A Proposed Classification of Anisotropic Engineering Behaviour of Cement Treated Clayey Mixtures
B.Sc., M.Sc., Ph.D., F.G.S., Lecturer, Laboratory of Soil Mechanics and Foundation Engineering, Geotechnical Engineering Division, Department of Civil Engineering, Aristotle University of Thessaloniki, Thesssaloniki, Greece
The existence of some difficult soils (non-durable), often present difficulties in construction operations. However the engineering properties of these clay soils, can be enhanced by the addition of cement, thereby producing an improved construction material.
In all practical cases, the primary ingredient necessary for stabilizing soils is calcium (% of cement). In addition to plasticity reduction, portland cement, by its inherent nature of producing strength - developing hydration products, provides improved strength and durability. Therefore the effectiveness of stabilization is based on the number of positions of exchangeable ions (mineralogical composition which is related to liquid limit) of a clay and the amount of liberated calcium ions from cement (% of cement, % of compaction and curing time) which influence the durability (bonding effect) and unconfined compressive strength (bearing capacity).
In an effort to characterize and study the suitability of silty - clayey soils for cement - stabilization both slaking and unconfined compressive strength tests were carried out on clayey - sand mixtures stabilized by cement, consisted of two types of clays, kaolin and natural bentonite.
Finally diagrams were prepared to study and classify the variation of slaking and strength of cement-stabilized clayey - sand mixtures (silty-clayey soils) due to compaction, curing time, cement percentage and liquid limit (of clayey-sand mixtures) and also to predict areas of efficient cement - stabilization.
Keywords: cement, clays, stabilization, classification, liquid limit, strength, durability.
With the growth of human activity and economic development, geotechnical engineers sometimes encounter different kinds of problems caused by natural disasters as well as environmental pollution which are related to the behaviour of soils (problematic) beneath foundations (low strength, durability and high compressibility).
To control and solve these problems, a suitable ground improvement technique is needed, for surface and deep excavations in problematic soils for stability, durability and deformation. Cement stabilization is one of the alternatives. The conventional cement stabilization methods are used mainly for surface treatment, while the use of cement has recently been extended at greater depth in which cement columns were installed to act as a type of soil reinforcement (deep soil - cement mixing and permeation grouting), (Pinto et al, 2003).
The soil stabilization and reinforcement of a soft soil by mixing with cement have a basic target, to find the most efficient and economical method so that the properties of a problematic soil become more like the properties of a soft rock. Conclusively these methods are ground modification techniques and are used:
The soil - cement mixing (deep or surface application), has been used for many diverse applications including building and bridge foundations, retaining structures, liquefaction mitigation, temporary support of excavations, water control and structures to protect the natural environment, pollution control by using stabilization/solidification (S/S) techniques, (Sällfors et al, 2002).
The strength and durability of a cement - stabilized soil depend obviously upon the effectiveness of the cement and on the mineralogical composition of a clayey - soil, (Kamruzzaman et al, 2000). Cement - stabilization is a function of mineral type and of previous stress history. These clays which have been subject to diagenetic changes imposed by high overburden pressure react quite differently from geologically "young" (well crystallized) clays with respect to cement stabilization. The reason of the above mentioned in a clayey - cement mixture, is that calcium ions liberated from the cement during hydration and hydrolysis occupy the positions of exchangeable ions on the clay minerals. The number of these positions depends upon the portion of clay on this soil - cement mixture, the type of clay minerals (active or inactive), the number of diagenetic bonds and the behaviour of weathered clays (degrated minerals) which are related to the liquid limit reflected the mineralogical composition of soil (Croft, 1968).
Tests, for the evaluation of a stabilized soil by cement such as compression combined with durability should be made (Yoder, 1967). Also, if the main objective of classification is to control the engineering behaviour of cement stabilized clayey soils then the liquid limit of the natural soils is useful for predicting and studying suitability for stabilization so that lesser amounts of stabilizing agents will be adequate (Croft, 1968).
For the above reasons diagrams were prepared, to classify and study the influence of cement percentage, degree of compaction, curing time and liquid limit on strength and durability of cement stabilized clayey - mixtures, leading to characterization of areas for efficient cement - stabilization. Therefore unconfined compressive strength and slaking (durability) tests were carried out on different cement stabilized clayey mixtures consisted of natural bentonite, kaolin and sand, (Stavridakis, 1997).
