Site Map

Lime piles, which essentially consist of holesin the grounfd filled with lime, have been used for two distinct purposes for the treatment of clay soils in situ. The first concerns the treatment of soft soils to improve their bearing capacity and in this case uses relatively large diameter quicklime piles at close spacings. The result is a significant reduction int he water content of the soil, causing densification and concomitant increases in its strength and stiffness. The secondapplication ia int he stabilisation of failing slopes, for which both quicklime and lime slurry piles have been used withthe intention of causing ion migration and subsequent lime-clay reactions int he surrounding soil.

However, although they have been successfully used worldwide, their usage has been relatively limited in relation to other techniques and the applications in which they have been used diverse. For this reason the literature ont he subject has tended to be inconsistent at best, and in some cases directly contradictory. There is thus an apparent lack of understanding of how lime piles work. This paper aims to produce some clarity by interpreting the literature int he light of recent research.

A summary of the stabilisation mechanisms that the current authors believe to operate is presented and the evidence from the literature that supports or contradicts these mechanisms is discussed. The result of this process thus provide a basis of design, albeit using parameters that need further definition for site specific application. For illustration, the design process in discussed in terms of UK application. © 1997Elsevier Science B.V.

Keywords: Lime stabilisation; lime piles; Lime columns; Soft soil treatment; Ground improvement; Spole stabilisation

Clay presents problems to geotechnical engineers due to its complex nature. This derives from its plasticity, its low permeability and thus the time dependency of pore water pressure ( hence shear strength) change and volume change, its structure, and its chemistry and mineralogy. Two particular facets of its behaviour that cause problems are its propensity to shear and/or compress excessively when stressed, and its change in stability in cuttings or embankment slopes with time. Improvement of the clay by treatment in situ to a considerable depth is nessecary if the often more expensive structural solutions to such problems (piles, ground anchors, nails) are to be avoided. Chemical alternation of clay has been shown to be effective in increasing its strength and stiffness and, when combined with long-term cementing reactions, considerable improvements can be achieved. Lime provides one such means.

The use of lime to improve the engineering properties of clay soils is well established, if not fully understood. The basis of lime treatment is firstly cation exchange (modification) followed by a reaction with the siliceous components of clay to cause stabilisation (TRB, 1987). Modification occurs within 24-72 h of mixing lime with clay and is manifested by rapid flocculation of the clay particles. This fundamentally changes its „cohesive nature” to a friable and granular nature, and itsstrength is considerably improved ( Rogers andGlendinning, 1996a). Stabilisation concerns the long-term additional strength development caused by crystallisation of calcium silicate hydrate and calcium aluminate hydrate gels that form following the dissolution of clay minerals in a highly alkaline environment.
Lime has been widely used in road construction by intimate mixing with clay subgrades to improve workability, shear strength and bearing capatity. Its use as a deep stabilising technique is less common, although three discint methods exist. In China, Japanb, Singapore and Scandinavia, for example, lime columns have been used to prepare soft ground for foundations. The general principle of the technique is to create, in situ, colums of intimately mixed lime and clay (Fig. 1.), which react to produce colums of material of greater strength and (initially) permeability than the surrounding soil.

Fig. 1. Procedure for construction of lime columns (after Broms and Boman, 1979).

Lime slurry pressure injection, developed int he USA (Blacklock and Wright, 1986), uses specialistequipment (Fig. 2). It has been used to treat expansive clay soils, silts other soft ground, as well as failed embankment slopes, although the problem of induced pore water pressures could exacerbate stability problems.
The third technique concerns lime piles, which consist, very basically, of holes int he ground filled with lime. Lime piles have been used int the USA, Thailand, Sweden and Austria as a method of slope stabilisation and in China, Japan and Russia as a ground improvement technique for soft soil.
This paper addresses deep stabilisation of clay using lime piles. The paper aims to summarise and discuss the experimental and field data presented int he literature. These are interpreted in the light of the stabilisation mechanisms postulated by ongoing research at Loughboroungh University, the experimental programme for which is reported elsewhere (Glendinning, 1995, and Rogers and Glendinning, 1994). Thereafter the potencial of the technique is illustrated in terms of its potential application int he UK by way of general illustration.

