Deep col—IIpaction with theVibre-Wing rIIethodby BENGT B. BROMS" 8 OSTEN HANSSON>

DURING THE CONSTRUCTION of anextension of the harbour at Rostock, GermanDemocratic Republic, for handling generalcargo, a 1.2 million m hydraulic fill has beencompacted by the Vibro-Wing method downto a maximum depth of 15m. The extensionconsists of a 1 250m long wharf constructedof precast concrete caissons and anchoredsheet pile walls and a 25 000m2 storage areadesigned to carry a uniformly distributed loadof 300kPa. Within the area are also severalportal cranes and a number of warehouses.The method has also been used below waterto compact the sand beneath the concretecaissons. The work is seen in progress in Fig.1.

The Vibro-Wing unit is shown in Fig. 2. Itconsists in principle of a 15m long steel rodwhich has been provided with a number ofwings welded to the rod. The unit is drivendown into the fill using a high capacityvibratory hammer and then slowly pulled outduring continued vibration. The time fordriving and withdrawal of the unit is aboutone and five minutes, respectively, the sandbeing compacted both during the driving andwhen the unit is retrieved. The time requiredfor compaction is thus short compared withother deep compaction methods such asdynamic compaction using heavy weights orvibro-flotation.

Compaction was done in a triangularpattern with a spacing of 1.5m to 5m

'Professor, Nanyang Technological Institute, Singapore2263.>Tyrttns Foretagsgrupp AB, Malmo, Sweden.

between insertion points. Effect of thecompaction is controlled preferably by staticcone penetration tests (CPTs) using anelectrical or mechanical cone penetrometer;it is advantageous to measure both the pointand the shaft resistance. The weightsounding method can also be used. Thepenetrometer tests indicated that a highrelative density can be obtained in medium tofine sand.

In Fig. 3 is shown the results from cone andstandard penetration tests carried out beforeand after compaction using the Vibro-Wingmethod. A considerable increase of thepenetration resistance was observed at alllevels except close to the ground surfaceabove the ground water level.

The cone penetration tests indicate that thethickness of the poorly compacted surfacelayer is relatively small (1.0-1.5m) with theVibro-Wing method. At the ground surfacecompaction therefore has to be supple-mented by other methods; compaction ofthis layer is normally done with a 8-10 tonvibratory roller after the deep compaction.

Compaction mechanismPore water pressure increases rapidly

during the compaction around the Vibro-Wing unit as it is driven down into the soil.This increase of pore water pressures hasbeen observed with pore pressure gaugesplaced at different levels in the soil. Theeffective stress is thereby reduced andindividual sand particles can be displaced;thereby a higher relative density can beobtained. When the pore water pressurecorresponds to the total overburden pressure

the effective stress in the sand is reduced tozero and the soil is liquified. Liquefactionoccurs at first locally around the vibratingwings. It spreads gradually as the pore waterpressures increase around the vibrating unit.

The high local pore water pressures aroundthe wings cause a mainly radial flow of waterto the surrounding soil. There is also anupward flow, at first locally around the rodwhich can be observed at the ground surface.The resulting reduction of the water contentcauses a reduction of the void ratio and of theporosity. Accordingly the relative compactionand the relative density of the soil areincreased. For a saturated soil the reductionof the porosity and the resulting increase ofthe relative density correspond to thereduction of the water content.

The most important effect of the compac-tion is the increase of the pore water pressureby the vibrations and the reduction of theeffective stress in the soil. The size of theliquefied zone around the Vibro-Wing unit isaffected mainly by the permeability of thesurrounding soil and the relative density. Inthe case where the permeability is high(coarse sand) the liquefied zone will berelatively small and localised around thevibration rod because the excess pore waterpressures dissipate rapidly. The permeabilityof a coarse sand can be 100-1 000 timeshigher than that of a fine sand. Driving andwithdrawal of the Vibro-Wing unit has to becarried out relatively rapidly; otherwise theunit may become stuck in the sand or bedifficult to pull out. Spacing of the insertionpoints should be relatively small in this case.

In fine sand a relatively large zone is

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Fig. 1. Vibro-Wing workin progress on thehydraulic fill at Rostock

34 Ground Engineering

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affected around the Vibro-Wing unit. Acomparatively long time is required for thedissipation of the excess pore waterpressures, the reduction of the water contentand the increase of the relative density.Driving and withdrawal should be donerelatively slowly to allow sufficient time forthe dissipation of the excess pore waterpressures and the reduction of the watercontent.

If the soil contains layers with low permea-bility, which is often the case in hydraulic fills,the effectiveness of the method is increasedif the sand is relatively coarse since a longer

Fig 2 The VrbroWing equipment

time will be required for the dissipation of theexcess pore water pressures. A larger zonewill thus be effected by the liquefactioncompared with the case when the soil isuniform.

