4.4 Dynamic Compaction, Consolidation & Replacement

4.4.1 IntroductionDynamic compaction is one of the oldest forms of ground

improvements in existence. The Romans reportedly utilized avariation of this technique and it was used in the United States asearly as 1871. Although the dropping of weight on the soil hadprobably been used for centuries, in 1970 Louis Menard patented thetechnique in France and reintroduced to the profession under itspresent form, into sporadic use in the United States in the early1970's. The use of dynamic methods for the densification of granularfill is well documented in the literature, particularly thetechnique of dynamic compaction. Useful information on techniquesand equipment employed and ground response to dynamic compaction maybe found in Mayne, et al. (1984), Varaksin (1981), Liausu (1984) andFindlay and Sheevood (1986).

As the availability of suitable construction sites decreases dueto developments of the urban areas, the need to utilize the siteswith poor bearing and settlement characteristics for foundationsupport increase. Dynamic compaction has proven to be an economicalalternative to other available methods such as excavation andreplacement, surcharging, compaction grouting and other soilimprovement techniques. One of the most common and effective uses ofdynamic compaction is to compact man made deposits of waste andrubble fills which are frequently placed in the old quarries or claypits, mine spoil, and landfills for both old and recent sites.Dynamic compaction was developed and successfully used fordensification of loose, saturated, cohesionless soils and has provento be particularly effective for liquefaction potential reduction.The densification process is similar to that of vibro-compaction.Although used also in fine cohesive soils, its success in thesesoils is uncertain and requires special attention to the generationand dissipation of pore pressures. On occasion, other groundimprovement techniques such as stone columns are used in conjunctionwith dynamic compaction (Bayuk and Walker, 1994).

Although developed for densification for loose natural soils, themajority of the dynamic compaction work in the US has recently beenperformed at sites of solid waste, questionable or uncontrolled oldfills and mine spoils. Another common application in recent yearshas been the stabilization of collapsible soils which are stiff anddry in their natural state, but lose strength and experiencesignificant settlement when they become wet (Rollins and Kim, 1994).

Table 6 illustrates soil classification based on themechanism of compaction.The method of dynamic compaction and replacement has been

reported by Liausu (1984), and Findlay and Sherwood, (1986)leading to the following definitions:

a) Dynamic Compaction: the compaction by heavy tamping ofunsaturated or highly permeable saturated granular materials.The response to tamping is immediate.b) Dynamic Consolidation: the improvement by heavy tamping ofsaturated materials in which the response to tamping is largelytime dependent. Excess pore water pressures are established asa result of tamping and dissipate over several hours or daysafter each tamping pass.c) Dynamic Replacement: the formation by heavy tamping of largepillars of imported granular soil within the body of the softsaturated soil to be improved. The original soil is highlycompressed and consolidated between the pillars and the excesspore pressure generated requires several hours to dissipate.The pillars are used both for soil replacement and drainage.

Figure 35 illustrates grouping of soils by sieve analysis fordynamic compaction (Lukas, 1992). It is worth noting that themethod is suitable for pervious and semi-pervious soils withfine contents less than 20% (Zone I, and Zone II). Zone III isrelatively impervious soils such as clays and organic deposits.When these deposits are saturated, excess pore pressuresdevelop quickly but because of the low permeability, longperiods of time are required for dissipation, which makedynamic compaction impractical. Dynamic compaction has beensuccessful in zone II deposits, but the construction procedurehas to be carefully planned so as to allow excess porepressures to dissipate between impacts. As a guide tocategorize the soil deposits into one of the three categoriesdescribed above, either field permeability tests or laboratorygrain size gradation tests could be undertaken.