DESCRIPTION OF MATERIALS
The variation of physical and engineering properties (compressive strength, durability, liquid limit, permeability, swelling etc) of clayey soils depends mainly upon two groups of clayey minerals present and dominant: montmorillonite and/or kaolinite. The liquid limit reflects the composition of a soil and in particular the nature and properties of the principal clay mineral (Morgenstern and Eigenbrod, 1974; Bell, 1976; Stavridakis and Hatzigogos, 1999). The number of exchangeable positions (for exchangeable cations) in illite and kaolinite give little variation in the liquid limit. This is however, significant as far as montmorillonite is concerned (Bell, 1976).
For these reasons compressive strength and slaking tests were carried out on clayey admixtures, stabilized by cement, consisting of two clayey soils namely Bentonite and Kaolin. Bell (1976), Croft (1967) and others have characterized the behaviour of bentonite (montmorillonite) as active and that of kaolinite as inert. Kaolinite and well organized (well crystallized) soil minerals appear to have little effect on the hydration of cement and hardening proceeds normally, after short curing periods by using small amounts of cement.
Table 1. X-rays results
Table 2. Index properties of clays
|Liquid limit (%)||34||111.50||—|
|Plastic limit (%)||29.61||42.19||—|
|Plasticity index (%)||4.39||69.31||—|
|Moisture content (%)||0.6||12.37||0.20|
|% finer than 74µm||100||100||0.48|
By contrast clay minerals with an expansive lattice (i.e. bentonite), have a profound influence on the hardening of cement and require large amounts of cement, to develop satisfactory strength and durability (Bell, 1976; Croft,1967).
In this research commercially available kaolin and sand were used, while the bentonite was from a natural source.
The qualitative characteristics (x-rays), of the above mentioned clayey soils are shown in Table 1. The sand used was fine to medium grained (74µm/840µm) with a uniformity coefficient 2.19. The basic properties of kaolin, bentonite and sand are given in Table 2.
The range of proportions of clays in the admixtures were:
Finally the particles of clayey admixtures retained on sieve No. 200 (74µm) are less than 65% and they can be described as a silty clayey material in accordance with the Highway Research Board Classification system (Yoder, 1967).
TESTING PROCEDURE - SAMPLE PREPARATION
The improvement of physical and engineering soil properties by cement protect the natural environment and earth - structures. Soil - cement mixing provides satisfactory performance under both static and dynamic loads (Andromalos et al. 2000).
The strength and durability of cement stabilized soil obviously depend upon the effectiveness of cement on the mineralogical composition of a clayey soil (Koncagül et al, 1999).
Bonding (clayey soil grains - cement) determines the ease with which micro - macrofractures during slaking process, can propagate through the specimens.
Durability (slaking), under environmental conditions of wetting - drying and potential stresses (i.e. during seismic events or movements due to landslides), (Tatsuoka et al, 1997) is an aspect of cement stabilized soils, behaviour that has been neglected in favour of other properties such as strength.
However it is an important feature of many commonly, encountered engineering problems concerning surface or deep soil - cement mixing (stabilization) when problems of durability arise in soil - cement mass because of water table fluctuations, in transportation engineering problems (Owttrim, 1988), in dam construction when the dispersive properties of clays (Na - montmorillonites) are not suitable accounted, (Shah and Ahmad, 2003).
For these reasons in this research work the following tests were performed:
Table 3. Classification and characterization of durability
(after Franklin and Chandra, 1972).
|Slake durability index Id2 (%)||Slaking
Table 4a. Clay mixtures stabilized with 4% cement and cured for 28 days
and their geotechnical characteristics.
The Portland Cement Association (1959) reported that 85% of all soils to be used for soil - cement stabilization can be adequately hardened by the addition of 12%-14% (in this research the maximum was 12% cement). As much as 5% addition of cement has been shown by Bell (1976) to be necessary if noticeable changes are to be brought about in the strength of montmorillonitic clays, whereas at least 2% was required in kaolinitic clays.
Table 4b. Clay mixtures stabilized with 12% cement and cured for 28 days
and their geotechnical characteristics.
CLASSIFICATION OF CEMENT STABILIZED SILTY - CLAYEY MATERIALS AND EVALUATION OF THEIR ENGINEERING AND CEMENT - STABILIZATION PARAMETERS
The objective of an engineering classification scheme is to categorize silty - clayey materials stabilized by cement according to their potential engineering behaviour. In this regard, an engineering classification is often oriented toward specific applications.
Classification of cement stabilized silty - clayey materials for engineering purposes has been particularly difficult. Many of these difficulties have resulted because of the transitional nature (from soil to soft rock - curing time) due to cement - stabilization of these silty - clayey materials. These transitional materials create further confusion for many geotechnical engineers who are accustom to viewing a silty - clayey material stabilized by cement as either like a rock or a soil, but not as a material that can have engineering properties of both.
