Hydrogeological impact assessment by tunnelling at sites of high sensitivity

A tunnel for the High Speed Train (HST) was constructed in Barcelona with an Earth Pressure Balance (EPB) Tunnel Boring Machine (TBM). The tunnel crosses Barcelona and passes under some famous landmarks such as the Sagrada Familia and the Casa Mill Both monuments are UNESCO world heritage sites and a committee appointed by the UNESCO acted as external observers during the construction. Concerns about soil settlements and the hydrogeological impacts of the construction were raised. These concerns were addressed during the design stage to forestall any unexpected events. The methodology consisted of 1) characterising the geology in detail, 2) predicting the impacts caused in the aquifer, 3) predicting the soil displacements due to water table oscillations produced by the construction, and 4) monitoring the evolution of groundwater and soil settlements. The main estimated impact on groundwater was a moderate barrier effect. The barrier effect, the magnitude of which matched the predictions, was detected during construction. The monitoring of soil settlements revealed short and long term movements. The latter movements matched the analytical predictions of soil displacements caused by the groundwater oscillations. This paper proposes a realistic procedure to estimate impacts on groundwater during tunnel construction with an EPB. Our methodology will considerably improve the construction of tunnels in urban areas. (C) 2015 Elsevier B.V. All rights reserved.Peer ReviewedPostprint (author's final draft

