Lime Columns

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Deep Soil Stabilization

Design and Construction of Lime and Lime/Cement

Columns

Bengt B. BromsRoyal Institute of Technology

Stockholm, Sweden

0. SUMMARY

0.1. Applications of Lime and Lime/Cement Columns

Soft normally consolidated clays and organic soils with a low shear strength areusually stabilized in Sweden and Finland by lime or lime/cement columns manufactured

by mixing the soil in-situ with lime, cement, fly ash, gypsum or granulated blast furnaceslag. This soil stabilization method is mainly used to increase the stability and to reducethe settlements of embankments constructed on soft clay and to stabilize trenches requiredfor sewer lines, heating ducts or water mains.

The diameter and the length of the columns have increased gradually since 1967when the method was first introduced and the time required for the manufacture of thecolumns has been reduced significantly as well as the costs due to the development ofefficient machines for the installation of the columns. Methods have been developed tocheck in-situ the shear strength and the stiffness of the columns.

Lime and lime/cement columns have also been used to reduce traffic vibrations, toimprove the stability of steep slopes and deep excavations. Gypsum, fly ash, granulated

blast furnace slag and other waste products have been added to increase the shear strength

and to reduce the costs. It has been possible to improve organic soils, primarily peat withcement in combination with granulated blast furnace slag.

0.2. Strength and Deformation Properties.

Soft or very soft inorganic clay or silty clay with a water content less than about100% to 120% can usually be stabilized with quicklime. An increase of 10 to 20 times theinitial shear strength can normally be expected. The maximum lime content is 10% to12% with respect to the dry weight of the soil. The shear strength of the stabilized soil isoften reduced if the lime content is increased further. The increase of the shear strengthwith quicklime is partly caused by a reduction of the water content due to slaking andevaporation and partly by an increase of the plastic limit. The long-term increase withlime and cement is mainly caused by pozzolanic reactions in the soil.

Lime/cement and cement are required to increase the shear strength of organicsoils, peat, gyttja and dy. The shear strength increases in general with increasing cement


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content and with decreasing water/cement ratio. The shear strength is usually higher withlime/cement and cement than with lime when the silt and the sand contents are high. Flyash and granulated blast furnace slag have been used together with cement to reduce thecosts and to stabilize organic soils, mainly peat.

Lime columns usually function as vertical drains in the soil due to the high

permeability of the columns. The function of lime/cement and cement columns as drainsis uncertain due to the low permeability of the stabilized soil.

0.3. Design of Lime and Lime/Cement Columns

The undrained shear strength of lime and lime/cement is calculated from thecohesive strength cu,col . The angle of internal friction φu,col is equal to 25 a 30 degrees at anormal pressure less than 150 to 250 kPa. The friction angle is assumed to zero ( φu,col

= 0)when the normal pressure exceeds 150 to 250 kPa. An effective angle of internal frictionφćol, which is 30 to 35 degrees, can be used to estimate the stability of slopes,embankments and excavations. The pore water pressure in the columns, which can be

high, corresponds initially to the pore water pressure in the unstabilised soil around thecolumns.The residual friction angle for the columns and for the unstabilized soil, φćol, res and

φu,col, res, can be assumed to φćol and φu,col, respectively. The residual cohesion cćol,res andcu,col,res is often neglected, when the long and short-term residual shear strengths are calculated.

The stability of embankments, slopes and excavations could be less than theaverage shear strength, when the shear strength is high and the columns behave as shortdowels. Single columns fail when the moment capacity of the columns is exceeded andone or two plastic hinges develop in the columns at the location of the maximum bendingmoments. The maximum undrained shear strength of lime and lime/cement columns,which can be utilized in design, is usually limited to 100 kPa. A design shear strength of150 kPa can only be used when the soil conditions are favorable.

It is important that an axial load, which is at least equal to the creep strength, can betransferred to and from the columns. The shaft resistance could be low and the length ofthe transfer zone can be large if the surface dry crust is missing or is poorly developed.The load in the columns could also be transferred to the soil below the bottom of thecolumns by end bearing and by shaft resistance.

The settlements of lime and lime/cement columns have been smaller than calculatedmainly due to the limitation of the maximum shear strength to 100 or 150 kPa, which isused in design. The compression modulus of the columns, Mcol is often assumedconservatively to 100 cu,col for clay, silty clay and clayey silt and to 50 c u,col for organic

soils where cu,col is the undrained shear strength of the stabilized soil.

0.4. Construction of Lime and Lime/Cement Columns

The lateral displacement of railroad and road embankments can be reduced withe.g. geofabric or geonet placed above the columns. The fabric reinforcement should bedesigned to resist the total lateral earth pressure in the embankment. Also the transfer ofload from the embankment to the columns is then improved.

It is preferable to locate the columns in the active zone below the embankmentsince the shear resistance of single columns in the active zone is at least two to threetimes larger than the shear resistance of the columns located in the shear or in the passivezones outside the embankment.

The extent of the checking should be more comprehensive when lime andlime/cement columns are used to stabilize embankments, slopes and deep excavations incomparison with columns, which are used to reduce the settlements.


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1. COTET OF LECTURE

The purpose of this lecture is to review

• Bearing capacity and compressibility of lime, lime/cement and cementcolumns,

• Methods to evaluate the stability and the settlements of embankments, buildings and other structures, where the soft soil has been reinforced by lime andlime/cement columns as well as the stability of excavations, trenches and slopes and

•

Field and laboratory methods to check the shear strength and thecompressibility of single columns, column rows, grids and column blocks.

2. APPLICATIOS OF LIME AD LIME/CEMET

COLUMS

2.1. General 2.1.1. Applications in Sweden and Finland.

Lime/cement (50% lime and 50% cement) are mainly used in Sweden and

Finland to increase the stability and to reduce the settlements of road and railroadembankments constructed on soft soil. Not less than 85% of all lime and lime/cementcolumns manufactured in Sweden the last few years are for this purpose as illustrated inFig.1. Only the dry method is used in Sweden and Finland to stabilize soft inorganicclays, silty clays or soft organic soils.

Lime and lime/cement columns have been found to be competitive in Swedenand Finland compared with other soil stabilization methods such as embankment piles,excavation and replacement and preloading. An additional advantage is that the stabilizedsoil can often be used as fill. The costs can frequently be reduced by combining lime andlime/cement columns with other soil stabilization methods such as lightweight fills,

preloading with and without vertical drains and berms.

2.2. Stabilization of Road and Railroad Embankments, Dikes, Airfields,

Fills and Caissons 2.2.1. Road and Railroad Embankments.

The main applications in Sweden and Finland of lime and lime/cementcolumns have been stabilisation of road and railroad embankments with a height of 2 to 4m. Also up to 9 m high embankments have been stabilised successfully (Edstam, 1996).Initially the reduction of the settlements of road and railroad embankments was the mainfunction of the columns.

Lime/cement columns have been used successfully to stabilize the

embankments for “Mälarbanan”, a large railroad project in the central part of Sweden(Axelsson and Larsson, 1994). Lime/cement columns have also been used in Gothenburg


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as discussed by Askmar and Henningsson (1998) to stabilize the embankments for a largeroad interchange (“Åbymotet”) where the soil consists of gyttja (organic clay) with an

Fig. 1. Applications of Lime/Cement Columns in Sweden 1991-1992(after Åhnberg et al. 1996a)

undrained shear strength of 8 to 13 kPa down to a depth of 7 m. The water content washigh, 100% to 200%.

Lime and lime/cement columns have to a large extent replaced other soilimprovement and soil stabilisation methods in Sweden and Finland including piles.

Lime and lime/cement columns can be combined with other soil stabilisationmethods to increase the stability and to reduce the settlements. Lime columns have also

been combined with a lightweight fill (Bjerin et al, 1984) and with berms (Carlsten and

Ouacha, 1993).

Fig. 2. Stabilization of Embankments using Lime and Lime/Cement Columns

The stability is usually analyzed by a cylindrical failure or slip surface through

the columns as illustrated in Fig.2. An average shear strength is normally used in theanalysis assuming that the columns and the soft soil between the columns behave as acomposite material. However, the shear resistance could then be overestimated due to


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progressive failure. It is also important to check the stability by a slip surface below thecolumns where the shear strength of the partly remoulded soil often is low.

The bearing capacity of the columns is reduced when the columns aredisplaced laterally and the shear strength is high due to the low failure strain of thecolumns. Centrifuge tests by Miyake et al (1996) and by Kitazume et al (1996a) indicate

that the bearing capacity of the columns located within the shear and the passive zonesoutside an embankment is low compared with the columns located in the active zone

below the embankment. The strain at the peak shear strength could be as low as 0.5% to2% for lime, lime/cement and cement columns, when the shear strength exceeds about300 kPa. The failure strain has a tendency to decrease with increasing shear strength.

The lateral displacements of the columns can be reduced and the bearingcapacity be increased by placing one or several layers with high strength woven geofabricwithin or just below the embankment or in narrow trenches just above the columns asillustrated in Fig. 3 (Broms, 1993). Thereby the transfer of load from the embankment tothe columns is improved. The soft upper part of the columns as well as any loose soil

above the columns should be excavated and replaced by compacted granular fill.Otherwise the settlements could be excessive since the columns are terminated 0.5 to 1.0m below the ground surface to prevent blowouts during the manufacture of the columns.

