SOIL TECHNOLOGY

Soil Technology 7 ( 1995) 319-325

Experimental system for one-dimensional freezing of undisturbed soil profiles

Holger Johnssona** , Bo Thunholmb, Lars-Christer Lundin” aDepartment of Soil Sciences, Swedish University of Agricultural Sciences, Box 7072, S-75007 Uppsala, Sweden

“Geological Survey of Sweden. Box 670, S-75128 Uppsala, Sweden CDepartment of Earth Sciences, Uppsala University, Viistra Agatan 24, S-75220 Uppsala, Sweden

Abstract

An inexpensive and easy-to-handle setup for freeze-thaw experiments was developed. The system mimics field conditions, with a relatively deep monolith of undisturbed soil and a soil-air interface as an upper boundary condition. The setup includes a freezing device for vertical freezing of a soil monolith and transducers at several depths in the soil monolith for continuous measurement of unfrozen water content (TDR), temperature and radial temperature differences. The setup makes rudimentary control of boundary conditions and sophisticated monitoring of soil water and heat conditions possible.

To study the performance of the system, soil temperatures and water contents in a clay soil monolith were measured during two freeze-thaw cycles. The setup was shown to be useful in terms of simulating freeze-thaw cycles in a lysimeter placed in the laboratory. One-dimensional heat flows could be simulated, with a thermal error (horizontal vs vertical heat flows) during freezing of about 5%.

The setup was used to test the hypothesis that the similitude between freezing/thawing and drying/ wetting holds for unsaturated clay soils. The results indicated a good agreement between water retention curves calculated from freezing-point depression data and from measurements, using pres- sure plates.

Keywords: Freezing; Water content; Heat flow

1. Introduction

Soil freezing affects water and solute transport in several ways. The influence on infiltra- tion capacity is of vital importance for the partitioning of snowmelt into percolation and surface runoff. Frost heave may result from an upward flow of water to the freezing zone.

* Corresponding author.

0933-3630/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDfO933-3630(94)00016-6

320 H. Johnson et al. /Soil Technology 7 (1995) 319-325

In addition, freezing also changes the solute concentration and influences solute redistri- bution in the soil. However, studying soil freezing processes in the field is difficult and time-consuming. Previous studies concerning soil frost and soil water flow, including field experiments and modelling approaches (e.g. Thunholm et al., 1989; Lundin, 1990; Johnsson and Lundin, 1991) , have pointed to the need for an experimental setup where soil boundary conditions (soil surface heat flow and infiltration) can be controlled.

Experimental studies of frozen soil samples have a long tradition (e.g. Koopmans and Miller, 1966; Burt and Williams, 1976). These experiments were often carried out on soil samples with a uniform temperature. A few studies have included temperature gradients in the soil samples (Penner, 1986; Yoneyama et al., 1983).

In this study, we follow the tradition of temperature gradient experiments and emphasize the importance of achieving one-dimensionality of fluxes (i.e. minimize lateral flow through vertical boundaries). Achieving reasonable one-dimensionality would make it possible to use the setup for testing one-dimensional soil water and heat flow models. Our objective was to develop and test an inexpensive and easy-to-handle setup for freeze-thaw experi- ments. We focused on reproducing field-like conditions as closely as possible, using a relatively deep monolith of undisturbed soil with a soil-air interface as an upper boundary condition. The setup made rudimentary control of boundary conditions and sophisticated monitoring of soil water and heat conditions possible. There are many topics which can be studied using this setup; infiltration into frozen soil, water and heat flux in frozen soil, frost heave, ion concentration and redistribution, etc.

To illustrate the behaviour of the system, soil temperatures and water contents, measured during two freezing/thawingcycles, are presented. The setup was used to test the hypothesis that the similitude between freezing/thawing and drying/wetting holds also for unsaturated clay soils (Miller, 1980).

2. Material and methods

The experimental setup was based on an undisturbed soil monolith coupled to a freezing device for vertical freezing of the profile (Fig. 1). The undisturbed soil monolith was sampled in the field using the drilling method described by Persson and Bergstrijm ( 1991). The casing consists of a PVC pipe with an inner diameter of 0.30 m and a thickness of 0.01 m. The length of the pipes can be varied. In our setup, the length was 0.60 m giving a soil monolith length of about 0.5 m. The bottom of the pipe was sealed off by a PVC lid with drain outlets, above which a thin layer of gravel was placed to provide free drainage. To reduce horizontal heat flow into the monolith, the side wall was covered with 20 cm of insulating mineral wool. The bottom of the soil monolith was not insulated.

