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Useful Lifetime and Suitable Thickness of Soil-Lime Mixture
Posgrado de Ingeniería, Universidad Autónoma de Querétaro J. B. Hernández Zaragoza, C. López-Cajun Posgrado de Ingeniería, Universidad Autónoma de Querétaro | |
TECHNICAL NOTE |
ABSTRACT
Among the several solutions suggested for foundations on expansive clay soils, one can cite the improvement of soils using Lime. Although this procedure has been efficient, its utilization has been constrained due to the un-knowledge of its lifetime or effectiveness in the long span (minimum time required for the project at hand) and the soil thickness that must be treated. In this work, the results observed for the lifetime of the mixture soil-lime and the minimum soil layer thickness for guaranteeing mixture functionality are discussed.
Keywords: Expansive soils stabilization, useful lifetime, Lime treated clays, soil aging, lime.
INTRODUCTION
Since long time ago, expansive soils have been causing great difficulties in the construction industry. This is due to the endless expensive damage they cause. Expansive soils are distributed worldwide in regions with semiarid climate. The main feature that defines its behaviour is its mineralogical structure which, in turn, is the one that influences its physico-chemical and mechanical behaviour.
Currently, there exists an endless number of techniques that have been proposed for solving the expansive soils problem. Among others, one can mention the stabilization techniques with different additives, pre-humidifying the material, replacing the expansive material, etc. One stabilization technique with chemical substances is the one using Lime. As a matter of fact, this has been used from ancient times and, in many cases, with satisfactory results. In fact, Lime stabilization has been used in ancient constructions such as the Chinese Wall and some Roman tracks during the flourishing Roman Empire. However, about thirty years ago, rational studies for the stabilizations mechanisms as well as the different modifications undergone for stabilized soils were undertaken in a rational fashion. Lime application is mainly concentrated to fine soils (Malagon, 1986).
Lime application has been used to fine soils in order to increase its resistance and decrease its sensibility to volumetric changes due to water content.
In the stabilization process of clay material with hydrated Lime, there are some chemical reactions mainly, because CaCO3 formation. This causes that clay soil remains relatively inert after treatment. Therefore, it is important a kinetic study (Castaño and López-Lara, 2001) to determine the amount of CaCO3 through time in order to support the application of this technique.
The characterization technique used in this study was X-ray diffraction via powders. This technique provides semi-quantitative information on crystalline forms and thus of the mineralogical soil composition. The technique allows to follow the changes between the soil and the additive used for stabilization. Moreover, it also allows to obtain all the mineralogical information even for small samples (approximately 1 gram) in relatively short times (about 30 min.) (Castaño and López-Lara, 2001).
On the other hand, the temperature at which the study was performed, was chosen as that which guarantee no changes which would produce that the reaction were destructed, either partially or totally, any of the minerals, since this is unlikely to occur in a natural-aged soil. Such temperature was determined via calorimetric techniques, specifically, thermal differential analysis and gravimetric thermal analysis. The first one provides exact information of the temperature at which a physical or chemical change could the sample undergo for example, water vaporization, losing of organic material, any structural transformation or else the formation or decomposition of some minerals. The second one, besides providing temperature information at which a change occurs, it also provides the amount of mass being lost. Results of calorimetric analysis, together with powders X-ray diffraction, provide worthy and reliable information of soil stability and composition when it is subjected to a given temperature (Castaño and López-Lara, 2001).
Furthermore, in this research work, a test section was built in field with the soil-lime mixture, using the same expansive soil of the site. The calcium oxide used was a commercial one previously reported Castaño, López-Lara and Zepeda (1999). In that study, it is proposed an 8% (approximately) of calcium oxide in relation to dry weight.
EXPERIMENTAL STUDY
Materials Used
This work was based upon the already known method of soil stabilization with a comercial Lime, i.e., to compensate for possible volumetric changes undergone for expansive clays under moisture variations.
Soil-Lime Mixtures Preparation for Studying Time Effectiveness
For performing time effectiveness, 12 soil samples were prepared. These were mixtures of 3 gr each, of soil and 8% (weight) of Lime. They were heated at 200ºC in an electric muffle for 1680 hours (about 10 weeks). Then samples were analyzed via X-ray for quantifying the amount of calcite being formed with time. Quantification was done by measuring the area under the peak with the highest intensity for calcite (2q = 29.45º, JCPDS No. 24-27) for different times. Afterwards, in order to determine quantitatively the amount of CaCO3 formed in each sample, at different times, a standard curve was built from the area quantification of X-ray diffraction peaks as related to composition. For that, mixtures of natural soil, washed with ammonia chloride to eliminate calcite, and different (weight) percentages of pure calcite were made. Mixtures were grounded in an agatha mortar and passed through a Tyler sieve 250. Then, X-ray diffraction patterns were obtained for computing the area under the diffraction peak located at 2q = 29.45º (Castaño and López-Lara, 2001).
