the influence of salts on the irreversible...

Scuola di Dottorato in Scienze della Terra,

Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2013-2014

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THE INFLUENCE OF SALTS ON THE IRREVERSIBLE SETTLEMENTS INDUCED IN CLAYEY SEDIMENTS BY THE THERMAL STRESS DUE TO A WORKING BHE

Ph.D. candidate: GIORGIA DALLA SANTA, II course Tutor: Prof. ANTONIO GALGARO

Cycle: XXVIII course

Abstract

In closed-loop geothermal systems, boreholes heat exchanger (BHEs) affect the surrounding subsoil prior thermal equilibrium. A laboratory program was accomplished to understand the effects of cyclic induced thermal stress on deformability and resistance parameters of cohesive sediments. Previous experiments demonstrated that, if freeze-thaw cycles are induced in normal-consolidated clayey sediments, a significant irreversible settlement occurs. The study case of Venice is considered, providing an example of densely urbanized areas in coastal/ transitional environments, with abundance of cohesive layers and brackish subsoil conditions. We investigated the effects of moisture salt concentrations on the induced settlement, testing a typical venetian silty clay at different salt contents. Results show that, despite the increasing salinity lowers the sediment freezing point preventing subsoil from freezing, the induced settlement increases with increasing salinity concentration. Furthermore, a mapping of territory’s geological thermal sensibility is shown, in order to regulate new BHE fields installation and their correct exploitation.

Introduction Considering the closed-loop systems, as the carrier fluid flowing inside a Borehole Heat Exchanger (BHE) exchanges heat with the ground, the thermal status of the surrounding subsoil is significantly altered. During the summer, heat is removed from the building and transferred into the ground, recharging the thermal resource extracted in winter. Our research deals with the effects on the subsoil of the exploitation of this resource, in particular considering the issues linked with the physical and mechanical consequences of the thermal footprint, and with sediments’ thermal properties. We consider a vertical BHE inserted into the ground reaching a depth of about 100m. The system is controlled by a reversible electrical heat pump, depending on the building’s thermal demand. In some cases when in winter season a thermal loads’ unbalance occurs, in order to provide the necessary thermal energy to the building, the thermal withdrawal from the ground could be improved lowering the carrier fluid temperature with the addition of some anti-freezing fluids even some degrees below 0°C (as low as -5°C in some cases). While the decreased fluid temperature enhances ground thermal withdrawal, it increases the induced thermal alteration, leading to temperature some degrees below 0°C also in the surrounding ground. In these conditions of daily and seasonal cycling variation of the carrier fluid temperature (between about -5°C and +55°C) freezing-thawing processes can occur in the sediments nearby the probe. The greater effects raise in cohesive sediments, where freezing processes affect directly the soil structure, owing to the strong electrical interactions between solid grains and interstitial water forming the adsorbed electrical double layer. Previous studies demonstrated that freezing-thawing processes modify irreversibly cohesive soil texture and its mechanical properties and deformability (Konrad, 1989; Konrad, 1990; Esch, 2004; Qi et al. 2006; Qi et al. 2008; Bing and He, 2010; Bing and Ma, 2011; Dashjamts and Altantsetseg, 2011). The area involved is quite delimited closed around the borehole. Previous modelling results showed that for an undersized system where the total BHE length is about 70% of the optimum, after 20 years of functioning the isotherm of 0°C has a diameter of about 25cm from the BHE centre. In previous works, we measured the irreversible settlement induced in normally-consolidated cohesive soils by the cycling thermal load related to an operating BHE. In this year we analysed the influence of soil salt content on the induced settlement due to the freezing-thawing processes, testing specimens characterized by different NaCl content. The case-study of Venice represents an example of the typical geological contest of coastal or transitional urban areas with abundance of clays and silts in the litostratigraphic sequence, inserted in a brackish environment.