4.5 Deep Dynamic Compaction

4.5.1 Basic procedureThe American Society of Civil Engineers (ASCE) ten years updates on

deep dynamic compaction (ASCE, 1997) provides a review of the stateof practice of dynamic compaction and its engineering applications.Dynamic compaction is applied in a systematically controlled

pattern of drops on a coordinate grid layout The initial impacts arespaced at a distance dictated by the depth of the compressiblelayer, depth to groundwater, and grain size distribution. Initialgrid spacing generally approximates the thickness of thecompressible layer. Typically, 5 to 15 blows per grid point areapplied.Often, the proximity of groundwater or excessive crater depth

limits the number of blows applied to each grid to avoid getting thetamper stuck, or to allow for pore water pressure dissipation.Standard practice is to curtail energy application when crater depthexceeds one and a half to two times the height of the tamper, orwhen the groundwater surface rises into the crater. When thisoccurs, additional passes after ground leveling, or backfilling thecrater are required to complete the required number of drops.This first phase of treatment is designed to improve the deeperlayers. Incorrect spacing and energy level at this stage couldcreate a dense upper layer making it difficult or impossible totreat loose material below. The initial phase is also called the"high energy phase" because the compaction energy is concentrated ona wider grid. Completion of the high energy phase is usuallyfollowed by a low energy phase, called "ironing," to densify thesurficial layers in the upper 1.5 m (5 ft). Here, the tamper is onlyraised from 5 to 6 m (15 to 20 ft), and is dropped on an overlappinggrid.

After each pass, the imprints are either backfilled with thesurrounding materials or with off-site material. In a situationwhere groundwater is at shallow depth, the craters should bebackfilled with imported materials to insure staying above thewater table. At least 1.5 m (5 ft) is generally requiredbetween the tamping surface and groundwater.In saturated fine-grained soils, the process is complicated

by the creation of excess porewater pressures duringcompaction, a phenomenon which reduces the effectiveness of thesubsequent compaction passes unless the pore pressure isadequately dissipated. For clayey soils, dynamic compaction isgenerally not recommended unless the craters are backfilledwith crushed stone and repounded, creating large diametercolumns of compacted stone (dynamic replacement).

4.5.2 Type of soil improvedThe single most determinative factor in the suitability of a soil

type to be improved by dynamic compaction is its ability todissipate the excess pore pressure generated by the DC process,During dynamic compaction, soil particles are displaced into atighter configuration or a tighter state of packing. If water ispresent in the soil voids, an instant rise in pore water pressureoccurs. It is necessary for this pressure to dissipate beforeadditional densification can occur under repeated high energy drops.If this isn't allowed to happen, then repeated drops from the tamperonly cause displacement of the ground, and not densification.

As with the increase in applications of dynamic compaction overthe last decade, the types of materials treated by dynamiccompaction have also increased dramatically. Originally, thepredominant soil types considered for dynamic compaction includedonly granular natural or fill soils. But because of the inherenteconomic advantages involved with the use of dynamic compaction, amultitude of materials have been improved. They include;

Uncontrolled fills: Soil types within old fills can include the entirespectrum of natural soils, manmade debris, byproducts, and anycombination of the three. Dynamic compaction works best, however, ondry granular fills, including sand, gravel, ash, brickbats, rock,shot rock, and steel slag.Dynamic compaction in granular fills is similar to a Proctor

compaction test, in that there is a physical displacement ofparticles into a denser configuration. Dynamic compaction produces alow frequency vibration, in the range of four to ten cycles persecond, and it is this low frequency excitation along with thisinput of impact energy that reduces void ratio and increasesrelative density resulting in improved bearing capacity and enhancedsettlement characteristicsFor deposits below the water table, the vibrations cause an

increase in pore pressure, and after a sufficient number of surfaceimpacts, cause a sufficient rise in pore pressure as to induceliquefaction, very similar to the process occurring duringearthquakes. Once this occurs, additional energy application isineffective until the pore pressure dissipates. Additional poundingfollowing pore pressure dissipation produces more low frequencyvibrations that reorganize the particles into a denserconfiguration.Dynamic compaction has been used more recently to improve fine-

grained fills as well. These Oils are much more difficult toimprove, and require much tighter field control and experience.Clays and silts tend to "heave" after repeated pounding, and ifadditional pounding continues, can have a detrimental effect on

compaction. If heaving occurs, pounding at that point should stop,and the number of passes should be increased with either a reduceddrop height or fewer drops per point.A more common technique that has been increasingly employed in the

