
68
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
W16 - Groundwater Modification: Stabilizing the Foundations ofPublic Works ProjectsSlide Number 2Causes of Unconsolidated SoilsSlide Number 4Slide Number 5Slide Number 6Slide Number 7Soil StabilizationMethods for Soil ImprovementSlide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Vertical DrainsSlide Number 22Slide Number 23Stabilizing TrenchesSlide Number 25Capillary Cut-Off Permeable Capillary Cut-Off Impermeable Capillary Cut-Off Methods for Soil Improvement-Soil NailingSoil NailingSlide Number 31Slide Number 32TiebacksShotcrete WallsStressing TiebacksThe Finished WallTop Down WallsGreater Span Without BucklingUnderpinningDefinitionsDefinitionsExcavation Support Wall MovementsStone ColumnsSlide Number 44Methods for Soil Improvement Jet GroutingElephant and CompactionCompaction and ObjectivesSlide Number 48Field Compaction Equipment and ProceduresEquipmentEquipment (Cont.)Equipment (Cont.)Equipment (Cont.)Equipment (Cont.)Equipment (Cont.)Equipment-SummaryVariables-Vibratory CompactionDynamic CompactionDynamic CompactionSlide Number 60Vibro Compaction Vibro Compaction VibroflotationVibroflotation ProceduresWhat is Benefit of Pressure Grouting?Duplex Drilling With AirDuplex Drilling with WaterHollow Bar Drilling with GroutSeepage Stress Important to Stabilize HoleChemical GroutingSlide Number 71Slide Number 72New York, July 18, 2007Slide Number 74A New York City policeman wears a mask as he walks past the scene of the steam pipe explosion.Slide Number 76Collapsed Sewer Line Erodes a Sinkhole in Tucson, ArizonaSt. Louis, MO 2007 Sinkhole collapse in Nixa, MO.Very Large SinkholeTaum Sauk ReservoirDecember 14, 2005Before the breachAfter the breachRemains of Home of Johnson Shut-ins Park Superintendent20-ft. Wall of Water Scoured the LandConclusionsQuestions
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