Interaction between groundwater and underground constructions

Underground constructions below the water table may be problematic if the role of groundwater is not properly acknowledged. Difficulties worsen in urban environments. Two aspects should be taken into account in the interaction between underground constructions and groundwater, 1) the impacts caused by the construction in the aquifers and 2) the difficulties that groundwater causes during the construction. Therefore, the design of an underground construction must be minimize the impacts in the underground environment as well as guaranteeing the safety of the workers and the integrity of adjacent structures. The adopted measures must not complicate the development of the construction and must not increase the total cost. To sum up, the construction must be efficient. These questions arose during the construction of the High Speed Train (HST) tunnel in Barcelona, which passes next to the Sagrada Familia. This thesis proposes answers to the questions which came up. Two problems may arise when a construction intersects an aquifer, the drain and the barrier effect. While the former has been widely studied, the second has not been adequately formalized and this is the first aim of the thesis. Analytical solutions are obtained to compute the head variations caused by an underground impervious structure. The solutions allow computing of the impact under different circumstances and to design corrective measures. The proposed equations were verified by using the data from real underground constructions. Subsidence caused by dewatering processes of deep excavations is feared. This fact affects the design. One option to reduce subsidences consist on deepen the enclosures (diaphragm walls, piles, jet-grouting piles) in order to avoid or minimise the dewatering. The second objective is to discuss the effectiveness of this measure, which rise the cost of the constructions, since drawdown caused by pumping are usually small and less dangerous (poorly differential) than is expected. Moreover, the pumping stabilizes the bottom of excavations. Therefore, to deepen the enclosures may be less efficient than combining short enclosures with deep pumping wells. Both alternatives must be compared. Thus, a number of dewatering scenarios, where the depth of the enclosures and the pumping wells are varied, are compared considering the safety, the outside affectations and the cost. Results show that combining deep pumping wells with short enclosures can become the most efficient method to perform excavations in preconsolidated soils. ii Regardless of the method used to perform an excavation, the enclosure, always, plays an important role since it guarantees the stability of the excavation walls and prevents the entrance of lateral flow. The presence of small defects may lead to disastrous consequences, which would invalidate all the previous work oriented to develop an efficient construction. Therefore, given that the defects are relatively common, that the techniques used to detect defects are limited and that the groundwater behaviour taking into account underground structures can be predicted, the third objective of the thesis is to develop hydraulic methods to assess the state of an enclosure. These methods, specifically the Watertightness Assessment Test (WTAT), are used as much to estimate the effective parameters of the enclosure as to locate the defects. Finally, the steps followed during the construction of the HST tunnel in Barcelona demonstrate the importance of the geological characterisation. If the soil is well known, all the aspects associated with the construction can be predicted accurately, which is crucial for designing an efficient underground construction. The geology, the hydrogeology and the historical processes suffered by the soil must be characterized accurately.Las construcciones subterráneas realizadas por debajo del nivel piezométrico pueden ser problemáticas si no se reconoce el papel del agua subterránea. Las dificultades aumentan en ambientes urbanos. La interacción con el agua subterránea tiene lugar en las dos direcciones 1) los impactos causados por la construcción sobre el acuífero y 2) las dificultades que, durante la construcción, causará la presencia de agua subterránea. Por ello, el diseño de una construcción subterránea debe minimizar los impactos en el medio subterráneo y garantizar la seguridad de los trabajadores y la integridad de las estructuras adyacentes. Las medidas adoptadas no deben complicar en exceso el desarrollo de las obras ni sobrecargar el coste total de la obra. En resumen, la construcción debe ser eficiente. Estas preguntas surgieron durante la construcción del túnel para el Tren de Alta Velocidad (HST) en Barcelona, adyacente a la Sagrada Familia. En esta tesis se proponen respuestas a las mismas. Los impactos sobre el acuífero pueden ser de dos tipos: el efecto dren y el efecto barrera. Mientras que el primero ha sido ampliamente estudiado, el segundo no ha sido formalizado adecuadamente, lo que constituye el primer objetivo de esta tesis. Para calcular las variaciones de nivel causadas por una estructura subterránea impermeable, se derivan ecuaciones para diversas condiciones de obra y para medidas correctoras. Se han verificado con datos de construcciones reales. La subsidencia causada por el drenaje de excavaciones profundas es uno de los temores más condicionantes del diseño. Una de las maneras de reducirla, consiste en profundizar los recintos (pantallas, pilotes, columnas de jet-grouting) con el fin de evitar o minimizar el bombeo. El segundo objetivo de esta tesis es cuestionar la eficacia de estas medidas, que aumentan el coste de la construcción, ya que los asientos causados por el bombeo suelen ser pequeños y menos peligrosos (poco diferenciales) de lo temido. Además, el bombeo estabiliza la base de las excavaciones. Por ello realizar recintos más