The major deficiency of most engineering classification schemes of silty - clayey materials stabilized by cement is the absence of time factor (curing time), degree of compaction (percentage of voids) and amount of cement used (bonding effect).
For the above mentioned transitional material it is important to not only consider how these materials behave due to cement - stabilization but also how they will behave in a reasonable engineering time frame (curing time), degree of compaction and amount of cement.
A classification system must be basic and take into account the potential use. For example, if soil and all its potential uses are considered (i.e. soil - cement mixing - stabilization) such factors grain size, Atterberg limits (type and portion of clay minerals), % of cement, strength and durability of the cement stabilized soil should be considered. Conventional specifications stipulate a lower limit of 1754 kN/m2 unconfined compressive strength after 7 days of curing for soils stabilized by cement to pass the erosion tests successfully (material loss 7%-14% in 12 cycles of freezing and thawing or wetting and drying test as described in ASTM D560-03 and ASTM D559-03 respectively), (Akpokodje, 1986; Croft, 1967; PCA, 1959).
Table 4c. Clay mixtures stabilized with 4% cement and cured for 7 days
and their geotechnical characteristics.
Also according to literature clayey soils with liquid limit less than 40% and plasticity index less than 18%, are stabilized successfully by using economical amounts of cement (Godin, 1962). Although soils, with large liquid limits (> 60%) and plasticity induces (> 25%) invariably contain expansive clay minerals and react with large amounts of cement. These cement - stabilized heavy clayey - soils is difficult to attain the lower limit of 1754 kN/m2 unconfined compressive strength since ultimate rigidity is dependent upon the structure (bonding effect) of the stabilized material rather than on relative reactivities of the components (Croft, 1968).
For these reasons empirical relationships were considered between slaking and liquid limit as well as unconfined compressive strength and slaking (durability) of clayey admixtures stabilized with 4% and 12% cement, at 90%, 95% and 100% compaction and cured for 7 and 28 days (Table 5), (Stavridakis, 2003a).
Table 4d. Clay mixtures stabilized with 12% cement and cured for 7 days
and their geotechnical characteristics.
Finally, composite diagrams were drawn containing all the afore-mentioned parameters, to show the influence of liquid limit (composition of a soil) and amount of cement on the development of strength and durability of bentonite - kaolin - sand mixtures stabilized by cement at a constant degree of compaction and curing time (Fig. 1 - Fig. 6).
The proposed classification system includes the liquid limit and a range of cement percentages from 4% to 12% and defines areas on liquid limit of clay mixtures which influence the engineering (strength, durability) and stabilization parameters (amount of cement, degree of compaction and curing time) to be used for stabilization.
The above classification diagrams (Fig. l - Fig. 6) yield sufficient information on "antithesis" between strength and slaking values. In these diagrams, clay mixtures stabilized with cement show extremely high slaking (from low durability to complete disintegration) while exhibit strength 1754 kN/m2 (ultimate accepted value in Britain).
Finally these proposed classification diagrams assess strength values corresponding to ultimate slaking (durability) values, (Morgenstern and Eigenbrod, 1974), for efficient cement - stabilization.
The afore-mentioned specifications mean that for safety reasons in construction works (soil - cement mixing - stabilization) the slaking (durability) should be considered as a serious safety factor together with strength especially under environmental conditions of wetting - drying and/or potential stresses (i.e. horizontal stresses due to seismic events or movements due to landslides).
Table 5. Empirical relationships between unconfined compressive strength,
slaking and liquid limit (Figs. 1-6).
Figure 1. Influence of liquid limit and cement percentage on strength and slaking of
cement-stabilized clayey mixtures at 100% compaction cured for 28 days.
Figure 2. Influence of liquid limit and cement percentage on strength and slaking of
cement-stabilized clayey mixtures at 95% compaction cured for 28 days.
Figure 3. Influence of liquid limit and cement percentage on strength and slaking of cement - stabilized clayey mixtures at 90% compaction cured for 28 days.
RESULTS AND DISCUSSION
At the liquid limit, montmorillonite clays (bentonite) contain large amounts of water. The water is present in two states: 1) oriented water surrounding the clay mineral particles and 2) non-oriented water in the interstitial pores. It can be considered that a small amount of pressure is adequate to remove the pore water (high compressibility - deformability) and that considerable pressure is probably necessary to remove the oriented water [high number of positions of exchangeable ions - high zeta (z) potential - high magnitude of electrical charges (forces) on the surface of clay mineral].
