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1.0 GROUND IMPROVEMENT

Ground Improvement Subcommittee.

INTRODUCTION

During the past decade (1987-1997), soil improvement has come of age, and reached a new level ofacceptance in the geotechnical community. t is now routinely considered on most pro!ects where poor orunsta"le soils are encountered, especially on sites underlain "y suspect or uncontrolled fills.

#or sites underlain "y deep layers of fill or soft or loose soils, conventional practice was to either removeand replace the unsuita"le soils, or "ypass them with e$pensive deep foundations. %oday, in-situ improvement isa via"le alternative, and in most instances proves to "e the most economical means to mitigate an undesira"lesituation.

Dynamic compaction is "ecoming one of the more routinely used ground improvement techni&ues"ecause of its relatively low cost, especially when improving large areas. %he dynamic compaction mar'et is"ecoming a mature mar'et, with steady growth e$pected "ecause of the num"er of old fills site that werepreviously "ypassed when "eing considered for development

%he vi"ro-compaction mar'et, in con!unction with stone columns, has also achieved universalprofessional acceptance, and has seen a steady growth "ecause of more concerns a"out li&uefaction, andrepairing areas damaged "y the two recent alifornia earth&ua'es %he vi"ro-compaction industry now uses morepowerful vi"rators, that allow for wider pro"e spacings, and more economical improvement

%he compaction grouting mar'et is also in a mature mar'etplace, with many different applicationsavaila"le other than the more routine fi$es for soft ground tunneling and settlement related pro"lems, "oth to

remediate failures and in advance of new construction.%he use of prefa"ricated vertical drains (new terminology for wic' drains) have also witnessed

phenomenal growth during the past decade, especially on transportation pro!ects and civil pro!ects underlain "ythic' compressi"le deposits ore efficient e&uipment has made *+ drains more economical to install in 1997than in 198.

last-densification, "ecause of the small num"er of sites suited for the techni&ue, has not "een used asmuch in the nited /tates as other ground improvement techni&ues, however pro!ects completed this decade inassachusetts and 0ashington /tate indicate that "last-densification is effective in improving granular depositsto at least 2 m (122 ft) or more.

%he following sections present the fundamental design and installation concepts of these densificationtechni&ues, how the techni&ues have developed "etween 1987 and 1997, and recent and current researchconducted to advance our understanding of their design and performance.

1.1 DYNAMIC COMPACTION

INTRODUCTION

Due to its success in o"taining significant engineering property improvements, as well as the economics ofthe process, dynamic compaction has evolved into one of the more commonly used ground improvementmethods. ased on dropping a heavy weight from a large distance, dynamic compaction was introduced to thenited /tates in 199 "y the late #renchman, 3ouis enard. %he 19724s saw development of the techni&ue,particularly in the hicago area, "y a national geotechnical engineering firm,and nationwide, "y two specialtycontractors.

%he 19824s saw the startup of several other specialty contractors, with the techni&ue going through ma!orrefinements and gaining widespread professional acceptance throughout the geotechnical community .%hero"ust economy of the mid to late 19824s resulted in pro"a"ly 522 pro!ects completed during that time. %herecession of the late 19824s reduced the num"er of pro!ects dramatically, and it was not until 1995 that thedynamic compaction mar'et recovered. %he 19924s have witne6sed wide spread use of dynamic compactionthroughout the entire ./., with an estimated 22 pro!ects completed nationwide to date. %he strong economy ofthe mid 19924s, coupled with the realiation that most of the good "uilding sites have already "een used, hasresulted in larger and more diversified types of dynamic compaction pro!ects.

hanges in retail shopping ha"its in the 19924s have resulted in the need for large home improvement stores,super stores, supermar'ets, and discount shopping malls re&uiring large flat areas /ince large undistur"ed areas

are sometimes difficult to find in populated areas, dynamic compaction is "eing used routinely to improve old

fill sites that will "e used as large retail centers. t is estimated that dynamic compaction has "een used at least5 times for shopping centers such as ome Depots, 0 :3-:;% stores, and


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#ig 1.1-1. %ypical dynamic compaction e&uipment.

n addition to strengthening and compacting the e$isting fill materials, dynamic compaction, li'e proof-rolling, e$poses poc'ets of soft material or materials which are unsuita"le for compaction. %hese areas, when

identified during compaction, either need additional treatment or re&uire undercutting and replacement withcompacted fill.

