MEJORAMIENTO DEL SUELO - EUN C. SHIN.pdf

7/29/2019 MEJORAMIENTO DEL SUELO - EUN C. SHIN.pdf

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OVERVIEW OF RECENT SOIL IMPROVEMENT TECHNIQUES

E.C. Shin

Associate Professor, Dept. of Civil and Environmental System Engineering,

University of Incheon, Republic of Korea

ABSTRACT:In recent years, soil improvement works have been extensively implemented for the various marginallands development projects. A great amount soils are hydraulically filled by the dredging ship and various soil im-provement techniques are being used for stabilization of soft ground. Some ground modifications are required for rein-forcing the soil to have enough bearing capacity and confinement to support the load carried by the existing or super-structures. The paper presents the current construction methods of sand piling including sand compaction piles, sanddrains, strong sand pile , mammoth compaction pile, compaction grouting, stone column, vibro-compaction, shallowand deep dynamic compactions, vertical and horizontal vacuum consolidations, prefabricated vertical drains, and pro-gressive trenching method. Design procedures, construction sequences, and quality control for various soil improve-ment methods are presented. Finally, construction case histories of soil improvement are also discussed with construc-tion process, field performance for both sandy and clayey soils. The selection of soil improvement method is dependedon geological formation of soil, soil characteristics, cost, availability of backfill material, and experience in the past.

1 INTRODUCTION

In the past 40 years, various soil improvement methodssuch as deep chemical mixing method, construction ofcompacted granular columns, grouting, prefabricatedvertical drains (PVD), shallow and deep dynamic com-pactions, vertical and horizontal vacuum consolidations,progressive trenching method (PTM), and preloadinghave been used for onshore or offshore land developmentprojects. Sand compaction piles have been extensivelyused in Asian countries like Korea, Japan, Singapore foroffshore development projects. While vibro-compactionand deep dynamic compaction methods are morefrequently used in Middle Eastern countries as well asEuropean countries. Grouting techniques are morepopular in Japan and USA, and South American

countries. The vertical & horizontal vacuum consolida-tions and PTM are mostly applied to an ultra soft clayeysoil for the land reclamation project. Installation ofcompacted granular piles and PVD can accelerate theconsolidation process of clayey soil and hence increaseits shear strength. Construction of these vertical drainsalso increases the load bearing capacity and prevents thepossible damages by earthquake. Vertical drain methodsare fast and relatively inexpensive methods compared tothe other deep stabilization methods. Most of soilimprovement works are for the construction of airport,various storage tanks, embankment for roads, industrialcomplex, port and harbor facilities, and so on.

2 SAND COMPACTION PILE

Sand compaction pile technique was developed byMurayama (1958) in Japan. Since then several differentmethods of making sand compaction piles such as sanddrain, packed drain, strong sand pile, and mammothcompaction pile for different degrees of densification andsite condition have been developed. Sand compactionpiles are usually constructed in a soil which has anundrained shear strength varying from 5kN/

2m to

15kN/2

m to provide enough confining pressure to thecompleted sand compaction pile. Packed drain methodcan be possibly applied if the existing soil has less thanthe required undrained shear strength because it provides

additional confining pressure along required part of com-

pleted sand drain by means of flexible fiber sack.

Sand compaction piles on reclaimed land from the seausually have diameters varying between 0.6-0.8 m withan area replacement ratio, sa 0.3 0.5. Typical pilespacings range from 1.5 m to 2.5 m. Construction proce-dures of the sand compaction piles are shown in Fig.1

The steps for sand compaction pile construction are asfollows:

Step 1. Installing a casing pipe having an outside diameter of 0.4 m and a wall thickness of 16 mm at adesignated position on the ground with an allowable maximum horizontal deviation of 0.3 m .

Step 2. Driving the casing pipe into the ground by thevibro-hammer with an allowable deviation of two degrees with respect to the vertical. The vibrator used has the following characteristics: weight, 54 kN (6.1 ton); driven by a 90-kW (120 hp) motor and hada vibrating force of 443 kN (50 ton).When a localized hard spot is encountered withSPT N-values greater than 20, water jetting is usedto facilitate the penetration of soil layer.If thepossibility of mud to enter the casing, airpressure in the range of 294-392 kN/m2 is applied.