In the case of kaolinite clays the amount of water at the liquid limit is small and relatively large part of it is pore water. There would be less oriented water than in the case of montmorillonite clays and perhaps the orientation would be less perfect [low number of positions of exchangeable ions - low zeta (z) potential - low magnitude of electrical charges (forces) on the surface of clay mineral]. As a consequence, on the application of pressure, the water would be removed and the particles would find it easier to adjust themselves one to the other so that compressibility and deformability would be less than in the case of montmorillonite clays (Grimshaw, 1971; Grim, 1962).
Conclusively the liquid limit reflects the composition of natural clayey soils and in particular the nature and properties (physical and engineering) of the principal (dominant) clay mineral, (Stavridakis, 2004). According to the afore-mentioned the development of strength and durability of cement - stabilized clayey soils is strongly related to the composition of these natural clay soils. Also the above based and on the number of liberated calcium ions from the cement occupy the positions of exchangeable ions on the clay minerals. The number of these positions depends upon the proportion and type of clay (i.e. montmorillonite or kaolinite) and is related to the liquid limit of the clay soil.
In addition the calcium ions react with the silicates and aluminates from the soil minerals. The latter coat the grains to form a skeletal structure of considerable strength and durability. Therefore classification diagrams were prepared to relate the liquid limit (composition of a clayey - soil) with the engineering properties such as unconfined compressive strength and slaking (durability), (Fig. l - Fig. 6).
The used silt - clay mixtures contained more than 35% particles passing No200 (74µm) sieve according to HRB classification system, stabilized with 4% and 12% cement, at 90%, 95%, 100% compaction and cured for 7 and 28 days, (Stavridakis, 2003c).
These mixtures consisted mainly from bentonite (montmorillonite - active clay mineral) and/or kaolin (kaolinite - inactive clay mineral) which exhibit extreme engineering behaviour. In all Figs. (1-6) the main curves of LL = 40% and LL = 60% divide the siIty - clayey mixtures into three groups which are the following:
1) mixtures with LL < 40% which are stabilized successfully with economical amounts of cement (Godin, 1962),
2) mixtures with 40% < LL < 60% which are stabilized successfully with larger amounts of cement than in the first group,
3) in mixtures with 60% < LL the expansive clay minerals are dominant and are stabilized successfully with larger amounts of cement than in the other groups or lime (Croft, 1968).
Generally the development of liquid limit curves from lower to higher values (Fig. l - Fig. 6) is corresponded to a decrease of unconfined compressive strength and durability values. However all Figs. from 1 to 6 showed an "antithesis" between strength and slaking. Same values of slaking (durability) show an increase of strength as the percentage of cement and liquid limit values increase (improved cement - stabilization conditions). By contrast, same values of unconfined compressive strength show an increase of slaking (decrease of durability) as the percentage of cement and liquid limit values increase. It is obvious that the potential cement bonds developed between soil grains during this improvement have a profound influence on the development of slaking (durability).
In the afore-mentioned comments the developed cement bonds (% cement) between soil grains are not enough to decrease slaking (increase durability - bonding effect) when the value of strength (bearing capacity) is constant and the liquid limit increases.
Finally the comparison between the diagrams of unconfined compressive strength - slaking (Figs. 1-6), indicated that the values of strength decrease and slaking increase as the percentage of compaction and the curing time decrease, (Stavridakis, 2003b).
Figure 4. Influence of liquid limit and cement percentage on strength and slaking of cement - stabilized clayey mixtures at 100% compaction cured for 7 days.
Figure 5. Influence of liquid limit and cement percentage on strength and slaking of cement - stabilized clayey mixtures at 95% compaction cured for 7 days.
Figure 6. Influence of liquid limit and cement percentage on strength and slaking of cement - stabilized clayey mixtures at 90% compaction cured for 7 days.
The classification system in present work for silt - clay mixtures stabilized by cement defines the following:
(1) An ultimate slaking value (maximum acceptable slaking or minimum acceptable durability), for slake - durability index test, accepted for efficient cement - stabilization. This ultimate slaking value is calculated for each degree of compaction (90%, 95%, 100%) and curing time (7 and 28 days). Such an ultimate slaking value is achieved if clay - mixture having a liquid limit of 40% is treated using 4% cement. To achieve the same ultimate slaking value in clay - mixture having a liquid limit of 60% would require the addition of 12% cement. The study in present work shows that the liquid limit is entirely adequate for predicting the maximum amount of slaking that can be expected, (Morgenstern and Eigenbrod, 1974).