Ingles and Metcalf (1972) show one method of lime pile construction, illustrated in Fig. 3, in which a hollow tube is pushed into the soil to the required depth and quicklime is forced into the tube under pressure it is withdrawn. The pressure forces open the end of the tube allowing the lime to fill the cavity below. After each metre isfilled, the end of the tube is closed and used to compact the lime forming the pile. The altarnative method pf construction in a self-supporting soil is to auger holes to the required depth and subsequently, or via a central stem, to add quicklime and compact in layers. In some cases hydrated lime, or more commonly lime slurry, is added to the augered holes depending ont he application and perceived stabilisation mechanism.

Fig. 2. Slope stabilisation using lime slurry pressure injection (after National Lime Association, 1985).

Fig. 3. Procedure for construction of lime piles in soft soils ( after Ingles and Metcalf, 1972).

It should be noted that the term „lime column” and „lime pile” have been used interchangeably int he literature, resulting in confusion over stabilisation mechanism. Indeed, different mechanismsoperate with different combinations of construction technique and application. Following experimental and field studies at LoughboroughUniversity, the current authors believe the stabilisation mechanisms to be:

As quicklime draws in water from the surrounding ground it reacts, or slakes, to form hydrated lime:

Considerable expansion of the solid phase occurs if the quicklime is unrestrained since the relative volume of quicklime to slaked lime is 1:1.99. Thisexpansion is said to cause lateral consolidation of the ground surrounding the pile. However consid erations of stoichiometry of the hydration reaction as a whole (i.e. including the liquid phase) indicate that there is a small net volume loss. Thus the idea of physical expansion cannot be valid unless there is an external source of water causing hydration of the lime. Densification of the assemblage of soil particles will occur as a result of this process, since water is drawn out from between them to react with the quicklime. Howewer, Densification as a result of physical expansion causing water to be squeezed from the soil is believed not to take place as a result of the above logic.

The slaking reaction „uses uo” some water from the surrounding soil and, being highly exethermic, it creates steam. These combined processes are thought by some authors to cause a significant water content reduction Howewer, relatively large volumes of lime need to be added to achieve significant water content reductions in cases where steam generation is absent.

It is widely reported that lime migrates from the piles and reacts with, and hence stabilises, the surrounding (clayey) soil. For this to occer both calcium and hydroxyl ions must migrate through the clay. Saturation of the system by calcium ionsis nessecary for maximum cation exchange ont he available exchange sites of the clay minerals. Hydroxil ions produce the highly alkalinic (pH>12.4) conditions that are required for dissolution of silica and alumina prior to the formation of calciom silicate hydrate and calciumaluminate hydrete, the gels that crystallise to causestabilisatino. In incatc clay the migration distanceshave been found to be relativelly small (Rogers and Glendinning, 1996b).

The addition of quicklime to a soil results in negative pore water pressures which draw in water to the piles to react. In soft, wet and relatively permeable soils these suctions will be high only int he immediate vicinity of the pile and for a short time. In drier clays of low permeability, however, the suctions will be higher and will last far longer and, since they increase the mean normal effective stress, the soil will become stronger and morestable. This could be of considerable advantage to failing slopes, if only to provide time for other stabilisation mechanisms to come into operation. In addition these suctions will result in:

Consolidation of the clay int he remoulded shear zone of a failed slope can occuer as a result of the negative pore water pressures (i.e. increased mean normal effective stress) if they last long enough. This is discussed in detail by Rogers and Glendinning (1993).

A final benefit is the strength of the piles themselves, swich is said by several authors to provide an incrase in bearing capacity. The same observation could be applied to shear resistance along a shear plane in failing slopes as long as pile durability and strain compability, both during strength gain and after the pile has fully strengthened, are acconted for.

Two discints themes emerge from the literature concerting the mechanism of stabilisation and authors largely discuss one independently of the other. The first of these is the idea of combined pile expansion and clay dehydration. In general, the authors who propose this mechanism are using the lime piles to improve the bearing capacity and settlement characteristics of soft ground for foundations. The second theme concerns the migration of calcium ions from the pile into the surrounding clay and its subsequent stabilisation by lime-clay reaction. In general the authors who propose this mechanism are relying upon the lime piles to stabilise failing slopes in stiff clays or weathered shales and mudstones.
The literature will be discussed int he context of the stabilisation mechanism listed above and, where possible, contradictory evidence will be explained with the hindsight provided by the results of the research at Loughborough University. Where one paper discusses more than one mechanism, the work will be summarised under the first mechanism and referred to thereafter.

