The Effect of Freezing on the Strength of Silty-Clay – Sand Mixtures

 

Costas A. Anagnostopoulos

Research Supervisor, Laboratory of Soil Mechanics and Foundations, Geotechnical Engineering Division, Department of Civil Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece
kanagnos@civil.auth.gr

Ioannis Grammatikopoulos

Assistant Professor, Laboratory of Soil Mechanics and Foundations, Geotechnical Engineering Division, Department of Civil Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece
ygram@geo.civil.auth.gr

TECHNICAL NOTE

ABSTRACT

The application of the freezing method on soil formations provides instantaneous, short-term solutions in construction works. This method may be applied in cases of large constructions, such as, for example, temporary support for the excavation front of the foundation of a highway-bridge pier or for opening a tunnel front. It may also be applied in smaller constructions, depending on road layout. In situ soil freezing, through the use of liquid nitrogen, offers the following advantages: (i) high strength of frozen soil formation, due to application of very low temperature; (ii) opportunities for drastic intervention, by eliminating potential heterogeneity. For example, at an excavation site, the application of the soil freezing method at the shoulders and the bottom provides tightness and stability for the entire soil section between ground water level and foundation front. Knowledge of the mechanical properties of the frozen soil is necessary before the application of the soil freezing method and the start of construction. The experimental investigation reported herein aims towards the study of the mechanical properties of frozen soil specimens prepared with different proportions of silty-clay and sand. For the purposes of the laboratory project, soil specimens were frozen to a temperature of -14oC and tested under conditions of compression and tension. Experimental results have revealed that water content, consolidation pressure and composition of specimens significantly affect mechanical properties of frozen soil specimens.

Keywords: freezing method, compressive strength, tensile strength, cohesion

INTRODUCTION

The nature and mechanical behavior of unfrozen soils have been studied by many researchers, in contrast with the physical and mechanical properties of frozen soils, which exhibit a remarkable complexity. A frozen soil appears to have higher strength than unfrozen soil (Czurda and Hohmann, 1997). On the other hand, it displays a time-dependent creep behavior similar to that of ice and a frictional behavior similar to that of its unfrozen state (Ma and Chang, 2002). As opposed to unfrozen soil, the strength of frozen soil decreases under high confining stress after reaching a peak (Chamberlain, 1985). The strength of frozen soil may be considered as the result of the cohesion of the ice mass and the frictional resistance of soil grain (Guymon et al., 1980). However, this concept is not absolutely correct. Frozen soil is a complex, multiphase system consisting of grain, frozen water and air (Li et al., 2002). Its mechanical properties are indisputably affected by the presence of a thin film of unfrozen water around soil particles, since ice is adjacent to this adsorbed layer and not in direct contact with soil particles. (Torrance, 2003).

There has been extensive research concerning strength and rigidity of coarse soils, frozen at subzero temperatures (Sage and D’Andrea, 1988). Nevertheless, in the case of frozen cohesive soils, there hardly any remarkable research has been undertaken. The frozen system of clay and silt differs from that of granular soils. Cohesive soils consist of smaller grain with more surface area and, thus, higher unfrozen water content (Ono, 2002). In addition, frozen soil structure is highly affected by average density, grain orientation and mineral composition.

Assessing the strength of frozen cohesive soils and the parameters affecting them is of great importance. This is so, because in many cases of surface or underground structures, the existence of a soil formation of high strength and tightness is absolutely necessary and can be achieved with the freezing method. Figure 1 shows the application of this method during a tunnel excavation (Bielefeld, Germany) through alluvial deposits, consisting of layers of water-saturated clay-fine sand.

The main objective of this laboratory project was to investigate the mechanical parameters of frozen soil specimens containing silty-clay and fine sand in different proportions in relation to the consolidation pressure and, consequent water content. Experimental results have led to some significant conclusions about the efficacy and applicability of this method.


Figure 1. Application of the freezing method during a tunnel opening

MATERIALS AND LABORATORY PROCEDURE

Silty-clay used was taken from excavations at a depth of 10 to 15 m in the broader region of Thessaloniki, Greece. In situ characteristics are presented in Table 1. Grain size distribution is shown in figure 2. According to Casagrande classification, it is defined as inorganic clay of medium plasticity.

The sand used was collected from river deposits. Its grain size ranged between 0.42 and 0.074 mm, with a uniformity coefficient of 2.2 (figure 2). The sand had a dry unit weight of 14.85 kN/m2, a saturated unit weight of 19.35 kN/m2 and a porosity of 45%.

 

Table 1. In situ characteristics of silty-clay
Soil Property Value
Liquid Limit (%)43.54
Plastic Limit (%)25.32
Plasticity Index (%)18.22
Water Content (%)25.16
Activity0.67
Bulk Unit Weight (kN/m3)16.68
Dry Unit Weight (kN/m3)18.6

 


Figure 2. Grain size distribution of silty-clay and sand

Laboratory tests were performed using reconstituted specimens, of a sand content of 0, 10, 20, 30, 40, and 50% of the total weight of solid material. The various soil specimens were prepared by thoroughly mixing silty-clay - fine sand and a sufficient amount of distilled water to achieve saturation. The soil mixture was then placed into cylinders 3.5 cm in diameter and 7 cm high. Measurements of strength were performed on samples consolidated under pressure of 25, 50, 200 and 400 kPa. Following consolidation, specimens for freezing were placed into an appropriate freezing chamber and frozen to a temperature of -14oC for 24 hours. Both frozen and unfrozen specimens were subjected to an unconfined compression test. The direct tensile strength of unfrozen specimens was measured by using the apparatus and following the testing procedure suggested by Ibarra et al (2005). The tensile strength of frozen specimens was determined from splitting tensile tests according to ASTM C 496. Each of the reported water content, compressive strength and tensile strength values correspond to the average value of three specimens.