Layer of a Soil-Lime Mixture
For determining the functional effective thickness of an expansive clay soil treated with Lime, an experiment in situ was performed. The experiment was to stabilize a soil layer with Lime and then saturate it. The aim of this experiment was to determine the moisture content at different depths and to observe the thickness where changes of water content, as related to the originally placed, could have been changed, Fig. 1.
Figure 1. Experiment in-situ of a stabilized soil (Lime)
(Out of scale schema, A, B and C indicate the sampling location).
RESULTS AND DISCUSSION
Useful Lifetime of a Soil-Lime Mixture vs. Time
With the computed values of the area under the calcite diffraction peak for samples subjected to 200ºC, Figs. 2 and 3, the percentage and the amount, in grams, of calcite in each of the samples for different times, was computed by extrapolation. Results are shown in Fig. 4. In that figure can be observed that calcite formation follows an exponential behaviour. Moreover, such formation reaches its maximum value when it has been treated for 1008 hours. After 1176 hours of treatment, it can be observed that the amount of calcite remains constant. This indicates that the formation-decomposition cycle has reached equilibrium (Castaño, López-Lara, and Zepeda, 1999).
Figure 2. X-ray Diffraction for the soil treated with calcium oxide at 8% in weight and aged at different times
(Castaño and López-Lara, 2001).
Figure 3. X-ray Diffraction for the soil treated with calcium oxide at 8% in weight and aged at different times
(Castaño and López-Lara, 2001).
Figure 4. Plot of CaCO3 formed vs. Time of samples aged at the laboratory
(Castaño and López-Lara, 2001).
To relate the lab aging time with real aging of some soils treated with Lime, already placed under some constructions, areas under the peaks corresponding to calcite of the treated soils with Lime and naturally aged, were computed. This area was related directly to the weight percentage of calcite via an standard curve. In turn, the percentage was related to the grams of the formed calcite, and then determine how many treatment hours corresponds according to the graph shown in Fig. 4. The obtained results are shown in Table 1.
Table 1. Comparison of aging time with the real time of samples naturally aged
(Castaño and López-Lara, 2001).
From Table 1 it can be observed that a stabilized soil with Lime and naturally aged for 6 years corresponds, approximately, to 2 weeks of the total kinetics of 10 weeks of the stabilization determined at the laboratory
(Castaño and López-Lara, 2001).
Recommendable Layer of Stabilized Soil with Lime in situ
Moisture content results obtained at different depths for the soil-Lime layer are shown in Table 2 and plotted in Fig. 5. Mixture initial moisture content was 41%. From Fig. 5, it can be observed that in the first 60 cm moisture content diminished. It can be considered that such thickness is the one subjected to moisture content changes during rainy and dry seasons. Therefore, that thickness would be the minimum to be stabilized to assure that the underneath soil does not undergo moisture content variations, and thus volume.
Table 2. moisture content (w) for the soil-Lime layer
Figure 5. Profile of Moisture content vs. Depth for the soil-Lime layer for differents boreholes.
CONCLUSIONS
Useful life of Lime Treated Soils
During the soil-treated stabilization process at the laboratory for ten weeks, there was a calcite formation which increased exponentially, reaching its maximum value at 1008 h (6 weeks); from there on, it remained constant.
It was found that the Lime-stabilized soil and aged for 6 years corresponds approximately to 2 weeks in the stabilization kinetic process at laboratory level. Thus, one can conclude that the real situation of the soil-lime mixture (montmorilonite clay of Jacarandas, Querétaro, México) is developing still the Lime reaction. Therefore, the effectiveness of this treatment will last for more years.
According to the kinetics, it can be confirmed that clay soils of the montmorilonite type, treated with Lime since many years, there is no evidence indicating that the process would be reversible, i.e., that the stabilized soil recovers its initial plastic properties. This has been an speculation since long time ago (Castaño, López-Lara and Zepeda, 1999).
Recommendable Thickness of a Soil Stabilized with Lime in-situ
For thickness of soil-Lime at 7%: The moisture content profile at different depths shows that, starting from an initial moisture content of 41%, the functional effective thickness would be of, approximately, 60 cm, because it can be observed that this thickness is subjected to water content depending on weather conditions, i.e., either rainy or dry seasons. It can also be observed that this thickness of stabilized soil works as an impermeable barrier of the expansive soil underneath, because the variations of water content would remain inside the treated layer, without affecting the expansive clay soil.
REFERENCES
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