(2) The value of 1754 kN/m2 is accepted as ultimate strength value for efficient cement stabilization (acceptable limit in Britain) for cement < 4%. In particular the ultimate values for strength of 3600 kN/m2 for 28 days and 2200 kN/m2 for 7 days of curing, 2600 kN/m2 for 28 days and 1754 kN/m2 for 7 days of curing, 1800 kN/m2 for 28 days and 1754 kN/m2 for 7 days of curing, were determined for 100%, 95%, 90% compaction respectively, for cement ³ 4%.
Table 6. Developed relationships, between unconfined compressive strength and slaking
for liquid limits from 15 to 60% (Figs. 1-6).
In all Figs. (1-6) the experimental results indicate good correlation between unconfined compressive strength and slaking and strong influence of cement, degree of compaction and curing time on strength and slaking values. Also a strong influence of liquid limit on slaking is obvious in diagrams of liquid limit - slaking (Fig. l - Fig. 6). The higher the values of liquid limit are, the higher the values of slaking (lower values of durability).
Finally the diagrams (Fig. l to Fig. 6) were made as follows: Clayey admixtures of 15, 25, 35, 40, 45, 50, 55, or 60% liquid limit give a pair of slaking values on 4 and 12% cement curves.
These slaking values correspond to unconfined compressive strength values on 4 and 12% cement curves and form curves y = axb, where y = unconfined compressive strength end x = slaking (%) for the above mentioned liquid limit values respectively (Table 6). From the known values of unconfined compressive strength, at 4 and 12% cement could be calculated the values of unconfined compressive strength at 6, 8 and 10% cement by using an approximate linear relationship, according to literature (Ingles and Metcalf 1972), y = a + ßx where y = unconfined compressive strength and x = % cement. The unconfined compressive strength values on the liquid limits curves correspond to slaking values. The values of strength and slaking at 6, 8, 10% cement, define negative power curves y = axb where y = unconfined compressive strength and x = slaking (%) for each cement percentage (Table 7).
Table 7. Developed relationships, between unconfined compressive strength
and slaking for 6, 8 and 10% cement (Figs. 1-6).
Conclusively a direct approach to the solution of a soil engineering problem consists of first measuring the soil property needed and then employing this measured value in some rational expression to determine the answer to the problem, such as in soil - cement stabilization. For these reasons, sorting soils into groups showing similar geotechnical behaviour may be helpful. Such sorting is the classification of engineering behaviour (strength and durability) of cement treated clayey mixtures of present work. Most soil classifications employ very simple index - type tests to obtain the characteristics of the soil needed to place it in a given group (Leong and Rahardjo, 1998).
The most commonly used characteristics are particle size and plasticity. In the present research work the HRB soil classification system was used regarding the particle size and liquid limit groups. A combination and coincidence of the above soil's physical (liquid limit and grain size) properties with the limits of geotechnical parameters for efficient cement stabilization of theory (strength > 1754 kN/m2 and LL < 40%) and present work [ultimate strength, slaking (durability) and liquid limit], (Fig. l - Fig. 6), form rational expressions available for analysis of the solution for engineering problems in soil - cement stabilization [surface or deep (cement deep mixing - CDM)].
The conclusions derived from this study are described below:
(1) The influence of liquid limit on the development of strength and durability is obvious from lower to higher values of liquid limit curves in classification diagrams. The higher the values of liquid limit are, the lower the values of strength and durability.
(2) The liquid limit curves divide the silty - clayey mixtures in three groups concerning their stabilization by cement:
(a) the first group with LL < 40% includes mixtures which are stabilized successfully with economical amounts of cement,
(b) the second group with 40% < LL < 60% includes mixtures which are stabilized successfully with larger amounts of cement than in the first group,
(c) the third, group with LL > 60% includes mixtures which could be stabilized successfully with uneconomical (very large) amounts of cement or lime.
(3) Same values of slaking (constant bonding effect) correspond to increased strength as the percentage of cement and liquid limit values increase (improved cement - stabilization conditions).
(4) Same values of unconfined compressive strength correspond to increased slaking (decrease of bonding effect) as the percentage of cement and liquid limit values increase [importance of slaking (bonding effect) evaluation].
For safety reasons in construction works the slaking (durability) should be considered as a serious safety factor together with strength.
(5) The classification diagrams define ultimate values of strength and slaking (durability) for efficient cement stabilization.
The combination of these ultimate values contributes to the safety of construction works concerning the soil - cement stabilization.
(6) The experimental results indicate good correlation between unconfined compressive strength and slaking, giving power curves.
(7) The increase of liquid limit influences positively the slaking. This relation revealed power curves with good correlation coefficient.
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