Fig. 3 Embankment Stabilised by Geo-Anchor

(after Broms, 1993)

2.2.2. Dikes for Irrigation and Flood ProtectionLime and lime/cement columns can be used to increase the stability and to

reduce the settlements of dikes for flood protection and irrigation. Column grids orcolumn blocks are used below the dikes to increase the effectiveness of the columns.Stage construction could be required when the initial shear strength of the soft soil is low.

There is normally no need to increase the height of the embankments tocompensate the long-term settlements caused by consolidation since the settlementsusually are small with lime and lime/cement columns.


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2.2.4. Breakwaters and Quay Walls.Kawasaki et al (1981) have described a case where up to 40 m long cement

columns were used as foundation for an 18.9 m high quay wall. The undrained shearstrength of the stabilised soil was 200 kPa.

Breakwaters have been stabilised by cement columns as discussed by Porbaha

et al (1998b). A shear strength of 2 to7 MPa could be obtained at a cement content of 150kg/m3.

Cement columns have been used, for example, to increase the bearingcapacity of a 20 m thick silt layer at the Yantai Port in China (Min, 1996). Theunconfined compressive strength was 2.5 MPa after 90 days at a c/w-ratio of 1.3. Therequired cement content was 170 to 190 kg/m3. The in-situ shear strength, whichincreased with time, was 1.5 to 1.7 times the shear strength of laboratory samples. Theunconfined compressive strength after 90 days was 1.4 to 1.5 times the 28-day strength.

2.3. Low-Cost Housing, Housing Developments, Warehouses and Light Industrial

Buildings.2.3.1.Foundation of Buildings and Light Structures.Lime and lime/cement columns are mainly used in Sweden and Finland to

support relatively light structures, one to two story buildings, warehouses and lightindustrial buildings as illustrated in Fig. 4 (Broms and Boman, 1975b, 1979a, 1979b;Broms et al, 1981; Holm et al, 1981; Bredenberg, 1983; Bredenberg and Broms, 1983;Åhnberg and Holm, 1986). The main function of the columns is to reduce the total andthe differential settlements. The bearing capacity of the unstabilised soil for relativelylight structures is usually not a problem.

In Japan 1.0-m diameter cement columns have been used to support up to fivestory buildings (Hibino, 1996) where the columns are installed in a square or a

rectangular pattern. The unconfined compressive strength of the stabilized soil was 2 to 4MPa at a cement content of 200 to 300 kg/m3. The settlements have in general beensmall, 10 to 30 mm.

Cement columns have also been used in China in saturated loess to support upto 12 to 15 story buildings. The area ratio of the 0.5-m diameter columns was 0.22. The

bearing capacity of the columns was 520 to 650 kPa at a cement content of 20%(Yuewen, 1996).

The thickness of rafts supporting buildings can often be reduced with lime orlime/cement columns since the rafts will be supported at a large number of points by thecolumns. The span lengths and the bending moments will be small compared with a raft

supported by a few high-capacity bored or driven piles.An additional advantage with a lime, lime/cement and cement column is thatthe columns and the soil between the columns can be considered as a composite material.The number of the columns can then be reduced since the columns will carry only part ofthe weight of the building. The remaining part is carried by the unstabilised soil betweenthe columns.

The columns, which are located along the perimeter of the loaded area, willgovern the differential settlements. The differential settlements can be reduced byincreasing the length of these columns as discussed in the following.

2.3.2. Housing Developments.

Lime and lime/cement columns can be used to control the settlement of buildings, to stabilize the trenches required for water, sewer lines and other services, toimprove permanent and temporary roads, parking and storage areas, sidewalks and


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walkways. Lime/cement and lime columns can also provide a smooth transition from the buildings to the surrounding unimproved ground.

The excavated soil from trenches and other excavations can be used asfill due to the high shear strength and permeability of the soil when stabilised with limeor cement. The cost for the transport of the excavated soil and the environmental

problems associated with the dumping of soft soil are then decreased (Paus, 1998).

Fig. 4. Lime and Lime/Cement Columns for Buildings

2.4. Trenches and Excavations.2.4.1. Trenches.

Lime and lime/cement columns have been used in soft clay as shown in Fig. 5to increase the stability of the trenches required for sewer lines, heating ducts and watermains as discussed by Paus (1979a, 1979b), Boman et al (1979), Sahlberg (1979a;1979b), Ekström and Tränk (1979), Holm (1979b), Lahtinen and Vepsäläinen (1983) and

by Broms (1984a, 1985b). Bracing is required when the sides of the excavation arevertical or near vertical and the depth exceeds a few meters. Columns are often installed

below the bottom of the excavations to prevent failure by bottom heave. Sewer, water andheating pipes can be placed directly on the bottom of the stabilized excavation. Theconcrete slab, which is necessary when piles are used to support the pipes, is normally notrequired with lime and lime/cement columns because of the close spacing of the columnsand the increase of the bearing capacity of the soft soil with lime and cement. Failureshave occurred during or just after a heavy rainstorm, when the cracks behind the columns,are filled with water. The columns failed by overturning due to the high water pressure.


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Fig.5. Lime and Lime/Cement Columns for Stabilisation of Trenches

The stability can be improved by inclining the columns. The maximum depthis increased to about 1.8 m at an inclination of 1H:3V for 0.5 m diameter columns Themaximum depth is 2.2 m when the side slope is 1H:2V (Åhnberg and Holm, 1991).

Blom (1992) has described a case at Kållekärr on the Island Tjörn not farfrom Gothenburg in Sweden where cement columns were used to stabilize a deep trench.

The soil consisted from the ground surface of gyttja down to a depth of 5 m and of 25 mwith soft to very soft clay. Short overlapping columns with 5 m length were placed asarches with 4 m diameter next to the trench (Fig. 52). The length of the columns wasincreased to 15 m below the bottom of the trench to improve the stability with respect to

bottom heave.

2.4.2. Excavations Lime, cement and lime/cement columns have been used in Sweden to

stabilize deep excavations as illustrated in Fig. 6 and to increase the stability with respectto bottom heave. The columns located behind the sheet pile wall will reduce the lateralearth pressures as well as the settlements. The columns in front to the wall increase the

stability and the factor of safety with respect to bottom heave.Skauerud and Finborud (1984) used lime columns in Norway to stabilize a 4

to 6 m deep excavation supported by anchored sheet piles. A shear strength of 125 kPacould be obtained with 7% to 10% quicklime in spite of the low initial shear strength ofthe clay, 10 to 20 kPa, and the high sensitivity, above 100. The remoulded soil flowed uparound the columns as a fluid during the installation due to the high sensitivity of theclay. The shear strength of the stabilized soil was 100 to 200 kPa two weeks after theinstallation and over 350 kPa after four weeks compared with a required shear strength of90 kPa. It was observed that the shear strength of the columns was higher than the shearstrength of samples prepared in the laboratory.

Sahlberg (1979a, 1979b) has described a case in Sweden where a 3.0 to 4.5 mdeep excavation in soft clay was stabilized successfully by 0.5 m diameter lime columns.The shear strength of the untreated soil was low, 10 kPa. The water content was 70% to


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80% and thus higher than the liquid limit. The cost was reduced by 30% to 40%compared with the costs for sheet piles.

Fig. 6. Stabilization of an Anchored Sheet Pile Wall with Lime or Lime/CementColumns

A 4.7 m deep excavation has been stabilized successfully by lime columns atStenungssund, Sweden (Holm, 1979b). The initial shear strength of the soft clay was low,5 to 10 kPa and the sensitivity exceeded 100 below 4 m depth. The shear strength of thestabilized soil was 115 to 270 kPa with 8% lime.

Lime columns have also been used in Norrköping, Sweden to stabilize a 4.5 m deepexcavation for a tunnel. The shear strength of the soft clay was increased with lime from 8

to 11 kPa to 100 kPa. The factor of safety was improved from 0.66 to 1.6 by placingcolumn rows perpendicular to the axis of the tunnel. A slip occurred next to the stabilized


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excavation when the depth of the excavation was 3.5 m. The initial stability of theexcavation was low.

Adestam (1996) has described a case, where lime/cement columns (50/50) were usedsuccessfully to stabilize a 4.5 to 5.0 m deep excavation in soft clay where the inclinationof the slope was 1.0V/1.5H. The undrained shear strength was low, about 10 kPa. The

ground water table was located 1.0 m below the ground surface. The factor of safety ofthe slope could be increased to 1.6 when overlapping columns were placed in rows

perpendicular to the excavation. A local slip occurred outside the stabilized area duringthe excavation, which showed that the initial stability was low. Lime/cement columnshave also been used to stabilize a deep excavation at Arlanda. The slope of the stabilizedexcavation was steep, 1V:1.5H (Redlund, 1995).

Cement columns with 1.0 m diameter were used to stabilize an up to 20.9 m deepexcavation at the Tokyo International Airport (Shiomi et al, 1996; Miyahara et al, 1991,Tanaka, 1993). The overlap of the columns was 200 mm.