%he degree of soil improvement depends to a large degree upon the total amount of energy applied to thesoil, i.e., the more energy input to the soil, the greater the degree of improvement. %he results of treatment "ydynamic compaction are dramatic and immediate. /urface settlement is typically five to ten percent of thethic'ness of the material "eing treated and is noticed immediately *ore pressure increase is instantaneous, anddissipation usually occurs rapidly> often accompanied "y arising groundwater level or localied "oiling at thesurface /trength and compressi"ility, as measured "y in situ tests, are typically improved "y a factor of two tofour

%he depth of improvement is related to the tamper weight and drop height, with improvement depths ofto 9 m (12 to 2 ft) "eing common. %he depth of influence is a s&uare-root function of the tamper weight timesthe drop height, times an empirical factor. %he empirical factor ranges from 2. to 2.7, "ut averages a"out 2.. tis somewhat less (2.=) in landfills and cohesive soils. t is dependant on a num"er of factors including the soiltype and stratigraphic features, efficiency or energy loss of lifting?tamping e&uipment, the contact pressure of thetamper, and the method of application of energy : schematic of the process is shown in #ig. 1.1-5.

#ig. 1.1-5. %he dynamicompaction process(3ucas 199)

Dynamic compaction is typically performed over a pre-determined grid pattern, with multiple passes overthe grid on an offset grid common. /ince grid spacing, num"er of drops per impact point, applied energy andnum"er of passes depend upon soil conditions, ground response, and the dissipation of pore water pressure>comprehensive field monitoring and engineering !udgement of ground response is imperative

:/ *;@AD;ADynamic compaction is applied in a systematically controlled pattern of drops %he initial impacts are

spaced at a distance dictated "y the layer, depth to groundwater, and grain sie distri"ution on a coordinate gridlayout. depth of the compressi"le.

nitial grid spacing generally appro$imates the thic'ness of the compressi"le layer. %ypically, to 1 "lows per

grid point are applied. nitial grid spacing generally appro$imates the thic'ness of the compressi"le layer.

%ypically, to 1 "lows per grid point are applied. nitial grid spacing generally appro$imates the thic'ness ofthe compressi"le layer. %ypically, to 1 "lows per grid point are applied.@ften, the pro$imity of groundwater or e$cessive crater depth limits the num"er of "lows applied to each


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grid to avoid getting the tamper stuc', or to allow for pore water pressure dissipation. /tandard practice is tocurtail energy application when crater depth e$ceeds one and a half to two times the height of the tamper, orwhen the groundwater surface rises into the crater. 0hen this occurs, additional passes after ground leveling, or"ac'filling the crater are re&uired to complete the re&uired num"er of drops.

%his first phase of treatment is designed to improve the deeper layers. ncorrect spacing and energy levelat this stage could create a dense upper layer ma'ing it difficult or impossi"le to treat loose material "elow. %heinitial phase is also called the Bhigh energy phaseB, "ecause the compactive energy is concentrated on a wider

grid. ompletion of the high energy phase is usually followed "y a low energy phase, called Bironing,B todensify the surficial layers in the upper 1. m ( ft). ere, the tamper is only raised from to m (1 to 52 ft),and is dropped on an overlapping grid.

:fter each pass, the in1prints are either "ac'filled with the surrounding materials or with off-site material.f the sides of the craters are pushed in, the wor'ing surface is gradually lowered "y an amount proportionate tothe densification achieved during each pass n some circumstances, it is necessary to maintain the wor'ingplatform at a constant level throughout the wor'. #or instance, in a situation where groundwater is shallow, thecraters should "e "ac'filled with imported materials to insure staying a"ove the water ta"le. :t least 1. m ( ft)is generally re&uired "etween the tamping surface and groundwater.

f the e$isting ground contains poor "ac'fill materials, as with most landfills, it is desira"le to useimported gravel or crushed stone materials to drive into the trash, essentially producing large diameter columnsof compacted stone at the surface.

n saturated fine-grained soils, the process is complicated "y the creation of e$cess pore water pressuresduring compaction, a phenomenon which reduces the effectiveness of the su"se&uent compactive passes unlessthe pore pressure is ade&uately dissipated nless there are large voids within a clayey mass, dynamic

compaction is not recommended unless the craters are "ac'filled with crushed stone and repounded, creatinglarge diameter columns of compacted stone (dynamic replacement).

ecause of large voids generally associated with landfills, pore pressures dissipate rapidly, and porepressure "uildup is generally not a pro"lem.

