Presentation and Assessment of Clay Influence on Engineering Parameters of Cement-Treated
B.Sc., M.Sc., Ph.D., F.G.S., Lecturer, Laboratory of Soil Mechanics and Foundation Engineering Geotechnical Engineering Division, Department of Civil Engineering,
The improvement of physical and engineering soil properties by cement protect the natural environment. Through cement - stabilization the plasticity of a problematic soil (swelling or soft) is reduced while its compressive strength, load – bearing properties and durability are improved. While the conventional cement – stabilization methods are used mainly for surface treatment 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 cement – soil mixing). Cement – stabilization (surface or deep) methods have a basic target to find the most efficient and economical method of mixing cement with soil so that the properties of a problematic soil become more like the properties of a soft rock.
The clay mineral composition of a soil is one of the most dominant factor determining the chemical, physical and engineering properties of this soil. Mineralogy of bonding [type of clay (bentonite – kaolin) – cement] of cementing material (cement or other cementing additive) is an important property that controls strength and durability. The efficacy of cement – stabilization is a function of portion and mineral type (active or inactive). Study and prediction of potential deterioration in durability due to climatic wetting and drying (i.e. problem of durability in cement treated soil due to fluctuation of water table) could be made by using the slake durability test developed by Franklin. Also this test contributes to the improvement of engineering properties of soils by relating the cement stabilization parameters (percentage of cement, compaction, curing time) and composition of clayey soils with the potential bonds developed between cement and soil grains (Stavridakis, 2003b).
For these reasons, triangular and three dimensional diagrams were prepared on the basis of unconfined compressive strength, slaking and liquid limit experimental results of cement stabilized bentonite – kaolin – sand mixtures. These diagrams classify the cement stabilized clay mixtures in durable and non durable and also define areas of strength, slaking and liquid limit of clay mixtures for efficient cement stabilization. Finally empirical relationships between unconfined compressive strength and slaking indicate the strong influence of bentonite and sand on these geotechnical parameters.
KEYWORDS: cement, clay stabilization, strength, slaking, bentonite.
Problematic soils (swelling, soft and loose soils) have been ignored for long time, in favour of more quality soils with reduced technical difficulties and lower construction costs. Alternative areas for construction became however more and more important during the last decades due to growing shortage of better quality soils for construction (Pinto et al. 2003). Geotechnical engineering community have recognized that problematic soils may result in considerable distress and consequently in severe damage to overlying structures. Major geotechnical problems in construction involving the aforementioned soils are due to their low strength, durability and high compressibility.
Confronted with these problems, a suitable ground improvement technique is needed for surface or deep excavations in problematic soils for stability, durability and deformation control. Cement soil mixing (surface or deep) is one of the alternatives (Andromalos et al, 2000). An increment in strength and durability, reduction in compressibility, improvement of swelling and squeezing characteristics are the main aims of cement stabilization method. Techniques such as Soil Mixing, Cement Deep Mixing (CDM), Soil Mixed Wall (SMW), and Deep Soil Mixing (DSM) are well known (Kaushinger et al, 1992).
The cement - stabilization (surface or deep) has been used for many diverse applications including foundations (to halt the settlement of existing structures), retaining structures (for temporary support of excavations), liquefaction mitigation (to cut off water infiltration into excavations and sewer lines), water control (foundation curtains that reduce water loss under structures), pollution control by using stabilization/solidification (S/S) techniques that aims to immobilize the source of contaminants (remediation of contaminated land prior to redevelopment). Bonding (clayey soil grains – cement) determines the ease with which microfractures during slaking process, can propagate through the specimens by disrupting the structure and breaking the bonds within the cemented clayey mass.
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, in transportation engineering problems (Owttrim, 1988), in dam construction when the dispersive properties of clays (Na – montmorillonite) are not suitable accounted for the design of an earth dam or even in grouts where problems of durability in cement – chemical grouted soil arise by fluctuation of water table.
