mechanical method for soil compaction

8/14/2019 Mechanical method for soil compaction

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GROUND IMPROVEMENT TECHNIQUES Mechanical Methods

Dr. M. [email protected]


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Mechanical Methods Compaction, Impact, Dynamic Vibro-flotation and Vibro-replacement

Stone Column Explosives


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A. MECHANICAL METHODS Choice of Method

PurposeCompactionShallow Surface CompactionDeep Compaction- Dynamic Consolidation- Vibro Compaction- Vibro replacement


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CHOICE OF METHOD- Type & degree of improvement required

- Type of soil, geological structure- Cost- Available equipment

- Time- Damage to adjacent structures- Durability (whole life considerations)


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PURPOSE

- Increase shear strength- Reduce compressibility- Reduce liquefaction- Control swelling/ shrinkage- Prolong durability


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What is compaction? A simple ground improvement technique,

where the soil is densified through externalcompactive effort.

+ water =

Compactiveeffort

1-Compaction


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- to obtain the compaction curve and define the optimum water content

and maximum dry density for a specific compactive effort.

hammerStandard Proctor: Modified Proctor:

3 layers

25 blows per layer

5 layers

25 blows per layer

2.7 kg hammer

300 mm drop

4.9 kg hammer

450 mm drop

1000 ml compaction mould


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Compaction - Procedure

1

2

3

4

5


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Variation of Dry Density With Water


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D r y

d e n s i t y

( d

)

optimum water

content

d, max

Soil grains densely packed

- good strength and stiffness

- low permeability


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Compaction Effect

Solids

Air

Water

Solids

Air

Water

Loose soil Compacted soil


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All compaction points should lie to the left

of ZAV curve

- corresponds to 100% saturation

Water content

D r y

d e n s i

t y (

d )

Zero air void curve (S=100%)

s

w sd

wG

G

1 :Eq

S100% (impossible)


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Lower optimumwater contentHigher maximumdry density

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Increasing compactiveeffort results in:

E1

E2 (>E 1)

Water content

D r y

d e n s i

t y (

d )


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Higher water content or highercompactive effort gives more dispersedfabric.

more dispersed fabric

m o r e

d i s p

e r s e

d f a b r i c

Water content

D r y

d e n s i

t y (

d )


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15 Water content

D r y

d e n s i t y

( d

)

Compaction curves fordifferent efforts

Line of optimum


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Compaction Advantages

As compaction increases, the following occurs: Increase soil strength Increase bearing capacity Decrease potential for settlement Control undesirable volume changes Reduction in hydraulic conductivity


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Top from left: Gridroller, vibratory plates,vibrating roller andpneumatic rubber roller.

Bottom from left:Smooth-wheeled roller,power rammer andsheepsfoot roller.

SHALLOW SURFACE COMPACTION


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Field Compaction

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Compacts effectively only to 200-300 mm; therefore, place the soil inshallow layers (lifts)

Smooth Wheeled Roller


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Field Compaction

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for compacting very MHll areas

Vibrating Plates

effective for granular soils


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Field Compaction

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Provides deeper (2-3m) compaction. e.g., air field

Impact Roller


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Type of Plant Suitability UnsuitableSmooth-wheeled roller Well graded sand and gravels;

silts and clays of low plasticity.Uniform sands; siltysands; soft clays.

Grid roller Well graded sand and gravels;soft rocks; stony cohesivesoils

Uniform sands; siltysands; soft clays.

Sheepsfoot roller Sands and gravels with morethan 20 % fines; most finegrained soils

Very coarse-grainedsoils; gravels withoutfines.

Pneumatic-tyred roller Most coarse-grained and fine-grained soils

Very soft clay; soils ofhighly variableconsistency

Vibrating roller Sands and gravels with no

fines; wet cohesive soils

Silts and clays; soils

with 5 % or morefines; dry soils.

Vibrating plates Soils with up to 12 15 %fines; confined areas.

Large-volume work

Power rammer Trench backfill; work in small

areas or where access isrestricted.

Large-volume work


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Compaction Control Test

compacted ground

d,field = ?

wfield = ?

Compactionspecifications

Compare!

w

d


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Dynamic Compaction

Pounder (Tamper)Mass = 5-30 tonne

Drop = 10-40 m


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Dynamic Compaction

- pounding the ground by a heavy weight

Suitable for granular soils, land fills andkarst terrain with sink holes.

Crater created by the impact

Pounder (Tamper)solution cavities inlimestone

(to be backfilled)


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DEEP COMPACTION TECHNIQUES

Dynamic Compaction Applications- Reduce foundation settlements- Reduce seismic subsidence- Permit construction on fills- Densify garbage dumps

- Improve mine spoils- Induce settlements in collapsible soils


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Dynamic Compaction

Used for cohesive and cohesionless soils

Compacting buried refuse Not done by dropping weight randomly

Closely spaced grid pattern Preliminary work done to determine: Grid spacing Weight Height Number of drops (typ. 5 to 10 drops per grid point)


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Dynamic Compaction

Applicable Loose sands, fills, mine refuse, collapsible soil and

sanitary landfills Up to depths of 40-feet Not typically used in urban areas 25-50 meters clearance to any structure

GWT > 6 below grade or 2 below bottom ofcraters


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Dynamic Compaction

Advantages Relatively inexpensive

Disadvantages Extremely invasive Multiple passes required / progressive

consolidation Granular fill to stabilize loose surface soils Too many drops may cause adjacent heave Requires careful monitoring


