To: Sciences in Cold and Arid Regions (scar@lzb.ac.cn)
Permanent corresponding author's email: ??????
Thermal conductivity of reinforced soils: A literature review
Muge Elif Orakoglu 1,2, JianKun Liu 1*
1. School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China
2. Technical Education Faculty, Department of Construction, Firat University, Elazig, Turkey
*Correspondence to: Ph.D., JianKun Liu, Professor of School of Civil Engineering, Beijing Jiaotong University, Beijing
100044, China. Tel: +86-10-51684096; E-mail: jkliu@bjtu.edu.cn
ABSTRACT
This paper aims a review of the literature related to soil reinforcements to achieve lower soil thermal conductivity (λ). The use of
various natural and synthetic fibers, polymers, geosynthetics, agricultural waste/materials, and nanoclays is discussed and existing
prediction models that have been thought to affect low thermal conductivity are presented.
Keywords: reinforced soil; heat flux; soil thermal conductivity
1
Introduction
Soil thermal conductivity is defined as key parameter in heat flux in a unit time through a unit cross sectional
area of soil under a unit temperature gradient. The constitutive equation for thermal conduction based on the
temperature gradient was presented by Fourier in equation (1) (Fourier, 1955; Tritt, 2004):
Q   A
dT
dx
(1)
where, dT/dx is the temperature gradient, A is a cross-sectional area, Q is the heat flow, and λ is thermal conductivity of soil.
Soil thermal conductivity plays a significant role in heat flux. Several methods are available for determining
soil thermal conductivity, and those pertaining to reinforced soils are divided into two categories: steady-state and
transient state. The steady-state method is explained by Fourier’s solution based on the heat conduction equation in
a known heat flux and a known thermal gradient. There are various available techniques of the steady state method,
namely, guarded-hot-plate apparatus, heat flow meter apparatus, guarded comparative-longitudinal heat flow
technique, rhometer apparatus, and the Rapid k method (Mitchell and Kao, 1978). The transient state method is
based on the soil thermal conductivity while the temperature of the reinforced sample changes in a period of
heating or cooling. The transient methods include the thermal needle method, which is widely used in soils due to
its simplicity and commercial availability (Barry-Macaulay et al., 2013). Table 1 summarizes the literature pertaining to methods used in measuring soil thermal conductivity.
A number of studies suggest that determining soil thermal conductivity through in-situ measurement procedures is expensive and time-consuming. That is why many empirical relations have been developed to estimate the
soil thermal conductivity (λ) based on effective parameters such as soil texture, moisture content, temperature,
porosity, and saturation (Camillo and Schmugge, 1981; Campbell, 1985; Sundberg, 1988; Becker et al., 1992,
Tarnawski and Leong, 2000; Côté and Konrad, 2005).
2
Usowicz et al. (2006) developed a statistical-physical model based on a non-linear regression equation using
the parameters of water content, bulk density, organic matter, and content of quartz and other minerals (as determined based on content of sand, silt, and clay). Pei et al. (2013) established a thermal conductivity model of soil-rock
media by a multiple linear regression method (Equations (2) and (3)) based on the saturation, temperature, porosity,
dry density, and water content. They compared this with the other three primary general models, namely, the BP
neural model, the physical model and the CK model, and they developed a general regression model based on the
apparent heat capacity theory specifically relevant to in cold regions. The main multiple linear model equations
(Zhang, 2003; Zhang et al., 2012b) are as follows.
For the unfrozen state:
u  (1.913  10.313)  (0.127  0.118) w  (0.869  3.355) d  (7.110  12.595)n  (2.076  2.948) Sr
(2)
For the frozen state:
 f  (16.028  14.954)  (0.042  0.174) w  (4.581  4.866) d  (17.448  17.846)n  (0.882  4.273) Sr
(3)
where λu is thermal conductivity of unfrozen soil, w is water content, ρd is dry density, n is porosity, Sr is the saturation ratio, and λf is thermal conductivity of frozen soil.