Controlling water flow through soil is important for various reasons. Water leakage into underground openings with the lowering of the ground water table and consolidation settlements of clay layers as a result, can cause surface damage to buildings. Water flow through storage of waste material causes an undesirable contamination of the ground water (Hassler et al, 1987). In order to reduce the water flow through soil – cement mixing is a commonly used method. In particular, in dams construction, soil – cement stabilization (soil – cement mixing, surface or deep) is a very important method to ensure water – tightness and tightness because water seepage under dams causes energy loss and erosion of the dam core.
Soil – cement mixing is one of the principal techniques to improve the foundation of structures. The water intrusion during environmental conditions of wetting, in active clay minerals, such as bentonite, creates swelling and disrupts the interparticle contacts and cement bonds. In contrary to the above, in drying environmental conditions the clay – cement system shrinks as the water moves farther apart from this system with result of breaking the soil – cement bonds. Type of bonding agent (i.e. cement), degree of bonding (% of cement) and percentage of dominant active mineral, in present work, bentonite, are more important factors that control strength (bearing capacity) and durability [strength of cement – soil bonds, internal strength – (Lindner, 1976)]. From this point of view a research was carried out in order to study and classify the influence of composition of clayey soils on their stabilization by cement (Stavridakis, 2003a). In this effort both slaking and unconfined compressive strength as well as liquid limit tests were carried out on clay mixtures stabilized with 4% cement, compacted at 95% and cured for 28 days (Table 1).
Triangular and three dimensional (3D) diagrams were prepared to reveal the development of strength, slaking (durability) and liquid limit through all of these clay mixtures contained various percentages of bentonite, kaolin and sand (Arpàd and Làszlò, 1988). Diagrams of sections with 5%, 10%, 15%, 20%, 25% and 30% bentonite revealed the strong influence of bentonite on strength, slaking and liquid limit (Stavridakis, 2004). Also relationships between strength and slaking showed that the higher the percentage of bentonite the lower the strength and durability. The relationships revealed also the influence of sand on the above geotechnical parameters (Stavridakis 2003c). Finally the aforementioned diagrams defined areas of best cement stabilization effect between minimum and maximum percentages of bentonite.
DESCRIPTION OF MATERIALS
It seems that packing density (% of compaction), degree of bonding (% of cement), type of cementing additive (i.e. cement) and bonding material, existence of dominant clay minerals (active or inactive) and microfractures affect both unconfined compressive strength and slake durability index (durability) in a similar fashion, i.e. these factors cause both unconfined compressive strength and slake durability test (durability) results to increase or to decrease. Whereas particle size distribution (interlocking effect) and percentage of sand (bearing capacity) have a contradictory effect on unconfined compressive strength and slake durability results of cement treated soils. These physical factors cause slake durability index to increase and unconfined compressive strength to decrease or vice versa (Koncagül and Santi, 1999).
Also the various clay minerals in a clayey soil influence the amount of volume expansion or swell and the ease with which it can be slaked or stabilized with cement (Dhakal et al, 2002).
Table 1. Clayey mixtures designs and their geotechnical - engineering characteristics
|No||Clayey mixtures||Liquid limit (%)||28 days of curing|
(100 - Id2) (%)
|*30M62.5K7.5A 30M = 30% Bentonite, 62.5K = 62.5% Kaolin, 7.5A = 7.5% Sand|
Kaolinite and well organized (well crystallized) soil minerals appear to have little effect on the hydration of cement and hardening proceeds normally by using small amounts of cement. 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).
For these reasons unconfined compressive strength and slake durability tests were carried out on clayey admixtures stabilized with 4% cement, compacted at 95% and cured for 28 days, consisted of two clayey soils namely natural Bentonite – Kaolin and Sand (Table 1).
In this research commercially available kaolin and sand were used, while the bentonite was from a natural source (Table 2).
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|
The qualitative characteristics of clays and sand are shown in x – rays diagrams of Figures 1, 2 and 3.
The sand used was fine to medium grained (74µm/840µm) with uniformity coefficient 2.19. The particle size distribution of sand is shown in Figure 4.