Step 3. When the casing pipe reaches the desired depth(that is, penetrated the full depth of the soft clayeysoil which varied from 20 m to 25 m), about 2 m3of s

and is filled through the upper hopper of thepipe. The casing pipe is then drawn up at the speedof 9 m/min through a distance of 3 m .

Step 4. An air pressure of 588 kN/m2 is applied to the toof the sand for extrusion from the tip of the casing.

Step 5. The casing pipe is redriven through a distance of2 m under vibration to compact the sand pile.

Step 6. Steps 3, 4 and 5 are continued until the sand pileis built up to the ground level.

Construction procedures of sand drain are Steps 1, 2, 3, 4,and 6 outlined above. The diameters of sand drains are


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usually varying from 0.4 m to 0.5 m with a center-to-center spacing ranging from 1.8 m to 2.5 m. Therefore,density of compacted sand in sand drain is less than thatof the sand compaction pile described above.

Construction sequence of strong sand pile is similar tothat for a sand drain with the exception of horizontal andcircular vibration of the casing pipe in Step 4. Additionalcompaction is also applied directly to the sand dischargedout of the lower end of the casing pipe by a horizontal vi-brator. The density of compacted sand in strong sand pileis much higher than that of the sand compaction pile.

Strong sand pile can be constructed on reclaimed land orsea.

Construction procedure of the mammoth compaction pileis similar to that in sand compaction pile outlined previ-ously. This particular method is usually used in offshoreconstruction as shown in Fig.2. Typical diameters andarea replacement ratio range from 0.8 m to 2.0 m and

sa 0.5 0.8. Generally 2 to 4 mammoth compactionpiles are constructed simultaneously from a large barge.

Fig.1 Construction sequence for sand compaction pile

Fig.2 Construction of mammoth compaction pile

Shin, et al.(1992) reported the soil improvement case his-tory of steel mill complex in Korea A total of 23,217sand compaction piles and 27,224 sand drains were at-tempted, out of which only 0.62% of the sand compac-tion piles and 0.36% of the sand drains were not success-ful. The cases for which sand pile construction was notsuccessful were due to one or more of the following rea-sons:

1. The slope of the sand pile deviated more than twodegrees from the vertical direction.

2. The pre-estimated length of the sand pile was notachieved due to localized hard spots.

3. The casing pipe did not reach the required depth pastlocalized hard spots due to the malfunctioning of wa-ter jet equipment.

4. Horizontal deviation of the actual sand pile locationwas more than 0.3 m from the designated location.

5. The sand pile was broken during the constructionprocess. The reason for breakage are (a) malfunction-

ing of the lower end of the casing (shoe) and (b) care-less operation of the construction equipment.

6. The level of sand inside the casing drawn on theautomatic recording graph was more than 0.3 m dueto the incomplete extrusion of sand from the bottomend of the casing. This was due to insufficient airpressure.

7. Automatic recording graphs were not drawn well dueto malfunctioning of the recording system.

8. The gradation of the sand was not in agreement withthe design requirements.

3 COMPACTION GROUTING

Grouting methods are categorized into : (a) permeatinggrouting, (b) compaction grouting, (c) hydro-fracturegrouting. Among them, compaction grouting method iscommonly used for ground reinforcement. Compactiongrouting uses displacement to improve ground conditions.A very viscous(low-mobility), aggregate grout is pumpedin several stages to form grout bulbs, which displace anddensify the surrounding soils. The high degree of soil im-provement can be achieved by sequencing the groutingwork from primary to secondary to tertiary locations.

Compaction grouting technique can be applied in the ar-eas of karstic regions, rubble fill and poorly placed fill,loosened soil, liquefiable soils, and collapsible soils to fillthe void and to compensate for ground loss during tun-neling work and reinforcement. The construction processof the compaction grouting process is shown in Fig.3.

Fig.3 Construction of compaction grouting

Several geotechnical considerations must take into ac-count to form a good quality of compaction grouting(Hayward Baker, 2005).


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1. The in-situ vertical stress in the treatment stratum mustbe sufficient to enable the grout to displace the soilhorizontally. In this case, the grout injection pressure should be carefully selected to prevent the uncon-trolled heave of the ground surface.

2. The grout injection rate should be show enough toallow pore pressure dissipation. Pore pressure dissipa-tion should also be considered in hole spacing andsequencing.