US over the past decade to improve fine-grained sites is dynamic"replacement" technique. This technique consists of producing acrater by conventional heavy tamping, and filling the craters with a"boney" or granular backfill material to create in-situ highlycompacted large diameters granular pillars, which is either floatingor driven to a firm strata.This boney material can be gravel, shot rock, brick bats,

reprocessed concrete, or anything that will lock together underadditional heavy tamping. Because of the higher permeability of thisbackfill, pore wafer pressure from the underlying and adjacent fine-grained soils will dissipate more quickly. This process is repeateduntil a noticeable decrease in crater formation occurs. Thistechnique essentially results in large diameter columns of compactedstone underlying a site or individual column locationsDynamic compaction is often used in conjunction with other ground

improvement techniques. A retail site in New Jersey was constructedover an old fill which was underlain by organic soils is an example.Here, a vibroflot was used to install stone columns at each interiorfooting location, and then the surface deposits at each of thesecolumn locations was dynamically compacted (Bayuk and Walker 1994).

There have been several old steel mill sites that have beenunderlain by steel slag (Troy, NY; Youngstown, OH; Trenton, NJ, St.Louis, East Chicago, IN) Steel slag is generally quite granular, andresponds very well to dynamic compaction.

Municipal Solid Waste (MSW): Post-construction settlements of sanitaryand rubble landfills under embankments are difficult to predict.Without site improvement, settlements can sometimes range from 1.5-4.6 m (5- 15 ft). The main causes of settlement in landfill depositsare due to:

• Mechanical compression due to distortion, reorientation of thematerials under self-weight,

• Biological decomposition of organic wastes,• Physio-chemical change such as oxidation, corrosion, and

combustion,• Ravelling of fines into larger voids

Dynamic compaction has been used extensively on MSW to remediatethe above causes and for a multitude of reasons. Again, experienceis essential in improving MSW, in that grid spacing, weight contact

pressure, and number of passes are crucial in achieving the desiredresults. Highway embankments, roadways, parking lots, and evenretail structures have increasingly been constructed on dynamicallycompacted MSW.In sanitary landfills, settlements are caused either by compression

of the voids or decaying of the trash material over time, Dynamiccompaction is effective in reducing the void ratio, and thereforereducing the amount of immediate and long-term settlements afterconstruction. It is also effective in reducing the decaying problem,since collapse of voids means less available oxygen for decayingprocess. Future settlements, however, can still be expected due to asecondary consolidation process, and future decaying of the trashmaterial.A distinction must be made between older landfills and more recent

landfills when considering the long-term settlement of the landfillafter improvement with dynamic compaction. Organic decomposition hasgenerally already taken place in older landfills, and the land5.1lusually consists of a dark-colored soil matrix containing varyingamounts of bottles, metal fragments, wood: and debris. Decompositiongenerally takes more than 25 to 30 years to occur.For deposits where biological decomposition is complete, dynamic

compaction has its greatest benefit. Densification results in higherunit weight and reduction incompressibility under load with littlelong-term subsidence under load.For recent landfills where organic decomposition is still underway,

dynamic compaction increases the unit weight of the soil mass bycollapsing voids and decreasing the voids ratio. It will nothowever, stop the biological decomposition, which may result in aloosening of the soil structure followed by long term settlement.

Coal Mine Spoil: Drumheller and Shaffer (1997) discussed 19 coal spoilsites in the US that have been improved by dynamic compaction.Dynamic compaction methodology in coal spoils varies with theconsistency of coal spoil. Some spoil are predominately shotrockwith minor amount of cohesive material, whereas some spoils havemuch higher concentration of fines.Contact pressure of tamper, size of weight, and grid spacing are

generally considered important factors in coal spoils. Dependingupon the nature of the proposed structure, dynamic "replacement" issometimes used following the DC area pass at column locations tofurther reduce the risk of intolerable settlement.

Collapsible Soils: Rollins and Kim (1994), Drumheller and Shaffer (1996),and Davis (1996) discussed eleven sites in Western States wheredynamic compaction was used to improve collapsible soils. Settlement

associated with collapsible soils can lead to expensive repairs,either in highway or structure construction.In 1982, FHWA conducted an extensive field test program of various

ground improvement techniques to improve collapsible soils in NewMexico. The various techniques included vibroflotation, deep mixing,pre-wetting, and dynamic compaction. Dynamic compaction was found tobe the most cost effective, and was selected to improve threeseparate sections of I-25 and I-40 around Albuquerque.