Several authors describe the mechanism of lateral consolidation when using lime piles for the improvement of soft soils, although the most notable exponents are the Japanese. Kitsugi and Azakami (1982) describe the „Chemico-pile method” in which granulated quicklime piles are constructed by driving a closed-ended casing into the ground and then forcing quicklime (using compressed air) into the cavity that is formed as the casing is withdrawn. Improvements from field measurements are given in Tables 1 and 2.
The main stabilising processes propounded in this paper are those of dehydration and lateral consolidation caused by pile expansion, which is said to „improve cohesion” int he soil. This improvement is calculated by estimating the change in void ratio due to the expansion, deter mined by multiplyng the reduced water content by the specific gravity. This is used to find the increase in stress, using the void ratio-effective normal stress relationship, which is multiplied by a „strengthening factor” to reach the change in „cohesion”. Reduction in vertical settlement is calculated by considering that part of the settlement capacity of the soil is taken up with lateral consolidation. Equations quantifyng these ideas allow determination of the length of the piles required.
The source of the lateral movements seems to be entirely attributed to the expansion of the lime. Piles of significantly increased volume in comparison with the casings used have been observed int he field and this is quoted as proof of the argument. While some expansion of the pile diameter is inherent int he assumption of no discernible net volume change, since the water from the soil becomes bound int o the pile, the values of diameter quoted exceed this expansion. This additional diametrical increase can, however, be attributed to the driven casing causing lateral soil movements and the lime applied under pressure, and subsequently compacted, expanding further the cavity in ground as soft as the type mentioned. Thus physical displacement caused by the construction process, rather than lime pile expansion on lime hydration, occurs.
Wang (1989), from China, describes identical design equations to those of Kitsugi and Azakami (1982), and yet neither paper references the other. Tystovich et al. (1971) of Russia describe the use of lime piles to aid compaction of 30 m of saturated loess. A closed-ended tube system was used to create 250-500 mm diameter piles from compacted lumps of quicklime at 2 m centres, causing the improvements shown in Table 3. The pile diameter was observed to increase by 26-60%, representing a volume increase of 160-260%, and radial cracking occured int he surrounding loess. The volume increase is attributed to loess compaction caused by the driving of the closed-ended tube and expansion. Of the quicklime pile as a result of slaking.

Table 1.
Improvements in soft clay properties due to Chemico-Pile treatment (after(Kitsugi and Azakami, 1982))

* Notes: w = water concent of soil; y = saturated unit weight; e = void ratio of soil; Cu = undrained shear strength from unconfined compression tests (Cu = 0,5 qu): qu = undefined compressive strength.

Adding int he volume increase caused by compaction of the quicklime ithe soft, weak loess mass (i.e. forced lateral movement of the lime), the observed diametrical increases appear to be adequately explained. The observed cracks indicate a net volume loss, thus supporting the observations made earlier.
Chui and Chin (1963) examined the expansive behaviour of quicklime piles int he treatment of Taipei Silt for foundation purposes. The experimental procedures adopted are unclear, although a photograph indicates that the clay was compacted into boxes and small quicklime piles were formed within it. An effective water supply and vertical surcharge were thought nessecary for the full expansive force ont he clay to be realised. This latter comment is important, the introduction of water being recommended by others (although generally without justification in this context).
Int he cases where water was added to a quicklime pile, for example by surface inundation, then lateral consolidation caused by physical expansion of the pile is likely. Another way in which this could, in effect, occur is where water is removed via pores in a relatively rigid soil structure (e.g. as would be caused in a weak sandstone), thus creating the lateral stresses postulated. Otherwise lateral consolidation by physical expansion caused by the lime should not be possible. In soft soils the physical displacement of the soil by a driven tube followed by compaction of the lime in the piles can cause physical expansion and consequently lateral consolidation. Thus the effect of soil densification can occur by several means, but the mechanisms differ and the term lateral consolidation is, perhaps, confusing. The successful use of lime piles in loess indicates that the densification sought can be achieved without a significant clay content, this type of improvement thus being applicable, for example, to alluvium in general.

The concept of lateral expansion has often been couped with water content reduction (?w). Kitsugi and Azakami (1982) developed the following equation relating ?w to the original water content of the ground for estimation of pile diameter and spacing.