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Freezing-Thawing Processes Of Soils And Effects Of The Pore Solution Salt Concentration The temperature at which a sediment starts to freeze is lower than the freezing point of pure water (few degrees below 0°C) owing to the high pressure and the dispersal of the moisture in fine pores inside the solid matrix (Bing and Ma, 2011). Concentration and kind of solutes dissolved in the interstitial water influence the freezing point of a given soil, as well as the water content and the applied load (Bing and Ma, 2011). The effects of the presence of salts in the interstitial water inside a cohesive soil are multiple because they participate to the electrical interactions between soil particles and interstitial solution (Perfect, 1991). The strong electrical field generated by the negatively charged clayey particles is enhanced by the positively charged (hydrated) ions dissolved in the interstitial solution, affecting the migration of water molecules along the thermal gradient towards the crystallization centres (Dashjamts and Altantsetseg, 2011). Hence, salts and ions affect the amount of unfrozen water content at a given temperature (Konrad, 1989, 1990; Marion, 1995), and lower the sediment freezing-point (Esch, 2004). Moreover, the freezing process has been shown to exclude salts from the ice phase, resulting in an increase of the salt content in the remaining unfrozen water, lowering the soil freezing point at the subsequent freezing cycle (Henry, 1988; Farouki, 1991). As regards how the freezing point depression of a soil is affected by the type of soil in addition to water content, ion sort and salt content, Bing and Ma (2011) highlight that the most important factor which influence the freezing point of a given soil is the salt concentration (more than the water content). The temperature at which a soil freezes decreases with increasing salt content, regardless to the kind of salt is dissolved. The salt that greatly depress the freezing point is NaCl. As for the grain size, at the same water content and salt concentration, the freezing point of cohesive sediments is lower than that of coarser ones (sand) (Bing and Ma, 2011). As for moisture and salts movement in freezing and frozen soils that is provided only by the presence of the interconnected unfrozen water films, despite salt may be expected to improve the hydraulic conductivity because it lowers the sediment freezing point, increasing the amount of unfrozen water and because it improve the attraction gradient to the freezing front, the dominant consequence of salts presence in the interstitial solution is a reduction in soil permeability (Perfect, 1991; Marion, 1995). Soil properties and testing procedures The tested material was a silty clay collected in the neighborhood of the Venice lagoon at a depth of 7.0-7.5 m below mean sea level. All the measured parameters confirm the data reported in previous studies on similar lithologies in the area. During sample preparation, firstly the tested soil was deprived of its natural pore water in order to eliminate its salt content by means of a sort of soil washing repeated 4 times. Afterwards, new pore solutions were prepared dissolving in 1l of deionized water a specific amounts of NaCl (0g/l, 35g/l, 70g/l, 140g/l), which is the salt that greatly depress the soil freezing point, according with literature (Bing and Ma, 2011). Thermal tests were achieved by means of a standard oedometer inserted in an expressly made thermostatic box where temperature can be controlled. The tests were performed applying a constant vertical load of 40kPa and cycling temperature conditions between -5°C and +55°C over a 24-hour period. The temperature and the vertical settlement of the sample were continuously measured. Results and discussion The most important experimental evidences are that the interstitial water salinity affects the shape of the freezing curve phase, confirming that different kind of molecules are subsequently involved in the freezing process. As for the compaction effect, with increasing salinity the total thermal induced settlement increases. On the other hand, the effect of freezing point depression is evident in the measures of change phase temperature and it is increased by increasing salt content. Moreover, this effect is enhanced at every cycle owing to salt exclusion from ice formation combined with the decreasing of water content, resulting in a sort of protection to the ground with respect to the freezing process. For example the saltiest specimen experienced only 4 cycles, because at the end its freezing point was lower than the imposed freezing temperature (-6°C). Consequently, the total settlement is lower than the one accumulated by the others. Measures of electrical conductivity (hence of total salt concentration ) of the



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interstitial fluid extracted from the specimen by means of a spin-dryer, show that the unfrozen water salinity increases of about 30% from the beginning to the end of the cyclic freezing-thawing process. The resulting freezing point depression effect is almost of 0,2°C at every freezing-thawing cycle.