Liquefiable Soils: dynamic compaction is a useful ground improvement toolto reduce liquefaction potential as it increases the relativedensity as well as lateral earth pressure. Disc et al., (1994) withthe US Bureau of Reclamation, discussed three large projects wheredynamic compaction was used to remedial liquefiable soils andimprove the seismic stability of several embankment dams.20 to 30 tone tampers were employed at all of these locations. Wick

drains and surface drainage were installed in conjunction with thedynamic compactian at the sites.

4.5.3 Dynamic ConsolidationFor soft cohesive soils, the densification of soil following heavytamping is attributed to;

(a) Compressibility of saturated soil due to the presence ofmicro-bubbles;(b) The gradual transition to liquefaction under repeated impacts;(c) The rapid dissipation of pore pressures due to highpermeability after soil fissuring;(d) Thixotropic recovery.

With successive tamping, energy is imparted to the soil, a certainamount of immediate volumetric strain is mobilized, and excess porepressure is generated. The level of energy input into the system iscalled the ‘saturation energy’ when the pore pressures equal 100%liquefaction pressure. No further volume change can be achieved byimparting additional energy to the soil. Dissipation of porepressures with time leads to consolidation and gain in strength ofthe soil. The process of densification under a number of passes withtime delays between each pass can be visualized from Fig. 3. Thebackground of the analysis of the heavy tamping mechanism has beendescribed in detail by Van Impe (1992).For low-velocity impacts on soft cohesive soils the impact energy

is used efficiently to improve the soil only in a thin layer. If theimpact energy is very high, as in the case of common dynamicconsolidation of normally consolidated soils, the depth of influenceand the final compacted density are greater, although the energypartly dissipates due to radiated longitudinal stress waves.

Beneficial effects such as inhibiting heave and greatly increasingthe impact efficiency have been obtained recently in Belgium by theSoils company patented impact block, capable of extending theduration of the pulse on the soil being treated, towards a more‘plastic collision', and allowing implementation of variable blockstiffness by prestressing the anchors (Van Impe, 1992). The extentto which heavy tamping improves the in situ soil is one of theprimary parameters studied.

4.5.4 Design and Analysis Considerations:The design of a dynamic compaction project involves determination oftamper weights, grid pattern, drop heights, and depth of influence.The following section briefly discuss theses design parameters(Lukas, 1986; Menard and Boris, 1975; Van Impe et al, 1997; Mayne etal, 1984).

Depth of Improvement, D, Prediction of the depth of influence and thelevel of improvements are the primary concern when using the dynamiccompaction method. These, however, depend on several other factorswhich include: the soil conditions, energy per drop, the contactpressure of the tamper, grid spacing, number of passes and the timelag between each pass.

Impact Energy, E, The energy induced by the dropping of the tamper issimply the weight of the tamper times the height of the drop. Theserepresent the main design parameters in determining the depth ofimprovement when using dynamic compaction. Menard and Broise, (1975)proposed that depth of influence was simply proportional to the

square root of the energy per blow, the equations was modified laterby Lukas, 1986.

D= n (WH)0.5 (1)

Where,D = Depth of Influence (meters)W = Weight of Tamper (tonnes)H = Height of Drop ( meters )n = empirical coefficient that depends on the type of soil (0.3 to0.6)

This equation is based on the free falling of the weights.The factor n, is to account for the applied energy, tamper contact

pressure, influence of cable drag, presence of energy absorbinglayers and ground water table. Table 7 lists the proposed values ofn, for applied energy with the range of 34 to 100 ton.ft/ft2. Figure37 graphically shows the range for various case histories (Leonardet al, 1980).The grid spacing is related to the impact energy by the following

equation,

E = (NWHP)/S2 (2)

Where, E is the average applied energy over the treated area,N, is the number of drops, P is the number of passes, and S isthe grid spacing. Lukas (1986) ves typical impact energy valuesper unit volume of treated soils. These values can bemultiplied by the thickness of the treated soil to estimate therequired applied energy at the surface. The estimated energy isused in the above equation to determine either the number ofdrops for a specific spacing or the minimum spacing for aparticular number of drops. The grid spacing usually used isabout 1.5 to 2.5 times the dimensions of the tamper (Munfakh,1997).