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Dynamic Compaction

Practical Considerations Drops from 10-40 meters

Weight 40(+) tons; shape doesnt matter Stratographic profiling for tamping pattern Max economic limit = 10 drops/location Requires horizontal pumps or drains Vibration sensitivity analysis recommended Crane Safety Program


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Important Terms


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Important Terms

Effective Depth -- Maximum depth of ground improvementZone of Major Densification -- About upper 2/3 of effectivedepthEnergy Level -- Energy per blow (weight times drop height)Energy Intensity Factor -- Involves energy level, spacing, andnumber of blows

Typical Dynamic Compaction ProgramInvolves

Weights of 10 to 30 tons Drop heights of 17m to 35 m Impact grids of 2.5 x 2.5 m to 7.0 x 7.0 m


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Applied Energy Requirements


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pp gy q

Li i i


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Limitation


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Suitability of Deposits for Dynamic Compaction

Dynamic Compaction Design Steps


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Dynamic Compaction Design Steps 1. Perform site investigation

2. Develop settlement influence diagrams3. Develop initial Dynamic Compaction program4. Develop numerical performance prediction5. Develop QA/QC plans

Dynamic Compaction Performance Prediction Requires

Depth of influence of dynamic


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Depth of influence of dynamiccompaction


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Dynamic Compaction Acceptance Testing


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y p p g

Large-Scale Load Test (where CPT & SPT are unreliablei.e. construction rubble and cobbles)

Standard Penetration Test (SPT) Cone Penetrometer Test (CPT) Pressuremeter Test (PMT) Dilatometer Test (DMT) Shear-Wave Velocity Profile


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Explosive compaction

Dr/ [email protected]


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Aftermath of blastingFireworks?

For densifying granular soils


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A Ground Improvement Technique

First used in 1936 (unsuccessfully)

Generally used to improve density of silty sands-sandy gravels (non-cohesive soils)

Makes use of dynamic/undrained loading conditions

to cause liquefaction-induced settlement


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The Mechanism

Sudden dynamic loading breaks cohesionand any cementation

Shockwave temporarily liquefies soil layer Settlement occurs as Du 0

Typical vertical strain between 2% and 10%(Narin Van Court, 1995; GeoDesign, 2002)

Principle


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Aftermath of blastingFireworks?

For densifying granular soils

Detonation of the explosives in a predetermined pattern causes liquefaction, followedby the expulsion of pore water and subsequent densification of the ground (Mitchell,1970). Gas and water escape to the surface forming sand boils, but cratering can beavoided by a suitable arrangement of the explosives.

Principle

D fi iti


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Definition* Explosion or blasting is the process of detonating small charges within

loose cohesionless soils for the purpose of densification.

* Used to modify loose sands, rock, special soils (increase density index to0.7 0.8) (loess), (soils with open skeletons)

Action shear stresses breaks down soil structure

reorientation of particles volumetric reduction (up to 10%)

saturated soil temp. high pore pressure liquifaction (@ use vertical drains)

Caution - do not cause local slips


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A Basic Procedural Overview

1. Site Investigation2. Survey3. Drill, Set Charges, and Blast

1. Survey, Further Site Investigation2. (Repeat 3, 4 if further settlement is required)

The usual procedure for blasting is as follows:* Jet or otherwise install a pipe to the required depth* Home the explosive charge* Withdraw the pipe* Backfill the bale* Fire the charge.


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Why Blast?

Inexpensive Compare to remove/ replace ordynamic compaction, grouting, etc.

Improves soil atdepth

Maximum depth implemented: 40m

Low impact tosurroundings

Controlled blasts leave nearbystructures unaffected


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1936 Russian Railway 1940 Franklin Falls Dam,

NH

1954-62 Karnafuli River,Kaptai, Bangladesh

1983 Jebba Dam, NigerRiver, Nigeria

1990s: Sakhalin Island,Russia

1990 Westover Airpark,

Chicopee, MA 1992 Coldwater Creek, Mt.

St. Helens, WA 2002 Westover AFB,

Chicopee, MA

Notable Field Applications

(Gandhi et al, 1999; Shakti, 2002; GeoDesign, 2002)


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Three to five detonations are usual Figure 1 0 illustrates ground settlement as a


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Three to five detonations are usual. Figure 1.0 illustrates ground settlement as afunction of the number of charges, based on Prugh (1963), 000 also suggested that thefirst firing (marker 1) caused 50 per cent settlement, the second (marker 2) 25 percent, the third 15 per cent, the fourth 5 per cent. Kummeneje and Eide (1961)

contradict this opinion, finding that similar settlements occurred after each detonation(Figure 2.0). A typical firing pattern for pad footings is shown on Figure 3.0. To assistthe densification process, a 1.0 m surcharge should be used in conjunction with the

blasting, but the upper 1-2 m of the ground is not compacted and will requirereplacement and compaction in layers using a vibrating roller.

Mitchell (1970) suggested that piezometers should be used to monitor pore-water pressures during the blasting operations.


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Excess pore pressures & settlement are related to the ratio.


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N h = Hopkins No.W = wt. of explosive (kg TNT)R = radial dist. of points of explosive m.

R

W N

h

3/1

where N h < 0.09 0.15 ;little or no liquefaction safe distance from explosion

To treat a given thickness of soil, H, with single detonation with spacing of holes, S ;

5.1055.0

3 H W

S = 2R = 2K 3

W

Values of K depend on grain size distribution and initial density

mechanical method for soil compaction

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