The columns can be reinforced to increase the moment resistance and thus the

stability of column walls with respect to overturning (Kitazume et al, 1996a, 1996b).Bamboo has been used in Shanghai, PRC as reinforcement to increase the momentcapacity (Chen et al, 1996). There up to 8-m deep excavations in soft clay have beenstabilized by double column rows. The reinforcement should be placed in the columns

just after the installation, when the shear strength of the stabilized soil still is low. Thecolumns can also be connected at the ground surface as proposed by Dong et al (1996)

2.5. Slope Stabilisation.2.5.1. Stabilisation of Slopes Using Lime, Lime/Cement and Cement Columns.

Lime columns have also been used successfully to restore areas which have been

affected by landslides e.g. at Tuve in Sweden (Hansbo and Torstensson, 1978) and atÖdesby (Engström et al, 1984). There the lime columns were combined with berms. Theexcess pore water pressures were observed to dissipate rapidly after the installation of thecolumns since lime columns functioned as vertical drains in the remoulded soil.

Lime/cement columns have also been used successfully to stabilize the riverbanks ofthe Göta River at Bohus located in the southwestern part of Sweden. There the factor ofsafety was low, 1.1 to 1.3 (Johansson and Norup, 1996). High excess pore-water

pressures were observed during the installation of the columns as well as relatively largelateral displacements, 0.15 to 0.25 m. Some heave also occurred. The spacing of thecolumns was increased in order to reduce the excess pore-water pressures and the lateral

displacements, which increased during the slaking of the lime. The lateral displacementsreduced the shear strength the unstabilized soil as well as the bearing capacity of previously installed columns.

2.5.2. Stability of Slopes. Several slope failures have occurred. Engström et al (1984) have e.g. described a

failure of a slope next to a river, where lime columns were used to increase the stability.The installation of the columns was interrupted, when about 85% of the columns had

been installed. A berm was constructed in addition to the lime columns to increase furtherthe stability of the slope since cracks had been observed outside the stabilized area.

Reinforced cement columns with 0.4 m diameter have been used as soil nails to

stabilize steep slopes in Japan as discussed by Tateyama et al (1996). The columnsfunctioned as short dowels along potential slip surfaces in the soil. Also steep slopes in


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Austria have been stabilized successfully by lime columns (Brandl, 1973). The columndiameter was small, 0.08 to 0.5 m.

2.6. Pipe Jacking.2.6.1.Excavations for Pipe Jacking.

Driven sheet piles have been used to increase the stability of the excavations requiredfor pipe jacking. However, accidents have occurred, when the holes, which were requiredfor the pipe jacking, were cut through the sheet pile wall. The soft soil around theexcavation flowed into the excavation through the hole due to the low shear strength ofthe clay. This type of failure occurs when the shear strength of the soft soil is low and thetotal overburden pressure at the level of the hole is about 6 cu,soil where cu,soil is theundrained shear strength of the soft clay at the level of the hole (Broms and Bennermark,1968).

Lime and lime/cement columns can be used as shown in Fig. 7 to increase thestability. The pipes can be jacked trough the column wall, when the shear strength of the

columns is less than about 300 to 500 kPa. Columns could also be required below the bottom of the excavation to prevent failure by bottom heave.The circumferential force in the column wall will be high when the diameter of the

column wall is large. A double column wall might be required to improve the transfer ofthe circumferential load through the columns. It is important that the overlap of thecolumns is sufficient, at least 60 to 100 mm, depending on the diameter of the columns. Itis often an advantage to use large diameter columns when the diameter of the excavationis large. Wale beams are normally required around the perimeter of the circular columnwall to prevent failure of the column wall if the overlap is not sufficient. The wale beamsare usually designed to resist the total circumferential force.

Lime/cement columns have been used e.g. in Stockholm at Smista Park and at

Ärvinge by Sellgren (1991) to support an up to 8 m deep excavation in soft clay. Thediameter of the lime/cement columns was relatively large, 0.8 m, to improve the transferof the lateral earth pressure to the columns. Columns were also placed below the bottomof the excavation to increase the stability with respect to bottom heave. The average shearstrength of the soft clay was low, about 15 kPa.


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Fig. 7. Lime and Lime/Cement Columns for Pipe Jacking

2.7. Bridge Abutments, Retaining Walls, Quay Walls and Revetments

2.7.1. General. Lime, cement and lime/cement columns have been used to stabilize soft clay behind

sheet pile walls, bridge abutments, keys and breakwaters. The main function of thecolumns has been to prevent failure of the excavation by bottom heave. The lime and thelime/cement columns will also reduce the settlements behind sheet piles and bridgeabutments. The columns will also contribute to the stability.

2.7.2. Quay Walls.Cement column blocks have stabilized two wharves at the Tiensin Port in China. The

area ratio was high, 0.5. The water content of the organic silt and clay (muck and mucky

clay) exceeded the liquid limit of the soil. The unconfined compressive strength was 3MPa after 60 days with a standard deviation of 1 MPa. The cement content was 150 to170 kg/m3. The water/cement ratio was 1.1 to 1.5 (Choa, 1991; Hosomi et al, 1996).

2.7.3. Bridge Abutments. Ekström and Tränk (1979) and Bengtsson et al (1991) have described cases where

overlapping lime columns were used to stabilize the soft clay behind bridge abutments.The initial shear strength of the soft clay was low, about 10 kPa. At an average shearstrength of the stabilized soil of 97 kPa, the factor of safety was over 2.0. The shearstrength as determined by field vane tests exceeded 200 kPa. It was observed that the limereduced the average water content of the stabilized soil from about 60% to 45%.

The settlements of rigid frame bridges, which traditionally are supported by piles,have been reduced with lime columns. Hartlén and Carlsten (1992) have described a casewhere the spacing of the 7 m long lime columns was 0.8 m below the bridge and 0.9 m


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below the abutments. The stabilized soil was preloaded for three months by a load, whichcorresponded to the total weight of the bridge. The maximum settlement after the

preloading was less than 5 mm.Also Bengtsson et al (1991) have described a case where lime columns were used

to support a rigid frame bridge. The settlement of the bridge after one year was 30 to 80

mm. The costs for the lime columns in 1992 was about 50% of the costs for driven precast concrete piles.

Lime and lime/cement columns could be combined with preloading, which should becarried out as soon as possible after the installation of the columns to reduce the timerequired for the consolidation of the soft clay between the columns. Lime and possiblylime/cement columns will function as vertical drains. Thereby the time required for theconsolidation is reduced. There is some uncertainty, however, about the function oflime/cement columns as drains because of the reduction of the permeability with cement.

The preloading should correspond to at least the weight of the bridge or of theapproach fill to reduce the differential settlement between the bridge and the fill behind

the abutments. A surcharge load in addition to a preloading, which corresponds to theweight of the structure, is sometimes required to reduce the time for the consolidation andthe creep settlements.

2.7.4. Retaining Walls. The dry method using cement was used to stabilize the soft soil below a retaining

wall at the Haneda Airport in Tokyo (Miyahara et al, 1991). The area ratio was high sinceoverlapping columns were used for the column block below the wall. The lateraldisplacement of the stabilized retaining wall, which was analyzed by FEM, was small, afew mm.

The wet method using cement slurry was also tried at the Kansai International

Airport to reduce the settlements of a retaining wail constructed on soft clay. It was possible to reduce the settlements within the reclaimed area as indicated by Kamon(1997).

Cement columns have been used in Shanghai as retaining walls to support up to 8 mdeep excavations. The cement content was 200 to 250 kg/m3. The area ratio of the 4.2 mwide wall was 0.6 to 0.8. Bamboo with 60 mm diameter was used as reinforcement in thecolumns (Chen et al, 1996). The factor of safety with respect to a circular slip surface

below the column wall was low, 1.1. The factor of safety of the cement column retainingwall was 1.2 and 1.3 with respect to sliding and overturning, respectively.

Overlapping cement columns have been used to increase the stability as discussed by

Shintani et al (1991). The stability has been checked by measuring the settlements and thelateral displacements of the wall. Also the rotation and the distortion of the stabilized block were determined. Shintani et al (1991) concluded that the best indications of thestability of the stabilized soil are the axial and the lateral displacements.

2.8.Tunnels.

2.8.1.Stabilization of Tunnels.At the Trans-Tokyo Bay Highway Project, where two bored tunnels with 14.1 m

diameter and a length of 9.5 km have been constructed, the soft clay was stabilized bycement columns (Unami and Shima, 1996; Uchida et al, 1994, 1996; Tatsuoka et al,1996, 1997; Kamon, 1997). The maximum depth of the soft clay was 60 m. The water

content was high, 80% to 120%. Up to eight slurry shield machines were used at the sametime for the construction of the tunnels. Totally 3.77 Mm3 of the soft clay were improvedusing cement slurry with a water/cement ratio of 1.0. The cement content was 140 kg/m3


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and 70 kg/m3. A relatively low cement content was used to limit the maximum shearstrength of the stabilized soil to 500 kPa. The area ratio of the treated soil was high, over0.99. It was specified that the shear strength of the stabilized soil as determined byunconfined confined compression tests should be between 0.3 and 1.5 MPa down to adepth of 30 m. It was expected that the stabilized soil could be difficult to excavate when

the unconfined compressive strength exceeded 1.5 MPa. The lower limit, 0.3 MPa, wasgoverned by the required stability of the tunnels. The diameter of the overlappingcolumns was large, 1.1 m. The spacing of the columns was 0.83 m, which correspondedto an overlap of 0.37 m.