References
Steady State
Nikolaev et al., 2013
Table 1 The methods used for measuring soil thermal conductivity
Methods
Abuel-Naga et al., 2009
Barry-Macaulay et al., 2013
Ekwue et al., 2006
Alrtimi et al., 2014
Mottaghy et al., 2008
Low et al., 2014
Transient State
Mochizuki et al., 2008
Nusier and Abu-Hamdeh, 2003
Komle et al., 2007
Akinyemi et al., 2006
de Zárate et al., 2010
Khedari et al., 2001
El-Rawi and Al-Wash, 1995
Canakci et al., 2007
Abu-Hamdeh, 2000
Waite et al., 2002
Guarded Hot plate; Ottawa sand (with 99% quartz) content, and Richmond Hill fine sandy
loam
Steady-state method (divided bar) , Transient-state method (needle probe); Soft saturated
clay (Bangkok clay)
Steady-state method (divided bar); Transient-state method (needle probe): Coode island silt,
Basaltic clay, Brighton Group sand, Brighton group clayey sand, Fishermens bend silt, Residual siltstone, sandstone, siltstone and basalt rocks.
KD2 sensors based on steady-state methodology; Piarco sandy loam, Maracas clay loam, and
Talparo clay
New thermal cell that utilizes steady-state methods; fine sand, coarse sand, ground glass, and
fluorspar with cement (20%)
Divided bar method; Crystalline rocks
Guarded hot plate method in accordance with BSEN 12664 (2001); clayey soil, Portland
cement, newspaper, aerated lightweight concrete compositions
Heat probe method by Kasubuchi; natural Toyoura sand, glass beads, NaCl solution
Single-probe and dual-probe methods; Sand and loam
Two needle probes; a grid of temperature sensors, evenly distributed inside the sample; two
additional thermal probes; fine-grained reddish sand, coarse-grained moist sand, gravels with
various grain size distributions (1 cm- to 6 cm), and for comparison and calibration pure
water (with convection suppressed by adding agar-agar), compact ice, and compact granite.
Single needle sensor; soils from Nigeria, heavy metals, namely, Fe, Cu, Ni, Cr, Cd, Zn, Md,
Pb, As.
Multi-current hot wire method; two different clays, namely, magnesium bentonite (BEMG)
and sabhasana (SABA)
According to JIS 2618, hot-wire method: compositions of cement, sand, and fiber waste of
fiber coconut and durian
Upper and lower thermo-couple wires; soil- cement mixtures
Single heater and thermocouple probe; clayey and chalky limestone
Single probe device; Jordanian clay loam and loam soils
Von Herzen and Maxwell’s needle-probe method; four porous mixtures of quartz sand and
methane gas hydrate
3
Park and Hartley, 1992
Bovesecchi and Coppa, 2013
2
Line-heat-source method based on hot-wire methods; unbounded and bounded sand
A stainless-steel tube inside of which a Pt wire heater and T sensor (type T thermocouple);
Pozzolanic soil (a fine sandy, unconsolidated, alluvial soil from Lazio, Italy, based on volcanic ash) and blue marlstone rocks
The thermal conductivity of reinforced soils
As is known, reinforced soils are solid composites which consist of a combination of two or more different
structures in which their identities are preserved. The advantages of newly produced composites are higher specific strength, stiffness, and lower thermal conductivity (Kazemian et al., 2010). Reinforced soil techniques are
based on the example of the bricks made from clay reinforced with straw by the Egyptians in the third millennium
BC (Mathews and Rawling, 1994).
Various soil types blended with waste-created bricks, fibers, geosynthetics, and polymer have been examined
to determine the effects on thermal properties. Table 2 shows various reinforcement materials for blending soils.
Polyesters have the widest range of use because of their simple handling and cheap price. Many researchers
have focused on determining the different thermal parameters of polyester composites with variable fibers. For
example, Sultana et al. (2013) studied sand polyester composites, and their experiments showed that the thermal
conductivity decreased with an increase in sand content from 10% to 60%.
Al-Malah and Abu-Jdayil (2007) studied bentonite (natural clay)-based unsaturated polyester composites.