3. Sequencing of grout injection is also important. If thesoil is not near saturation, compaction grouting canusually be effective in most silts and sands.

4. The loss of the shear strength of soil during remoldingfor the case of saturated, fine-grained soils, and sensi-tive clays should be avoided.

5. The great amount of soil displacement could be oc-curred in weaker soil strata. Exhumed grout bulbs con-firm that compaction grouting focuses improvementwhere it is most needed.

6. Collapsible soils can usually be treated effectively withthe addition of water during drilling prior to compac-tion grout injection.

7. Stratified soils, particularly thinly stratified soils, canbe caused for difficult or reduced improvement capabil-ity.

8. In case of compensation grouting, rate of tunnel ad-vance and tunneling method must be considered to

avoid a void space in the grouted area. The ade-quate range of grain-size distribution for various soils forcompaction grouting is shown in Fig.4.

Fig.4 Adequate range of grain-size distribution forcompaction grouting.

There are several advantages of compaction groutingsuch as pinpoint treatment, fast installation, and widerange of application. This technique can be effectivelyapplicable in a variety of soil conditions. It can be per-

formed in very tight access and low headroom conditions.Compaction grouting is non-hazardous and yields nowaste spoil disposal. There is no connection needed tothe footing or column. This method is used in the mannerof non-destructive way and adaptable to existing founda-tions. There is no necessary to removal and replacementor piling, so it is cost effective ground reinforcementtechnique and minimized the surface environment.

Quality control of compaction grouting can be achievedby following the procedural inspection and documenta-tion of the work activity, testing to ensure proper mix de-sign/injection rates, and verification of ground improve-ment where applicable.

Ground improvement can be assessed by in-situ test likestandard penetration test (SPT), cone penetration test(CPT), and geophysical methods. Data recording systemfor monitoring the important grouting parameters can beused on sensitive projects to have a good quality control.

4 COMPACTED STONE COLUMN

Compacted stone columns are stiffer than the surround-ing soil which replaced. Because the stone column is co-hesionless, its stiffness depends upon the lateral supportgiven by the soil around it. If that support is inadequate,the stone column fails by bulging. Compacted stone col-umn method is normally used the soil which has anundrained shear strength varying from 15kN/

2m to

45kN/2

m to provide enough confining pressure to thecompleted stone column. Densification of the soil withstone columns is accomplished by either top-feed or thebottom-feed method.

The stability of a soil-stone column composite systemalso depends on whether skin friction develops betweenthe column and the soil surrounding it. Soft compressiblesoils undergo much lower settlements when they arestiffened by stone column. Bulging of the stone columnunder the applied load causes horizontal compression ofthe soil between columns which provides additional con-finement for the stone. An equilibrium is eventuallyreached resulting in reducing probable settlement, in-creasing the shear strength of soil and hence the bearingcapacity, and mitigate the potential for liquefaction whencompared to unreinforced soil..

The two primary construction methods of compactedstone column are practiced : (a) wet, top feed method, re-placement and displacement method, (b) dry, bottom feedmethod, displacement method.

In the first method, that is, replacement and displacementmethod, jetting water is used to remove soft material,stabilize the probe hole, and ensure that the stone backfillreaches the tip of the vibrator. This is the most commonlyused and most cost-efficient of the deep vibratory meth-ods. However, handling of the spoil generated by theprocess may make this method more difficult to use onconfined sites or in environmentally sensitive areas. Thelatter method, that is, dry and bottom feed method usesthe same vibrator probes as standard compacted stonecolumns, but with the addition of a hopper and supplytube to feed the stone backfill directly to the tip of the vi-brator. Bottom feed displacement method is a completely

dry operation where the vibrator remains in the groundduring the construction process. The elimination of flush-ing water in turn eliminates the generation of spoil, ex-tending the range of sites that can be treated. up to adepth of 24 m and is not inhibited by the presence ofgroundwater. The construction process of compactedstone column is shown in Fig.5.


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Fig.5 Construction process of compacted stone column

A blanket of sand or a semi-rigid mat of reinforced earthis usually placed above the stone column-reinforced soil.