It has been proposed by Broms (1986) that overlapping lime, lime/cement andcement columns could possibly be used instead of jet grouting to increase the stability oftunnels constructed in soft clay. The columns could be installed as blocks or rows

perpendicular to the tunnel axis. Some of the columns should extend down to anunderlying firm layer in order to reduce the settlements. The shear resistance of the blockor of the column rows should be sufficient to resist the total overburden pressure. The

required spacing of the column rows is usually three to four column diameters.A drilling machine has been developed in Japan (Suguki and Maeda, 1996) where thesoft soil can be stabilized by horizontal cement columns. The diameter of the boreholes,which can be varied, is up to 0.8 m. The maximum length of the boreholes is 22 m.

2.9. Traffic Vibrations and Vibrations Caused by Blasting and Pile Driving.2.9.1.Reduction of Traffic Vibrations.

Lime and lime/cement columns have been used to reduce traffic vibrations in softclay (Beigler, 1982; Boman and Tholén, 1979; Massarsch and Sanaee,1993) as well ascement columns (Takemiya et al, 1996). Double column rows or grids with overlapping

columns are usually required for safety. The reduction of the vibration level has beenfound to increase with increasing number of columns. The reduction is generally largerfor point bearing than for floating columns.

It has been possible, for example, to reduce the maximum vibration velocity caused by traffic vibrations by about 50% in soft clay with lime columns for a two-story buildinglocated about 20 m from a heavily trafficked road. Massarsch and Sanaee (1993) havereported a reduction by 45% of the maximum vibration velocity 10 days after theinstallation of lime/cement columns. Boman and Tholén (1979) observed that thevibration level with lime columns was reduced by 30% to 50% at Skå-Edeby close toStockholm in Sweden. A large reduction of the maximum velocity has also been reported

by Takemiya et al (1998). The reduction was the largest at frequencies between 4 and 20Hz.Cement columns have also been tried close to Kungsbacka, Sweden to reduce the

vibrations of a railroad embankment constructed on a 5 to 13 m thick clay layer (Ekström,1992). A special mixing toll was developed to inject cement slurry close to the bottom ofthe mixing toll. Overlapping columns with 0.8 m diameter were used to increase theefficiency of the columns. The unconfined compressive strength with 70 kg cement permeter was 500 to 1000 kPa. The reduction of the vibration level was almost as good as bya pile supported concrete deck. The columns also contributed to the stability of theembankment and to a reduction of the settlements (Ekström, 1992).

2.10. Stabilisation of Excavated and Dredged Materials.

2.10.1 Stabilisation of Excavated Material and Spoil.


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The excavated material from trenches stabilized by lime columns has been usedsuccessfully as fill. The lime content of the columns has been sufficient to increasesubstantially the shear strength and the permeability and to reduce the water content. Thesavings of both time and money have been considerable since it was not necessary totransport and to dump the excavated soft clay.

2.10.2. Stabilization of Dredged MaterialLime and cement have been used successfully to stabilize dredged materials (Endo,

1976) so that the treated soil can be used as fill. Lime and cement reduce thecompressibility while the shear strength and the apparent preconsolidation pressure of thestabilized soil are increased. The unslaked lime increases also the permeability while the

permeability is reduced with cement.

2.11. Prevention of Liquefaction.

2.11.1. Reduction of Liquefaction PotentialCement columns have been used in Japan to reduce the liquefaction potential ofloose sandy soils (Matsuo et al, 1996b; Hatakeda and Fukazawa, 1996). The increase ofthe pore water pressure caused by earthquakes is thereby reduced. With lime columns thehigh excess pore water pressures, which develop in interbedded sand layers dissipaterapidly since the columns function as drains.

The columns are installed in a grid to increase the effectiveness of the columns (Takiand Yang, 1991; Kamon, 1997). Overlapping cement columns have been used also inUSA at the Jackson Dam in Wyoming to reduce the liquefaction potential (Jasperse andRyan, 1987).

3. PROPERTIES OF SOFT SOIL STABILISED WITHLIME, LIME/CEMET, CEMET AD OTHER

ADDITIVES

3.1. Physical Properties.

3.1.1. General. Several factors affect the shear strength, the compressibility and the permeability of

soft soils stabilized by lime, lime/cement or cement such as water content, grain size

distribution, type of clay mineral (kaolinite, illite or montmorillonite), cation exchangecapacity, amount of soluble silica and alumina, pH-value of the pore water, organiccontent, plastic and liquid limits and plasticity index. The properties of the stabilised soilare very similar to those of an overconsolidated clay.

3.1.2. Grain Size Distribution. The grain size distribution of the soil as determined by hydrometer tests is affected by

lime. The average particle size is increased due to flocculation and the clay content asdetermined by hydrometer tests is reduced (e.g. Assarson, 1972, Wilhelmsson andBrorsson, 1987). The stabilized soil has a grainy, crumbly or blocky structure.

3.1.3. Density.


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The unit weight of organic soils, quick clay and peat increases with increasing limeand cement content, when the initial water content of the soil is high while the density ofinorganic soils, clay and silt, often is reduced.

3.1.4. Liquid and Plastic Limits, Plasticity Index. The plastic limit is increased by lime and also to some extent by cement due to cation

exchange, while the plasticity index is reduced. When the plasticity index is less than10% the clay fraction may not be sufficient for the lime to be effective and the increase ofthe shear strength will be low.

At least 2% lime is required to affect significantly the plastic limit and the plasticityindex (Bell and Tyrer, 1989). Sherwood (1967) has observed e.g. that the plastic limit forLondon Clay increased from 24% to 43% with 4% lime. The plastic limit increasesgradually with time and with increasing lime content. Also the liquid limit is increased,

but the increase is generally less than the change of the plastic limit.

The lime content which is required to increase significantly the shear strengthdepends to a large extent on the cation exchange capacity (CEC) and thus on the clayminerals present in the soil. The exchange capacity is high for the clay mineralmontmorillonite, intermediate for illite and low for kaolinite.

3.1.5. Water Content. The water content, which is determined by drying the soil for 24 hours at 105oC, can

be overestimated, when the soil contains the clay mineral holloysite, which can loose partof the hydration water at a relatively low temperature (Newill, 1961).

The reduction of the water content during the slaking of the quicklime affects boththe shear strength and the compressibility. The reduction of the water content corresponds

to 32% of the dry weight of the quicklime. With 10% lime the average water content isthus reduced by 3.2 percent.

The amount of water required for hydration of cement is about 20% of the dryweight. The average water content is thus reduced by 2% when the cement content is10%. The average reduction of the water content with 10% lime/cement (50/50) is 2.6%,which is less than the reduction with lime alone.

The increase of the shear strength of the remoulded soil can be large when the plasticity index of the soil is low. The increase could be low when the water content islow and the free water available in the soil is not sufficient for hydration of the quicklime.The shear strength will in that case increase when water is added during the mixing.

The water content of the stabilized soil is also reduced by evaporation due to the hightemperature during the slaking and the consolidation of the soil. The reduction can belarge. Ekström and Tränk (1979) have, for example, reported that the average watercontent was reduced by 15% when 10% lime (CaO2) was mixed with soft clay. The initialwater content of the clay was 60%.

3.1.6. Volume Changes.Lime and lime/cement columns expand laterally during the slaking when water,

which is required for the slaking, is drawn from the unstabilized soil around the columns.The volume increase is about 85% of the dry weight of the quicklime. The resulting highlateral pressure around the columns will consolidate the surrounding soft soil. A few days

after the mixing the volume is reduced. The reduction, which can be relatively large,increases the permeability of the columns due to cracks and fissures in the stabilized soil.


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Åhnberg et al (1995a) have reported that the shrinkage of silty soils both with limeand with cement has been much larger than for other soil types. It has also been observedthat the permeability of lime and lime/cement columns has been much higher than the

permeability of laboratory samples probably because of cracking during the slaking of thequicklime in the columns. The columns could be separated from the unstabilized soil

around the perimeter close to the ground surface due to shrinkage after the initialexpansion by the slaking. It is well known that concrete shrinks during the curing. Opencracks in and around the columns will likely close after a few weeks or months. It has

been observed that the average permeability of the columns decreases with time.Volume changes have been observed at both drained and undrained triaxial tests.

After an initial increase of the volume the partly saturated samples has a tendency todilate. The dilatancy decreases with increasing confining pressure. The dilatancycontributes to the high friction angles φu,col and φ´col, which have been observed at bothundrained and drained triaxial and direct shear tests.

. Fig. 8. Stabilization with Unslaked Lime

3.2. Shear Strength

3.2.1. General.The short-term increase of the shear strength with lime (CaO2) is partly caused by

• Reduction of water content caused by hydration and evaporation during the

slaking• Ion exchange and the resulting increase of the plastic limit and the reduction of the

plasticity index• Pozzolanic reactions


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The increase of the short-term shear strength depends mainly on the reduction of thewater content during the slaking, on the increase of the plastic limit and the reduction ofthe plasticity index while the long-term shear strength with lime and cement is mainlygoverned by pozzolanic reactions in the soil as illustrated in Fig. 8.

3.2.2. Undrained Shear Strength. The undrained shear strength of the unstabilized soil can be determined by

unconfined compression tests, by fall-cone and vane tests, by undrained triaxial or directshear tests (UU-tests). Sometimes consolidated-undrained triaxial tests (CU-tests) areused, where the confining pressure during the consolidation of the stabilized soilcorresponds to the estimated effective lateral pressure in-situ. The undrained shearstrength as determined by field vane tests and by fall-cone tests should be reduced whenthe plasticity index is high before the shear strength of the unstabilized soil can be used toanalyze the stability of embankments, slopes and excavations (Bjerrum, 1972,

Helenelund, 1977, Aas et al, 1986).The undrained shear strength of the stabilized soil is in general determined byunconfined compression tests and/or by undrained triaxial tests (UU-tests). Unconfinedcompression tests have been found to be an effective and economical method todetermine the lime and the cement content, which is required to obtain the specified shearstrength

The undrained shear strength cu,col is usually assumed to be half of the unconfinedcompressive strength qu,col (qu,col = 2 cu,col). It is then assumed that φu,col is equal to zero. Itshould be noted that φu,col > 0 when the normal pressure on the failure plane is less than 150to 250 kPa.