Where
?w = reduction in soil water content (%)
w? = original water content (%)
yt = wet density of soil in situ (t/m3)
as = area ratio of the Chemico-pile
as = ?d2/4s2
d = diameter of pile (m)
S = pitch of piles (square arrangement) (m)
h = equivalent of water absorption of lime column
yc = apparent saturated unit weight of lime pile (t/m3)
nt = porosity of slaked lime (%)
Ev = coefficient of volumetric expansion in slaking (%)
St = degree of saturation of slaked lime (%)
Yw = unit weight of water (t/m3)

The equation appears to assume that water enters the pile solely to fill the void space within it, and is in turn dependent ont he pile formation method. Similar result may be achieved simply by calculating the amount of water required to produce the same content int he pile as that int he surrounding soil. Field result are compared with those calculated, buta re only broadly comparable.
For ground with water contents of the orders quoted (50-150%) the equation predicts large reductions. However, applied to overconsolidated clays more typical of the UK the reductions are far more modest (?w = 1% for w? = 30%, d=0,1 m and S=1,0m) necessitating large at relatively small spacings for significant reductions. This is, in fact, what the paper advocates, but the higher cost of lime production int he UK could make it prohibitively expensive unless applied to wetter alluvial soils for ground improvement since they can cause considerable problems. In addition the facility for rapid ground improvement, which this technique provides, wold influwncw the decision. Furthermore for water content reduction there is no need for the soil to have a significant clay content, as evidenced by Russian loess stabilisation described later.
Kitsugi and Akazami (1982) present a case study in which lime piles were used to reduce settlements beneath a 1.5-3.0 m high embankment.The result show that the changes in water content are less significant than those discussed earlier int he paper (Table 1) and that the water content int he piles is signifacantly lower than that int he surrounding soil. This could indicate an incomplete reaction. Factors affecting water content transfer to the pile thus need further consideration. The relationship between change in void ratio and water content does not appear to hold in this case and therefore the dependent change in undrained shear strength (?cu) cannot have been accurately calculated. Since ?cu was shown to increase with depth, this could indicate that the effects recorded were due to vertical consolidation caused by reduced pore water pressures increasing the mean normal effective stress. However insufficient data are presented for full validation of the equations or explanation of observed phenomena.
Tystovich et al. (1971) of Russia describe how water content reduction is effected by the heat of the slaking reaction causing evaporation. They state that the best results are archieved in porous soils such as saturated loess. These conditions would undoubtedly enhance water movements within the soil, thus promoting a more complete and rapid reaction. Int he field trials steam was observed and cracks up to 40 mm wide and 480 mm long developed around the 250-500 mm diameter piles. Temperatures observed at the centre and edge of the piles peaked at 390o and 50 Co , respectively around 4 h after construction. These were remarkably similar to measurements arround 1000-1500 mm diameter quicklime piles in peat having water content of 400% to 460% (Kitsugi and Azakami, 1982). Samples teken 1 m away from the pile edge had a significantly reduced degree of saturation and increased strength parameters (Table 3). A plate load test carried out int he area containing the pile group deformed substantially less than that on untreated ground. It is unclear at what depth the tested samples were taken and, hence, if the water content reduction occured over the entire pile length or if it was a surface effect attributable to the steam.
The volume of water used up int he slaking reaction is thus simple to calculate, the calculation showing that large diameter piles at relatively smallspacings, as used in the soft soil treatment of the far east, are necessary to generate large water content reductios. Large piles and a source of rapidly availabla water (a sin soils with a high permeability and/or high water content) will create the conditions for significant steam generation and increased water loss from the soil. The presence of clay is not necessary, and since is would reduce the permeability of the soil it could prove a hindrance to stabilisation by this mechanism.