;A3A+:C% /%DA/ and %AC:3 *:*A;/%he most widely &uoted and referenced document in dynamic compaction is generally considered to "e

the 198 #0: pu"lication "y ;o"ert 3u'as entitled BDynamic ompaction for ighway onstruction, +ol.1.Design and onstruction uidelinesB. : more recent follow up pu"lication to the 198 manual was released in199 as #0: eotechnical Angineering ircular Co.1. %his manual is also "y 3u'as, and provides a moreBcoo'"oo'B approach for use "y the various state D@%s.

%he decade following the first %en Eear pdate has seen several other specialty conferences and severalnota"le papers on the state of practice of dynamic compaction in the nited /tates.

%he "i"liographic and reference sections list most of the papers relevant to dynamic compaction presented

since 1987. %he main specialty conferences or :/A :nnual meetings that included papers on dynamiccompaction were.Soil Improvement for Earthquake Mitigation, :/A :nnual onvention, /an Diego, Cov. 199, :/Aeotechnical /pecial *u"lication Co =9.

In-Situ Deep Soil Improvement, :/A :nnual onvention, :tlanta, Cov. 199=, :/A eotechnical /pecial*u"lication Co.=.Grouting, Soil Improvement and Geosynthetics, :/A /pecialty onference, Cew @rleans, #e". 1995, :/Aeotechnical /pecial *u"lication Co.2.:/A :nnual onvention, Cew Eor', #all 1991

In-Situ Soil Improvement Techniques, %as' #orce 57, %ransportation ;esearch oard?::/%@, 1992

%here have also "een numerous sectional :/A meetings that sponsored mini symposiums on ground

improvement, most nota"ly Cew Eor', 0ashington, hicago, Detroit, adison, 01, and several @;+/ (@hio

;iver +alley) meetings.

APPLICATIONS%he significant developments in dynamic compaction over the past decade are the e$pansion of the

applications, uses, and purposes of dynamic compaction. %he list of applications continues to grow, "ut dynamiccompaction has "een used in the following applications.

F;educe settlement under new loading

Fncrease "earing capacity allowing higher load supporting capacity ;educe need for grade "eams in

sla"s-on-grade

F:llow use of spread footings in lieu of piles

F;educe li&uefaction potential for structures, landfill liners, dams, andem"an'ments

F;educe voids in old fills and collapsi"le soils

Fmprove su"grade for liner applications


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Fain air space in sanitary landfills

Follapse soil voids in limestone terrain and 'arst topography ollapse old mining features

(deep shafts and drifts)

FDensify and consolidate landfills for construction of highways or em"an'ments

FDensify landfill cells prior to fmal cap to reduce ris' of cap crac'ing (has "een used for conventional landfill

cells, as well as for low level radioactive landfill cells)

FDensify collapsi"le soils underlying highway right-of-ways in order to reduce pavement distress

F;educe the ris' of encountering voids underlying future column locations

FDensify dredge material in-situ

FDecrease post-placement consolidation time of recently placed deep controlledfills

F*rovide level of assurance for certification purposes that structural fill was placed properly if field density test

results are inade&uate or missing

Fncrease lateral support for future pile foundations for a large shopping mall over a sanitary landfillF;educe ris' of pavement and infrastructure failure over landfills or undesira"le soils

F:llow for wet weather fill placement in wet su"grades "y pounding in gravelF3ower a municipal landfill final elevation to permitted limitsFncrease slope sta"ility of future water par' constructed on old la'e "ed

F%reat environmentally &uestiona"le sites "y not uncovering or e$posingpotentially haardous materials