They determined the lowest thermal conductivity of an unsaturated polyester composite with 50% filler content
and 40% polyester content in weight.
Natural materials
Fibers
Fibers
Synthetic materials
Geosynthetics
Table 2 Various soil reinforcement materials
Coconut (Coir), Sisal, Palm, Jute, Flax, Barley Straw, Bamboo, Cane, Wood Fibers,
Broom, Sawdust, Sugar Palm, Pineapple leaf, Sunn hemp, Rice husk, Kenaf
Geomembrane polymers
Polyethylene (HDPE, VLDPE etc.)
Polypropylene (PP),
Polyvinyl chloride (PVC)
Polyester (PET),
Impermeable
Cholorosulphonated polyethylene
Polyethylene (PE),
(CSPE)
Glass, Nylon, Steel,
Ethylene Interpolymer alloy (EIA)
Polyvinyl alcohol (PVA),
Rubber)
Polyurethane, Polyvinyl chloride
Nonwoven
Needle punched
Chemical bonded
Geotextile
Geotextiles, Geogrids, Geocells, Permeable
Heat bonded
Geonets, Geocomposites
Knitted
Woven
Processed types of agriculture waste/materials have been widely tested by blending them (various types of soils,
cement, sand, and water) to create new composites. Raut et al. (2011) reviewed the use of waste materials in different compositions to develop waste-created bricks, and they determined that this approach was economical,
environmentally sustainable option. Zhang et al. (2012a) revealed the thermal conductivity of soil in earthen ruins
by adding different concentrations (5%, 10% and 15%) of potassium silicate (PS). Their test results showed that the
4
larger the PS concentration, the lower thermal conductivity (Figure 1).
Bederina et al. (2007) studied the effect of the addition of wood shavings (wastes of woodworking activities)
on thermal conductivity of sand concretes made from dune sand and river sand. They also compared their experimental results with a theoretical approach based on the auto-coherent method. They found that an increase in
waste contents decreased the thermal conductivity of sand concrete. Additionally, Xu and Chung (2000) determined the thermal conductivity of cement pastes and mortars decreases with the addition of sand, in contrast to
opposite effects of silica fume addition. Rim et al. (1999) studied the thermal and mechanical properties of a
clay-cement-wood composite with different proportions of wood aggregates. Their test data showed that an increase wood aggregates (from 10% to 50%) decreased the thermal conductivity (from 0.24 to 0.08 W/(m·K))
(Figure 2).
The use of natural cellulose fibers such as flax, palm, sisal, kenaf, straw, and jute in polymeric matrices in soil
reinforcement methods has gained a great of attention in civil engineering applications due to their desirable
properties (Alamri et al., 2012). Millogo et al. (2014) determined the engineering and thermal properties of pressed
adobe blocks (PABs) fabricated with Hibiscus cannabinus fibers. Their test results showed that thermal conductivity decreased with increasing amounts and lengths of the fibers. There was a greater decrease in the thermal
conductivity with long fibers (60 mm in length) than with 30 mm long fibers (Figure 3). Khedari et al. (2005)
produced soil-cement blocks reinforced with coconut fibers in 18 different mix ratios. Their results showed that the
thermal conductivity decreased when the fiber content increased (Figure 4).
Khedari et al. (2001) figured out the effects on thermal conductivity of 18 coconut fiber specimens and 12
durian specimens prepared with different fiber lengths, grain sizes of sand, and mixing proportions (cement, sand,
fiber). They found that packing fibers in composites creates voids in the composites, and short-length fibers are
more difficult to coordinate and pack than longer ones. Thus, the short fiber lengths create many voids in composites, resulting in lower thermal conductivity (Figures 5, 6).