The mechanism of the compactionthrough local liquefaction implies that anhydraulic fill will be considerably moredifficult to compact above than below theground water level provided the soil is notwatered. The effective stress is increasedabove the ground water level due to the falsecohesion of the soil. This false cohesion canbe eliminated by watering.

The hydraulic fill in the concrete caissonswas also compacted by the Vibro-Wingmethod. However the caisson walls reducedthe drainage and the consolidation of the fill

during the vibration. The excess pore waterpressures were higher and the size of theliquefied zone was increased. Since radialdrainage was reduced by the caisson walls alonger vibration time was required comparedwith the case of free radial drainage.

The pattern and order of insertion of theVibro-Wing unit are particularly important forsilty sand when the silt content is relativelyhigh and a long time is required for acomplete dissipation of the excess porewater pressures. For these soils it would beadvantageous to carry out the compaction in

stages. First, a relatively large spacing shouldbe used and a relatively long vibration time.After a few days to allow the soil toconsolidate the compaction is repeated. Thespacing of the insertion points is thenreduced, as well as the time required for thevibration. The procedure is repeated until asufficiently high relative density has beenobtained which can be checked with staticcone penetrometer tests.

The efficiency of the Vibro-Wing method isalso affected by the size and location of thewings. In coarse sand the wings should berelatively short and be placed relatively farapart because of the high permeability of thesoil. If the wings are large it will prove difficultto retrieve the Vibro-Wing unit because ofrapid dissipation of the excess pore waterpressures in the soil. A relatively intensivevibration will be required to cause localliquefaction of the sand. In fine sand relativelylarge and closely spaced wings can be used inorder to increase the effectiveness of themethod.

The compaction locally increases thelateral earth pressure in the soil which mayeven approach the total overburden pressure.This means that adjacent structures have tobe designed to resist locally a relatively highlateral earth pressure especially in fine or siltysand.

The driving and retrieval of the Vibro-Wingunit can be regarded as a penetration test.The pore water pressure is increased duringthe driving which reduces the driving resis-tance. By measuring the time required forthedriving of the Vibro-Wing unit at a given

Static cone penetration testStandard penetration test

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—1—S

~cone test before compaction,, rZf2 Z) P/J~

—2—S ~5PTr f p

i<P <~<~ —after compaction

"~Vi~N4S s3

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p GW0

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Static cone penetration test

ce) 20 MPae point

Cone test before compaction

after compaction

///////////////////////~ Cone res i stan

necessary for

—120 2 4 6 8 10 12 14 16 18 20 Cone resistance, MPa

0 10 20 30 N3Q (blows/0.3m) (Point 10cms )

0 2 4 6 8 10 12 14 16 18 20 Cone resistance, MPa

Fi .3.Statfcc'g.. 'ne and standard penetration test results before and after the Vibro-Wing corn action at R

(Point 10cmt )

hydraulic sand fill back of the caissons on the right t th f'll 'di, o t e i inside a caisson

July 1984 35

0—12—3—4—5—6—7—8—9—10—

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0—1—2—3—4—5—6—7—8—9—10—

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Cone resistance ) 20 MPaturning of the pointnecessary forpenetration

0 50 80 100 Retrieval force, kN 0 2 4 6 8 10 12 14 16 18 20 Cone resistance, MPa

(Point teems )

Fig. 4. Retrieval force (less weight of the eqfjipmentj, showingits relationship to the penetration resistance

applied axial load (the weight of the hammer)and the variations of the penetration rate,deviations in the composition of the soil canbe detected.

The force required to retrieve the Vibro-Wing unit at a given rate is measured. Thisforce is an indication of the pull-out resis-tance when the retrieval rate is kept constantand of the friction resistance along the wingsand the rod. This force is thus indirectly ameasure of the relative density of the soil. Thefriction resistance increases gradually asaverage lateral earth pressure increases inthe compacted soil and the excess pore waterpressures dissipate. In Fig. 4 is shown acomparison between the retrieval force andthe penetration resistance.

The greatest advantage of the method liesin the speed and low cost compared withother deep compaction methods. Anotheradditional advantage is that the thickness ofthe poorly compacted surface layer isrelatively small. A limitation of the method isthat water may in many cases have to beadded above the ground water level. Themethod has not so far been applied in silty orclayey soils when the content of finesexceeds 5-10% or in coarse sand or gravel.The permeability of silty or clayey soils isprobably too low for the method to beefficient; the time required for consolidationof the soil would probably be excessive.