Many authors propose that the pozzolanic clay-lime reaction, initiated by the „migration of lime” from the piles into the surrounding soil, is a major stabilising process int he treatment of failing slopes, and this is discussed below. Kitsugi and Azakami (1982) discuss how the „lime” content int he ground diminishes with distance from the pile as a result of thier studies of soft soil improvement. However, it is likely that lime being forced into very soft ground under pressure would tend to cause significant displacement of the surrounding soil and yet no account is apparently taken of this Tystovich et al. (1971) also include chemical reaction between the relatively small quantity of clay held within (soft, wet) loess and migrating lime as a stbilishing mechaninsm, but present no supporting data. Chui and Chin (1963) similarly mention the possibility of clay-lime reactions int he context of soft soil improvement, altough again without alaboration. The literature on soft soils thus provedes few leads.
Several American authors refer to the mehanism in slope stebilisation. Handy and Williams (1967) describe a case study in which 150 mm diameter quicklime piles were successfully used to stabilise a failing clay fill slope where more conventional treatments had failed (Fig. 4). The clay fill had a mineralogy of predominantly calcium montmorillonite, properties int he active slippage zone ofØ = 17o, c’=4 kPa nd a water content of 27,2%. Design methods were entired based ont he prdicted lime migration distance and ont he soil typein relation to the subsequent clay-lime reaction. The piles were constructed on a 1,5 m grid to penetrate the zone situated within a perched water table. Holes ware augered, and quicklime was poured in and „watered”. Sampling indicated a water content reduction of 4% after 3 months, although the sample locations were not described. After 1 y the unconfined compressive strength had risen relative to the 3-month strength by approximatelly 55%. Acid soluble aluminate and silicate were said to be indicative of a reaction having occured 300 mm from a pile. After 2 y a soil sample taken 300 mm away from a pile and 2 m below its top had Ø’ = 21 o and c’= 9,5 kPa. No lime movement could be detected above the water table, thus indicating the need for water int he movement of the lime. Tests ont he lime piles indicated that very little of the lime had reacted. A subsequent slip occured underneath the treatment area, highlighting the need for comprehensive analysis of the unstable region prior to treatment. Over the 2-y period after treatment, cracks int he houses founded ont he slipped area closed up and no further cracking took place. This could be explained initially by the expansion of the piles due to the introduction of additional (i.e. external) water. Int he longer term, the strengthening of the lime placed througn the shear zone prevented further movement. If some hydration of the piles was caused by water from the softened zone around the slip plane, improvement of properties could have been affected by dehydration and consolidation (see Rogers and Glendinning, 1993). The improved soil properties could be explained by lateral consolidation, although the time dependence of the improvement is difficult to attribute, or even natural soil property variation. However lime migration assisted by hydraulis transport („watering” remains likely.

Fig. 4. The use of lime piles in slope stabilisation in Iowa ( after Handy and Williams, 1967).

The most interesting result is the measurement of acid soluble aluminates and silicates, which are indicavite of the clay-lime reaction. The equation of Davidson et al.(1965):