Fncrease point-to-point contact of "oulders in de"ris fillsF;ender old mining pits as "uilda"le sites

F/ave money "y eliminating conventional undercut and replacement when fills are deeper than a"out ft.F*repare par'ing or product storage su"grade in coal mine spoil.

ecause of the increase in marginal sites within the country, the main application of dynamic

compaction, however, is still to minimie settlement of compressi"le deposits under new loading. %hisprimarily .means treatment of old fills, whether they "e granular, slightly cohesive, ru""le, de"ris, or municipal

solid waste. %he intent of improving fills is to ma'e a heterogeneous mass, either manmade or natural, ahomogeneous mass that has a lower void ratio, a higher shear strength, and therefore a higher "earing capacity.

TYPES OF SOIL IMPROVED

%he single most determinative factor in the suita"ility of a soil type to "e improved "y dynamiccompaction is its a"ility for e$cess pore pressure to dissipate. During dynamic compaction, soil particles are

displaced into a tighter configuration or a tighter state of pac'ing. f water is present in the soil voids, an instant

rise in pore water pressure occurs. t is necessary for this pressure to dissipate "efore additional densification canoccur under repeated high energy drops. f this isn4t allowed to happen, then repeated drops from the tamper only

causes displacement of the ground, and not densification

:s with the increase in applications of dynamic compaction over the last decade, the types of materialstreated "y dynamic compaction have also increased dramatically. @riginally, the predominant soil types

considered for dynamic compaction included only granular natural or fill soils. ut "ecause of the inherenteconomic advantages involved with the use of dynamic compaction, a multitude of materials have "een

improved. %hey include.ncontrolled fills. /oil types within old fills can include the entire spectrum of natural soils, manmade

de"ris, "yproducts, and any com"ination of the three. Dynamic compaction wor's "est, however, on dry granularfills, including sand, gravel, ash, "ric'"ats, roc', shot roc', and steel slag.

Dynamic compaction in granular fills is similar to a *roctor compaction test, in that there is a physicaldisplacement of particles into a denser configuration Dynamic compaction produces a low fre&uency vi"ration,

in the range of four to ten cycles per second, and it is this low fre&uency e$citation along with this input ofenergy that reduces void ratio and increases relative density resulting in improved "earing capacity and enhanced

settlement characteristics.#or deposits "elow the water ta"le, the vi"rations cause an increase in pore pressure, and after a

sufficient num"er of surface impacts, causes a sufficient rise in pore pressure as to induce li&uefaction, verysimilar to what occurs during earth&ua'es, @nce this occurs, additional energy application is ineffective until the

pore pressure dissipates, :dditional pounding following pore pressure dissipation produces more low fre&uencyvi"rations that reorganies the particles into a denser configuration,


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Dynamic compaction has "een used more routinely to improve fine grained fills as well, %hese fills are much

more difficult to improve, and re&uire much tighter field control and e$perience, lays and silts tend to BheaveBafter repeated pounding, and if additional pounding continues, can have a detrimental effect on compaction, f

heaving occurs, pounding at that point should stop, and the num"er of passes should "e increased with either areduced drop height or fewer drops per point,

: more common techni&ue employed in the ./ over the past decade to improve fine grained sites is to use a

dynamic BreplacementB techni&ue. %his techni&ue starts out "y producing a crater "y conventional heavytamping, "ut instead of either pushing in the craters or "y adding additional imported "ac'fill, the craters are"ac'filled with a

B"oneyB or granular "ac'fill.%his "oney material can "e gravel, shotroc', "ric' "ats, reprocessed concrete, or anything that will loc' together

under additional heavy tamping. ecause of the higher permea"ility of this "ac'fill, pore water pressure from theunderlying and ad!acent fine grained soils will dissipate more &uic'ly. %his process is repeated until a noticea"le

decrease in crater formation occurs. %his techni&ue essentially results in large diameter columns of compactedstone underlying a site or individual column locations.

Dynamic compaction is often used in con!unction with other ground improvement techni&ues. : retail site in

Cew Gersey was constructed over an old fill which was underlain "y organic soils is an e$ample. ere, a

vi"roflot was used to install stone columns at each interior footing location, and then the surface deposits at each

of these column locations was dynamically compacted (ayu' and 0al'er 199=).