Figure 1 Change of thermal conductivity with addition of potassium silicate to the soil of earthen ruins (Zhang et al., 2012a)
5
Figure 2 Influence of weight percentage of wood aggregates in a clay-cement-wood composite on
thermal conductivity (Rim et al., 1999)
Figure 3 Change of thermal conductivity with adding coconut fibers to soil-cement block (Khedari et al., 2005)
Figure 4 Change of thermal conductivity with adding Hibiscus cannabinus fibers to pressed adobe
blocks (Millogo et al., 2014)
6
Figure 5 Change of thermal conductivity in composites with adding young coconut fibers of different lengths
and different sand sizes (Khedari et al., 2001)
Figure 6 Change of thermal conductivity in composites with adding durian fibers of different lengths and
different sand sizes (Khedari et al., 2001)
Gorhan and Simsek (2013) investigated the thermal conductivity of clay bricks made with rice husks. Their
results showed that the thermal conductivity coefficients increased in samples with added rice husks and increasing
firing temperatures as compared to reference samples. Also, the thermal conductivities of the samples with added
coarse rice husks were lower than those of the samples with added ground rice husks. Alami (2013) determined the
thermal conductivity of two composites different filler materials and different dispersed-phase materials.
Clay-fronds-crushed olive pits and clay-straw-olive husks were also tested and the results revealed that the
clay-straw-husks specimen had a great reduction in thermal conductivity. Bal et al. (2013) revealed a decline in the
thermal conductivity of laterite samples (10cm×10cm×3cm) with added millet waste.
Baniassadi et al. (2011) investigated the mechanical and thermal properties of polymer (polyamide resin)/nanoclay (organo-modified montmorillonite) composites with added nanoclay. Their experimental test results
showed that the addition of nanoclay increased the thermal conductivity of polyamide resin, and the higher the
amount of nanoclay, the higher the thermal conductivity. Additionally, Hostler et al. (2009) studied the thermal
conductivity of clay-based aerogels. The thermal conductivity of samples was determined with and without added
poly (vinyl alcohol). The test data revealed that the polymer decreased thermal conductivity, and this reduction was
related to a more complex interaction between the polymer and the clay.
3
Conclusions
7
This paper provides a review of thermal conductivity in soils reinforced with different materials. Fibers, polyester composites, agricultural wastes/materials and clay nanoparticles have been successfully used in various
soils to reduce the thermal conductivity which enables them to be used as a thermal insulator.
Clearly the thermal conductivity of soils reinforced with various natural and synthetic fibers and agricultural
waste/materials decreases with increased added material contents, added material surface frictions, and existing
chemical binders. Conversely, the thermal conductivity increases with addition of nanoclay to polymer/nanoclay
composites, increased fiber contents and higher firing temperature.
Our literature review shows that, both experimental and modeling, adding various fibers and materials produces lower soil thermal conductivity. Future research should assess reinforced soils with more waste materials
and various fiber matrices (two or more) and polymer composites to address problems arising from clumping effects, fiber aspect ratios and weight fractions, gradation, and particle shapes and sizes.
Acknowledgments:
This research is supported by the National Natural Science Foundation of China (Nos. 51378057, 41371081, and
41171064) and the National 973 Project of China (No. 2012CB026104).
References: (Note: Manuscripts with references less than 15 will not be accepted for publication in SCAR!)
Abuel-Naga HM, Bergado DT, Bouazza A, et al., 2009. Thermal conductivity of soft Bangkok clay from laboratory and field measurements.
Engineering Geology, 105(3–4): 211–219.
Abu-Hamdeh NH, 2000. Effect of tillage treatments on soil thermal conductivity for some Jordanian clay loam and loam soils. Soil and
Tillage Research, 56(3–4): 145–151.
Akinyemi OD, Olowofela JA, Akinlade OO, et al., 2006. Thermal conductivity of soils with heavy metals concentration from the Niger Delta
region of Nigeria. Journal of Zhejiang University Science B, 7(8): 615–618.
Alami AH, 2013. Mechanical and thermal properties of solid waste-based clay composites utilized as insulating materials. International
Journal of Thermal & Environmental Engineering, 6(2): 89–94.
Alamri H, Low IM, Alothman Z, 2012. Mechanical, thermal and microstructural characteristics of cellulose fibre reinforced
epoxy/organoclay nanocomposites. Composites(Part B), 43: 2762–2771.
Al-Malah K, Abu-Jdayil B, 2007. Clay-based heat insulator composites: Thermal and water retention properties. Applied Clay Science, 37:
90–96.