Influence of Cable Drug, since dynamic compaction is a repetitiveprocess, substantial amount of time is required to manuallyrehook the weight after each drop. As a result, drops areconducted with the cable attached to the tamper This howeverposes another problem due to the influence of cable drag whichis due to friction of the cable unwinding over the spool drumand reduction in tamper velocity due to air resistance. Lukas(1992) bases his observations on five separate dynamiccompaction projects indicating that whenever tampers are reusedand dropped with a single cable with a free spool, the measuredvelocity was found to range from 0.88 to 0.93 of thetheoretical velocity. Results were, however, encouraging since

the influence of the cable drag on the energy applied wasrelatively constant and, hence, does not have to be measuredfor each equipment or tamper weight.

Equipment Limitations, The type of equipment used will also have aneffect on depth of influence. Conventional crawler cranes witha rated capacity of 136 tons are commonly used for dynamiccompaction for drop heights up to 24 meters. The usual mass oftamper used however is in the order of 10 to 20 tons with dropheights usually ranging from 10 to 20 meters. Higher dropenergies have been achieved with tamper masses of up to 150tons and drop heights as high as 40 meters with special cranesor tripods.

Influence of Tamper Size, Tamper size is instrumental in controllingthe contact pressure at impact. Contact pressure which isdefined as the weight of tamper divided by the contact area iscommonly used in the range from 30 to 75 kN/m2. Low contactpressure could develop a crust of soil and prohibit any soilimprovements below this crust Conversely contact pressurehigher than those indicated above could result in the temperpunching into the ground upon impact, which reduces energyefficiency.

Grid Spacing, The print spacing (the spacing between thecompaction points) used in dynamic compaction has a significanteffect on the soil improvement within the grid (Chow, et al,1994). The first pass is designed to improve the deeper level,and is dependent on the thickness of the compressible layer,grain size distribution and depth of the groundwater. Initialgrid spacing is usually at least equal to the thickness of thecompressible layer. Other passes that follow are aimed atdensifying shallower level, which may also require lesserapplied energy. Finally, an "ironing" pass to densify the toplayer of the ground is conducted by dropping of a square orrectangular tamper over the entire surface area with relativelylow drop energy.

Time Delay between Passes, Where pore-water pressure can develop,the timing between each pass must be such that it will allowfor the pore-water pressure to dissipate Piezometers can be

installed to monitor the dissipation of pore-water pressurefollowing each pass

Soil Conditions, As described before, dynamic compaction is bestsuited for densification of deposits grouped as pervious andsemi-pervious (Lukas 1986). In addition, the position of thewater table and the amounts of fine contents generallyinfluence the effectiveness of dynamic compaction. Presence ofclay content greater than I5% fines by weight, generallyrenders this method less effective (Luongo, 1992)

Degree of Improvement, Main factor controlling the degree ofimprovement is the applied energy. Menard and Broise (1975)stated that the applied energy used for Dynamic Compaction,should produce a minimum static load of 2 - to 3 t/m2 at thedepth corresponding to the water table level. Other suggestionswere also made for magnitude of the applied energy. Lukas(1992) based his recommendations on different types of soilconditions as shown in Table 7.

Although increasing the number of drops in each pass and icenumber of passes can be used to achieve a greater depth ofinfluence, there is a point for which the further applicationof energy produces only minimal gains. This threshold is calledthe "saturation energy". Lukas, (1992) presented a typicalgraph of depth of improvement versus the number of drops.Depending on the soil type, increase in the number of dropswill have very little gain in improvement of the lower levels.For cohesive soils, Charles et al. (1981) proposed an

influence depth which takes into consideration other parameterssuch as soil type, surface area and shape of the pounder.