Finally the particles of most clayey admixtures retained on sieve No200 (74µm) are less than 65% and they can be described as silt - clay materials in accordance with the Highway Research Board Classification System (Yoder, 1967).
Figure 1. X-ray diffraction traces of sand
The test programme included 66 (22x3) specimens of unconfined compressive strength as well as 242 (22x11) specimens of slake durability index tests for the clayey mixtures.
Figure 2. X-ray diffraction traces of Kaolin
Figure 3. X-ray diffraction traces of Bentonite
TESTING PROCEDURE – SAMPLE PREPARATION
The behaviour of bentonite (swell effect), regarding the cement stabilization, is characterized by Croft (1967) and others as active and requires large amounts of cement for efficient cement stabilization.
Figure 4. Particle size distribution of sand
The behaviour of kaolinite is characterized as inert and generally develops satisfactory strength and durability with economical amounts of cement after short curing periods (early strength and durability).
The slake durability test (Franklin and Chandra, 1972) used to predict the potential deterioration of durability due to climatic wetting and drying.
In this test the apparatus combines the effect of both wetting – drying and abrasion (bonding effect) in order to accelerate the rate of weathering that can be attained by water immersion.
In relation to the above the development of strength and durability of cement stabilized soils depends upon the effectiveness of cement on the mineralogical composition of a clayey soil (Koncagül et al, 1999).
For these reasons in this research work the following tests were performed:
a) The slaking (100-Id2) was measured using the device and testing procedure developed by Franklin and Chandra (Photograph 1 and Figure 5), (Table 3).
Table 3. Classification and characterization of durability (after Franklin and Chandra, 1972)
|Slake durability index Id2
Photograph 1. Slake-durability test equipment
Figure 5. Critical dimensions of Slake-Durability test equipment
The cylindrical specimens tested in slake durability test had a diameter of 35.5mm and were 23.7mm in length.
Finally the slake – durability index (Id2 – second cycle) was calculated as the percentage ratio of final to initial dry sample weight.
b) The unconfined compressive strength was measured using a commercially available device named Versa Tester (Soil test Inc).
The cylindrical specimens tested in unconfined compressive strength had a diameter of 35.5mm and were 71mm in length. The displacement rate was 0.6604mm/min.
- The specimens were prepared at the optimum moisture contents and maximum dry densities (Standard Proctor test) according to BSI 1377 d2 test 12 and BSI 1924.
- The cylindrical samples were prepared according to ASTM 1632-96.
- The Atterberg limits were estimated according to BSI 1377.
- The percentage of cement was selected to give noticeable change in strength and durability (Bell, 1976).
- A compaction level of 95% of the Standard Proctor maximum dry density obtained in the laboratory is often taken as the minimum degree of compaction that should be obtained in field projects. This compaction level was adopted for this study.
- The curing time was related to the acceptable compressive strength for satisfactory stabilization (Ingles and Metcalf, 1972; Bell, 1978).
Finally the clay – sand mixtures were cured at approximately 95.5% relative humidity and 21oC temperature.
PRESENTATION AND DISCUSSION OF RESULTS
Clay soil is a variable and complex material, but because of its availability and low cost it frequently is used for construction purposes. At a particular location, however, a clay soil may not be wholly suitable for the desired purpose.
This soil usually possesses medium or high or even very high plasticity and therefore presents soil which would be likely to be more suspect as far as construction is concerned (Bell and Tyrer, 1989).
In relation to the above problematic soils (swelling or soft) contain clay minerals that exhibit medium to high volume change upon wetting. The large volume change upon wetting causes extensive damage to structures, in particular, light buildings and pavements (Fredlund and Rahardjo, 1993).
This volume change depends upon the portion and type (i.e. bentonite) of expanding clay mineral in a clayey soil.
Also pollution of the environment due to leakage from waste repositories is a well – known and wide spread problem.
The choice of liner depends on the design of the waste deposit and its water balance.
Many different barrier materials exist, for example: plastic membranes, geotextiles, bentonite, compacted clay, cement stabilization etc.