profundos puede ser menos eficiente que combinar recintos cortos y pozos de bombeo profundos. Es obvio que ambas alternativas deben compararse adecuadamente. Para ello, se han estudiado una serie de escenarios de drenaje que son comparados teniendo en cuenta la seguridad, las afecciones externas y el coste. Las variaciones entre los diferentes escenarios son las profundidades de los recintos y de los pozos de bombeo. Los resultados muestran que combinar pozos de bombeo profundos con recintos cortos es el método más eficiente para llevar a cabo excavaciones en suelos preconsolidados. Independientemente del método utilizado para llevar a cabo una excavación, el recinto, siempre juega un papel importante, ya que garantiza la estabilidad de las paredes de la excavación y evita la entrada de flujo lateral. La presencia de pequeños defectos puede tener consecuencias desastrosas, lo que haría inútil todo el trabajo previo orientado a desarrollar una construcción eficiente. Por lo tanto, dado que los defectos son relativamente comunes, que las técnicas utilizadas para detectar defectos son limitadas y que el comportamiento del agua subterránea puede ser predicho teniendo en cuenta las estructuras subterráneas existentes. El tercer objetivo de la tesis es desarrollar métodos hidráulicos para evaluar el estado del recinto de una excavación. Se muestra que estos métodos y, en particular, el ensayo de caracterización de impermeabilización permiten tanto estimar los parámetros efectivos del recinto como localizar los defectos. Por último, los pasos seguidos durante la construcción del túnel para el Tren de Alta Velocidad en Barcelona evidencian la importancia de una buena caracterización geológica. Si el suelo es bien conocido, todos los aspectos asociados con la construcción pueden ser estimados con precisión, lo que resulta crucial para diseñar una construcción subterránea eficiente. La geología, la hidrogeología y los procesos históricos sufridos por el suelo deben ser bien conocidos.Les construccions subterrànies realitzades sota el nivell piezométric poden ser problemàtiques si no es reconeix el paper de l'aigua subterrània. Les dificultats augmenten en ambients urbans. La interacció amb l'aigua subterrània té lloc en els dos sentits 1) els impactes causats per la construcció sobre l'aqüífer i 2) les dificultats que, durant la construcció, causarà la presència d'aigua subterrània. Per això, el disseny d'una construcció subterrània ha de minimitzar els impactes al medi subterrani i garantir la seguretat dels treballadors i la integritat de les estructures adjacents. Les mesures adoptades no han de complicar en excés el desenvolupament de les obres ni sobrecarregar el cost total de l'obra. En resum, la construcció ha de ser eficient. Aquestes preguntes van sorgir durant la construcció del túnel per al Tren d'Alta Velocitat a Barcelona, adjacent a la Sagrada Família. En aquesta tesi es proposen respostes a les mateixes. Els impactes sobre l'aqüífer poden ser de dos tipus: l'efecte dren i l'efecte barrera. Mentre que el primer ha estat àmpliament estudiat, el segon no ha estat formalitzat adequadament, la qual cosa constitueix el primer objectiu d'aquesta tesi. Per calcular les variacions de nivell causades per una estructura subterrània impermeable, es deriven equacions per a diverses condicions d'obra i per a mesures correctores. S'han verificat amb dades de construccions reals. La subsidència causada pel drenatge d'excavacions profundes és un dels temors més condicionants del disseny. Una de les maneres de reduir-la, consisteix a aprofundir els recintes (pantalles, pilotis, columnes de jet-grouting) amb la finalitat d'evitar o minimitzar el bombament. El segon objectiu d'aquesta tesi és qüestionar l'eficàcia d'aquestes mesures, que augmenten el cost de la construcció, ja que els seients causats pel bombament solen ser petits i menys perillosos (poc diferencials) del temut. A més, el bombament estabilitza la base de les excavacions. Per això realitzar recintes més profunds pot ser menys eficient que combinar recintes curts i pous de bombament profunds. És obvi que ambdues alternatives han de comparar-se adequadament. Per a això, s'han estudiat una sèrie d'escenaris de drenatge que són comparats tenint en compte la seguretat, les afeccions externes i el cost. Les variacions entre els diferents escenaris són les profunditats dels recintes i dels pous de bombament. Els resultats mostren que combinar pous de bombament profunds amb recintes curts és el mètode més eficient per dur a terme excavacions en sòls preconsolidats. vi Independentment del mètode utilitzat per dur a terme una excavació, el recinte, sempre juga un paper important, ja que garanteix l'estabilitat de les parets de l'excavació i evita l'entrada de flux lateral. La presència de petits defectes pot tenir conseqüències desastroses, la qual cosa faria inútil tot el treball previ orientat a desenvolupar una construcció eficient. Per tant, atès que els defectes són relativament comuns, que les tècniques utilitzades per detectar defectes són limitades i que el comportament de l'aigua subterrània pot ser predit tenint en compte les estructures subterrànies existents. El tercer objectiu de la tesi és desenvolupar mètodes hidràulics per avaluar l'estat del recinte d'una excavació. Es mostra que aquests mètodes i, en particular, l'assaig de caracterització d'impermeabilització (WTAT) permeten tant estimar els paràmetres efectius del recinte com localitzar els defectes. Finalment, els passos seguits durant la construcció del túnel per al Tren d'Alta Velocitat a Barcelona evidencien la importància d'una bona caracterització geològica. Si el sòl és ben conegut, tots els aspectes associats amb la construcció poden ser estimats amb precisió, la qual cosa resulta crucial per dissenyar una construcció subterrània eficient. La geologia, la hidrogeologia i els processos històrics soferts pel sòl han de ser ben coneguts