The undrained shear strength, τfu,col increases with increasing normal pressure σf onthe failure plane through the columns

τfu,col = cu,col + σf tan φu,col (3.1)

where φu,col is the undrained angle of internal friction of the stabilized soil, which varieswith the soil type and with the water content and cu,col is the undrained cohesion A valueon φu,col = 30 degrees can be used up to a normal pressure of 150 to 250 kPa for lime andlime/cement columns. It should be noted that an estimated undrained shear strength cu,col = 0.5 qu,col is too high when the normal pressure is less than 150 to 250 kPa since

cu,col = q u,col a K (3.2)

The ratio 2 cu,col / qu,col is shown in Table 2.1 as a function of the friction angle φu,col.It should be noted that the ratio 2 cu,col / qu,col could be less than 1.0 and that the cohesioncu,col will be less than 0.5 qu,col which is the value, which is generally assumed in theanalysis of the unconfined compression test. At e.g. φu,col = 30 degrees and σf,col = 0 then2 cu,col / qu,col = 0.577. The shear strength is in this case only 57.7% of the assumed shearstrength,

When φu,col = 0, the critical normal pressure σf,crit is equal to

σf,crit / qu,col = (0.5 – cu,col / qu,col)/ tan φu,col = 0.5 (1 - a K ) / tan φu,col (3.3)


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It can be seen in Table 2.1 that the ratio σf,crit/qu,col decreases with increasing value onqu,col. The critical value on σf,crit as calculated in Table 2.1 varies from 69.6 kPa at φu,col =25 degrees to 150 kPa at φu,col = 45 degrees when qu,col = 150 kPa. This normal pressurecorresponds to the total overburden pressure at a depth of 4.35 m with respect to ahorizontal slip surface at φu,col = 25 degrees and 9.38 m at φu,col = 45 degrees.

In Table 2.1. is also shown the shear resistance τfu,col which increases from 69.9 kPaat φu,col = 25 degrees to 150 kPa at φu,col = 45 degrees when qu,col = 150 kPa. The ratio cu,col/ 0.5 qu,col, which corresponds to the relative increase of the shear strength caused by φu,col is 0.906 at φu,col = 25 degrees and 0.707 at φu,col = 45 degrees. The undrained shearstrength is thus 9.4% to 29.3% lower than the shear strength as determined by unconfinedcompression tests, qu,col, when the normal pressure on the failure plane is 150 kPa.

Table 2.1. Undrained shear strength as determined by unconfined compression testsat φ u,col > 0.

φu,col cu,col / 0.5 qu,col σf,crit / qu,col 150 tan φu,col τfu,col / 0.5 qu,col

25o 0.637 0.389 69.9 0.90630o 0.577 0.366 86.6 0.86635o 0.520 0.342 105.6 0.91940o 0.466 0.318 125.9 0.76645o 0.414 0.292 150.0 0.707

The increase of the undrained shear strength of remoulded clay from cuo to cu1 caused by a reduction of the water content by ∆ω can be estimated by the following equation

log cu1/cuo = 2 ∆ω / PIo (3.4)

where PIo is the plasticity index. When the reduction of the water content is e.g. 0.15 PIo then log cuo/cu1 = 0.3 and cu1 = 2cuo. The undrained shear strength of the remoulded soil isthus doubled by a reduction of the water content by 0.15 PIo (Fig. 9).

The shear strength of the remoulded soil is also affected by an increase of the plasticlimit from w po to w p1 and by a reduction of the plasticity index from PIo to PI1.

log cu2/cu1 = 2(ω p1 -ω po) (ωLo - ω1) / (PIo x PI1) (3.5)

where ωLo is the initial liquid limit and ω1 is the water content. At e.g. PIo = 40, PI1 = 20,ω1 = 65%, (ω p1 -ω po) = 20, and ωLo = 80 then cu2 = 5.62 cu1. The shear strength is thusexpected to increase from e.g. 10 kPa to 56.2 kPa at a reduction of the plasticity indexfrom 40 to 20.


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Fig. 9. Effect of a Reduction of the Water Content on the Undrained Shear Strengthof a Remoulded Clay

The undrained shear strength of the remoulded clay is also affected by a change of

the liquid limit from ωLo to ωL1 and by an increase of the plasticity index from PI1 to PI2(Fig. 10).

log cu3/cu2 = 2(PI2 - PI1)( ω1 - ω p1) / PI2 x PI1 (3.6)

where ω1 is the water content and ω p1 is the plastic limit. At e.g. PI2 = 25, PI1 = 20, ω1 =65% and ω p1 = 60 then cu3 = 1.26 cu2. The shear strength is expected to increase by 26% inthis case.


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Fig. 10. Effect of a Change of the Plastic Limit on The Undrained Shear Strengthof a Remoulded Clay

The undrained shear strength as determined by unconfined compression tests with10% lime, cement or lime/cement (25/75) is shown for a silty clay in Fig. 11a and for aquick clay in Fig. 11b. The increase of the shear strength for silty clay and quickclay isoften large the first month with lime/cement and cement. Thereafter the increase isgenerally small. The undrained shear strength for clays and silty clays with low tomedium sensitivity with lime/cement and cement, is usually higher after several monthsthan the shear strength with only lime.

The undrained shear strength with quicklime alone exceeded after 90 days the shearstrength with lime/cement and cement. After nine months the shear strength of thequickclay, which had been stabilised with lime, was more than twice the shear strength

with lime/cement or cement. It can be seen that the increase was initially slow with lime.The reason for this effect is not known.


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Fig.11. Shear Strength of Lime, Lime/Cement and Cement Columns(after Åhnberg et al, 1995a)

The design of lime, lime/cement and cement columns is usually based on theestimated shear strength 28 days after the installation of the columns. The shear strengthof lime columns is often underestimated since a large part of the long-term increase of theshear strength occurs after 28 days. It is therefore proposed for lime columns, that thedesign strength should be the estimated shear strength after 90 days

A relatively high friction angle φu,col has been reported for lime and lime/cement,which has been attributed to dilatation when the confining pressure is low (Serra et al,1983). A negative pore water pressure can therefore be expected when the load is appliedrapidly and the confining pressure is less than about 20 kPa (Åhnberg et al, 1995a). Ahigh undrained friction angle φu,col > 0 can also be caused by air present in the columns,since compressed air is used to force the lime and the cement into the columns during theinstallation. It is expected that the degree of saturation will increase and that φu,col mightdecrease with time. After a few months it is expected that the stabilized soil will be fullysaturated and that the shear strength of the unstabilized soil around the columns will

increase with time as well as the lateral earth pressure when the columns are loaded. Theundrained friction angle φu,col decreases with increasing confining pressure It is expected,however, that the friction angle φu,col = 0, when the soil is fully saturated.


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Åhnberg et al (1994, 1995a) have found that the undrained shear strength asdetermined by unconfined compression tests on cores from actual columns has been aboutthe same as the undrained shear strength of samples prepared in the laboratory when theundrained shear strength is 100 to 200 kPa. The laboratory samples often indicate a lowershear strength than the average shear strength of the columns, when the shear strength is

low.When the shear strength exceeds about 200 kPa, the shear strength of laboratory

samples is often higher than the in-situ shear strength. Kamon (1991, 1997) has reportedthat the in-situ shear strength of cement columns for on-shore projects was only one-halfto one-fifth of the shear strength of laboratory samples. The difference increased ingeneral with increasing shear strength of the stabilized soil.

A relatively high friction angle φu,col has been reported for soils stabilized by lime andlime/cement, when the confining pressure is low The stabilized soil has a tendency todilate at a low confining pressure as pointed out by e.g. Serra et al (1983). A negative

pore water pressure can therefore be expected when the load is applied rapidly and the

confining pressure is low. A negative pore water pressure has been reported by e.g.Balasubramaniam and Buensuceso (1989).The undrained friction angle φu,col decreases with increasing confining pressure when

the degree of saturation of the soil increases. Tatsuoka (1983) and Åhnberg et al (1995a)have e.g. reported that the increase of the shear strength is small when the confining

pressure is high. It is expected that the friction angle φu,col = 0, when the soil is fullysaturated and the total normal pressure on the failure plane exceeds 150 to 250 kPa.

Shear strengths of 140 to 280 kPa have been reported by Baker et al (1997) from in-situ field tests while laboratory tests indicated a shear strength of 220 to 420 kPa. Thelaboratory values were almost twice the in-situ shear strength. The difference increases ingeneral with increasing shear strength of the stabilized soil.

The shear resistance of the stabilized soil in the overlapping zone has been low.Yoshida (1996) has reported that the shear strength as determined by vertical direct sheartests (σf = 0) was 23.3% of the unconfined compressive strength. Similar observationshave been made in Sweden. This reduction of the shear strength had to be considered inthe design of column walls.