As bentonite is a material with a very low hydraulic conductivity, it is obvious that the higher the percentage of bentonite, the lower the permeability.
However, the permeability is not the only criterion to fulfil the requirements to create waste repositories with cement stabilized clay; strength and durability of this barrier material are also very important so that this construction will remain intact over a long period of time.
Normally the amount of bentonite, in a sand - bentonite mixture used as a barrier constitutes 4-13% of the dry weight of this sandy material (Sällfors and Öberg, 2002).
Greater amounts of bentonite tend to form around the sand grains and the mixture becomes plastic and consequently difficult to compact developing simultaneously low strength and durability.
So these limits of bentonite from 4 to 13% define areas for efficient cement stabilization of a clayey sand mixture by using economical amounts of cement.
In present research work the prepared diagrams define areas of efficient cement stabilization and classify the cement stabilized clay mixtures in durable and non durable.
However the main task of this present work is to determine the minimum and maximum percentages of bentonite – kaolin and sand necessary to fulfil the strength and durability requirements for best cement stabilization effect.
The triangular diagram of Figure 6 shows the designs and development of clay – sand mixtures, the positions of experimental results of strength, slaking and liquid limit, the sections of 5%, 10%, 15%, 20%, 25%, 30% bentonite (Figures 10, 11, 12) and the sections of experimental results.
Figure 7 shows the development of unconfined compressive strength.
In these diagrams maximum strength values appear along the area of 60 – 65% sand.
Figure 8 shows the development of slaking.
Figure 6. Representation in a triangle of schemes for the development of Figures 7-12
Maximum values of slaking appear on 20% sand and minimum on 60 -65% sand.
Comparing Figure 7 with Figure 8 unconfined compressive strength values increase and slaking decrease from 20% to 60 -65% sand.
Figure 9 reveals the development of liquid limit values of clay mixtures.
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) and giving an ultimate unconfined compressive strength value of 1754 kN/m2 (acceptable limit in Britain) after 7 days of curing.
The above mentioned soils, 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).
Means and Parcher (1963), Croft (1968) suggest that soils with large liquid limits (>60%) and plasticity indices (>25%) invariably contain expansive clay minerals such as montmorillonite and react with large amount of cement.
Figure 10 shows the influence of sand and bentonite on the development of unconfined compressive strength of clay mixtures.
The above results indicate that the greater is the amount of sand the higher is the strength (bearing capacity).
Figure 7. Development of unconfined compressive strength of clay-sand mixtures treated with 4% cement, compacted at 95% and cured for 28 days
Also this figure shows that the increase of bentonite content decreases strongly the values of strength.
Figure 11 shows the strong influence of bentonite content on slaking values. These sections indicate that the greater is the amount of bentonite the higher is the slaking.
It seems that the amount of sand has a contradictory effect on slaking in relation to unconfined compressive strength (Koncagül and Santi, 1999), from 0% to 20% of sand the strength increases (Figure 10) and slaking increases (Figure 11), this could be attributed to the high amount of kaolin. Therefore the higher the percentage of kaolin the larger the number of contacts on grain – kaolin and the greater the amount of cement needed to develop strong bonds for enough durability. It is obvious that 4% cement is not enough to control slaking in cement stabilized clay mixtures containing 0 – 20% sand, 0 - 30% bentonite and kaolin.
The sections of Figure 12 revealed the strong influence of bentonite and kaolin on liquid limit values, the higher is the amount of bentonite and kaolin the higher is the liquid limit.
By comparing the triangular diagrams of Figures 7, 8, 9 with Figures 10, 11, 12 an area of clay – sand mixtures, with LL£40%, showed maximum cement stabilization effect, between 20% and 60-65% of sand and bentonite content less than 15% exhibiting high strength values between 2800 – 6200 kN/m2 and slaking less than 60% approximately.
Figure 8. Development of slaking of clay-sand mixtures treated with 4% cement, compacted at 95% and cured for 28 days.