RESULTS AND DISCUSSION

Laboratory results show that the higher the clay content, the higher the reduction of water content (w) for all values of consolidation pressure (sc) (figure 3). The most appreciable difference was noted in the case of specimens with 100% clay content, where, for sc = 25 kPa, w was 38.66% and for sc = 400 kPa, w was 24.57%. Nevertheless, w values of specimens with

 


Figure 3. Water content of the various silty clay-sand mixtures plotted against the consolidation pressure.

high silty-clay content remained higher in comparison to those of specimens with low silty-clay content. Figures 4 and 5 demonstrate the effect of sc on the compressive and direct tensile strength of unfrozen specimens, respectively. Mechanical properties were substantially improved as sc increased, due to higher compaction and the consequent development of a flocculated structure with stronger clay particle bonds. This explains the fact that the higher the clay content, the higher the mechanical properties of specimens for all sc values.

Figures 6 and 7 show the compressive and splitting tensile strength of frozen specimens plotted against sc. Both strengths were remarkably increased, although a wide range of values, depending on the composition of specimens and sc, was observed. In fact, the development of compressive and splitting tensile strength was significantly affected by sand content and sc and, therefore, by the water content, i.e. the higher the water content, the higher the strength. This phenomenon can be attributed to the extended periods of bond formation between ice

 


Figure 4. Compressive strength of unfrozen soil specimens plotted against the consolidation pressure.

 


Figure 5. Direct tensile strength of unfrozen soil specimens plotted against the consolidation pressure.

 

crystals and soil particles, even though a film of unfrozen water is interposed, due to the formation of large ice crystals with high rigidity and resistance to recrystallization and pressure melting (case of high water content). In the case of low water content, partial thawing leads to the quick crack formation and propagation through the medium, resulting in an overall reduction of strength. This is also supported by the failure modes observed for specimens of different water content. Specimens with high water content, when subjected to compression stress, failed along a major and / or some inferior discernible surfaces caused by

 


Figure 6. Compressive strength of frozen soil specimens plotted against the consolidation pressure.

 


Figure 7. Splitting tensile strength of frozen soil specimens plotted against the consolidation pressure.

secondary tension. On the contrary, in cases of specimens with low water content shear failure occurred, confirming the significant loss of cohesion during loading with subsequent shear fracturing. The optimum strength improvement was obtained for frozen specimens of 100% silty-clay content, with values ranging from 5460 to 3440 kPa for compressive strength and from 2149 to 1515 kPa for splitting tensile strength. On the contrary, specimens with 50% silty-clay content exhibited the lowest strength values, ranging from 1400 to 600 kPa for compressive strength and from 856 to 440 kPa for splitting tensile strength.

CONCLUSIONS

Soil freezing has a considerable effect on improving its mechanical properties, due to the formation of a rigid ice-soil matrix. Experimental results reported in this paper indicate that this improvement depends directly on water content and it is inversely proportional to the increase in consolidation pressure and sand content. The higher the water content, the higher the mechanical properties after freezing. Taking into account that reduction in temperature increases frozen soil strength, it is easily understood that freezing the ground with liquid nitrogen (-196oC) leads to the formation of a frozen soil mass of especially high strength. This strength is much higher than that observed during laboratory tests in specimens frozen at -14oC and provides suitable solutions for many construction geotechnical problems.

REFERENCES

  1. ASTM Standard C 496-93, “Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA.

  2. Chamberlain, E.J. (1985) “Shear strength anisotropy in frozen saline and freshwater soils,” Proc of the 45th Int. Symp., Balkema, Rotterdam, pp 189-194.

  3. Czurda, K.A., and M. Hohmann (1997) “Freezing effect on shear strength of clayey soils,” Applied Clay Science, Vol.12, pp 165-187.

  4. Guymon, G., R. Berg, and T. Hromadka (1980) “A one-dimensional frost heaven model based upon simulation of simultaneous heat and water flux,” Cold Regions Science and Technology, Vol.3, pp 253-263.

  5. Ibarra, S.Y., E. Mckyes, and R.S. Broughton (2005) “Measurement of tensile strength of unsaturated sandy loam soil,” Soil and Tillage Research, Vol.81, pp 15-23.

  6. Li, N., F. Chen, B. Su, and G. Cheng (2002) “Theoretical frame of the saturated freezing soil,” Cold Regions Science and Technology, Vol.35, pp 73-80.

  7. Ma, W., and X. Chang (2002) “Analyses of strength and deformation of an artificially frozen soil wall in underground engineering,” Cold Regions Science and Technology, Vol.34, pp 11-17.

  8. Ono, T. (2002) “Lateral deformation of freezing clay under triaxial stress condition using laser-measuring device,” Cold Regions Science and Technology, Vol. 35, pp 45-54.

  9. Sage, J.D., and R.D. D'Andrea (1988) “Long term mitigation of frost deterioration of existing road ways,” Final Report, NSF Grant No ECE 85 18813, Worcester, USA, pp 1-105.

  10. Torrance, J.K. (2003) “A conceptual analysis of chemical factors in soil freezing and frost heave,” Electronic Proceedings, 56th Canadian Geotechnical Conference, Manitoba, Canada.

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