Used to calculate rates of movement appears to be invalid when there is a movement of water (see later discussion). However, purely considering permeabilití and water flow, a rate of ion migration of the order quoted would indicate a coefficient of permeability for the clay of 5x10-9 m/s, which seems reasonable. The anomaly appears to be that the treatment was designed ont he basis of the quantity of lime required to promote a chemical reaction and very little of the lime had reacted. As it is believed that a certain pH (>12.4), hence concentration of „lime”, is required before aluminates and silicates go into solution, it is questionable as to whether a reaction was occuring. It may be possible that the acid used int he test is breaking down the clay and giving these results.
Anon (1963) and Lutenegger and Dickson (1984) describe similar usage of lime piles where a flow of water was thought to be necessary to promote lime migration.
As part of expensive Indian studies, Chummar (1987) reports model laboratory experiments on marine and moderately expansive clays using 20 mm diameter quicklime-sand piles at 200 mm spacing. He also describes their successful fielduse, including the stabilisation of a slip plane in a lateritic clay embankment. Unfortunately no design criteria are discussed. Further field trials are described by Shanker et al. (1989). Again, the flow of water to transport the lime was felt essential, so the piles were „watered” for 3 weeks after installation. Also, a 50 mm diameter sand core was provided int he soil-lime pile as it was thought this would aid drainage.
Ayyar et al. (1989) examined the effect of pile shape on its efficiency via a combination of field trials and laboratory trials on clay compacted into a 560 mm diameter, 850 mm deep tank. The mineralogy of the clay used was predominantly smectite, with liquid and plastic limits (LL and PL) of 60 % and 21 %, respectively. Augered holes were filled with quicklime and allowed to reach equilibrium with a thin layer of water ont he surface. Pile sizes were calculated so as to achieve 2 % by dry weight of lime. Clay samples taken after 1, 2, 3 and 6 weeks were analysed for calcium content, shearstrength and water content. The study concluded that more calcium than was required for ion exchange had migrated into the surrounding soil, although again without justification or even information on how far the ions had migrated.
Venkatanarayama et al. (1989) used similar laboratory models with surface inundation to study the effects of pile spacing. Tests on samples taken from between the piles demonstrated that smallaer spacings gave greater degress of improvement although once more detailed data were lacking. One interesting aspect is their attempted comparison between the effects of the piles and traditional mixing. It is, however, considered inappropariate by the current authors to compare directly percentages of lime between the different treatment methods and this graph should be used with caution. Using a mix technique, the modification process relies partly on pulverisation, flocculation and recompation to achive improvements in plasticity, strength and workability. The cementation reactions then bind small „lumps” of (modified) clay together. These processes do not happen via ion migration. Improvements from the clay-lime reaction must be derived solely on a molucelarlevel, the main stabilising mechanism being derived from other processes entirely.
Brandl (1981) describes lime pile usage in Austria since 1968. Laboratory tests and field trials have shown that long-term strength is derived from diffusion of lime from the piles. Their efficiency for any particular application is thought to be controlled by the clay mineral content, ion exchangeability and the presence of movable silica. The most suitable soils are thought to be „loosely packed” and of low plasticy, since they have an increased natural permeability and remove the risk of rapid cementing reactions preventing further diffusion by „sealing in” the lime. Piles of 80-500 mm in diameter at spacings of 0,5-3,0 m (depending on soil charasterics) have been employed to give changes in Ø’ of as much as 15o. No long-term data were available, but Brandl doubts if the reactions can be reversed with time.
Ruenkrairergsa and Pimsarn (1982) describe the stabilisation of a loose clay shale fill embankment in Thailand. Hand augered, 150-mm diameter holes were formed on a 3-m grid through to the natural ground (Fig. 5). Lime and water were simultaneously poured into the holes, the mixture being topped up four times daily for 2 months. Pile construction occured immediately prior to the rainy season as it was thought that limemigration would not occur without the flow of water. Lateral movement was monitored by means of aluminium rods driven through the madeground into the natural ground below (Table 4). Shear strength was monitored in situ using an „Iowa borehole shear device” (Hnady and Fox, 1967), water content and consistency limits being measured ont he samples thus recovered (Table 5). Samples were taken immediately after the rainy season.
The authors state that lateral movement has been reduced, although the result show that movement is still occuring. The strength changes are attributed to improved drainage caused by the lime migration. The significance of the results is difficult to ascertian since no „pre-treatment” lateral displacement or climatic data are available: a drier rainy season eould reduce movement if water were the trigger of the slip. The plasticy index reduced from a range of 20-30% to an average of 12% due to a rise in PL, a change that could reasonably be attributed to a clay-lime reaction (Rogers and Glendinning, 1996a). Although apparently successful, the topping up of lime slurry in each hole on 240 occasions results in a extremely labour intensive operation and one that is likely to prove fruitless in intact clays.
Fosh and Kinter (1972) carried out a laboratory study using high calcium hydrated lime to treat small blocks of montmorillonitic and kaolinitic clay.Free calcium ion concentrations were detected by means of acid extraction and titretion with Ethylenediaminetetracetic Acid (EDTA). Migration was found to be limited to less than 40 mm after 180 days. They concluded that water movement and ion migration were separate processes, with the most concentrated solution of limecontributing most lime to the soil independent of clay water content.
Katti and Gupta (1970) studied lime migration in Black Cotton Soil (LL = 80%, PL = 40%) using models of quicklime piles in soil and soil piles in quicklime. Unconfined compressive strength and shrinkage limit were used as measures of migration.

Cross section of clay shale embankment at km. 115-500 on the Chiangmai – Chiangdao Fang National Highway.

Fig. 5. Application of lime piles to a clay shale embankment (after Ruenkrairergsa and Pimsarn, 1982).

It was concluded that lime had travelled to the centre of a 150 mm soil specimen after 120 days. Although both in extreme proved detrimental, migration was thought to be aided by lower water contents and higher soil densities via promotion of capillary action.
Noble and Anday (1967) constructed 25-mm diameter lime paste piles within 150-mm diameter compacted clay speciments of unknown mineralogy. The tops were plugged with clay and the samples sealed. Migration was monitored by leaching soil samples with acid titreting with EDTA to determine the free calcium ion content.