3.2.3. Undrained Shear Strength with Lime.The shear strength increases in general at the same lime content with increasing clay

fraction. It is thus expected that the lime content will govern the shear strength when theclay content is high. If the water content is low, less than about 20%, the water available

in the soil might not be sufficient for the slaking of the quicklime. In that case the shearstrength could even increase when water is added and the water content is increased. Themaximum undrained shear strength with unslaked lime is usually 200 to 300 kPa. Theshear strength with lime is often low for clays with a low salt content. The increase of theshear strength is also low when the silt content is high.

The shear strength is often low when the clay content is less than about 15% sincelime mainly affects the clay fraction. The required lime content to obtain a certainspecified shear strength increases in general with increasing water content. About 7% to8% quicklime is required at a water content of 40% to 80% and 12% to 15% when thewater content is 100% to 140% (Wilhelmsson and Brorsson, 1987)

The increase of the shear strength with 5% to 10% lime is typically 10 to 20 times theinitial shear strength for normally consolidated or slightly overconsolidated inorganicclays or silty clays with an initial shear strength of 5 to 15 kPa. Balasubramaniam et al(1991) have reported that a five-fold increase of the shear strength can be obtained with


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lime. Sabry et al (1996) found that the shear strength increased 100% to 200% after 35days at an area ratio of 0.16 to 0.29. The authors indicate that the behavior of thestabilized soil was brittle. Mitchell (1981) found that an unconfined compressive strengthof up to 70 kPa could be obtained with well-mixed and compacted soil with 3% to 8%lime with respect to the dry weight of the soil.

The undrained shear strength increases in general with increasing lime content up toabout 10% to 12% with respect to the dry weight (e.g. Broms, 1984a, 1991, 1992). Thereis usually no further increase when the lime content exceeds 10% to 12%. The undrainedshear strength may even decrease. Balasubramaniam and Buensuceso (1989) have, forexample, reported that the shear strength of Bangkok clay was reduced with 15% and25% lime after three months. Brookes et al (1997) found that the undrained shear strengthof Gault and London Clay in the U.K., which was compacted in the laboratory, decreasedwith increasing lime content when the lime content was high.

The undrained shear strength is also affected by the clay fraction and thus by the clayminerals present in the soil. A high lime content is often required to obtain a certain shear

strength, when the specific surface area of the clay is high as it is for the clay mineralmontmorillonite.The long-term increase of the undrained shear strength, which depends on the

pozzolanic reactions in the soil, varies greatly. Eriksson and Carlsten (1995) havereported that the highest undrained shear strength is obtained with lime, when the claycontent of the clay is high and the silt content is low.

Consolidation of the stabilized soil in the columns reduces the water content whilethe shear strength is increased. The volume increase caused by the slaking of thequicklime, when water is drawn from the soft soil around the columns or from pervioussilt and sand layers, increases the lateral pressure around the columns. The resultingvolume increase of the columns reduces the water content of the soil around the columns

and increases the shear strength. The time required for the consolidation is short since thelime increases the permeability of the stabilized soil.

3.2.4. Undrained Shear Strength with Lime/Cement.Usually the undrained shear strength is higher with cement and lime/cement than

with lime especially when the clay content and the plasticity index are high. The shearstrength increases in general with increasing lime/cement content and with increasingcontent of fines (silt and clay). The increase of the shear strength is often poor for organicsoils when the water content exceeds 200% (Babasaki et al, 1996). Gotoh (1996) hasfound that the pH-value of the soil also affects the shear strength. The ignition loss and

the initial water content are also important. The maximum shear strength is about 500 kPawith 10% to 20% lime/cement (50/50).The silt and the sand fractions are mainly affected by cement while lime affects

mainly the clay fraction. For many organic soils only cement is effective. Kivelö (1994b,1995a) has reported shear strength of 200 to 300 kPa with 100 kg/m 3 (50% lime and 50%cement). Åhnberg et al (1994) found that a much higher shear strength could be obtainedwith lime/cement for clayey silt than for clay and gyttja.

The additional increase of the shear strength is often small when the lime/cementcontent exceeds 150 to 200 kg/m3. The shear strength is expected for clayey silt to besomewhat higher with cement after four months than with lime/cement while for clayeygyttja the shear strength is usually higher with lime/cement than with cement (Åhnberg et

al, 1995a, 1995b).The increase of the undrained shear strength of the remoulded clay by the reduction

of the water content during the slaking and the mixing can be estimated from Eq. (3.4) as


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well as the increase caused by an increase of the plastic and liquid limits by Eqs. (3.5) and(3.6).

Green and Smigan (1995) have reported values on φu, col of 30 to 40 degreesdetermined by direct shear tests for soils stabilized with lime/cement (50/50 and 80/20)while values of 41 and 33 degrees were obtained by triaxial tests. Kivelö (1996) obtained a

value of 45 degrees by triaxial tests and by direct shear tests (UU-tests).Axelsson and Larsson (1994) obtained an average angle of internal friction (φu,col),

which at direct shear tests was 42 degrees for lime/cement columns from two differentlocations and for different orientation of the samples. A friction angle (φυ,col) of 43 to 45degrees was determined by Björkman and Ryding (1996) and by Kivelö (1996) at directshear tests. The friction angle determined by undrained and drained triaxial tests (UU-and CD-tests) was 41 and 40 degrees, respectively.

The cohesion cu,col has been found by Kivelö (1996) to be lower at direct shear teststhan at triaxial tests, 160 kPa and 210 kPa, respectively. The shear strength as determined

by triaxial tests has in general been higher than the shear strength by direct shear tests.

Yoshida (1996) has reported that the shear strength from triaxial tests with soils stabilizedwith cement was 1.57 times the shear strength from direct shear tests.The variation of the reported undrained angle of internal friction φu,col has been large

since the quality of the columns and of the investigated samples has often been low. Alsothe variations of the shear strength and of the lime/cement content over the cross-sectionand along the columns have been large as well as the variation of the soil conditions.

Kivelö (1994a) found that the water content of the soft soil around the lime/cementcolumns was reduced. The water content was 32% at a distance of 10 to 30 mm from thesurface of the investigated column, 37% at 50 to 80 mm, 39% at 80 to 120 mm and 45%at a distance exceeding 0.5 m. The reduction of the water content was thus large, up to13% (45% - 32%) next to the columns. This reduction of the water content increasessubstantially the shear strength of the unstabilized soil next to the columns.

3.2.5. Undrained Shear Strength with Cement.Cement had to be mixed thoroughly with the soil compared with lime to obtain a

high shear strength. The increase of the shear strength depends mainly on the pozzolanicreactions in the soil, while for quicklime the shear strength depends to a large extent onthe reduction of the water content, on flocculation and a reduction of the plasticity indexand on an increase of the plastic limit due to cation exchange.

The undrained shear strength increases in general with increasing cement content andwith decreasing liquid limit (Nagaraj et al, 1996). The maximum undrained shear strength

usually exceeds 500 kPa with cement. In granular soils the cement binds together theindividual soil particles while in cohesive soils the cement affects mainly the particleclusters.

Åhnberg et al (1994) have reported that the shear strength increases with increasingcement content when the cement content is between 6% and 16%. The soil type, theinitial water content and the water/cement ratio also affect the increase of the shearstrength. The effectiveness of the cement will generally decrease with increasing

plasticity index and with increasing clay content. The increase of the shear strength isoften low when the activity ratio is high, which is the case for the clay mineralmontmorillonite.

The increase of the shear strength with cement is often low when the water contentexceeds 200% (Babasaki et al, 1996). The increase has also been low for organic soilswhen the ignition loss exceeds 15% even at a cement content above 20%. There is also atendency for the shear strength to decrease with decreasing pH-value.


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The cement type affects also the shear strength. Åhnberg et al (1995a) haveinvestigated the effect of different cement types on the shear strength and on thecompression modulus. Fast setting cement has been found to give somewhat higher shearstrengths and a higher bearing capacity than standard Portland cement (Åhnberg et al,1995a). Also Bergado et al (1996) have reported that the cement type affects the shear

strength.The wet method is common in Japan while in Sweden and Finland only the dry

method is used. A higher shear strength can generally be obtained by the dry than by thewet method at the same cement content (Chida, 1981) due to the increase of the watercontent and of the water/cement ratio (w/c-ratio) at the wet method. The w/c-ratio of thecement slurry is usually 0.6 to 1.3 in Japan at the wet method (Okumura, 1996). A w/c-ratio of 1.0 is common.

Sandros and Holm (1996) have reported that the undrained compressive strength ofsilty clay has been 1.7 to 3.2 times higher by the dry method than by the wet method. Theunconfined compressive strength 60 days after the mixing was about 1.0 to 1.6 MPa at a

cement content of 200 to 350 kg/m

3

. The increase of the shear strength after 60 days wassmall, about 10% up to 180 days after the mixing.The pozzolanic reactions are initially faster with cement than with lime especially

when finely ground cement is used. The pozzolanic reactions and the resulting increase ofthe shear strength with lime will continue for many months and years. The increase of theground temperature is less with cement than with lime, which will slow down the

pozzolanic reactions and the shear strength increase with time.Due to the difficulty to mix dry cement with the soil when the cement content is high,

the variation of the shear strength and of the stiffness of the columns is often less by thewet method than by the dry method especially when the water content of the soil is lowand the shear strength is high. Also the variation of the shear strength along the columns

is usually less with cement and lime/cement than with lime.Asano et al (1996) have reported that the shear strength of samples mixed in the

laboratory has been up to 2 to 5 times higher than the shear strength of samples obtainedfrom actual columns The difference of the shear strength is attributed to the difference inthe mixing in the laboratory and in the field.