The freezing device was built using a standard top-opened freezer. The top-cover was replaced with an insulated lid, extending 50 cm horizontally to cover the top of the soil monolith. The top of the soil monolith was in contact with the chilled air in the freezer through a duct in the lid. The lid was mounted in the same way as the top cover and was fitted to the soil monolith with a large isolating O-ring when closed. A fan was installed to increase circulation of air within the freezer and above the soil surface. Fan speed and freezer efficiency (intensity) could be varied. The experimental setup was placed in a cold-

321 H. Johnsson et al. /Soil Technology 7 (IYY5) 319-325

Fig. 1. The experimental setup

storage room to obtain a realistic lower boundary condition ( +4”C) and to avoid large horizontal temperature gradients (forcing heat flow) between the soil and the air outside the insulation material.

Measurements of unfrozen water content were made using time domain reflectometry (TDR) (Patterson and Smith, 1981; Stein and Kane, 1983). The method is based on measuring the dielectric constant of the soil, which is a function of the volumetric liquid water content (Topp et al., 1980). The dielectric constant has been observed to also have a small temperature dependence (e.g. Roth et al., 1990, calculated that a 10°C change in temperature resulted in changes in water contents between 0.1 and 1%) . In this experiment, where the temperature range was small, the temperature effect was considered negligible compared to other possible sources of error. Stainless steel two-rod-probes (5 cm distance between the parallel rods) were installed horizontally, at 5 cm intervals from the soil surface to 45 cm depth. The probes were inserted through holes drilled in the plastic casing of the monolith and fixed using silicon rubber stoppers. The length of the probes in the soil was 15 cm, i.e., half of the diameter of the soil monolith. The probes were connected to the TDR device (Tektronix 1502B) through coaxial cables and a multiplexer (Campbell Inc.). The TDR was controlled by a programmable logger (Campbell CRlO) with a data logger prom including evaluation algorithms and a calibration function (Topp et al., 1980). Measure- ments of unfrozen water content were made automatically every hour and stored in the logger.

Measurements of temperature were made within the freezer, at the soil surface, outside the insulation material, and at 5 cm depths from the soil surface to a depth of 30 cm. In addition, radial temperature differences were measured at depths of 5, 10 and 20 cm. These measurements were made using specially designed probes containing thermocouples in pairs, 11 cm apart, enclosed in a rubber tube (diameter 5 mm). The probes were inserted through drilled holes in the plastic casing of the monolith and fixed using silicon stoppers. Thus, radial temperature differences between 12 and 1 cm from the centre of the monolith were measured. The probes were connected to the logger system. The measurement interval was 2 seconds and mean values for every half hour were stored.

322 H. Johnsson et al. /Soil Technology 7 (1995) 319-325

- Surface --- Scm

10

c

------ ,OCrn - ZOcm

Fig. 2. (Top) Temperature outside the insulation material and temperature at different depths in the centre of the monolith. (Middle) Horizontal temperature gradients at different depths calculated from the radial temperature difference between 12 and 1 cm from the centre of the monolith. (Bottom) Unfrozen water content at different depths during the two freeze-thaw periods.

The soil monolith used in this experiment was sampled close to Ultuna experimental station in Uppsala (59”50’N, 17’41’E). The soil, described by Wiklert et al. ( 1983), is a well aggregated clay with a mean clay content at O-50 cm depth of 47%, an organic matter content in the topsoil of about 6% and a mean porosity of about 60%.

Water retention curves were calculated by transforming soil temperatures (mean values of the two measurements at each depth) to matric potentials using the Clapeyron equation and assuming zero gauge pressure in the ice phase;

Pfp- T(Ll273.15)

where P is the matric potential, p is the density of water, T is temperature (in “C), and L is the latent heat of freezing (Fig. 3). This approach gives a conversion factor between

H. Johnsson et al. /Soil Technology 7 (1995) 319-325 323

Freezing-thawing, 5 cm

- Freezing-thawing, 20 cm

cl%4 1 E+O lE+l 1 E+2 1 E+3 lE+4

Soil water pressure head (-cm)

I 1 E+5 1 E+6

Fig. 3. Transformed water retention curves using the Clapeyron equation (conversion factor between temperature and matric potential = 1.22 MPa/K) and water retention curves measured using pressure plate apparatus and undisturbed soil cores sampled close to the site where the soil monolith was sampled (Data from Wiklert et al., 1983).

temperature and matric potential of 1.22 MPa/K and is based on an assumption of similitude between freezing/thawing and drying/wetting as proposed by e.g., Miller (1980). The similitude holds theoretically for saturated colloidal soils and is often assumed to hold also for unsaturated colloidal soils. In practice, the assumption is extensively made for all kinds of natural soils and degrees of saturation (e.g. in models of water and heat flows in frozen soil: Harlan, 1973; Jansson and Halldin, 1979). The investigated clay soil was considered to approximate a colloidal soil.