Lateral movement of a clay shale slope after lime pile (after (Ruenkrairergsa and Pimsarn, 1982))

Test depth (m)	Water content, n (%)					Cohesion, c (kN/m2)					Angle of shearing resistance Ø (deg)
	1977		1979			1977		1979			1977		1979
	Range	Avg.	Range	Avg.		Range	Avg.	Range	Avg.		Range	Avg.	Range	Avg.
1	30.2-55.0	43.4	27.4-48.1	37.4	-6.0	0-19.3	7.1	0-17.9	7.2	+0.1	22.0-44.5	33.9	29.5-48.0	39.8	+5.9
2	30.5-51.2	42.6	23.5-53.8	38.3	-4.3	0-37.9	12.9	0-30.3	13.9	+1.0	22.1-39.4	31.2	28.0-47.0	39.3	+8.1
3	31.1-52.8	42.4	26.8-54.0	38.9	-3.5	0-35.0	11.3	4.1-27.6	15.5	+4.2	20.0-40.2	32.3	31.5-46.5	38.8	+6.5
4	28.5-54.5	42.5	22.3-51.8	38.0	-4.5	0-38.6	12.2	3.4-29.7	1.57	+3.5	20.1-44.0	33.3	27.5-47.6	37.5	+4.2

Changes in water content, cohesion and angle of shearing resistence after lime treatment (after [Ruenkrairergsa and Pimsarn, 1982])
(+) means increase, (-) means decrease; 100 kN/m2 = 14.5 lb/m2.

Ions migrated 45 mm, the limit of the experiment, in 44 days. At shorter distances ion concentrations increased over 99 days. Some of the early readings indicated that migration had occured, whilst later data at the same distance did not. This was attributed to migration occuring in paths of lower density material. Although not mentioned, it is possible that free calcium ions became unavailable due to cation exchange.
It is thus widely reported reported that lime migrates from the piles and reacts with, and hence stabilises, the surrounding ground. In order for this to occur both calcium ions and hydroxyl ions must migrate through the clay. Saturation of the system by calcium ions is neseccary for maximum cation exchange ont he available exchange sites of the clay minerals. Hydroxyl ions produce the highly alkalinic (pH > 12,4) conditions required for dissolution of silica and alumina prior to the formation of calcium silicate hydrate and calcium aluminate hydrate, the gels that crystallizse to cause stabilisation. Laboratory experiments have shown that the migration in clays is restictec to a relatively small distance, as would be expected from consideration of the operation of clay barriers. Water transport of ions from quicklime piles inintact clay dous not appear to be a valid mechanism, and indeed it is suggested that capillarity provedes the mechanism such that drier clays are better in this respect.
The actual mechanism is probably a combination of ion diffusion and mass transport depending on water flow conditions. With respect to diffusion, a possible explanation for the short distances observed is that it is controlled by ion exchange. Ions may radiate away from the pile by a process of ion exchange, an equlibrium position being achieved when the potential for ion exchange is balanced by a reverse osmotic potential. The potential for ion exchange must, in turn, depend on mineralogy. In cases where lime slurryhas been used to create the piles, migration over significant distances is reported, presumably as a result of hydraulic transport. „Watering” of quicklime pileshas also been suggested in order to improve hydraulic transport of ions. The measurements of ion migration are, however, suspect in many cases and in some cases migration is claimed without measurements to support the claim. The self-limiting nature of the reactions, in which stabilisation of the soil adjacent to a pile inhibits further migration, must also be considered int he migration process. These ideas are expanded further by Rogers and Glendinning (1996b).

Although not mentioned int he literature, this mechanism has been observed during the research at Loughborough University. Generation of negative pore water pressure is caused by the strongly hydrophilic quicklime int he piles drawing in water from the surrounding soil. Reductions of the order of 20 kPa have been observed in both laboratory trials and, importantly, field measurements. This mechanism represents an important short-term process.

Again not mentioned int he literature, overconsolidation of remoulded clay along a shear plane, or zone, as a consequence of the negative pore water pressure leads to increased strength. The caly int he shear zone is particularly susceptible to this form of improvement since water contents are typically higher than int he remainder of the soil. Improvements are small, but when applied to shallow slips they are significant in terms of stability. This is discussed in greater detail in a later section.