3.2.6. Undrained Shear Strength with Gypsum and Fly Ash.Stabilization of soft soil with gypsum and fly ash has been investigated by Nieminen

(1977) and by Viitanen (1977). Kujala (1983a) has reported a friction angle φu,col of 23degrees by triaxial tests for gypsum and lime and 40 degrees by direct shear tests. Kujala

and Nieminen (1983) found that the friction angle was about 10 degrees higher for soilsstabilized with lime and gypsum than for soils stabilized only with lime.Also fly ash has been used for soil stabilization (Wu et al, 1993). An increase by 40

times was obtained with 30% fly ash after 28 days mainly due to a reduction of the watercontent. The increase of the plastic limit and of the reduction of the plasticity index of thesoil contributed also to the shear strength. One uncertainty is the possible pollution of theground water by the heavy metals in the fly ash. This risk can be reduced, however, bymixing the fly ash with lime or cement.

3.2.7. Undrained Shear Strength with Blast Furnace Slag.Blast furnace slag does not have any hydraulic properties unless activated by lime

and cement. Often a high shear strength can be obtained with cement and blast furnaceslag also for soils with a high water and organic content such as peat.


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3.2.8. Undrained Shear Strength with Rice Husk Ash.Ali Jawaid (1997) has found that silty clay could be stabilized with 10% rice husk

ash and 6% lime. The required lime content is thus low. It might be possible to use ricehusk ash to stabilize also other soil types since the silica content of the ash is high. InIndia over about 10 Mt rice husk ash is produced annually. The available amount is thus

very large

3.3. Drained Shear Strength.

3.3.1. GeneralThe drained shear strength τfd of the stabilised soil, which governs the long-term

stability of e.g. a slope can be estimated by the following equation

τfd = cćol + σf ́ tan φćol (3.7)

where σf ́ is the normal effective pressure on the failure plane.

The drained shear strength parameters (φćol and cćol) can be determined bydrained triaxial or drained direct shear tests (CD-tests) or by consolidated-undrainedtriaxial tests (CU-tests) with pore pressure measurements. A backpressure of about 300 to400 kPa or higher is often required at the triaxial tests to saturate the samples.

When the confining pressure is high the shear strength as determined by triaxial UUand CD-tests is about the same. With cement the friction angle φćol is constant up to aneffective confining pressure of at least 500 kPa.

The long-term bearing capacity could be lower than the short-term bearing capacitydue to creep. The lowest shear strength and the lowest bearing capacity of the columns areexpected just below the stiff crust, when the clay is normally consolidated or slightlyoverconsolidated .

The effective friction angle φćol can be assumed to 30 degrees for lime columns, 35degrees for lime/cement columns and about 40 degrees for cement columns. It should benoted that the shear strength as determined by drained triaxial tests is often lower than theundrained shear strength determined by direct shear tests, when the normal pressure islow. When the confining pressure is high the shear strength as determined by triaxial UU-and CD-tests is expected to be about the same.

3.3.2. Drained Shear Strength with Lime.The effective friction angle φ’col is relatively high for lime columns. The friction angle

usually increases with increasing lime content. Rogers and Lee (1994a) have found that the

friction angle φćol increased a few degrees when the lime content was increased. The effectivefriction angle φ’col increased also with increasing lime content and with increasing time. Thefriction angle φćol is often lower for lime columns than for cement and lime/cementcolumns. The difference is usually small. The highest values have been reported forclayey silt with both lime and cement (Ekström, 1992). The lowest values on φćol have

been for organic soils.Balasubramaniam et al (1989) have reported values on φćol of 38 and 35.8 degrees at a

lime content of 5% to10%. With 15% lime the friction angle was 40.1 degrees. Also Brandl(1981) has reported very high values on φu,col as determined by triaxial tests (UU-tests), 32to 37 degrees. Göransson and Larsson (1994) determined φ’col to 31 to 36 degrees from

triaxial tests (CD-tests). A friction angle φu,col of 33 degrees as determined by direct sheartests has been reported by Reiment (1978).


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Brookes et al (1997) found that φćol was 30 to 42 degrees by consolidated-drained triaxialtests with Gault Clay from the U.K. at a lime content of 5% to 15%. The friction angle φćol forthe unstabilized Gault Clay was 23 to 25 degrees. For London Clay it was reported that thefriction angle φćol was 31 to 41 degrees with 5% to 15% lime. The highest friction angle, 41degrees, was observed with 5% lime. The friction angle for the unstabilized weathered London

Clay was 17 to 23 degrees.

3.3.3. Drained Shear Strength with Lime/Cement and Cement.The friction angle φćol as determined by drained triaxial tests has been of 34 to 36

degrees for organic clay, 34 to 39 degrees for clay and 40 to 44 degrees for clayey siltwith lime/cement. Åhnberg (1996) found from drained triaxial tests that φćol was 34 to 44degrees. The friction angle has been higher for clayey silt than for clayey gyttja.

Björkman and Ryding (1996) have reported a value of 40 degrees on φćol by triaxialtests with samples from actual lime/cement columns. Triaxial tests with 0.5 m diametersamples by Steensen-Bach et al (1996) and by Rogbeck (1995) indicated values on φćol of

37.5 to 39.3 degrees. Ekström (1992) has reported that φćol was 40 degrees for a silty clayand 25 to 29 degrees for a clay stabilized by lime/cement or cement. Bergwall andFalksund (1996) determined a value of 57 degrees on φćol. With lime/cement Ekström(1994b) observed values on φćol of 38 to 40 degrees at consolidated-undrained triaxialtests (CU-tests). Values of 40 to 45 degrees have been reported by Wada et al (1991) forcement columns.

The friction angle for lime/cement and cement columns has a tendency to increasewith time (Åhnberg et al, 1995a, 1995b). The friction angle φćol has been constant withcement up to an effective confining pressure of at least 500 kPa.

.

3.3.4. Drained Shear Strength with Gypsum.Kujala (1983b) has reported that φćol was 40 degrees at triaxial tests with gypsumand lime and 23 degrees with lime alone. The friction angle for gypsum and lime has beenhigher than the friction angle with only lime. Brandl (1981) determined values of 35 to 40degrees by direct shear tests on clays stabilized with lime and with lime and gypsumwhen the normal pressure was low. Huttunen et al (1996b) found for peat that φćol was36.8 to 41.3 degrees for cement and granulated blast furnace slag. Huttunen and Kujala etal (1996) have reported that φćol was 37.1 to 60.6 degrees for peat with cement andgypsum (Finnstabi) at a stabilizer content of 250 kg/m3. The water content was 174% to198%. The unconfined compressive strength was 130 to 234 kPa.

3.3.5. Effective Cohesion c´ col and c´ soilThe effective cohesion cćol, has been found to be about 25% to 30% of the undrained

shear strength with lime and about 35% to 45% with lime/cement and cement (Åhnberget al, 1994b, 1995a). The effective cohesion cćol was found to be higher for clayey gyttjathan for clayey silt and clay, which has been stabilised with lime/cement. Åhnberg (1996)has reported that the cohesion cćol varied between 50 kPa for clays stabilised by lime and1600 kPa for clayey silt stabilised by cement. The effective cohesion cćol was 46.2 to 83.6kPa with cement and gypsum (250 kg/m3) and 37.4 to 48.5 kPa with cement and granulated

blast furnace slag.The effective cohesion c´soil varied between 26 and 55 kPa for the Gault Clay and

between 42 and 77 kPa for the London Clay with 5% to 15% quicklime (Brookes et al,1997). The effective cohesion c´soil was 12 to 18 kPa for the weathered London Clay and


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10 to 14 kPa for the weathered Gault Clay. Reiment (1978) has reported that the cohesionintercept cu,col was 25 kPa.

3.3.6. Water/Cement Ratio. The shear strength of organic soils stabilized with cement is to a large extent

governed by the water/cement ratio as discussed e.g. by Åhnberg et al (1995a), Kukkoand Ruhomäki (1995), Rathmayer (1997), Asano et al (1996) and by Nagaraj et al (1996).Asano et al (1996) found that the increase of the shear strength depended mainly on thew/c-ratio and that fly ash and gypsum did not contributed much to the increase of theshear strength.

Kujala et al (1993) have reported that the shear strength decreased with increasingw/c-ratio. Test results reported by Åhnberg et al (1995a) suggest that the undrained shearstrength τfu increases with decreasing water/cement ratio (w/c) including the water fromthe stabilized soil. It is expected that the shear strength of mainly silty or sandy soils willincrease with decreasing w/c-ratio according to the following equation

τ fu = τfou / (w/c) (3.8)

where τfou is a reference shear strength of the stabilized soil as determined by unconfinedcompression tests after 28 days at a w/c-ratio of 1.0 including the initial water content ofthe soil. It is thus expected from Eq. (3.8), that the increase of the shear strength is about100% when the water/cement ratio is reduced by 50% e.g. from 8 to 4.