This mat facilitates transfer of superimposed loads to thestone columns by arching over the in-situ soil. With time,as the surrounding clay consolidates, further load trans-fer takes place from the native soil to the stone columnsby negative friction resulting in additional reduction insoil settlement (Munfakh et. al. 1984). The critical depth(which does not contribute of bearing capacity) of stonecolumn is usually about four column diameters (Mattesand Poulos, 1969).Design of stone column-soil structure is subject to theseveral parameters such as column diameter, spacing,stone size, internal soil friction angle, undrained shearstrength, degree of saturation, and permeation.

The diameter of stone column is depended on the desiredlevel of improvement, the method of installation, thestone size and the strength of the in-situ soil. The columndiameters range from 0.5 to 1.2 m. Square or rectangulargrid patterns with center to center column spacing of 1.5to 3.6 m are used. It is a function of the desired im-provement, construction process, and the sensitivity ofthe existing soil. Angle of internal friction of the stone isdepended on the size and shape of the stone, the installa-tion process and the infiltration of the native soil between

stone particles. Greenwood (1970) assumed an angle ofinternal friction of 35 degrees for evaluation of stone col-umn horizontal resistance. High values of 40 to 45 de-grees have been used, based on the results of direct sheartests performed in the field on constructed columns(Munfakh et. al. 1984.

Fig.6 Ultimate bearing capacity of stone columns(Greenwood and Kirsch, 1984)

The friction angle, may have a big effect on the horizon-tal shear resistance of the stone reinforced soil. Modulusvalues in the range of 40 to 70 MN/

2m . Settlement andstability calculations were also performed using effectivestress parameters of the soil (Goughnour, 1983 ; Priebe,1976 ).

Design values of 20 to 30 tons per column are typical forcolumns in soft to stiff clays (Mitchell, 1981). Both limitanalysis (Hughes and Withers, 1974) and experience(Thorburn, 1975) indicate that the allowable verticalstress, on a single column can be expressed by

=SC u

N c /F.S

Whereuc is the undrained shear strength of the soft

ground and F.S is the factor of safety. A value of 3 isrecommended for the F.S. Mitchell (1981) recommendsusing an

SCN of 25 for vibro-replacement stone columns.

Barksdale and Bachus (1983) propose a range foruc be-

tween 18 to 22, depending on the stiffness of the soil.Brauns (1978) compared the results of a number of ana-lytical approaches for determining ultimate capacity ofstone columns which is reproduced by Greenwood andKirsch (1984) as shown in Fig.6. Estimated settlement ofstone column treated group as a function of soil strengthand column spacing is shown in Fig.7. To theses esti-

mated should be added any anticipated settlements con-tributed by the underlying strata. The installation of stonecolumns led to a reduction in settlement to about 30 to40% of the values to be expected on unimproved ground.

Fig.7 Effect of stone column on anticipated foundationsettlement (Greenwood, 1970)

Fig.8 shows that the soil improvement factor with thearea replacement ratio and the internal stone fric-tion angle.


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The degree of soil improvement is proportional to theangle of stone friction in the stone column and reverse tothe area replacement ratio. Fig.8 shows these relation-ships.

Quality control of compacted stone columns can be pro-ceed by means of the stone column location, resistancelevel as measured by amp meter (Vibrator draws morecurrent in denser soils), quality and quantity of stoneadded friction angle and diameter of completed stonecolumns. Several field tests can be done for quality as-surance such as, SPT, CPT, DMT(Dilatometer Test),

load test, and shear wave velocity test with respect tothe depth of stone column.

5 VIBROFLOTATION

This technique was developed in Germany in the 1930sand has been practiced in the United States since 1940s.Vibroflotation also knows as Vibro-compaction is used todensify clean, cohesionless soils. The performance ofthis techniques in loose granular soil is excellent. How-ever, it is marginal to good for silty sand and mine granu-lar spoils. The action of the vibrator, usually accompa-nied by water jetting, reduces the inter-granular forces

between the soil particles, allowing them to move into adenser configuration, typically achieving a relative den-sity of 70 to 85 percent. Compaction is achieved aboveand below the water table. Fig.9 shows the process of soildensification by vibroflotation method. The vibroflotwith a diameter of 350 to 450mm is lowered into theground with the help of water jetting at the bottom of thevibroflot. Granular material is poured into the top of thehole. The vibrating unit is lowering with a rate of 1 to 2m per minute and gradually raised in about 0.3m lift perminute and held vibrating for about 30 seconds at a time.

Fig.9 Construction of the vibro-compaction process.

