Freezing characteristics of a highway subgrade

Teltayev Bagdat,Suppes Elena

Kazakhstan Highway Research Institute,2А,Nurpeissov St.,Almaty,050061,Republic of Kazakhstan

Freezing characteristics of a highway subgrade

Teltayev Bagdat*,Suppes Elena

Kazakhstan Highway Research Institute,2А,Nurpeissov St.,Almaty,050061,Republic of Kazakhstan

Temperature and moisture variations were experimentally investigated for the subgrade of the Astana−Burabai Highway, located in the northern region of Kazakhstan.Temperature and moisture were measured by means of specially manufactured sensors,including an element for temperature measurement based on the effect of thermal resistance and an element for moisture measurement through diamagnetic permeability.It was determined that the long-term average depth of frost penetration was 237 cm.In the upper part of the subgrade with 1 m of thickness,the average duration of a frozen condition decreases toward the bottom from 131 days to 117 days,with an average rate of 0.133 day/cm.The unfrozen water content decreases with the negative temperature reduction.In the case of high values of initial moisture(12%～22%),the unfrozen water content does not depend on the initial moisture;and it decreases under the power law,with reduction of negative temperature.In the case of the lowest values of moisture(9%～12%),the unfrozen water content depends also on negative temperature;but its values are considerably lower.

cold period;temperature;moisture;freezing depth;unfrozen water

1 Introduction

The climate of Kazakhstan is sharp continental. Winter is usually cold in northern regions of the country;very often,air temperature falls to minus 40～45°C.At the same time,the maximum depth of freezing reaches 250～280 cm.

It is known that deformation and strength properties of a frozen soil differ greatly from those of a soil in its usual(unfrozen)condition(Liu,2013;Shinet al.,2013).The frozen soil contains not only frozen (ice)but also unfrozen water.The unfrozen water content of a soil depends both on the value of the negative temperature reached and on other factors.For definition of the physical,mechanical,and other necessary characteristics of soils,one must have sufficiently accurate information on the duration of a frozen condition,the negative temperature reached,the moisture value,and the phase structure(liquid,ice)of water at points of a subgrade in specified climatic conditions.In considering the characteristics mentioned,it is necessary to carry out tests of soils under laboratory conditions.Thus,an important stage in the process of evaluating soil condition in a highway subgrade during the winter period is experimental research on temperature and moisture variation(Teltayev,2012; Teltayev,2013;Teltayevet al.,2015).

For a highway subgrade located in the northern region of Kazakhstan,this work shows the results of long-term experimental research for such important freezing characteristics as the freezing depth, duration and rate of reduction for freezing duration, and dependence of unfrozen water content on the negative temperature.

The section of the Astana−Burabai Highway studied is located close to Astana city(about 10 km distant).In the winter season,the minimum temperature of the coldest days is equal to−41°С(a probability of 98%);average wind speed is 5.2 m/s;precipitation(November to March)is 88 mm.In the summerseason,the maximum temperature of the hottest days is equal to 29.5°С(a probability of 98%);average wind speed is 3.7 m/s;precipitation(April to October)is 288 mm.

2 Experimental section of highway

The section of the Astana−Burabai Highway with asphalt concrete(km 76+30)pavement was selected for long-term monitoring of variation in temperature and moisture in pavement structure layers and points of the highway subgrade under climatic conditions of the northern region of Kazakhstan in November 2010. The highway has six lanes,with a width of 3.75 m each.The permitted speed for cars along this highway is 140 km/h,and for trucks 110 km/h.Reconstruction of the highway was completed in November 2009.

The pavement structure of the section with asphalt concrete(Figure 1)consists of the following layers: (1)stone mastic asphalt concrete,6 cm;(2)dense asphalt concrete,9 cm;(3)crushed stone treated with bitumen,12 cm;(4)crushed stone and sand mix treated with cement(7%),18 cm;(5)crushed stone and sand mix,15 cm;and(6)sand,20 cm.The subgrade is constructed from heavy,sandy clay loam:moisture in the plastic limit,WP=18.7%;moisture in the liquid limit,Wt=34.8%.The underground water is deep (more than 3.0 m below the surface).

Figure 1 Scheme for the location of sensors in the pavement structure and subgrade for the section of theAstana−Burabai Highway with asphalt concrete pavement:1−6,numbers of pavement layers;7,subgrade;temperature and moisture sensors

3 Temperature and moisture sensors

The Interpribor company(Chelyabinsk,Russia) produced temperature and moisture sensors to meet the specifications of the Kazakhstan Highway Research Institute(KazdorNII).Each sensor,fashioned in the form of a metal capsule,contains an element for temperature measurement based on thermal resistance and an element for moisture measurement through diamagnetic permeability(Figure 2).The design allows simultaneous measurement of temperature and moisture at pointsthroughoutthepavementandsubgrade.

The temperature elements of the sensors were calibrated by the producer,and the moisture elements were calibrated in the laboratory of KazdorNII.Calibration of sensors was performed using soils taken from the areas where the sensors were to be installed. The measurement ends of the sensors were put on the surface of the highway and fixed in the measurement chamber of the land system of the set(Figure 3).

Figure 2 One set of temperature and moisture sensors

Figure 3 Measurement(land)system for a set of temperature and moisture sensors

Each set had eight pairs of temperature and moisture sensors,three of which were installed in the pavement layers and five within the highway subgrade. The depths for installation,calculated from the pavement surface,were equal to 6,15,45,80,115,150, 185,and 220 cm.