D = 0.4 (EdB/Apcu)0.5(3)

where B is the width or diameter of the pounder, Ed/Ap is thetotal impact energy applied per unit area of the pounder and cu

is the undrained shear strength of the soil.An exhaustive compilation of data from over l20 sites was

presented by Mayne et al. (1984). Moreover, useful correlationfor normalized crater depth, D/(WH)0.5, overall subsidence ofthe ground, peak particle velocity, and maximum depth ofinfluence, D, have been presented. The normalized crater depth

increases with the number of passes (Fig. 5a), the trendshowing a limit for this parameter. The overall groundsubsidence increases with applied energy (Fig. 54), while peakparticle velocity decreases (Fig. 5c) with scaled distance, d.The maximum depth of influence is proportional to the energyper blow.

Site Preparations, The site to be consolidated must first beprepared to support the weight of the tamping machine (60-200t). Occasionally 1 to 2 meters of granular materials areapplied to the ground surface, particularly in landfills andother soft ground conditions, to provide bearing surface forthe machine. It must also be safeguarded against bad weather.If sensitive to rain water (alluvia and clays ), and removal ofwater rising to the surface during the consolidation processmust be facilitated by means of peripheral trenches, drains,and so on.

4.5.5 Environmental ConsiderationsOne major concern with heavy tamping is the high impact

energy that generates damaging ground vibrations The use ofsuch methods of ground improvement is therefore much morelimited in the urban surroundings. Ground vibrations caused bydynamic compaction not oniy can be damaging to the nearby

structures, under ground utilities and electrical or mechanicalequipment, and are also disturbing to people.

Vibrations are normally quantified in terms of the PeakParticle Velocity (PPV). Peak particle generally used to definethe damage criteria for buildings and the annoyance tolerancelevels to people. Figure 39 shows the relationship betweenparticle velocity is and scaled energy factor. It illustratesthat well-constructed buildings can tolerate a PPV of 50mm/sec, however, a limit of12.5 mm/sec is often used as amaximum value for safety margin. To facilitate comparisonbetween various projects, the peak particle velocity has beenplotted against the inverse scaled distance as shown in Figure39. The inverse scale distance is the square root of compactionenergy, (WH)0.5, divided by distance from the impact point.

4.5.6 Other Design Considerations

Many dynamic compaction sites have irregular subsurfaceconditions particularly boulder and rubble deposit where it isdifficult to interpret verification test results and assesssoil properties after dynamic compaction improvement. Based onextensive laboratory model study of dynamic compaction of drysand with measurements included tamper acceleration and soilpressure during impact, a procedure was presented by Poran et

al (1992) to use Dynamic Settlement Modulus (DSM) to determinethe degree of improvement during construction. DSM was definedas the slope of the tangent of the loading portion of impactstress-relative settlement curve where relative settlement, ej,is defined as the tamper settlement dt, divided by itsdiameter, D.

DSM = pt / (dt/D) (4)

where, pt, is the impact stress.

Also, DSM values is a function of tamper drops, As it can beobserved, for most of the tests, the rate of DSM increase wassignificantly reduced from the 12' drop on This relative changein the DSM values was found to be proportional to the rate ofdensification (density increase as a function of Number ofdrops).Although not widely used, other design methods have been

developed in order to make a better prediction of the effect offactors affecting dynamic compaction and the required or thedesired level of ground improvement -Lo, et al (1990) introduced a DC design method based on their

findings that a relationship exists between the saturationenergy and enforced settlement. The authors presented a plot(Figure 40) of enforced settlement versus total applied energyintensity, pointing out that for a given initial soilconsistency and energy per blow of pounder, a hyperbolic curvemay reasonably be fitted to the field results Thus, asaturation energy can be defined beyond which furtherenforcement of settlements would be relatively insignificant.Furthermore, saturation energy intensities were plotted againstthe ratio of energy per blow of tamper Ey (energy per blow) tothe initial pressuremeter limit pressure PL of each site. Asshown in Figure 41, a unique relationship may reasonably beconstructed between parameters Is (saturation energy) and EB/PL.The authors also emphasized that in view of scarcity of well-

documented tamping projects to draw upon, the assumed trendsfor IS in Fig 8 have been inferred on the basis of ratherlimited data. Additional research would no doubt refine theresults, establishing more reliable design curves.