Also the above comparison revealed a second area of clay – sand mixtures, with liquid limit between 26% and 40% which showed minimum stabilization effect between 20% and 60-65% of sand and bentonite content between 15% and 30% exhibiting low strength values between 3700 kN/m2 and 2400 kN/m2 as well as slaking between 39% and 91% (extremely high slaking – very low durability of cement stabilized clay – sand mixtures).
Finally a third area of clay – sand mixtures appeared, with liquid limit higher than 40% between 20% and 60-65% of sand and bentonite content between 15% and 30% exhibiting low strength values lower than 2800 kN/m2 as well as slaking between 60% and 100% (complete disintegration of cement stabilized clay – sand mixtures). The third area reveals a minor cement stabilization effect on the clay – sand mixtures.
In accordance with the above the diagrams of Figures 7 and 8 yield sufficient information on “antithesis” between strength and slaking values. In these diagrams, clay mixtures stabilized with cement contained 30% of bentonite showed extremely high slaking (100% - complete disintegration) while they exhibited strength values varying between 1500 kN/m2 and 2500 kN/m2.
The above mentioned, means that for safety reasons in construction works the slaking (durability) should be considered as a serious safety factor together with the strength especially in environmental conditions of wetting – drying or soaking and initiation of potential horizontal stresses (i.e. during seismic events or movements due to landslides).
Figure 9. Development of Liquid limit of clay-sand mixtures treated with 4% cement, compacted at 95% and cured for 28 days.
In the previous mentioned first area the bentonite content is coincided with the percentage proposed from Sällfors and Öberg regarding the use of bentonite as barrier material.
It is obvious that greater amounts of bentonite than 15% tend to form around the soil grains and the mixture will become plastic.
In relation to cement stabilization clay mixtures contained more than 15% bentonite, revealed lower strength and durability as it is appeared in the previous mentioned second area.
The reason for the aforementioned results is that the greater the percentage of bentonite in a clay – sand mixture, the greater the number of positions of exchangeable ions and the greater the amount of cement to be used for maximum stabilization effect.
The above mentioned related with the number of calcium ions liberated from the cement of a soil – cement mixture, occupied the positions of exchangeable ions on the clay minerals.
The number of these positions depends upon the proportion of clay (% bentonite) in a soil cement mixture and the type of clay minerals (i.e. bentonite) present and is related to the liquid limit which reflects the composition of this soil.
Figure 10. Sections according to Figure 6 and 7 for the assessment of bentonite and sand influence on strength
Figure 11. Sections according to Figure 6 and 8 for the assessment of bentonite and sand influence on slaking
Figure 12. Sections according to Figure 6 and 9 for the assessment of bentonite and sand influence on liquid limit
Figure 13. Effect of bentonite and sand on strength and slaking of cement stabilized clay mixtures
The results plotted in Figure 13 show negative power relationships between the unconfined compressive strength and slaking (Table 4).
The influence of sand on strength and slaking it is obvious, same values of slaking (constant bonding effect) correspond to increased values of strength (bearing capacity) as the amount of sand increases.
Table 4. Empirical relationships between unconfined compressive strength and slaking (Fig.13)
|No.||Sand (%)||y = axb
y = qu (kN/m2)
x = slaking (100 - Id2) (%)
|N.B. *cc = correlation coefficient|
Same values of strength correspond to increased values of slaking (decrease of durability) as the amount of sand increases. The reason for this is that the developed cement bonds between soil – sand grains are not enough to control slaking.
Figures 14 and 15 revealed in three dimensions the developed surfaces of strength and slaking. The developed surface of strength values is the opposite of slaking surface.
Figure 14. Development of strength surface of cement stabilized clay-sand mixtures (Fig. 7)
Figure 15. Development of slaking surface of cement stabilized clay-sand mixtures (Fig. 8)
Clay mixtures, contained less than 15% of bentonite revealed high values of strength and medium to low values of slaking (effective cement stabilization).
In contrary to the above, clay mixtures contained 15 – 30% of bentonite exhibited lower values of strength and high – extremely high (disintegration) values of slaking.
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