3. Results and discussion

Results from two freezing/thawing cycles are presented. After starting the experiments (on day 1 and 11, respectively) the soil surface temperature reached between - 12 and - 15°C within a few hours (Fig. 2). The instability in the soil surface temperature was a result of the thermostat actions of the freezer. The freezing front (O’C isotherm) subse- quently moved downwards to reach a depth of about 25 cm below the soil surface after 4 days. After turning off the power of the freezer device (after 4 and 6 days, respectively), soil temperatures increased rapidly at first to between - 2 and - 1°C and thereafter more slowly. Thawing was completed after 5 days.

The horizontal temperature gradients at 10 and 20 cm depths ranged from 0.01 to O.O3”C/ cm (Fig. 2). At 5 cm depth, the horizontal gradient was about O.OS”C/cm during freezing and decreased to about O.Ol”C/cm at thawing.

During freezing, the vertical temperature gradients were fairly stable and inversely related to depth, varying from 0.2Wcm at 20 cm depth to 1.4”C/cm at 5 cm depth. During thawing, the situation was reversed. The gradient at 5 cm depth changed sign, while the gradient at 10 cm depth was close to zero.

324 H. Johnsson et al. /Soil Technology 7 (1995) 319-325

The ratios between horizontal and vertical gradients were about 5% during freezing and somewhat higher during thawing (Fig. 2). This means that about 5% of the heat flow through the lysimeter was lost as horizontal heat flow, giving a thermal error of about 5% if one-dimensionality is assumed. Note that, for very small vertical heat flows, the relative error became very large (e.g. during thawing). The absolute error, however, was negligible, sometimes even below the accuracy of the thermocouples. If smaller horizontal heat flows were required, this could be achieved by increasing insulation thickness or by a variable adjustment of the temperature outside the insulation material to minimize horizontal tem- perature gradients. The latter would require a heat flow source at the bottom of the profile to regulate the lower boundary temperature.

The unfrozen water content clearly responded to temperatures below zero during both freeze-thaw periods. Temperatures increased rapidly following the freezing period, whereas water contents increased more slowly (Fig. 2).

Water retention curves calculated using freezing point depression data (Fig. 3) were compared with water retention curves measured on undisturbed soil cores using a pressure plate apparatus. The cores were sampled at a site close to the sampling site for the soil monolith (Kungsangen no. 1: Wiklert et al., 1983). A common reference volume was used for the comparison. This simplification was motivated by the low frost susceptibility of the chosen soil. Frost heave was not observed in this experiment. Initial water contents at 5 and 20 cm depths were considerably below saturation in the soil monolith. The water retention curves calculated from the freeze-thaw experiment coincided with the drying curves at soil water pressure heads below ca 103.5 cm at 20 cm depth and below ca 104.’ cm for the 5 cm depth (Fig. 3). The part of the freeze-thaw retention curve which is to the left of the drying retention curve represents the freezing front passing through the layer influencing the measurement probes. The deviation between the curves is due to the different measurement volumes of the TDR probes (ca 2-3 cm in the vertical direction) and the temperature probes (3-5 mm in the vertical direction). Thus, as the freezing front passes, the layer influencing the TDR probes gradually freezes causing a gradual decrease in the unfrozen water content recorded. When the whole layer influencing the TDR probe is frozen, the recorded unfrozen water content is close to the actual water content at the temperature measurement point. This is also the point for which the soil water pressure head is calculated.

A strong hysteresis was noted, with larger unfrozen water contents during freezing than during thawing. The hysteresis effects were similar to the behaviour reported for drying/ wetting cycles.

4. Conclusions

The experimental setup was shown to be useful in terms of simulating freeze-thaw cycles in a lysimeter placed in the laboratory. The setup was inexpensive and easy to make. It can be used, for example, to study infiltration into frozen field soils.

A thermal error (horizontal vs vertical heat flows) during freezing of about 5% was found. Therefore, heat flow could be considered one dimensional. The importance of the deviation from the vertical heat flow depends largely on the objective of the study. Depend- ing on the specific requirements, this deviation can be reduced.

H. Johnsson et al. /Soil Technology 7 (1995) 319-325 325

It was concluded that the experimental setup was promising for further studies of freezing- point depression. Studies of different soils could prove useful in modelling work where the information on freezing-point depression has to be obtained from the soil-moisture char- acteristic curves.

The results indicated that a good agreement can be achieved for a clay soil between water retention curves calculated from freezing-point depression data and measurements made using pressure plates. The assumption of similitude appeared to be valid for the unsaturated clay soil investigated in this study.

Acknowledgement

We would like to thank Hans Bonde for valuable help concerning the construction of the freezer device and Nicholas Jarvis for valuable comments on the manuscript. This investi- gation was supported by grants from the Swedish Natural Science Research Council.

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