Edstam (1997) indicates, however, that there is no definite increase of the undrainedshear strength with decreasing water/cement ratio, when the water content is low possiblydue to poor mixing. The scatter of the results is large. Also the organic content and the

pH-value of the pore water have been found to affect the increase of the shear strength.The unconfined compressive strength is often 2 to 4 MPa at a water/cement ratio of

1.0 (Asano et al, 1996; Matsuo et al, 1996a). When the water/cement ratio is 1.2 to 1.5the expected unconfined compressive strength is about 1 MPa. Holm (1994) has reportedthat the shear strength increases approximately linearly with increasing cement contentand that an unconfined compressive strength of 2 MPa can be expected at a cementcontent of 200 kg/m3.

3.3.6. Residual Shear Strength.The undrained and the drained shear strengths are reduced when the peak strength is

exceeded. The reduction has been large when the peak shear strength is large. This

reduction had to be considered when the peak shear strength of the unstabilised soil isused in design.

The reduction of the undrained shear strength as reported by Kivelö (1996) at directshear tests with a clay stabilized by lime/cement are shown in Fig. 12. The shearresistance decreased rapidly when the peak strength was exceeded. Mainly the cohesion isaffected. The residual friction angle φćol,res, 45 degrees, is not affected much when theeffective cohesion is neglected. It can thus be assumed that φćol,res is equal to φćol.


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Fig. 12. Peak and Residual Undrained Shear Strength as Determined by DirectShear Tests (after Kivelö, 1996)

The residual shear strength is typically 70% to 80% of the peak shears strength(Hansson, 1991). A residual shear strength, which was 74% to 100% of the peak shearstrength, has been reported by Göransson and Larsson (1994). Kivelö (1996) andBjörkman and Ryding (1996) found that the residual shear strength was 50% to 70% ofthe peak shear strengths as determined by undrained direct shear tests with samples fromlime/cement columns (Fig. 13) and 80% to 90% of the peak shear strengths at triaxialtests (UU-tests). The reduction of the shear strength has been larger at direct shear teststhan at triaxial tests. The difference decreases with increasing normal pressure at thedirect shear tests and with increasing confining pressure at the triaxial tests. The reductionis typically 50% at drained triaxial tests when the confining pressure is 160 kPa and about70% at a confining pressure of 80 kPa for soils stabilised with lime/cement (Åhnberg etal, 1996).

Axelsson and Larsson (1994) found for lime/cement columns, that the residual shearstrength as determined by undrained direct shear tests (UU-tests) varied between 60% and96% of the peak shear strength and that the behavior of the soil was similar as that of anoverconsolidated clay. The reduction increased rapidly with increasing shear strength.

When the undrained shear strength exceeds 500 kPa the reduction can be as high as 80%to 90% when the confining pressure is low as reported by Åhnberg et al (1996). Åhnberget al (1994) have indicated that the residual strength for clayey silt is typically 50% to


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60% of the peak undrained shear strength when the shear strength is low. The reductionof the shear strength is generally relatively small, 10% to 30%, when the shear strength isless than 100 to 150 kPa.

Fig. 13. Direct Shear Tests with Lime/Cement Columns and Unstabilised Clay(after Kivelö, 1996)

Unami and Shima (1996) observed at undrained triaxial tests that the residual shearstrength was about 85% of the peak shear strength, 600 kPa. Similar results have beenreported by Bergwall and Falksund (1996). Tatsuoka and Kobayashi (1983) found that theresidual undrained shear strength is typically 75% to 100% of the peak shear strength. Aresidual shear strength of 600 kPa has e.g. been reported by Holm (1994) for a soilstabilized with lime/cement with a peak shear strength of 5 MPa. The reduction wasalmost 90%.

The reduction of the effective cohesion cćol has a large effect on the shear strengthwhen the confining pressure is low. Åhnberg et al (1995) have reported values on c´ col of10 to 30 kPa for soils, which had been stabilized with lime, lime/cement and cement. Thereduction of the angle of internal friction φu,col and φćol is relatively small, less than 5

degrees.Steensen-Bach et al (1996) have proposed that φćol could be assumed to be the same

as the residual angle of internal friction φćol, res when the residual cohesion cćol, res is


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equal to zero, due to the reduction of the bonding at large deformations of the stabilizedsoil.

3.3.7. Failure Strain and Ductility.The strain εf at the peak shear strength can be 10% or higher for lime columns when

the unconfined compressive strength is low, less than about 100 kPa. Sabry et al (1996)have pointed out that the stabilized soil is brittle when the shear strength is high. Thefailure strain has a tendency to decrease with increasing shear strength and withdecreasing confining pressure. The failure strain of lime/cement and cement columns can

be as low as 0.5% to 2% when the unconfined compressive strength exceeds about 200kPa as shown in Fig. 14 (Åhnberg, et al, 1995). Ekström (1994b) has reported a failurestrain of about 2% when the shear strength exceeded 150 kPa.

A failure strain of only 0.1% to 1% has been reported by Tatsuoka et al (1996),Tatsuoka and Kobayashi (1983) and by Terashi and Tanaka (1981). Unami and Shima(1996) found a failure strain of 1% at an unconfined compressive strength of

Fig. 14. Failure Strain (after Åhnberg et al, 1995a)

1 MPa for soils stabilized with cement. A failure strain of only 0.77% has e.g. beenreported by Holm (1994) at a peak shear strength of 5.0 MPa.

The failure strain has been 0.7% to 0.8% for lime/cement columns with anunconfined compressive strength of 520 to 760 kPa (Kivelö, 1994a). Ekström (1994b)found that the failure strain at unconfined compression tests with lime/cement columns ata shear strength of 130 kPa was 1.8% to 2.2%. Above 1.0 MPa εf was about 1%.

The failure strain of column cores has generally been less than the failure strain oflaboratory samples (Ekström, 1994b). The failure strain is often higher for organic than


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for inorganic soils due to the low shear strength when the soil is organic. Åhnberg et al(1996) have reported a failure strain of up to 17% for clayey gyttja.

The axial strain at the peak shear strength increases with increasing confining pressure. Tatsuoka and Kobayashi (1983) have reported that the failure strain increasedfrom less than 1.1% to more than 15% when the confining pressure was increased from

20 kPa to 686 kPa. Terashi and Tanaka (1981) found that the failure strain increased from1.5% at a confining pressure of 50 kPa to about 8% at 400 kPa. Balasubramaniam et al(1989) has reported that the failure strain for soils stabilized with lime increased from2.5% at a confining pressure of 50 kPa to about 10% at a confining pressure of 400 kPa.Rogers and Lee (1994) indicated that the failure strain increased with 3% lime from about2% to about 13% when the confining pressure was increased from 0 to 600 kPa. Ekström(1994b) has reported for lime/cement columns that the failure strain increased from 7.5%when the confining pressure was 60 kPa to more than 11% when the confining pressurewas 200 kPa.

The failure strain is about the same for lime, lime/cement and cement columns at the

same shear strength (Ekström, 1994b). The failure strain εf is usually higher for limecolumns than for lime/cement and cement columns because of the high shear strength oflime/cement and cement columns. The failure strain at the wet method is often higherthan the failure strain at the dry method at the same cement content due to the highershear strength with the dry method. The failure strain increases with increasing watercontent due to the reduction of the shear strength with increasing water content. Thefailure strain has also a tendency to decrease with time since the shear strength increaseswith time (e.g. Brandl, 1995; Sandros and Holm, 1996).

The failure strain is also low for soils stabilized with fly ash (Brandl, 1995) evenwhen the unconfined compressive strength of the stabilized soil is low. Kujala and

Nieminen (1983) have e.g. reported that the strain at the peak shear strength was reduced

from 4.1% with lime to 1.4% with lime and gypsum.A failure strain of 5% is recommended in the design of lime columns when the

undrained shear strength is less than 100 to 150 kPa and 3% when the shear strengthexceeds 150kPa. For lime/cement and cement columns a failure strain of 1% can likely beused. The failure strain can be estimated by triaxial tests (CU- or CD-tests) with aconfining pressure, which corresponds to the in-situ total overburden pressure.

3.3.8. Increase of Shear Strength with Time.The shear strength and the bearing capacity of lime, lime/cement and cement

columns increase with time as well as the modulus of elasticity. Initially the increase is

much faster with cement than with lime. After 30 days the increase with cement isgenerally small. With lime the increase continues for several years.Okamura and Terashi (1975), Bredenberg (1979), Åhnberg and Holm (1986) and

Kujala et al (1993) have reported that the undrained shear strength of the stabilized soil

increases with t while Brandl (1981, 1995) and Nagaraj (1996) have proposed that theshear strength increases with log t when the time after the mixing exceeds three days.Also Sherwood (1993) has found that the shear strength with cement increases with log twhen the clay content is low. The difference between the two methods can be large.

The increase of the shear strength is often slow initially with lime/cement for clayswith a sulphide content larger than 1% to 3%. The shear strength increases gradually withtime and could eventually be about the same as that for clays with a low sulphide content.

The increase of the shear strength is generally fast when the columns are loaded justafter the installation (Wilhelmsson and Brorsson, 1987). Considerable care is required,however, so that the bearing capacity of the columns will not be exceeded.


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The pozzolanic reactions increase rapidly with increasing ground temperature andwith increasing pH-value. The chemical reactions are accelerated since the solubility ofsilicate and aluminate in the clay minerals increases with increasing pH-value and withincreasing ground temperature. The lime affects the clay particles mainly along the edgeswhen the pH-value is high (>12). The rate is reduced when the water content of the soil is

high (Saitoh et al, 1996).Often the increase of the shear s

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