Alrtimi A, Rouainia M, Manning DAC, 2014. An improved steady-state apparatus for measuring thermal conductivity of soils. International
Journal of Heat and Mass Transfer, 72: 630–636.
Bal H, Jannot Y, Gaye S, et al., 2013. Measurement and modelisation of the thermal conductivity of a wet composite porous medium: Laterite
based bricks with millet waste additive. Construction and Building Materials, 41: 586–59.
Baniassadi M, Laachachi A, Hassouna F, et al., 2011. Mechanical and thermal behavior of nanoclay-based polymer nanocomposites using
statistical homogenization approach. Composites Science and Technology, 71: 1930–1935.
Barry-Macaulay D, Bouazza A, Singh RM, et al., 2013. Thermal conductivity of soils and rocks from the Melbourne (Australia) region.
Engineering Geology, 164: 131–138.
Becker B, Misra A, Fricke B, 1992. Development of correlations for soil thermal conductivity. International Communications in Heat and
Mass Transfer, 19(1): 59–68.
Bederina M, Marmoret L, Mezreb K, et al., 2007. Effect of the addition of wood shavings on thermal conductivity of sand concretes: Experimental study and modeling. Construction and Building Materials, 21: 662–668.
Bovesecchi G, Coppa P, 2013. Basic problems in thermal-conductivity measurements of soils. International Journal Thermophysics, 34:
1962–1974.
Camillo P, Schmugge T, 1981. A computer program for the simulation of heat and moisture flow in soils. NASA Technical Memorandum
82121, NASA Goddard Space Flight Center, Greenbelt, MD.
Campbell GS, 1985. Soil Physics with Basic Transport Models for Soil-Plant Systems. Elsevier Science, New York.
Canakci H, Demirbog R, Karakoc MB, et al., 2007. Thermal conductivity of limestone from Gaziantep (Turkey). Building and Environment,
42: 1777–1782.
Côté J, Konrad JM, 2005. A generalized thermal conductivity model for soils and construction materials. Canadian Geotec hnical Journal,
42(2): 443–458.
De Zárate JMO, Hita JLuis, Khayet M, et al., 2010. Measurement of the thermal conductivity of clays used in pelotherapy by the multi-current hot-wire technique. Applied Clay Science, 50(3): 423–426.
Ekwue EI, Stone RJ, Bhagwat D, 2006. Thermal conductivity of some compacted trinidadian soils as affected by peat content. Biosystems
Engineering, 94(3): 461–469.
El-Rawi NM, Al-Wash AAA, 1995. Strength and thermal properties of plain and Reinforced soil-cement. Journal of Islamic Academy of
Sciences, 8(3): 107–118.
8
Fourier JB, 1955. Theorie analytique de la chaleur, English trans. by A. Freeman. Dover Publications Inc., New York.
Gorhan G, Simsek O, 2013. Porous clay bricks manufactured with rice husks. Construction and Building Materials, 40: 390–396.
Hostler SR, Abramson AR, Gawryla MD, et al., 2009. Thermal conductivity of a clay-based aerogel. International Journal of Heat and Mass
Transfer, 52: 665–669.
Kazemian S, Huat K, Prasad A, et al., 2010. A review of stabilization of soft soils by injection of chemical grouting. Australian Journal Basic
Applied Science, 4: 5862–5868.
Khedari J, Suttisonk B, Pratinthong, et al., 2001. New lightweight composite construction materials with low thermal conductivity. Cement &
Concrete Composites, 23: 65–70.
Khedari J, Watsanasathaporn P, Hirunlabh J, 2005. Development of fibre-based soil-cement block with low thermal conductivity. Cement &
Concrete Composites, 27: 111–116.
Komle NI, Bing H, Feng WJ, et al., 2007. Thermal conductivity measurements of road construction materials in frozen and unfrozen state.
Acta Geotechnica, 2(2): 127–138.
Low KS, Ng SC, Tioh NH, 2014. Thermal conductivity of soil-based aerated lightweight concrete. KSCE Journal of Civil Engineering, 18(1):
220–225.