4 Temperature and moisture in the highway subgrade

Figures 4 and 5 show graphs with the distribution of temperature and moisture within the subgrade in the cold season,constructed from the experimental data obtained by the temperature and moisture sensors. It is obvious(Figure 4)that the temperature graduallydecreases over time during the cold season—at all points of the subgrade,located up to the depth of 220 cm;and the depth of freezing increases.Thus,temperature was approximately equal to 2°С on the surface of the subgrade(80 cm)on November 16,2014;and it decreased to−10°С on January 30,2015,i.e.,temperature decreased by 12°С over 75 days.For the same period of time,the temperature decrease at the depth of 220 cm was 5°С.

Figure 4 Temperature distribution at subgrade depths during the cold season

It is clearly seen(Figure 5)that moisture significantly decreases at all depths of the subgrade with the temperature decrease.So,during the time(75 days) mentioned above,the moisture at the surface of the subgrade and at the depth of 220 cm decreased by 12%and 3%,respectively.

Figure 5 Moisture distribution at subgrade depths during the cold season

It is clear from the graphs for temperature and moisture variation on the surface of the subgrade(80 cm)and at the depth of 150 cm(Figures 6 and 7) there is a sharp decrease of moisture approximately at the time of temperature transition into the minus range;and moisture also decreases with the further fall of the temperature.And upon the transition of the temperature from minus to plus,a stepwise increase in moisture occurs.Certainly,these phenomena show the phase transitions,occurring at a temperature approximately equal to 0°С.

Figure 6 Temperature and moisture variation at the depth of 80 cm(2015 to 2016)

Figure 7 Temperature and moisture variation at the depth of 150 cm(2015 to 2016)

5 Freezingcharacteristicsofthehighwaysubgrade

5.1 Unfrozen water

The analysis of the values obtained for moisture and temperature showed that the unfrozen water content at high values of initial moisture(W0=12%～22%), practically speaking,does not depend on the initial moisture(Figure 8);and its reduction,depending on the absolute value of the negative temperature,is well described by the power function(Figure 9).The unfrozen water content at low values of initial moisture (W0=9%～12%)also depends on negative temperature, but its values are considerably lower(Figure 10).To explain this fact,one should turn to Figure 5.As can be seen,the unfrozen water content for all moments of time at the depth of 150 cm for 1%～7%is less than for the depths of 80 cm and 115 cm.A paper by Teltayev and colleagues(Teltayevet al.,2015) shows that for this section of the highway the initial moisture at the depth of 150 cm was considerably lower than for other points of the subgrade.Therefore,it turns out that the unfrozen water content is less at this depth.

Figure 8 Dependence of the unfrozen water content on temperature:W0=12%～22%

Figure 9 Fitting of the dependence of the unfrozen water content on temperature:W0=12%～22%,|θ|≥0.5°С

Figure 10 Dependence of the unfrozen water content at the depth 150 cm on temperature and initial moisture

5.2 Depth and duration of freezing

Table 1 shows the values of the maximum depth for freezing of the highway,and Figure 11 shows the average duration of the frozen condition for subgrade soil at various depths.Within six winter periods,the maximum depth of freezing was from 225 cm to 243 cm,the average of which was 237 cm.It should be noted that,for practical considerations,the maximum deviation from the average value(5%)is considered of minor importance.It can be seen from Figure 11 that the average duration of the cold period near the experimental section is 156 days.The average duration of the frozen condition in the upper part of the subgrade with a thickness of 1 m decreases at lower depths from 131 days to 117 days,with an average rate of 0.133 days/cm.At lower depths,the average duration of frozen conditions decreases significantly. So the average values at the depths of 185～220 cm and 220～237 cm are 1.114 days/cm and 4.588 days/cm, respectively.

Table 1 Maximum depth of freezing for the highway

Figure 11 Average duration of a frozen condition at various depths of subgrade

6 Conclusion

The results of experimental research examining temperature and moisture variations in the subgrade of the Astana−Burabai highway,located in the northern region of Kazakhstan,allow us to make the following conclusions:

(1)Temperature decreases and the depth of freezing increases over the course of time at all points of the subgrade,located at the depth of 220 cm,with an oncoming cold period.The average long-term depth of freezing for the highway is 237 cm.In the upper part of the subgrade with thickness of 1 m,the average duration of a frozen condition decreases with the depth from 131 days to 117 days,with an average rate of 0.133 days/cm.

(2)At all depths of the subgrade,soil-moisture decreases with the transition of temperature into the minus and with its further decrease.It was determined that a sharp decrease of moisture occurs at the time of temperature transition from plus into minus,and a sharp increase of moisture at the time of temperature transition from minus into plus.These phenomena show the phase transitions,occurring at a temperature approximately equal to 0°С.

(3)The unfrozen water content at high values of initial moisture(W0=12%～22%),practically speaking, does not depend on the initial moisture;and its reduction depending on the absolute value of the negative temperature is well described by the power function. The unfrozen water content at low values of initial moisture(W0=9%～12%)depends also on negative temperature,but its values are considerably lower.

Acknowledgments:

This research is supported by the Committee of the Ministry for Investments and Development of the Republic of Kazakhstan(Agreement No.36 dated July 21,2016).

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10.3724/SP.J.1226.2017.00325.

November 22,2016Accepted:December 26,2016

*Correspondence to:Teltayev Bagdat,Professor,President of Kazakhstan Highway Research Institute,2А,Nurpeissov St.,Almaty 050061,Republic of Kazakhstan.E-mail:bagdatbt@yahoo.com