The 100-HP unit are normally used in the vibroflot. The

weight of a vibroflot is about 20kN and induced themaximum centrifugal forces of 160kN. The vibrationamplitude can be up to 25mm with the operating fre-quencies of 30 to 50HZ. The use of vibroflotation methodon granular fills increases bearing capacity and hence re-duces foundation size. It can be reduced the foundationsettlement and mitigated the liquefaction potential duringearthquake

The configuration of treated and untreated soil profilesfor sandy ground is shown in Fig.10. The important pa-rameters related to this technique are the type of soils inthe ground and its gradation as well as relative density.

The quality control and quality assurance of this tech-nique is somewhat similar to the case of compacted stonecolumn. Fig.11 shows the degree of soil compaction withrespect to the site surface area per compaction probe insandy soils.

Fig.10 Treated and untreated soil profiles for granularsoil

Fig.11 Soil treatment requirements for densification

6 DEEP AND SHALLOW DYNAMIC COMPACTIONS

Deep dynamic compaction is the dropping of heavy deadweights on the ground surface to densify soils at depth.

The degree of soil compaction involves a number ofblows and the weight of hammer, drop height, the spac-ing of drop spot, and types of soil to be compacted. Thismethod is used to reduce the probable foundation settle-ments and also reduce seismic subsidence. It can be usedin the area of garbage dumps, mine spoils, and collapsi-ble soils. The typical weight of hammer, dead weightwhich normally consisted with thin metal plates, rangesfrom 10 to 30 tons and its drop heights of 15 to 30 m.Impact grids are normally in of 2.1 x 2.1 m to 6.0 x 6.0 m.

Important geotechnical engineering parameters for deep

dynamic compaction are such as soil conditions, ground-water level, relative density of soil, degree of saturation,and permeability of soil to be compacted. The coarsegranular soils such as gravel and sand(PI=0) are excellentmaterial to be compacted and applied energy of 20-25ton/m3 is normally required. The compaction effi-ciency of fine grained soil like silty soil(0 PI 8) ismoderated to good and applied energy of 25-35ton/m3 isnormally required. For cohesive soil which has a plastic-ity index greater than 8, the deep dynamic compactionmethod is not applicable.


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The maximum depth of improvement by this method canbe estimated as

D = 1/2 (WH)0.5

Where D is the maximum depth of influence in meter, Wis a falling weight in metric tons, and H is the drop heightin meter.

The compaction area should be 30-45 m away from anystructures and sensitivity of vibration must be evaluated.

The number of blows per per area is depended on thetype of soil and depth of improvement required. The

tamping is normally executed in square pattern and 2-43overages of an area is required. Meanwhile the porewater pressure is checked by utilizing the piezometer.

The standard penetration tests in the deep dynamic com-pacted area for Incheon International Airport(IIA) in Ko-rea is shown in Fig.12. The depth of soil improvement isabout 5.0 6.0 m

Fig.12 SPT results for IIA site.

Dynamic compaction quality control can be made bymeasuring the crater depth, surface elevation monitoring,pore water pressure, SPT, CPT, geophysical monitoring.

The shallow dynamic or heavy hydraulic hammer com-paction method was adopted in the area where the heightof the hydraulic fill was 2.0 6.1 m. The diameter oframmer in this compaction equipment is 1.5 m and itsweight are 10 metric ton for runway and taxiway area and7 metric ton for the rest of area (Fig.13).

(a) Hydraulic hammer

(b) Ground condition after tamping

Fig.13 Equipment and ground condition of heavyhydraulic hammer compaction for IIA site

The number of blow per one spot was 20 blows with thedrop height of 1.5 m. The spacing of tamping was 2.1 mand the pore water pressure was measured during thetamping period. Comparison of soil improvement effectsis tabulated in Table 1.