It should be possible to compact mediumto fine sand down to a depth of at least 40m.In general it is only required to compact a soilbelow a depth of 20-25m in exceptionalcases because the compressibility of co-hesionless soils decreases with depth andwith increasing overburden pressure. Thesettlement of cohesionless soil is in generalsmall and negligible below a depth of20-25m for most conventional structures.Also the risk of liquefaction caused by earth-quakes is reduced with increasing depth andwith increasing overburden pressure.Normally the risk is small at depths exceeding15m even when the relative density of the soilis low.

Applications of theVibro-Wing method

There are several possible applications ofthe Vibro-Wing method such as in founda-tions, harbours, hydraulic fills, damsespecially tailings dams subjected toearthquakes, machine foundations and forspecial structures.Foundations

There are large areas in many parts of the

world where deep deposits of medium to finesand and of silty sand occur and where theVibro-Wing method can be used to compactthe soil e.g. along coasts or major rivers. Themethod should be particularly useful in areassubjected to earthquakes, to avoid lique-faction.

HarboursWharfs and piers are to a large extent

located in areas where there are deepdeposits of cohesionless soil and where therelative density is low. The Vibro-Wingmethod should be especially suited for suchstructures since compaction is often alsorequired below the ground water level orbelow water. The efficiency of the method isincreased below ground water level or whenthe compaction is done in water.

Soils along coasts normally have a compo-sition and grain size distribution which suitsthe Vibro-Wing method (fine to mediumcoarse sand or silty sand). Clay, muck or peatlayers may, however, reduce the effective-ness of the method since these layers act asmore or less watertight membranes in thesoil. Such layers reduce the drainage andconsolidation of the soil. The soil will,however, be homogenised to a certain extentby the compaction which reduces differentialas well as total settlements.Hydraulic fills

Hydraulic fills are used extensively in, forexample, Japan and Holland for industrialsites and for housing developments. Otherapplications include airfields and harbours.The relative density of hydraulic fills is ingeneral low without compaction. In mostcases compaction has to be carried out belowthe ground water level or below water. Pre-loading, dynamic compaction or the sand pilemethod are frequently used. However thesemethods are relatively slow and expensivecompared with the Vibro-Wing method formedium to fine sand or for silty sand.Preloading is a suitable method often incombination with vertical drainage in clayeyand silty soils where the permeability is toolow for the Vibro-Wing method, dynamiccompaction or the sand pile method.

DameIt could be advantageous to use the Vibro-

Wing method for earth or rockfill dams whichare founded on a deep deposit of medium tofine sand. The method could be used belowthe upstream or downstream slopes in orderto reduce the risk of liquefaction during anearthquake. The main advantage with themethod is that the compaction can be donebelow water or below an existing earth dam

since the Vibro-Wing unit can be driventhrough even a compacted earth fill. Theresults of the compaction can be checked,e.g. with static cone penetrometer tests.

Tailings from mines are normally storedbehind special tailings dams. The coarserfractions are used in the main body of thedam. The tailings in the dam structure are notcompacted at present. A number of land-slides have occurred which have been causedby liquefaction in, for example, South Africa(gold mines), Great Britain, and USA (coalmines), Canada, Peru, Chile and Mexico. Itshould be possible to use the Vibro-Wingmethod to increase the stability of existingtailings dams, especially during earthquakes.Machine foundations

Another possible area where the Vibro-Wing method could be used is for machinefoundations which otherwise would have tobe placed on piles to reduce settlementscaused by vibration. The damping of amachine foundation founded on piles is oftenvery low and the vibration level is often highsince a gap is normally formed between thepiles and the surrounding soil. A largenumber of machine foundations in Swedenwhich are supported on piles haveexperienced difficulties. By placing thefoundation block directly on the compactedsoil settlements can be reduced andexcessive vibrations can be avoided.

For such foundations located on slowlyconsolidating soil, e.g. clay, a very economicsolution could be to excavate the clay underwater in order to keep the slopes stable. Afterthat the excavation could be filled withhydraulic sandfill in one layer up to and abovethe groundwater level. From surface the fillcould be compacted with the Vibro-Wingmethod and the foundation placed directly onthe compacted soil.Special structures

The Vibro-Wing method can be used alsoto compact medium to fine sand in caissonsin order to increase stability. The compactionfrequently has to be done below water.

Other possible applications of the methodare the artificial islands that are constructedof dredged material in the Arctic for thedrilling of oil. Such islands are also used inwater to provide a link between a bridge and atunnel. Other applications are offshorestructures, such as drilling and productionplatforms, lighthouses, moorings etc.constructed on sand. However the relativedensity is often sufficiently high so that anyadditional compaction of the soil is notrequired.

36 Ground Engineering