Artificial ground freezing is a ground improvement technique that originated in the nineteenth century and is now widely known and used in various fields of geotechnics, mainly as a support for underground or open-air excavations, with structural or waterproofing functions. Over the years, this technique has developed greatly due to the continuous modernization of the technologies used. In addition, scientific research, which began by studying the behavior of naturally frozen soil, has wisely guided its growth and development. The work carried out in this project came from the collaboration between Groutfreezlab srl and a number of specialists involved in some AGF interventions carried out starting in 2019 in Italy, which are the “Isarco River underpass” and cross-passages of “Line 4 of the Milan subway”. The opportunity to interact with these professional figures, who designed and followed the AGF interventions, and to frequently visit the building sites allowed to create an innovative comprehensive study that provided support from the preliminary design phase to the control and monitoring phase during tunnel excavation operations. In fact, many geomaterials coming from the above sites (soil, concrete lining of tunnel and cement-mixture treated soil), and others such as saline soils, coming from other sites not subject to AGF, were taken into consideration. These geomaterials were subjected to mechanical and thermal tests to identify the mechnical and thermal parameters under different thermal conditions, and the data provided an important database upon which to base the final design. The data obtained show great consistency with previous work. A constructive comparison also identified some shady areas that represented points of early development, such as performing point load tests in situ on frozen soil samples to estimate the true strength of the frozen soil shell or monitoring the temperature at the excavation face with an infrared imaging camera to assess intervention quality and frozen wall thickness. The collection furthermore of frozen material from a site gave the opportunity to study undisturbed samples, through a workflow consisting of storage stages, investigation by Computed Tomography (CT), and dry coring, and to compare the results with test values inherent in the design phase to assess their reliability at the preliminary stage. These type of interventions is supported by numerical modeling that allows the effects of a process within a domain to be calulated in a distributed manner. Similarly, the AGF makes use of FEM-based thermal analyses that allow to predict cold front propagation within the ground and to perform possible "adjustments" to the arrangement of the freezing pipes to reach the established temperature targets, furthermore such a tool allows to estimate execution times, and in an indirect way also costs. Based on these considerations, a simple and fast laboratory test of unidirectional freezing of a soil sample was developed to be able to provide useful data for validation of commercial numerical codes. The use of LN2 at visited construction sites as a coolant has prompted a search for methods to numerically simulate the freezing effect induced by an LN2-fed freezing pipe, since the thermodynamic processes that takes place within it are very complex. In the state of the art, there are oversimplified methods that are based on the often sparse data referring to what happens in the probe in vertical position. This has led to the design and construction of an innovative freezing pipe with a large number of temperature sensors positioned to capture the level of liquid nitrogen inside the pipe, external and internal temperatures, and to compare the distribution of temperatures in vertical and horizontal positions, with dry or wet sand, and to monitor liquid nitrogen consumption. The data collected are of great value for detailed numerical simulation.

Il congelamento artificiale del terreno è una tecnica di miglioramento del terreno nata nel XIX secolo e oggi ampiamente utilizzata in geotecnica, principalmente come supporto per scavi sotterranei o a cielo aperto, con funzioni strutturali o di impermeabilizzazione. Nel corso degli anni, questa tecnica ha avuto un grande sviluppo grazie alla continua modernizzazione della tecnologia. Inoltre, la ricerca scientifica, partita dallo studio dei terreni naturali ghiacciati, ne ha guidato lo sviluppo. Questo progetto è nato dalla collaborazione tra Groutfreezlab srl e specialisti coinvolti in interventi AGF realizzati a partire dal 2019, quali il "sottopasso del fiume Isarco" e i cross-passages della "Linea 4 della metropolitana di Milano". L'opportunità di interagire con queste figure professionali, che hanno progettato e seguito gli interventi AGF, e di visitare frequentemente i cantieri ha permesso di realizzare uno studio innovativo e completo che ha fornito un supporto dalla fase di progettazione preliminare a quella di controllo e monitoraggio durante le operazioni di scavo delle gallerie. Infatti, sono stati presi in considerazione geomateriali provenienti dai siti sopra citati (terreno, calcestruzzo della galleria e terreno trattato con miscele) e altri, come i terreni salini, provenienti da siti non soggetti ad AGF. Questi geomateriali sono stati sottoposti a prove meccaniche e termiche per identificare i parametri tecnici e termici in diverse condizioni termiche, e i dati hanno fornito un'importante base di partenza per la progettazione. I dati ottenuti mostrano una grande coerenza con i lavori precedenti. Un confronto costruttivo ha permesso di individuare zone d'ombra da sviluppare, come l'esecuzione di PLT in situ su campioni di terreno congelato per stimare la reale resistenza dell'involucro di terreno congelato o il monitoraggio della temperatura al fronte di scavo con una termocamera per valutare la qualità dell'intervento e lo spessore della parete congelata. La raccolta di materiale congelato da un sito ha inoltre fornito l'opportunità di studiare campioni indisturbati, attraverso un flusso di lavoro costituito da fasi di stoccaggio, indagini mediante TAC e carotaggio a secco, e di confrontare i risultati con i valori di prova inerenti alla fase di progettazione per valutarne l'affidabilità in fase preliminare. Questi interventi, supportati dalla modellazione numerica, permettono di calcolare gli effetti di un processo all'interno di un dominio. Allo stesso modo, l'AGF si avvale di analisi termiche basate su FEM che permettono di prevedere la propagazione del fronte freddo nel terreno e di effettuare "aggiustamenti" alla disposizione delle sonde congelatrici per raggiungere i target di temperatura stabiliti; inoltre, tale strumento permette di stimare i tempi di esecuzione e, in modo indiretto, anche i costi. Sulla base di queste considerazioni, è stato sviluppato un semplice e veloce test di laboratorio di congelamento unidirezionale di un campione di terreno, in grado di fornire dati utili per la validazione dei codici numerici commerciali. L'uso di LN2 come refrigerante nei cantieri ha spinto a cercare metodi per simulare numericamente l'effetto di congelamento indotto da una sonda congelatrice alimentata a LN2, poiché i processi termodinamici che avvengono al suo interno sono molto complessi. Esistono metodi eccessivamente semplificati che si basano su dati spesso scarsi relativi a ciò che accade nella sonda in posizione verticale. Questo ha portato alla progettazione e alla costruzione di un innovativa sonda dotata di un gran numero di sensori di temperatura posizionati per rilevare il livello di azoto all'interno del tubo, le temperature esterne e interne, e per confrontare la distribuzione delle temperature in posizione verticale e orizzontale, con sabbia asciutta o umida, e per monitorare il consumo di azoto liquido. I dati raccolti sono utili per una simulazione numerica dettagliata.