Table 1. Comparison of soil improvement effectsIn-situ test Pre-

investigationAfter

tampingSPT N value 16 24 42 44

DCPTd

N value 36 43 59 99Degree of compaction

(%)84 86 94 97

Void ratio 0.719 0.765 0.526 0.572

7 PREFABRICATED VERTICAL DRAIN

This method was first developed in 1937, Sweden, byKjellman. Since then it is very popular to use in soft clay

to improve the ground, particularly in South East AsianCountries. Construction without soil treatment is usuallyimpractical due to unpredictable long-term settlement.Simple surcharging as a soil consolidation method cantake many years. Soil consolidation using prefabricatedvertical drains (PVD or wick drains) can rapidly increasesettlement rates with dissipation of water and cut con-struction period drastically. The prefabricated verticaldrain core is made of high quality flexible polypropylenewhich exhibits a large water flow capacity in the longitu-dinal direction of the core via preformed grooves or wa-ter channels on both sides of the core. Each vertical wickdrain can provide a greater vertical discharge capacitythan a 15 cm diameter sand column. The prefabricatedvertical wick drain core is tightly wrapped in a geotextile

filter jacket of spun-bonded polypropylene which has avery high water permeability while retaining the finest ofsoil particles. Both the core and geotextile filter jackethave high mechanical strength, a high degree of durabil-ity in most environments, and high resistance to chemi-cals, micro-organisms, and bacteria.

The width of drain board is 100 mm with its thickness of3 mm. It can be installed in triangular or square patternwith an anchor plate and quantity of installation is about40 80 m per day. Fig.14 shows the installation of PVDin a square pattern for the construction site of Incheon In-


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ternational Airport. The approximate settlement trendsfor various ground conditions are depicted in Fig.15.

Prefabricated vertical drains are installed vertically todepths exceeding 65 m. The inclination of installed PVDshould be less then 5 % and the slope of ground level isno more than 2%.The water, under pressure in excess ofhydrostatic, flows through the filter fabric of the prefab-ricated vertical drain and into the channels of the verticaldrain core where it can flow vertically out of the soil.

This flow mat be either up or down to intersecting naturalsand layers or to the surface where a sand drainage blan-

ket of 50 cm thick (k=2.0 x 10-2 cm/sec) or prefabricatedhorizontal strip drains are provided. The water in the soilhas only to travel the distance to the nearest prefabricatedvertical drain to reach a free drainage path.

Fig.14 Installation of PVD in IIA site

Fig.15 Settlement versus time with or without PVD

The PVDs are usually placed in 1.0 -2.0 m interval de-pending on the desired consolidation time. As a result ofthis method of accelerating the consolidation process, un-evenpost - construction settlements can be virtuallyeliminated.

8 VACUUM CONSOLIDATION

Vacuum Consolidation is an effective means for im-provement of saturated soft soils. The soil site is coveredwith an airtight membrane and a vacuum is created un-derneath it by using a dual venturi and vacuum pump.

The technology can provide an equivalent pre-loading ofabout 4.5 m high conventional surcharge fill.Instead of increasing the effective stress in the soil massby increasing the total stress by means of conventionalmechanical surcharging, vacuum-assisted consolidationpreloads the soil by reducing the pore pressure whilemaintaining a constant total stress.

Fig.16 Schematic diagram of vertical vacuum consolida-tion method

Typical layout of a vacuum-assisted prefabricated verti-cal drain consolidation scheme is shown in Fig.16.

Construction process of the vertical vacuum consolida-tion is described as below.

Step 1. Placing a free drainage sand blanket (60-80 cmthickness) above the saturated ground in order toprovide for a working platform.

Step 2. Installation of vertical drains, generally of 5 cmin equivalent diameter, as well as relief wellsfrom the sand blanket.

Step 3. Installation of closely spaced horizontal drainsat the base of the sand blanket using a special la-ser technique to maintain them horizontal.

Step 4. The horizontal drains in the longitudinal andtransverse directions are linked through connec-tions.

Step 5. Excavation of trenches around the perimeter ofthe preload area to a depth of about 50 cm belowthe groundwater level and filled with an impervi-ous Bentonite Polyacrolyte slurry for subsequentsealing of the impermeable membrane along theperimeter.

Step 6. The transverse connectors are linked to the edgeof the peripheral trench. They are then connectedto a prefabricated module designed to withstandfuture pressure due to the vacuum.

Step 7. Installation of the impermeable membrane onthe ground surface and sealing it along the periph-eral trenches. The membrane is delivered to thesite folded and rolled in elements of approxi-mately 1000m2. The membrane elements arewelded together and laid in the peripheral trenchwhere they are sealed with the Bentonite Polyac-rolyte slurry. The trenches are backfilled andfilled with water to improve the tight sealing be-tween the membrane and the bentonite slurry.