Dewatering of a deep excavation undertaken in a layered soil

In order to carry out deep excavations under the water table in urban environments, the safety of the work site and of the adjacent buildings is a major cause for concern. One of the most common and effective methods of undertaking these excavations involves combining the cut and cover method with a dewatering system. The success of a construction depends on the stability of the excavation bottom, the effects produced outside the excavation by dewatering (soil movements) and/or the state of the enclosure (defects in the diaphragm walls). This study proposes a realistic multidisciplinary procedure to address these issues. The work emphasizes the importance of soil characterisation and underlines the need to perform a Watertightness Assessment Test (WTAT) before the excavation stage. The procedure was applied to the excavation of a deep shaft of the High Speed Train (HST) tunnel in Barcelona. An earlier geological characterisation at large scale ruled out the use of deep pumping wells. However, a subsequent hydrogeological characterisation, which involved borehole logging, grain size analyses, Natural Gamma Ray and pumping tests, revealed the presence of thin transmissive layers inside the low hydraulic conductivity materials. The dewatering system was designed by considering different model scenarios and the safest design was selected for the excavation. Depths of the enclosure and of the pumping wells differed in accordance with the scenarios. The impacts (settlements due to pumping) and the stability in each scenario were computed. The state of the enclosure underwent a WTAT before the start of the excavation, but after constructing the enclosure, to verify its low permeability. The test consisted in pumping inside the enclosure and monitoring the groundwater behaviour outside the enclosure. Numerical interpretation of this test showed a defect in the diaphragm walls below the excavation bottom. Since this defect was not repaired because of its location (below the bottom of the excavation), the dewatering system had to be redesigned to ensure safety. Surface settlements, which were also a source of concern, were small. They were computed using coupled hydro-mechanical models.Peer Reviewe

Dewatering of a deep excavation undertaken in a layered soil

In order to carry out deep excavations under the water table in urban environments, the safety of the work site and of the adjacent buildings is a major cause for concern. One of the most common and effective methods of undertaking these excavations involves combining the cut and cover method with a dewatering system. The success of a construction depends on the stability of the excavation bottom, the effects produced outside the excavation by dewatering (soil movements) and/or the state of the enclosure (defects in the diaphragm walls). This study proposes a realistic multidisciplinary procedure to address these issues. The work emphasizes the importance of soil characterisation and underlines the need to perform a Watertightness Assessment Test (WTAT) before the excavation stage. The procedure was applied to the excavation of a deep shaft of the High Speed Train (HST) tunnel in Barcelona. An earlier geological characterisation at large scale ruled out the use of deep pumping wells. However, a subsequent hydrogeological characterisation, which involved borehole logging, grain size analyses, Natural Gamma Ray and pumping tests, revealed the presence of thin transmissive layers inside the low hydraulic conductivity materials. The dewatering system was designed by considering different model scenarios and the safest design was selected for the excavation. Depths of the enclosure and of the pumping wells differed in accordance with the scenarios. The impacts (settlements due to pumping) and the stability in each scenario were computed. The state of the enclosure underwent a WTAT before the start of the excavation, but after constructing the enclosure, to verify its low permeability. The test consisted in pumping inside the enclosure and monitoring the groundwater behaviour outside the enclosure. Numerical interpretation of this test showed a defect in the diaphragm walls below the excavation bottom. Since this defect was not repaired because of its location (below the bottom of the excavation), the dewatering system had to be redesigned to ensure safety. Surface settlements, which were also a source of concern, were small. They were computed using coupled hydro-mechanical models.Peer Reviewe

A methodology for analysing the drainage system in excavations between sheet pile walls

The development of an underground station below the water table requires rigorous and careful planning. As such an in depth knowledge of the station construction designs and the related hydraulic behavior are required in order to design the most appropriated drainage system. Moreover, it is important to ensure the correct drainage of the excavation site to avoid liquefaction and to minimize water seepage. As an example of underground construction we consider the tunnel for the new line in the Barcelona metropolitan area (Línia 9) through the town of Prat de Llobregat in the Llobregat River’s delta, which is currently under development. Most of the tunnel sections are constructed with Tunnel Boring Machines (TBM) and the stations and the ventilation shafts are constructed using “Cut & Cover” technology between sheet pile walls. A typical station on the L9 line lies partially between the shallow and the main aquifer. Specifically, it is located in the middle layer which in made up of silts and clays. In order to design the most appropriated drainage system we present a methodology whereby we generalize the hydraulic problem. This method enables rapid and efficient estimation of fluxes as well as a general understanding of the problem in a simplified manner.Peer ReviewedPostprint (published version