%here have "een several old steel mill sites that have "een underlain "y steel slag (%roy, CE> Eoungstown, @>%renton, CG, /t, 3ouis, Aast hicago, C), /teel slag is generally &uite granular, and responds very well to

dynamic compaction.unicipal /olid 0aste (/0)H *ost-construction settlements of sanitary and ru""le landfills under

em"an'ments are difficult to predict 0ithout site improvement, settlements can sometimes range from 1. -=.m ( -1 ft). %he main causes of settlement in landfill deposits are due toH

Fechanical compression due to distortion, "ending, crushing, and reorientation of the materialsunder self weight,

.Fiological decomposition of organic wastes,

F*hysio-chemical change such as o$idation, corrosion, and com"ustion,F;avelling of fines into larger voids.

Dynamic compaction has "een used e$tensively on /0 to correct the a"ove

causes and for a multitude of reasons !Igain, e$perience is essential in improving /0, in that grid spacing,weight contact pressure, and num"er of passes are crucial in achieving the desired results ighway

em"an'ments, roadways, par'ing lots, and even retail structures have all "een constructed on dynamically

compacted /0.n sanitary landfills, settlements are caused either "y compression of the voids or decaying of the trash material

over time. Dynamic compaction is effective in reducing the void ratio, and therefore reduces the amount of

immediate and long-term settlements after construction. t is also effective in reducing the decaying pro"lem,since collapse of voids means less availa"le o$ygen for decaying process. #uture settlements, however, can still

"e e$pected due to a secondary consolidation process, and future decaying of the trash material@f the 2 or so highway dynamic> compaction pro!ects (Drumheller 1995), completed in the ./., over half were

underlain "y landfills> either ru""le or sanitary.%hey include pro!ects located in.

anchester, +% iramar, f3 %inton *alls, CG/pringdale, :; Avansville, C 3afoursche, 3:

oc'eysville, D Avansville, C #t 3auderdale, #3%ulsa, @< Davie, #3 Denver, @

%ampa, #3 @range ity, : @'lahoma ity, @< %renton, CG ;aleigh, C

;u""le landfills generally have lesser amounts of organics than the sanitary landfills, and are more suita"le todynamic compaction "ecause of the lower ris' for long-term organic decomposition.

: distinction must "e made "etween older landfills and more recent landfills when considering the long-termsettlement of the landfill after improvement with dynamic compaction @rganic decomposition has generally

already ta'en place in older landfills, and the landfill usually consists of a dar'-colored soil matri$ containingvarying amounts of "ottles, metal fragments, wood, and de"ris Decomposition generally ta'es more than 5 to

2 years to occur


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#or deposits where "iological decol1l*osition is complete, dynamic compaction has its greatest "enefit.

Densification results in a higher unit weight and a reduction in compressi"ility under load with little long-termsu"sidence under load.

#or recent landfills where organic decomposition is still underway, dynamic compaction increases the unitweight of the soil mass "y collapsing voids and decreasing the void ratio. t will not, however, stop the "iological

decomposition, which may result in a loosening of the soil structure followed "y long-term settlements.

t is desira"le in highway landfill pro!ects to place a 2. to 1. m (5 - ft ) stone or granular "lan'et on thelandfill surface prior to the densification process. *lacing a crushed stone "lan'et over a landfill and "ac'filling

the resulting craters with additional stone performs several functions. t provides a safe, sta"le wor'ing platformfor the crane, it prevents e$posure of the landfill, and acts as a reinforcing mat to "ridge or reduce differential

settlement that might occur due to future decomposition within the landfill. %he tamper also drives columns ofstone into the gar"age. %hese Bstone columnsB or Bdrive plugsB, although short in length, help to confine the

gar"age, forcing it to compress rather than move laterally.oal ine /poilH Drumheller and /haffer ( 1997) discuss 19 coal spoil sites in the . / . that have "een

improved "y dynamic compaction. %hey include.%russ Goist *lant Gac'son,


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ollapsi"le soils are stiff and strong in their dry natural state, "ut lose strength and undergo significantsettlement when they "ecome wet /ettlements associated with collapsi"le soils can lead to e$pensive repairs,

either in highway or structure construction.n 1985, the Cew e$ico ighway Department conducted an e$tensive field trial program of various ground

improvement techni&ues to improve collapsi"le soils. %he various methods included vi"roflotation, deep mi$ing

and compaction, pre-wetting, and dynamic compaction. Dynamic compaction was found to "e the most costeffective, and "ecame the "asis for production wor' to improve three separate sections of - 5 and 1-=2 around:l"u&uer&ue.