Step 8. Vacuum pumps are connected to the prefabri-cated discharge module extending from the

trenches. The vacuum station consists of specifi-cally designed high-efficiency vacuum pumps act-ing solely on the gas phase in conjunction withconventional vacuum pumps allowing liquid andgas suction.

The vacuum consolidation technique is often combinedwith surcharge preloading either by placing an additionalsurcharge by backfilling or using water placed on the topof the impervious membrane. The major practical advan-tage of the vacuum consolidation is that it generates inthe granular layer an apparent cohesion due to the in-crease of the effective stress and the granular layer pro-


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vides a useful working platform to accelerate the sur-charge backfilling process. Experience indicates thatwithin days after vacuum pump is turned on, construc-tion vehicles can maneuver on the top of the membrane.

Yeocheon industrial complex extension project in Koreabegan in May 1996 and was completed in February 2002.

Total area of this project was 8,485,000 m2. It is locatedon the South Sea about 300 km south of Seoul. The firststage of this project was reclaiming the land from the seawith the construction of earth dikes and hydraulically fill-ing the soil by dredging ship with a capacity of 12,000

HP. The filled soil was silty clay in a slurry state. Prior tothe soil improvement work, it was necessary to obtain thetrafficability of subsurface soil layer for soil improve-ment equipment. Schematic diagram of horizontal vac-uum consolidation technique was adopted by using asmall barge-ship as shown in Fig.17 (Cheon 2000). Thedepth of soil improved in this area is about 5 m with thenatural water content of 80%.

Fig.17 Schematic diagram of horizontal vacuumconsolidation method

9. PROGRESSIVE TRENCHING METHOD

Progressive Trenching Method(PTM) is executed priorto any deep soil improvement work for strengthening the

surface of dredged clay fill layer by making a system oftrench network. The dredged-extremely soft clay filllayer at the site had excessive water soon after pumpingactivity. The success of this improvement method de-pends on the effective removal of excessive water fromthe layer. Heat energy of sun was one of main sources forsurface water removal from the dredged-clay fill layer bythe action of evaporation and eventually made the hardcrust at the surface.

Fig.18 Schematic diagram of working mechanism ofPTM

Once hard crust was made, the effectiveness of evapora-tion process was minimized by the presence of crust. Toexpedite the evaporation activity, a system of trench net-work was formed and later deepened for the drainage ofexcessive water from the vicinity of surface. This trenchturned out to be also useful for draining of rain runoff(Fig.18).

The adopting of PTM in soil improvement work is togain the shear strength of ground surface with a certainthickness to support the soil improvement equipment.

Youlcheon industrial complex located on the South Sea ,300 km away from Seoul, was also constructed on landreclaimed from the sea. The dike is devided in severalblocks and it is filled by means of the hydraulically fill-ing technique with utilizing the dredging ship. It has acapacity of 12,000 HP cutter suction pump dredger withdredging capacity of 1.2 mil. m3per month.

When the self-weight consolidation of soft clay pro-gressed for about 3-4 months after completion of soil par-ticle sedimentation, PTM was applied over the softground which had a water content of about 150 %. Thismethod is also one of the soft soil improvement tech-niques in the upper soil layer to obtain the trafficability ofdeep soil improvement equipment. Perimeter trench was

constructed along the earth dikes and then shallow inte-rior trench was constructed (Geotechnical Eng. 2000).

The fundamental process of PTM is shown in Figure 16.Normally, the contact pressure of soil improvementequipment is about 1 t/m2. Therefore, the required shearstrength of clay for obtaining the trafficability could be0.60 t/m2 with the corresponding water content of 70%.

The natural water content on the surface ground withelapsed time is shown in Figure 17. The required naturalwater content of 70% was achieved 1 year after filling thedredged soft clay at the site.

PTM construction procedures are categoried into threemajor parts: (1) construction of containment dyke and di-

vision of the area inside dyke into several blocks, (2)pumping dredged-clay material into blocks in two phases,and (3) removal of extra water through the weir and leav-ing the fill to rest for 3.5 months and shallow-trenchingwith Amfirol and deepening with disc wheel and ditchcutter.

