In-situ and laboratory determination of small-strain stiffness-suction-moisture relationship and its application

(Even though it’s a technical article, I shall describe it in a mixture of a formal and a narrative tone.)

The title of my Master’s degree research was, “Insitu and Laboratory determination of small-strain stiffness-suction-moisture relationship and its application”. First, I’d like to explain the technical terms used so that my research becomes apparent, and I shall try to explain in brief about the whole research.

Suction (ψ) is defined as the potential of water to attract water and comprises of two parts namely matric suction and osmotic suction. For engineering purposes, matric suction is of importance and in general, the drier the soil higher is its matric suction. This property of soil has a significant effect on its stiffness, strength and permeability. For engineers, since we often design in worst-case scenarios which is a saturated state, suction is often neglected but its capacity to alter the strength and permeability properties after the construction stage is vital.

Small-strain shear modulus (G0) of a body is defined as the resistance of the body against deformation at small strains, below 0.001%, under which the soil strain is considered elastic. To understand the engineering behaviour under dynamic loading like from earthquakes, vibrations from machine foundations, traffic, pile driving, wind, etc. it is imperative to define the parameter G0. Even in a static condition, G0 is a useful parameter to characterize the nonlinear stress-strain behaviour of soil. It also provides a direct relationship with different soil parameters like density and liquefaction potential.

The third parameter is moisture content (w) which is simply the water content present in the soil. It is relatively easy to comprehend and measure than the suction which is measured in terms of pressure requiring a special instrument (tensiometer). Once we quantify moisture content we can specify its corresponding suction value through a curve commonly known as soil water retention curve (SWRC).

So my research was to examine how these parameters changed relative to each other for different types of soil and its stress states. Therefore, three sites possessing different structures and soils were chosen in the study. Two of the sites were present in the Kanchanaburi province of Thailand. The first one being a Mechanically Stabilized Earthen (MSE) wall comprised of clayey sand (SC). The other was a natural slope lying along the roadside and was characterized by a nonplastic silt (ML). The third site consisted of a clay dyke (CL) and was located at the Asian Institute of Technology (AIT), Bangkok, Thailand.

As for the investigation of the relationship under laboratory conditions, free-free resonant frequency (FFR) test was conducted on recompacted soil sample collected from those sites. Similarly, spectral analysis of surface waves (SASW) was done in-situ at different time periods to study the change in G0 relative to suction (measured in the field through various instrumentation). Soil water retention curve (SWRC) was determined in the laboratory to relate the suction measured in the field with respect to the moisture content.

Through the FFR test, in the tested suction range, G0 and suction were found to be positively correlated for all the soil sample, though the rate of increase in G0 with suction varied. Through the SASW test, it was found that the effect of suction on G0 was dominant only for the initial depths of about 1 m, after which the effect of overburden pressure was more pronounced. Also, for the same suction values, G0 obtained from SASW test on an intact ground was higher than that from FFR test because of the presence of confining pressure. In addition, it was found that the stiffness of the MSE wall which was reinforced with geosynthetics and geotextiles remained fairly constant for different time periods, whereas in natural slope stiffness varied proportionately with suction for about a depth of 1m. Intriguing results were found through the series of SASW tests on clay dyke as the presumption that G0 increased with suction was not valid and it was later explained by the presence of shrinkage cracks.

Let’s observe these findings, site wise.

  1. MSE Wall
G0-ψ relationship for MSE wall site

At MSE wall, it can be seen that the stiffness of the structure remained quite constant despite the change in suction. KU tensiometer were installed at different depths but here the results of 0.85 m, 2.2 m and 6.15 m has only been presented to avoid the cumbersome in the chart. Its rest assured that even plotting the data from other tensiometers installed at other depths, the pattern is similar to the ones presented i.e. no change in stiffness despite change in suction at the same depth. The stiffness at 6.15 m is higher than that at 0.85 m and 2.2 m despite the similar suction values, which can be attributed to the overburden pressure which is higher at 6.15 m. Similarly, it is the very reason that for the same suction value, the result of G0 obtained from FFR is quite lower to that obtained from SASW test, since in FFR test there is no net confining pressure.

2. Natural Slope

G0-ψ relationship for natural slope site

It can be seen that the suction and stiffness are proportionate for that depth but for the deeper soil profiles, suction seems to have very less effect on the small strain shear stiffness. For the deeper depths, the overburden pressure has greater role than the suction. In FFR test, the soil sample is devoid of external confining pressure, and therefore demonstrates small value of G0. It can also be seen that since in FFR test, the role of confining pressure is eliminated, the role of suction is quite distinct, since for all the tested suction range, the relationship is proportionate.

3. AIT – Clay Dyke

G0-ψ relationship for AIT clay dyke site

At the AIT dyke site, during the periods of SASW tests, shrinkage cracks were observed. The results from the SASW tests could be distinguished as before and after the appearance of shrinkage cracks. When the cracks appeared, the modulus of the soil dropped compared to when it was at intact condition. The G0 –ψ relationship demonstrated here depicted the effect of shrinkage cracks on the small strain stiffness and SASW along with suction measuring devices has been proposed as a method to determine the unseen cracks below the soil.

Details of instrumentation, methodology as well as results has not been presented here. Similarly, soil water retention curve, modelling of shrinkage crack depth as well as concept of suction stress has not been illustrated .The aim of this post is just to provide a gist of the relationship between these properties and various factors affecting it during the laboratory and in-situ testing methods.

Readers who are interested in details of these are requested to contact the author for his complete thesis.

Following excerpt has been taken from the thesis which represents the conclusion of the research:

  • All the SWRCs determined in the study demonstrated bimodal nature which can be attributed to the dual porosity associated with the soil structure.
  • Through the FFR results, the relationship between suction and G0 was found to be proportionate, i.e. with increase in suction there was corresponding increase of G0 value for all the soils.
  • In SASW tests, the observed relationship holds true only for the initial depths (about 1 m) after which various other factors play important role like the overburden pressure as was observed from the results of SASW in natural slope site, Kanchanaburi.
  • On the other hand, SASW results from MSE wall suggests that  G0 stays fairly constant and role of suction is limited for such structures which is well compacted and reinforced with geosynthetics and geotextiles.
  • Similarly, results from the periodic SASW test on the clay embankment, reflected the effects of crack on G0 and how despite the increase in suction, G0 did not increased but instead decreased from the effect of propagating shrinkage cracks. SASW along with suction measuring devices has been proposed as a method to determine the shrinkage cracks underneath the soil layer.
  • The G0 values obtained from the FFR test were less than the values obtained from SASW test for the same suction because of the presence of net confining pressure which is absent in FFR test.
  • Semi-empirical procedure with appropriate parameters can be used to model the shrinkage crack depth with acceptable accuracy.

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