%a"le 1.1-1 summaries soil conditions and dynamic compaction parameters of si$ pro!ects involvingcollapsi"le soils (;ollins and


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of initial wor' at Gac'son Dam, all were done with specifically designed 3ampson thumper 3D- 2 liftingdevices. 0ic' drains and surface drainage trenches were installed in con!unction with dynamic compaction atseveral sites, and noticea"le enhancement of the results was noted when pore pressures had a chance to dissipate.

%he results of the ureau of ;eclamation4s wor's can "e summaried as.

.F0ic' drains in con!unction with dynamic compaction enhanced results if installed to full depth re&uiring

treatment. ;esults were considered to "e site specific since only small enhancement was noted at Gac'sonDam, whereas considera"le and undenia"le "enefits were noted at /teina'er Dam.

.F/urface drainage trenches not only enhanced results, "ut provided e$ploratory and construction o"servationtools.

.Fetter results were o"tained when applied energy applied in numerous phases.

.F;educe energy-per-drop (applied energy) re&uirements in shallow or confined areas

.F%he greatest improvement depth for reducing li&uefaction potential was from the surface to 9 m (58 ft).

.F:dditional energy can increase depth of influence to 1 m (=9 ft).

.FCoticea"le increases in shear wave velocities and penetration resistances were noted with time. #iner-grainedmaterial increases less with time than coarser grained materials. aterials e$hi"iting plasticity increased lesswith time than non-plastic materials.

Dynamic compaction was used at numerous other locations to reduce li&uefaction in the west, as well as

the midwest and east. ;epresentative pro!ects included

.F0ater treatment plant at the e$ican "order !ust south of /an Diego

.F*erimeter sand di'e around a "usiness par' in :lameda (itchell and 0ent 1991)

.F:dult Detention enter, /anta ru, : ( tidewater +irginia>

metropolitan Cew Gersey> south #lorida> :l"any, CE> olum"us, @> and /t 3ouis. Aach of these areas has hadat least five dynamic compaction pro!ects completed, with hicago, metropolitan Cew Gersey, south #lorida, and

the altimore-0ashington area each having more than forty pro!ects completed over the past 52 years #ig 1.1-=shows representative locations of at least 22 'nown pro!ects (Drumheller 1997)

.


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#ig. 1.1-= 3ocations of some dynamic compaction *ro!ects

%he ma!ority of the dynamic compaction wor' in the east and idwest involves improvement of someform of man-placed fill. %he fill sites pro"a"ly outnum"er loose natural soil sites four to one

%he wor' in the western ./. generally involves collapsi"le soils or earth&ua'e mitigation to reduceli&uefaction potential. ecause of scale, the larger pro!ects generally occur in the west, however there have "eenseveral million s&uare foot pro!ects in the east

A%@D@3@A/ :CD AJ*AC%

%he ma!or changes in e&uipment during the past decade are in lifting devices. %he 3ampson3D-2thumpers were introduced in the mid 19824s, and have "een used on 1 to 52 !o"s re&uiring heavier tampers.%hey routinely lift tampers heavier than 18 tonnes (52 tons), and have "een used e$clusively on the ureau of;eclamation pro!ects since the ureau usually specifies heavier tampers than are normally used throughout the./. %ampers heavier than 18 tonnes (52 tons) cannot "e lifted "y conventional cranes since 1 to 17 tonnes (1to 18 tons) is generally considered to "e "eyond the safe rated single line pull capacity of conventional ./.cranes. onsultants should consider this during their evaluation, and not specify weights heavier than 1 tonnes (1 tons) to ma'e the dynamic compaction method cost-effective.