When the pumping work was completed, trenching workwas started at the end of 3.5 month rest period. The depthof trenching was to deepen to 50 cm from the initial 20cm as the strength of surface layer increased with time.Different equipment combinations were used for thedeepening of trench. The main equipment Amfirol, whichcould travel on land and water content of soil less than150% was used at the site to form up the trench in thebeginning. Once the trench was formed in shape to about20 cm deep, then disc wheel was employed to deepen thetrench to about 30 cm deep (Fig.19 a) and ditch cutter to50cm deep (Fig.19 b). The spacing of trench constructedat the site was determined due to mainly turning radius ofequipment at the end of each path travel. The surfacenatural water content of clay soil with the elapsed timefor PTM is given in Fig. 20.

Original Subsoil (Sand)

Original Subsoil (Marine Clay)

Evaporation

CrustInfiltrationSeepage

DrainageTrench

Crust

Self-ConsolidationDrainage Dredged-Clay Fill (Marine Clay)

PBDSealing Cap

Earth Dike

Ship

Head Pipe

VaccumPump

Soft Soil

Membrane Sheet

Drainage

Earth Dike

OutletWater


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(a) Amfirol +Disc Wheel

(b) Amfirol +Ditch Cutter

Fig.19 Soil improvement work by PTM

Fig.20 Surface water content of clay with elapsed timefor PTM

9 CONCLUSION REMARKS

Soil improvement techniques have been practiced sincein the early 1960s with the exception of soil stabilization.

The primitive soil reinforced works were also practicedin the pre-history period. In this paper, only recent devel-opment of deep soil improvement methods are mostly in-troduced. Chemical soil stabilization and geosyntheticsreinforcement technology, slope reinforcement tech-niques such as soil nailing and micro-piles are not dis-cussed. Several case histories of deep soil improvementmethods are also described with a specific construction

and design critera. It is found out that any marginal landcan be possibly used for the foundation soil of the struc-tures as long as the water table is not so high. In the earlyperiod of the soil improvement techniques, the soil androcks are the major sources for the ground reinforcement.While the recent soil improvement technologies are beingused more artifical synthetics materials in soil. The ad-vanced new soil improvement technologies are being de-veloped with cost effective and environmentally soundmethods for the future generation.

10 REFERENCES

Bachus, R. C., and Barksdale, R. D. (1984). The behav-ior of foundations supported by clay stabilized by stonecolumn, Eighth European Conference on soil mechanicsand foundation engineering, Helsinki, 199-204.

Cheon, B. S. (2000). Field pilot test of horizontal vac-uum consolidation. Geotechnical Engineering, KoreanGeotechnical Society, Vol. 16, No. 6, 57-59.

Geotechnical Engineering (2000). Progressive trenchingmethod. Korean Geotechnical Society, Vol. 16, No. 6,66-68.

Greenwood, D. A. (1970). Mechanical improvement ofSoil below ground surface, Ground engineering, June,11-22.

Goughnour, R. R. (1983), Settlement of verticallyloaded stone columns in soft ground, Eighth EuropeanConference on soil mechanics and foundation engineer-ing, Helsinki, 235-240.

Hughes, J.M.O. and Withers A. J . (1974), Reinforcingof soft cohesive soils with stone columns, Ground engi-neering, Vol.7, No.3, May, 42-49.

Munfakh, G. A. (1984). Soil reinforcement by stone

columns varied case applications, International confer-ence on in-situ soil and rock reinforcement, Paris, 157-162.

Mattes, N. S. and Poulos, H. G. (1969). Settlement of asingle compressible pile, Journal of soil mechanics andfoundations division, ASCE, SMI, January.

Mitchell, J . K. (1981), State-of-the-art Report on soilimprovement, Tenth International Conference on soilmechanics and foundation engineering, Stockholm.


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Priebe, H. (1976), Abschatzung des Setzungsverhaltenseins durch Stopfuerdichtung verbessertan Baugrundes,Die Bautechnick, H. S.

Shin, B. W., Shin, E. C., K im, S. W., Yeo, B. C., andDass, R. (1993). Case history of soil improvement for alarge-scale land reclamation. Proceedings, 3rd IntalConf. On Case Histories in Geotechnical Engineering, St.Louis, MO., Vol. II, 955-960.

Shin, E. C., Shin, B. W., and Das, B. M. (1992). Siteimprovement for a steel mill complex, Specialty confer-ence on grouting, Soil improvement, and geosynthetics.ASCE, GSP No. 30, Vol.2, 816-828.

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