טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentPrimo Hadas
SubjectUse of ANSS as a Stabilizer for Fine Grained Swelling Soils
DepartmentDepartment of Civil and Environmental Engineering
Supervisors Professor Mark Lloyd Talesnick
Dr. Arieh Sidess
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Design and construction of pavements on swelling subgrade may induce sever problems due to volume changes of the subgrade. Variations in water content in a subgrade may induce differential deformations which result in cracking of the pavement leading to a reduction in life expectancy of the pavement and/or higher maintenance costs. The design and construction of pavements on subgrade materials with inferior characteristics such as low strength, high potential for volume change, high water content and poor workability is not trivial. Improvement of the subgrade properties could involve a combination of the following solutions: replacement of the problematic soils, installation of impermeable membranes to guard against changes in moisture content or the use of stabilizers such as cement based, lime based, fly ash, polymer or liquid based stabilizers. Stabilization of the subgrade material enhances the engineering properties of the subgrade layers which produces a structurally sound pavement and will allow for the design of a thinner overall pavement or alternatively extended life span and reduction in required maintenance.



The primary objective of this study is to gain insight to the effects of ANSS stabilizer on the behavior and performance of a fine grained soil of high swell potential. The specific tasks included determination of engineering properties influenced by the stabilization such as vertical and horizontal swell, resilient modulus, permanent deformation, volumetric strain and bulk modulus. The performance of the stabilized clay with ANSS was compared to the performance of clay stabilized with conventional stabilizers; cement and lime.


    The investigation is based on following sets of laboratory tests:-
  1. (a) Laboratory swell tests aimed to investigate the effect of ANSS content on the vertical volume change of swelling clay as a function of initial water content and vertical pressure. A secondary set included the definition and evaluation of the coefficient of swell (αω) as a function of ANSS content and initial water content. The coefficient of swell is defined as the ratio of vertical strain in response to a unit change in water content. The coefficient of swell is an indicator of the potential activity of the stabilized clay.
  2. Resilient modulus tests were performed in order to investigate the resilient response of the stabilized clay. The resilient modulus test is a repeated load triaxial test performed under constant confining stress. The resilient modulus is defined as the ratio of the maximum axial cyclic stress to the maximum recoverable axial strain of the specimen. Resilient modulus of the unstabilized and stabilized clay was tested also after permanent deformation and wetting process.
  3. Permanent deformation tests were performed in order to evaluate the influence of stabilization with ANSS, lime and cement on unrecovered accumulated deformation during repeated loading and unloading. The permanent deformation test was performed also after wetting process.

Soil characterization

The soil examined in the research was sampled from the Yizrael Valley in northern Israel. The tested clay is classified as A-7-6 according to AASHTO classification and as CH according to the USCS classification system. Atterber limits of the clay are as follows: Liquid Limit (LL), 77 %, Plastic Limit (PL), 28 % and Plasticity Index (PI) 49 %. The Specific gravity (Gs) of the clay is 2.72, the free swell is 150% and the California bearing ration (CBR) is 3.5%. The above properties indicate that the soil is of high swell potential and low strength.



Swell tests

The aim of measuring the vertical swell under an applied vertical surcharge is to examine the change of the vertical dimension of the specimen (swelling percentage) over time, as the clay specimen is allowed to absorb an unlimited amount of water. The apparatus used in this research was a standard rigid ring consolidometer. The specimen is laterally restrained and can absorb water freely through two porous stones placed at the upper and lower boundaries of the specimen. The change in height of the specimen is monitored during the entire test.



Tests for the determination of the coefficient of swell

Determination of the coefficient of swell was accomplished by measuring the vertical strain per unit water content change as the specimen wets up. The tests were performed under a vertical pressure of 1 kPa. Upon reaching a desired percent swell the specimen was quickly removed from the consolidometer ring and the water content measured. The resulting water content is the average water content since water absorbed into the specimen is not uniform from all sides. The tests included 3 sets, one for each nominal water content; each set included natural clay specimens and specimens stabilized with 2, 4 and 6% of the ANSS stabilizer. Every individual test was built from 4 points, each point representing the vertical swell that the specimen reached up until the point in time (percent swell) that the test was stopped and the water content determined. A linear regression of the four points going through the origin was computed. The slope of the regression represents the coefficient of swell per unit change in water content (αω) .



Resilient Modulus and permanent deformation tests

The resilient modulus tests were performed in accordance with the LTPP P46 Protocol test method. The procedure consists of applying 15 stress sequences using a cyclic haversine shaped waveform with duration of 0.1 seconds and a rest period of 0.9 seconds (fixed cycle duration of 1.0 sec). The haversine shaped load is considered the optimal waveform to simulate the induced load in pavement layers. For each sequence of the applied load, vertical and horizontal displacements were recorded by three linear variable differential transducers (LVDT) and a clip gage to measure horizontal deformation.
Unstabilized and stabilized specimens were tested for resilient modulus under the following

    conditions:
  1. after compaction and curing at initial nominal water content of the plastic limit (28%).
  2. after permanent deformation testing
  3. after wetting of the specimen. All the stabilized specimens were prepared with 4% stabilizer based on the dry weight of the soil.


The results were correlated to a non-linear log-log model (Uzan, 1985). This model was selected because it is incorporated to the new MEPDG for unbound materials. In this model resilient modulus is expressed as a function of bulk stress and deviator stress.