*ounders?tampers in the ./., particularly ones used "y the geotechnical specialty contractors, aregenerally constructed of steel. /pecifications for tampers in the east and idwest are generally in the 7 to 1tonnes (8 to 1 ton) range, with higher tampers specified as su"surface conditions warrant %he ma!ority of thetampers specified in ur"an areas are generally in the to 9 tonne ( to 12 ton) range so as to reduce off-sitevi"rations. %he heavier tampers are generally used in the western ./., where deeper and greater improvementsare re&uired, and there are no ad!acent "uildings or vi"ration sensitive facilities.

CONTRACTING PRACTICES

%he contracted wor' in the ./. is split somewhat "y performance specifications and method specifications./everal engineering firms 'nowledgea"le in dynamic compaction occasionally specify wor' on a method spec"asis where they specify theweight, drop height, num"er of drops, grid spacing, num"er of passes. In these instances,general contractors, demolition contractors, and crane vendors may "id on the wor' with the @wner andengineer sharing in the ris' of the outcome of the wor'. ethod specs are generally used when state andgovernment agencies are involved.

%he ma!ority of the wor', however is still "id on a performance "asis where the specialty contractor agreesto meet a specified acceptance criteria, either a tolera"le settlement criteria, or a specified testing re&uirement%here are a"out five to eight specialty contractors in the ./. that routinely perform dynamic compaction. %heyoffer the advantage of e$perience, as well as the availa"ility of different sie ("oth contact pressure and tonnage)

tampers. %hey are generally involved "y the geoteehnical consultant during the engineering investigation

?evaluation regarding the applica"ility of dynamic compaction on a particular pro!ect./pecialty contractors are responsi"le for the design of the program, which includes energy amounts,tamper?weight selection, grid spacing, drop patterns, and drop se&uencing, and control testing. +erification

testing generally is still the responsi"ility 4the owner or engineer.eneral contractors occasionally underta'e dynamic compaction, "ut generally under the supervision of an

e$perienced engineer or agent to the owner, n this manner, ey perform in a true BcontractorB sense, in that theydrop the tamper the specified num"er of times from the predetermined drop height, "ut are not responsi"le for

the degree of improvement achieved.+A;#:%@C %A/%C

ecause enard introduced Dynamic compaction, as well as the pressuremeter,the ./, in the early 19724s, much of the early verification testing involved the pressuremeter, %he use of the

pressuremeter over the past decade has dropped off somewhat, with the most common test "eing theconventional /tandard *enetration %est *%). one *enetration %ests (*%) and ec'er *enetration %ests (*%)

are also fairly common.


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*lacement of a static load test is also used, particularly in evaluations of landfills. Drumheller and /haffer (199)

and Drumheller and /haffer (1997) report results of surcharge loads used as static load test in coal spoils insouthwest +irginia and a ru""le 1 in inneapolis, oth indicate settlements of less than one inch.

/tevens et al. (199) discusses ureau of ;eclamation4s testing at three western ms, where they supplementedthe conventional testing with the following specialty testing techni&ues.

.Forehole /hear %est(/%)

.Fenard pressuremeter test (*%) .;oc' dilatometers

.F/tepped "lade test (/%)

.F/elf"oring pressuremeter test (/*%) .#lat plate dilatometer (D%)

.F/pectral analysis of surface waves (/:/0) .Cuclear "orehole geophysicsF/eismic topography

.Forehole sonic logging

.F/tress captors

onitoring of ground response, however, is pro"a"ly the most important control test during production wor'. fcrater depth continues to increase, if unusual ground heave occurs, or if vi"rations increase with drops are all

useful tools to determine if the desired achievement is occurring and in modifying a field program.