Mathews FL, Rawling RD, 1994. Composite Materials: Engineering and Science. Boca Raton: FL, Chapman and Hall/CRC Press.
Millogo Y, Morel JC, Aubert JE, et al., 2014. Experimental analysis of Pressed Adobe Blocks reinforced with Hibiscus cannabinus fibers.
Construction and Building Materials, 52: 71–78.
Mitchell JK, Kao TC, 1978. Measurement of soil thermal resistivity. Journal of the Geotechnical Engineering Division, ASCE, 104(GT10):
1307–1320.
Mochizuki H, Mizoguchi M, Miyazaki T, 2008. Effects of NaCl concentration on the thermal conductivity of sand and glass beads with
moisture contents at levels below field capacity. Soil Science and Plant Nutrition, 54: 829–838.
Mottaghy D, Vosteen HD, Schellschmidt R, 2008. Temperature dependence of the relationship of thermal diffusivity versus thermal conductivity for crystalline rocks. International Journal of Earth Sciences, 97(2): 435–442.
Nikolaev IV, Leong WH, Rosen MA, 2013. Experimental investigation of soil thermal conductivity over a wide temperature range. International Journal of Thermophysics, 34: 1110–1129.
Nusier OK, Abu-Hamdeh NH, 2003. Laboratory techniques to evaluate thermal conductivity for some soils. Heat and Mass Transfer, 39:
119–123.
Park S, Hartley JG, 1992. A model for prediction of the effective thermal conductivity of granular materials with liquid binder. KSME Journal,
6(2): 88–94.
Pei W, Yu W, Li S, et al., 2013. A new method to model the thermal conductivity of soil-rock media in cold regions: An example from permafrost regions tunnel. Cold Regions Science and Technology, 95: 11–18.
Raut SP, Ralegaonkar RV, Mandavgane SA, 2011. Development of sustainable construction materials using industrial and agricultural solid
waste: A review of waste-created bricks. Construction and Building Materials, 25: 4037–4042.
Rim KA, Ledhem A, Douzane O, et al., 1999. Influence of the proportion of wood on the thermal and mechanical performances of
clay-cement-wood composites. Cement and Concrete Composites, 21(4): 269–276.
Sultana R, Akter R, Md Alam Z, et al., 2013. Preparation and characterization of sand reinforced polyester composites. International Journal
of Engineering & Technology, 13(2): 111–118.
Sundberg J, 1988. Thermal properties of soils and rock. Ph.D. dissertation. Publication A57, Chalmers University of Technology, University
of Guteborg, S-41296, Guteborg, Sweden.
Tarnawski VR, Leong WH, 2000. Thermal conductivity of soils at very low moisture content and moderate temperatures. Transport in Porous
Media, 41(2): 137–147.
Tritt TM, 2004. Thermal conductivity: Theory, properties, and applications. Kluwer Academic/Plenum Publishers, New York.
Usowicz B, Lipiec J, Ferrero A, 2006. Prediction of soil thermal conductivity based on penetration resistance and water content or air-filled
porosity. International Journal of Heat Mass Transfer, 49: 5010–5017.
Waite WF, de Martin BJ, Kirby SH, et al., 2002. Thermal conductivity measurements in porous mixtures of methane hydrate and quartz sand.
Geophysical Research Letters, 29(24): 82-1–82-4.
Xu Y, Chung DDL, 2000. Effect of sand addition on the specific heat and thermal conductivity of cement. Cement and Concrete Research, 30:
59–61.
Zhang D, Wang T, Wang X, et al., 2012a. Laboratory experimental study of infrared imaging technology detecting the conservation effect of
ancient earthen sites (Jiaohe Ruins) in China. Engineering Geology, 125: 66–73.
Zhang MY, Min KH, Wu QB, et al., 2012b. A new method to determine the upper boundary condition for a permafrost thermal model: An
example from the Qinghai-Tibet plateau. Permafrost and Periglacial Processes, 23: 301–311.
Zhang RC, 2003. Multivariate Statistical Analysis. Science Press, Beijing.