Further, since IS is uniquely determined by EB/PL it should inprinciple, be reasonable to expect that a collective term ofthese parameters relating to the operational requirements ofeach tamping project would similarly determine the maximumdegree of ground improvement uniquely. This collective term oroperational factor maybe expressed as

= IS (EB/PL)(5)

where,IS = Saturation Energy (t-m / m‘)EB = Energy per Blow (t-m)PL = Pressure meter Limit Pressure (t/m‘)

The maximum degree of ground improvement maybe expressed interms of enforced settlement per unit thickness of treated soildeposits as follows:

= SE/Ht

(6)

where, Ht = total initial thickness of soil depositsrequiring treatment According to Figure 41, the relationshipwould then be applicable.

= /(30+3)(7)

Thus, given the initial ground conditions, it may in principlebe possible to specify a priori, with the characteristics ofFigure 41, energy per below of pounder print spacing, andnumber of below per print to achieve the required degree ofimprovement, and thereby rationalize performance designFurthermore, due to the subsequent thixotropic recovery anddissipation of pore-water pressure, long terms effects of heavytamping might result in greater ground improvement thaninferred herein.

4.5. 7 Dynamic ReplacementDynamic replacement is based on the same mechanism ofdensification as dynamic compaction and utilizes similar plant,essentially a tall rig with a drop weight. It furtherincorporates ground replacement techniques by progressivelyfilling the crater, or "prints" formed by the drop weight with

rock fill to create columns of strong, heavily compactedmaterial. The columns greatly improve the average stiffness ofnear surface zone, as well as better transmitting the impactforces from the drop weight to greater depths within the fill.The basic processes involved in dynamic replacement areindicated in Figure 42.

The main design parameters for this technique could be summarizedas (Barksdale and Bachus, 1983), i) shear strength h of thecomposite mass of vertical compacted granular material within theexisting soft soils; ii) stress ratio deemed as loads transferred tocolumns to that transferred to adjacent soils, which will be afunction of the soft material stiffness (cohesion), and the volumeof the replaced material per unit volume of the soft soils, iii)vertical capacity of the columns, governed by the diameter andallowable confining pressures provided by the surrounding soils; iv)drainage properties of the replaced soils and their influence on theconsolidation and strength gain of the soft soils.The main draw back of dynamic replacement is that the improved soil

may experience 40 % of the estimated settlement without improvementdue to the fact that soils between columns, while improved throughthe compaction induced lateral compression are not treated in thesame manner.As reported by Bevan, 1997 a major dynamic replacement project for

a multi-story building was carried out in Iran for ground

improvement to enhance settlement and strength characteristics ofsoft sandy clayey soils. Boreholes revealed either very siltycollapsing sandy soils above ground water or soft compressible sandyclay below ground water table overlying a marl bedrock at a depthbetween zero to 12 meters below finished facility ground level. Lowstandard penetration test ($PT) blow counts are observed just aboveground table. Ground table varied between about 3 to 4 meters belowfinished floor level. There appeared generally to be a stiff surfacecrust across the whole site with loose to very loose horizons justabove and below the ground water table. The initial proposal was toimprove the alluvial deposits by Dynamic Compaction. However, due tothe high content of fines, mainly the clayey fraction, the processbecomes dynamic replacement. In this particular site, plugs ofstone, consisting of imported backfill, which is repeatedly filledinto the craters by the pounder, were driven into the soil. Theseplugs were reinforcing the existing soil to a certain extent as loadsupporting elements. They reduced the settlement, increased thebearing capacity and accelerate the consolidation process.To test the effectiveness of the spread treatment from the tampingpoints, tests were carried out with various number of drops andCPT's made at the center of the print and at 1,2,3,4, and 5 meterRom the center of the print. Figure 43 illustrates that the effectis concentrated in the layer above the water table and at the centerof the tamping print area in the plan. Disruption of the originalsurface crust between 0 and 2 meter can be observed, which wouldneed careful restoration in ironing pass.