/%;AC% :C @+A; %A

3ucas (1997) discusses several case histories of strength gain of dynamically compacted soils over time. %his

increase in strength has "een o"served "oth in the la"oratory and in the field, and can "e measured not !ust overwee's or months, "ut over years. %his improvement follows completion of primary consolidation either in

natural soils or in deposits that have "een newly stressed such as "y dynamic compaction or other forms of siteimprovement. %he strength gain and reduction in compressi"ility occurs after e$cess pore pressures dissipate,

and it does not seem to "e related to primary compression under an effective stress.3ucas (1997) discusses dramatic strength gains in "oth granular and cohesive soils in Gac'sonville, #3> hicago>

Angland> 0yoming> and tah. /trength gains ranging from 1K to 522K have "een o"served, and Dise et al.(199=) indicates that more improvement was o"served in non-cohesive soils rather than fine grained cohesive

materials. Cumerous researchers have discussed the strength gain phenomenon. t has "een o"served in othernewly stressed soils, and it is not limited to !ust dynamically compacted soils. /econdary consolidation coupled

with cementation in granular deposits certainly contri"utes to strength gain> however, the improvements appearto "e much more than can "e attri"uted to these factors /chmertmann (1991) hypothesies that the transfer of

load from the pore fluid to the soil fa"ric s'eleton "y arching without hydrodynamic water flow appears to "e a

realistic e$planation for the "eneficial aging effects.#or conventional dynamic compaction pro!ects where soil improvement is measured immediately or only a shortperiod of time after ground improvement, consideration should "e given to the long term increase in soil

properties with time. mprovements of 2 to 122 percent appear reasona"le. #or pro!ects where dynamiccompaction is used to reduce li&uefaction and where the earth&ua'e event may not occur for decades,

improvements could "e greater than l@@ to 522 percent, depending upon the soil type. n the case of "uildings or

em"an'ments to "e constructed shortly after dynamic compaction, only a proportion of the eventual long term"eneficial effects will "e realied prior to construction with the magnitude of the increase in soil properties

depending upon the time frame "etween site improvement and the new construction.

3%:%@C/ :CD @##/%A ;A/%;%@C/

ecause of the inherent characteristics of a tamper hitting the ground, dynamic compaction does producevi"ration concerns. 0ith close monitoring, vi"rations can "e maintained well "elow vi"rations that would cause

any cosmetic or structural damage> however, pu"lic awareness of the vi"rations, particularly in residential areasoften precludes use of dynamic compaction.

3@;:*E

n addition to those pu"lications referenced in the preceding sections, the following pu"lications on dynamiccompaction are recommended.

:tu'orala, .D., 0i!ewic'reme, D., and utler, ;. (1991). Bround mprovement and %esting of ;andom ranular #ills and :lluvial /oils.B %ransportation ;esearch oard> 0ashington,

D.riaud, G.3., 3iu, .3., and 3epert, *.. (1992). B%he 0 :< %est to hec' the ncrease in /oil /tiffi1ess Due to

Dynamic ompaction.B :/% /%* 1272, eotechnic.s. of 0aste #ills -%heory and *ractice, pp. 127-155.

utler, ;. (1991 ). Bround mprovement sing Dynamic ompaction.B eotechnicalCews, pp. 51-57, Gune.

astro, . +.,


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*rogram for /teel ree' Dam.B /oil mprovement -12-year pdate, :/A

eotechnical /pecial *u"lication Co.15, pp. 1-1.how, E.


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3u'as, ;, and /tein"erg, / (198=) BDensifying a 3andfill for ommercial Development.B *roceedings of the

nternational onference on ase istories in eotechnical Angineering, niversityofissouri-;olla,+ol , pp. 1191199.

3uongo, +., (1991). BDynamic ompaction. *redicting Depth of mprovement.B routing, /oil mprovement,and eosyG>thetics, :/A eotechnical /pecial *u"lication 2.

3utenegger, :G. (198) BDynamic ompacticn in #ria"le 3oess.B Gournal of eotechnical Angineering, :/A,

+ol115, Co., pp. -7.arinescu,


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;eport, n-/itu /oil mprovement %echni&ues, pp. 1-1=.

ayu', :.:., and 0al'er, :.D. (199=). BDynamic ompaction. %wo ase istories tiliing nnovative%echni&ues.B n-/itu Deep /oil mprovement, :/A eotechnical /pecial *u"lication Co.=.

orden, ;. ., olt, ; D. and Guran, ., Aditors. (1995). routing, /oil mprovement and eosynthetics. :/Aeotechnical /pecial *u"lication Co.2, 1=88 pp.

Davis, /. (199)


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