The same specimen tested for resilient modulus was retested in the permanent deformation tests. Unstabilized and stabilized specimens were tested for permanent deformation. The permanent deformation test was performed at a single deviator stress level of 70 kPa and a single confining pressure of 20 kPa. The loads are of haversine shape and were applied over 100,000 repetitions.


Permanent deformation tests enabled to compute the bulk modulus (K) of the unstabilized and stabilized specimens. The bulk modulus (K) is a measure of the material stiffness to volume change. The bulk modulus can be obtained by linear elastic theory and is related to the elastic modulus (E) and Poisson ratio (ν).


FINDINGS AND CONCLUSIONS

    Swell tests under various vertical loads
  1. In general, addition of ANSS stabilizer to the tested clay reduces the vertical swell. The reduction is noted at all pressures and for all nominal water contents.
  2. The effect of ANSS content on the vertical swell is not linear. The effect of the stabilizer content on the vertical swell for stabilizer content of up to 4% is significant. This is true for different pressures and nominal water contents. On the other hand, stabilizing beyond 4% produces minimal additional effect beyond that produced by the 4% stabilizer content.
  3. Stabilizing the soil under consideration at contents greater than 4% ANSS does not yield any additional engineering benefit. The vertical swell at high stabilizer contents, i.e. 4% and 6% are close to zero. For vertical pressures between 1-50 kPa at all nominal water contents the vertical swell was limited to 0.04-1.1%.

    Coefficient of Swell tests
  1. (a) Addition of ANSS stabilizer to the soil produces a reduction in the coefficient of swell. The coefficient of swell for the unstabilized soil reaches a value of 1.0. Specimens with 6% stabilizer led to a coefficient of swell close to zero.
  2. (b) The effect of stabilizer content on the coefficient of swell is more significant than initial water content.
  3. (c) The engineering implication from the coefficient of swell tests is that the value of the coefficient of swell is a descriptor of the activity of the clay, and the sensitivity of the soil to changes in water content. Higher values of the coefficient of swell (αω) indicate that the soil is more sensitive to changes in water content. It can be seen that at higher ANSS contents the soil is less prone to swell in response to increase in water content.

    Resilient Modulus tests
  1. Resilient modulus increases as a result of stabilization. All three stabilizers improve the resilient modulus of the clay tested. The lime stabilized soil showed the highest improvement in resilient modulus at all different stages of the test. For example, the resilient modulus for the natural clay after compaction and curing was 63.0 MPa. Addition of 4% ANSS increases the resilient modulus by a factor of about 2.5, the resilient modulus reached a value of 141.0 MPa. Adding lime and cement to the clay increases the resilient modulus by a factor of 7 and 2.5 respectively to resilient modulus of 438.0 MPa with lime and 150.0 MPa with cement.
  2. All samples exhibit a decrease in resilient modulus after the wetting process. The degree of reduction in resilient modulus values varied with stabilizer type. Still, resilient modulus of stabilized specimens is less sensitive to moisture variations. Comparing resilient modulus before wetting to the resilient modulus after wetting yields a factor of 6.6 in unstabilized clay, 3.5 with clay stabilized with ANSS and 2.3 and 0.8 with specimen stabilized with cement and lime respectively.
  3. Regression equations were developed to estimate the resilient modulus. Predicted values were well correlated to measured values for all specimens and at all experimental stages.
  4. The deviator and confining pressure had little effect on magnitude of the resilient modulus for either stabilized or unstabilized specimens.

Permanent deformation tests

    The permanent deformation parameters were obtained by two methods as follows:
  1. Vesys model, (1978) for which the rutting parameters α and μ are determined. μ is a parameter representing the constant proportionality between permanent and elastic strain, α is a parameter indicating the rate of decrease in permanent deformation as the number of load applications increase.
  2. (2) A model that connects between accumulated plastic strain p) at N repetitions of load to the resilient strain (εr),
    pr
    =EPER) by a second order polynomial equation.
      The conclusions from the test results are as followed:-
    1. The permanent deformation parameters by Vesys model for the unstabilized and stabilized specimens at all stages of the tests were unrealistic. For example, the Vesys parameters for the stabilized materials after curing show that as a result of the stabilization α decreases and μ increases, which implies that rutting increases as a result of stabilization.
    2. The development of EPER with load cycles is similar to the accumulated permanent strain. Comparison between computed EPER and measured EPER yield a good correlation.

    Bulk modulus results
  1. (a) Bulk modulus increases as a result of stabilization. The lime stabilized soil showed the highest improvement in bulk modulus. For example, the bulk modulus of the natural clay after compaction and curing was 57.0 MPa. Addition of 4% ANSS increases the bulk modulus by a factor of about 2.3, the bulk modulus reached a value of 129.0 MPa. Adding lime and cement to the clay increases the resilient modulus by a factor of 6 and 1.7 respectively.
  2. (b) After the wetting process the ANSS stabilized specimens show a reduction in bulk modulus (from a value of 129 MPa in the dry condition to 59 MPa in the wet condition). A fact that indicates the sensitivity of the material tested to variations in water content.


In conclusion, a well designed stabilized subgrade can have a significant structural benefit in the design of a flexible pavement. A stabilized subgrade can improve the workability, reduce the swell potential, reduce the sensitivity to moisture variations and improve the support of the pavement foundation by increasing the stabilized layer's resilient modulus and strength.