Figure 1: Measured deformability due to cyclic thermal conditions for the same material (mineralogical and geotechnical

characterization in the box) added with solutions at different NaCl content. Otherwise, as regards the thermal exchange, also the sediment thermal properties are altered by freezing-thawing processes. Some measures of thermal conductivity of clayey sediments at different temperatures (-20°C, -8°C, 2°C, 20°C, 40°C) were carried out. According to literature (Farouki, 1981; Esch, 2004), we found out that thermal conductivity of a frozen clayey ground is higher than the one at 20°C of about 50% (from 1.32W/mK to 1.97W/mK). In order to avoid the highlighted issues, it is important to have knowledge of the territory geological sensibility, identifying the areas more suitable for this applications. In figure 2 a mapping of Venice thermal geological sensibility is shown, based on the distribution of cohesive sediments, obtained from high density stratigraphic information. This kind of map could be used by the administrations in order to regulate the installation of new BHE fields and their correct exploitation. Legend

percentage share of NON thermally sensitive material(20m)

0% - 20%

20% - 40%

40% - 60%

60% - 80%

80% - 100%

percentage share of thermally sensitive material (20m)

0% - 10%

10% - 20%

20% - 30%

30% - 40%

40% - 50%

50% - 60%

60% - 70%

70% - 80%

80% - 90%

90% - 100% Figure 2:Map of the thermal geological sensibility of the city of Venice. It is gathered from the calculation of the percentage

share of the entire stratigraphic column of thermally sensitive material present in the first 20m of depth. Conclusions and future works The main conclusions are that the presence of salt depresses the soil freezing point, providing a sort of protection to the soil against freezing. Moreover, the cyclic freezing-thawing process increases the unfrozen water salinity at every cycle improving the protection effect, so that at the end the soil freezing point is some degrees lower than at the beginning. However, the interstitial water salinity seems to improve the compaction effect induced by the freezing-thawing cycles. Although the phenomenon is quite localized just next to the probe, an induced settlement could result first of all in potential risk on buildings’ integrity due to induced differential settlements, depending on the type of buildings foundations and on the relative position between foundations and geothermal probes. Secondly, a compaction of the sediments just in contact with the probes could rise a sort of negative friction on the boreholes themselves disturbing their integrity, instead of providing support. In addition,

SJ 0 34,4% 0,1% 26,9% 0,1% -22% 74% 7,3%

SJ 35 35,3% 1,0% 27,2% 1,3% -23% 30% 8,4%

SJ 70 35,4% 1,5% 27,2% 1,9% -23% 28% 9,1%

SJ 140 36,1% 2,5% 27,4% 3,3% -24% 34% 7,0%

water

content

variation

[%]

salt

content

variation

[%]

TOTAL

THERMAL

SETTLEMENT

[%]

specimen

tested

initial water

content

Win[%]

initial salt

content

(NaCl

equivalent)

[%]

final

water

content

Wfin[%]

final salt

content

(NaCl

equivalent

)

[%]



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the induced freezing-thawing cyclic processes have been shown to higher the vertical permeability, leading to a possible hydraulic connection between different aquifers previously separated. Therefore from the ‘static’ point of view the obtained results suggest that also in coastal areas and brackish environments, despite the salinity of the interstitial solution lowers the soil freezing point, it is necessary to avoid inducing freezing-thawing processes in the subsoil surrounding the probes. Otherwise, the ground thermal conductivity has been shown to be enhanced in frozen conditions, improving the thermal withdrawal. Hence, supplementary studies about the heat exchange efficiency in frozen conditions are needed. In addition, further studies will analyze the effect of freeze-thaw cycles on cohesive sediments in terms of vertical hydraulic conductivity and mechanical properties as cohesion and friction angle, also considering cohesive sediments with different degrees of overconsolidation. References BING, H, MA,W.2011. Laboratory investigation of the freezing point of saline soil, Cold Reg Sci Technol67,79-88. BING H., HE P., 2010. Experimental investigations on the influence of cyclical freezing and thawing on physical

and mechanical properties of saline soil. Environ Earth Sci 64, 431–436. DASHJAMTS, D., ALTANTSETSEG, J., 2011. Research on consolidation of frozen soils upon thawing, in: Proc.

of The 6th International Forum on Strategic Technology (IFOST), Nangang District Harbin, Heilongjiang, China, August, 1295-1300.