Kitsugi and Azakami (1982) are the only authors who specifically mention the strength of the piles themselves, attributing improvements in bearing capacity of soft soils mainly to pile strength.
An average strength of 431 kN/m2 is quoted. Additivies of calcium silicate and/or calcium alluminate are pre-mixed with the quicklime, presumably to improve strengtn. Pile strength is said to depend upon the confining pressure provided by the surrounding soil. Such lateral support will increaseconsiderably with depth and thus the degree of vertical confinement of the clay close to the surface will be important in how such piles should be designed. In practice it will be the pile-soil systemthat resist the applied stress and an equilibrium condition will be reached.

The literature has thus provided much evidence of the cussess of lime piles in treating both soft groung to improve its bearing capacity and slopes to improve their stability. The mechanisms of stabilisation postulated by authors from several different countries are in some cases incorrect, and a significant proportion of the literature ont he subject is contradictory or misleading. Thus although the technique is of value, the need for a comprehensive programmme of researc, both int he laboratory to isolate the stabilisation mechanisms operating and int he field to prove the techniques, has been established.
The researc at Loughborough University examined the application of quicklime piles to stabilise slope failures and this will be discussed briefly here int he context of the above stabilishing mechanisms. The conclusions are draw from laboratory data and, more especially, observations of field performance over a period of more than 4 y. The research concentrated upon the shallow failures described by Perry (1989), 95% of which were less than 2 m deep and thus had effective normal stresses acting ont he failure plane of typically 10-40 kPa. No such considered research programme has been published ont he application of quicklime piles to soft soil improvement, and indeed this paper represents the first comprehensive critique of this promising potential application.

When analysing shallow slope failures it was evident that small changes in the effective cohesion (c`) dramatically altered the factor of safety of the slope . Although it can be argued that c` effects in soils are artificial and c` ?provides a por basis for analysis reliance on c` becomes more acceptable if the increase in strength is considered as a ” cementation” rather than a ”cohesion”. In the case of quicklime piles this additional c` is relatively large and derives from different sources , the effects of which are superimposed to generate significant improvements in safety factor at different times after construction . It is with this in mind that a definitive list of stabilising mechanisms was developed .
In the overconsolidated clay cuttings and embankments that are typical of the UK ,significant expansion of quicklime piles is not thought to occur due to the degree of confinement and relatively low initial water content . Even if some degree of external water ingress occurs , for example via the system of radial cracks generated by rapid clay dehydration . The high degree ofconfinement will tend to cause densification of the lime within the pile .Reducton in water content of the clay mass as a whole is also likely to be relatively modest . Field data show it to be confined to a small annular zone around each pile ,with differential dehydration causig radial cracking . Migration of lime(i.e. Ca2 + and OH – ions)in any significant quantity is likely to 20-50 mm from the pile or from lime-filled cracks. (Greater migration of Ca2+ ions in the much longer term should not be discounted , even though direct evidence is currently lacking , since Quigley et al ., 1990, have indicated this to occur in work associated with landfillv liners.) This necessarily limits the facility for subsequent clay-lime reaction. The reduction in pore water pressure generated by quicklime piles is relatively high.These suctions immediatety increase the stability of the slope, in the same way that considerable stability exists due to the high stress relief suctions that are generated whet creating slopes. Overconsolidation of the shear zone is dependent upon site –specific ground conditions, but improvements in effective cohesion would be expected to be of the order of 1-5 kPa if complete and effective overconsolidation were to occur. Significant pile strength is developed with time and is dependent (in part) upon the degree of confinement and (strongly) upon iniital clay water content. Undrained shear strengths of 400-500 kPa have benn recorded in the field after 2 y and in the laboratory (i.e. under ideal conditions) after 4 months . In soft, very wet alluvial soil found in an isolated buried stream valley, however, no apparent strength gain occurred in piles constructed without pressurised insertion of the quicklime.
For the above mechanisms to be relied upon, the technique should be restricted to seft-supporting clay soils in which relatively dri augered holes can be constructed, unless alternative construction techniques are developed. Hawever, there in no reason to believe that the application of lime piles to soft soils could not be adopted in the UK, albeit that researrch is not currently available to prove its efficacy.

The research programme conducted at Loughboroughm University was funded jointly by four industrial partners (Geotechnics Ltd, Cementation Piling and Foundations Ltd., British Waterways and Buxton Lime Industriest) and the Science and Engineering Research Council(now the EPSRC) via the LINK Scheme in Transport, Infrastructure and Operations. This funding, and the considerable assistance of the staff of the above organisations, is gratefully acknowledgedged.