Constrained interpolation using rational cubic splines

SIGLEAvailable from British Library Document Supply Centre- DSC:33940.2725(DU-DMCS-CS--90/01) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

A methodology for analysing the drainage system in excavations between sheet pile walls

The development of an underground station below the water table requires rigorous and careful planning. As such an in depth knowledge of the station construction designs and the related hydraulic behavior are required in order to design the most appropriated drainage system. Moreover, it is important to ensure the correct drainage of the excavation site to avoid liquefaction and to minimize water seepage. As an example of underground construction we consider the tunnel for the new line in the Barcelona metropolitan area (Línia 9) through the town of Prat de Llobregat in the Llobregat River’s delta, which is currently under development. Most of the tunnel sections are constructed with Tunnel Boring Machines (TBM) and the stations and the ventilation shafts are constructed using “Cut & Cover” technology between sheet pile walls. A typical station on the L9 line lies partially between the shallow and the main aquifer. Specifically, it is located in the middle layer which in made up of silts and clays. In order to design the most appropriated drainage system we present a methodology whereby we generalize the hydraulic problem. This method enables rapid and efficient estimation of fluxes as well as a general understanding of the problem in a simplified manner.Peer Reviewe

A methodology for analysing the drainage system in excavations between sheet pile walls

The development of an underground station below the water table requires rigorous and careful planning. As such an in depth knowledge of the station construction designs and the related hydraulic behavior are required in order to design the most appropriated drainage system. Moreover, it is important to ensure the correct drainage of the excavation site to avoid liquefaction and to minimize water seepage. As an example of underground construction we consider the tunnel for the new line in the Barcelona metropolitan area (Línia 9) through the town of Prat de Llobregat in the Llobregat River’s delta, which is currently under development. Most of the tunnel sections are constructed with Tunnel Boring Machines (TBM) and the stations and the ventilation shafts are constructed using “Cut & Cover” technology between sheet pile walls. A typical station on the L9 line lies partially between the shallow and the main aquifer. Specifically, it is located in the middle layer which in made up of silts and clays. In order to design the most appropriated drainage system we present a methodology whereby we generalize the hydraulic problem. This method enables rapid and efficient estimation of fluxes as well as a general understanding of the problem in a simplified manner.Peer Reviewe

Barrier effect of underground structures on aquifers

Impervious structures below the water table modify the natural groundwater flow in aquifers. They act as barriers, causing heads to rise upgradient and to fall downgradient. We define the barrier effect as the increase in head loss across the barrier with respect to the natural conditions prior to construction. We distinguish between regional (the minimum head loss observed at long distances) and local (the maximum head loss observed close to the structure) barrier effects. We use numerical and analytical methods to derive semi-empirical equations to quantify the two barrier effects for semi-permeable, partially penetrating (or fully penetrating but finite in length), and barriers with a by-pass in confined aquifers. The resulting equations depend on the barrier geometry and on the natural head gradient in the aquifer and they are easy to apply. We test their validity at two construction sites, obtaining excellent agreement between the computed and observed barrier effects.Peer Reviewe

Quantifying chemical reactions by using mixing analysis

This work is motivated by a sound understanding of the chemical processes that affect the organic pollutants in an urban aquifer. We propose an approach to quantify such processes using mixing calculations. The methodology consists of the following steps: (1) identification of the recharge sources (end-members) and selection of the species (conservative and non-conservative) to be used, (2) identification of the chemical processes and (3) evaluation of mixing ratios including the chemical processes. This methodology has been applied in the Besos River Delta (NE Barcelona, Spain), where the River Besos is the main aquifer recharge source. A total number of 51 groundwater samples were collected from July 2007 to May 2010 during four field campaigns. Three river end-members were necessary to explain the temporal variability of the River Besos: one river end-member is from the wet periods (W1) and two are from dry periods (D1 and D2). This methodology has proved to be useful not only to compute the mixing ratios but also to quantify processes such as calcite and magnesite dissolution, aerobic respiration and denitrification undergone at each observation point