ESCH, D.C., 2004. Thermal analysis, constriction, and monitoring methods for frozen ground, ASCE HENRY, K, 1988. Chemical aspects of soil freezing. CRREL Report 88-17. FAROUKI, O.T, 1981. Thermal properties of soil, CRREL Monograph 81-1. KONRAD,J.M.,1989.Physical processes during freeze-thaw cycles in clayey silts,Cold Reg Sci Technol16,291–303 KONRAD, J.-M., 1990. Unfrozen water as a function of void ratio in a clayey silt, Cold Reg Sci Technol 18, 49-55. MARION G.M., 1995. Freeze–Thaw Processes and Soil Chemistry. Special Report 95-12. QI, J., MA, W., SONG, C. 2008. Influence of freeze–thaw on engineering properties of a silty soil, Cold Reg Sci

Technol 53, 397–404. QI, J., VERMEER, P. A., CHENG, G. 2006. A review of the influence of freeze-thaw cycles on soil geotechnical

properties, Permafrost Periglac 17, 245–252. PERFECT E., GROENEVELT P.H., KAY B.D., 1991. Transport phenomena in frozen porous media. In Transport

Processes in Porous Media, Dordrecht: Kluwer Academic Publishers, p. 243–270. SUMMARY OF ACTIVITY IN THIS YEAR

Courses: FLORIS M.: “Introduzione al GIS”, Dipartimento di Geoscienze, Università degli Studi di Padova. Relatori vari. : “STEPS3” Confindustria Padova – Area università impresa

Journal publications and Proceedings Pubblications: GALGARO, A., DALLA SANTA, G., TEZA, G., TATEO, F., COLA, S.; DE CARLI, M., DI SIPIO, E., DESTRO, E.. Changes in the mechanical properties of clayey sediments due to thermal stress induced by the operation of a borehole heat exchanger. Submitted to Engeneering Geology 29/7/2014, under review DALLA SANTA, G.; GALGARO, A.; TATEO, F.. "Freezing time influence on clayey sediments settlements induced by a borehole heat exchanger." 5 th European Geothermal PhD Day. Darmstadt, Germany 1-2 April 2014. DALLA SANTA, G.; GALGARO, A.; TATEO, F.; DESTRO, E.; COLA, S.; BASSAN, V.. BHE geological hazard on clayey sediments induced by thermal stress. CONGRESSO SGI-SIMP 2014, Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31 (2014) p. 574. doi: 10.3301/ROL.2014.140 DALLA SANTA, G.; GALGARO, A.; TATEO, F.. Thermal Hazard on Clayey Sediments Induced by a Borehole Heat Exchanger, Proceedings World Geothermal Congress 2015. Melbourne, Australia, 19-25 April 2015, accepted Congresses and Workshops: REGIONE VENETO – “Progetto Legend: Geotermia a bassa entalpia: studi > normative > applicazioni”, Rovigo 29 May 2014 REGIONE VENETO – “Progetto Legend: Geotermia a bassa entalpia: esperienze a confronto”, Venezia 26 June 2014 Oral presentations: DALLA SANTA, G.; GALGARO, A.; TATEO, F.; DESTRO, E.; COLA, S.; BASSAN, V.. Bhe geological hazard on clayey sediments induced by thermal stress, SGI-SIMP 2014, Milano, 10 - 12 September 2014 Teaching activities - Co-supervisor of thesis: • BERTAGGIA GAIA, Effetti meccanici indotti da cicli gelo disgelo sulle argille veneziane - Laurea Triennale in Ingegneria Civile, DICEA, UNIPD. Supervisor: prof. S. Cola • GIULIANI NICCOLÒ, Relationships between mechanical properties and borehole heat exchangers thermal forcing in clayey subsoils - Laurea Magistrale in Ingegneria Ambientale – DICEA, UNIPD, 14 ottobre 2014, Supervisor: prof. R. Sassi

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