(2024). Innovation applied to artificial freezing of soils: an experimental study to enhance theoretical and numerical analysis. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2024).

Innovation applied to artificial freezing of soils: an experimental study to enhance theoretical and numerical analysis

BAVARESCO, NICOLA
2024

Abstract

Artificial ground freezing is a ground improvement technique that originated in the nineteenth century and is now widely known and used in various fields of geotechnics, mainly as a support for underground or open-air excavations, with structural or waterproofing functions. Over the years, this technique has developed greatly due to the continuous modernization of the technologies used. In addition, scientific research, which began by studying the behavior of naturally frozen soil, has wisely guided its growth and development. The work carried out in this project came from the collaboration between Groutfreezlab srl and a number of specialists involved in some AGF interventions carried out starting in 2019 in Italy, which are the “Isarco River underpass” and cross-passages of “Line 4 of the Milan subway”. The opportunity to interact with these professional figures, who designed and followed the AGF interventions, and to frequently visit the building sites allowed to create an innovative comprehensive study that provided support from the preliminary design phase to the control and monitoring phase during tunnel excavation operations. In fact, many geomaterials coming from the above sites (soil, concrete lining of tunnel and cement-mixture treated soil), and others such as saline soils, coming from other sites not subject to AGF, were taken into consideration. These geomaterials were subjected to mechanical and thermal tests to identify the mechnical and thermal parameters under different thermal conditions, and the data provided an important database upon which to base the final design. The data obtained show great consistency with previous work. A constructive comparison also identified some shady areas that represented points of early development, such as performing point load tests in situ on frozen soil samples to estimate the true strength of the frozen soil shell or monitoring the temperature at the excavation face with an infrared imaging camera to assess intervention quality and frozen wall thickness. The collection furthermore of frozen material from a site gave the opportunity to study undisturbed samples, through a workflow consisting of storage stages, investigation by Computed Tomography (CT), and dry coring, and to compare the results with test values inherent in the design phase to assess their reliability at the preliminary stage. These type of interventions is supported by numerical modeling that allows the effects of a process within a domain to be calulated in a distributed manner. Similarly, the AGF makes use of FEM-based thermal analyses that allow to predict cold front propagation within the ground and to perform possible "adjustments" to the arrangement of the freezing pipes to reach the established temperature targets, furthermore such a tool allows to estimate execution times, and in an indirect way also costs. Based on these considerations, a simple and fast laboratory test of unidirectional freezing of a soil sample was developed to be able to provide useful data for validation of commercial numerical codes. The use of LN2 at visited construction sites as a coolant has prompted a search for methods to numerically simulate the freezing effect induced by an LN2-fed freezing pipe, since the thermodynamic processes that takes place within it are very complex. In the state of the art, there are oversimplified methods that are based on the often sparse data referring to what happens in the probe in vertical position. This has led to the design and construction of an innovative freezing pipe with a large number of temperature sensors positioned to capture the level of liquid nitrogen inside the pipe, external and internal temperatures, and to compare the distribution of temperatures in vertical and horizontal positions, with dry or wet sand, and to monitor liquid nitrogen consumption. The data collected are of great value for detailed numerical simulation.
CASTELLANZA, RICCARDO PIETRO
FRATTINI, PAOLO
congelamento terreno; esperimenti avanzati; analisi termiche; validazione codice; test innovativi
ground freezing; advanced experiments; thermal analysis; code validation; innovative tests
GEO/05 - GEOLOGIA APPLICATA
English
21-feb-2024
36
2022/2023
open
(2024). Innovation applied to artificial freezing of soils: an experimental study to enhance theoretical and numerical analysis. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2024).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/461830
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