Over the past years, levee failures during floods have caused significant losses of lives and properties in the nation. Majority of these failures were induced by seepage through granular foundation soils underneath the top soil on which the levees were built. One of methods to mitigate this phenomenon includes the treatment of the granular soil deposits with an engineered fluid delivered by permeation (permeation grouting), resulting in a less pervious deposit. Since the conventional cement-based suspensions and chemical solutions may cause groundwater contamination due to long term reaction with groundwater, clay suspension such as bentonite suspension can be an alternative in terms of environmental friendliness and long-term safety. Moreover, the suspensions, after being permeated, are expected to be stable in the pore space due to the thixotropic nature of bentonite. The main challenge in this approach is being able to permeate a concentrated suspension through the pores of a granular material. To achieve a significant reduction in the hydraulic conductivity, concentrated bentonite suspensions should be used; however, concentrated suspensions can have low mobility, resulting in a low penetration depth and little practical application. The main objective of this study is to investigate the permeation of concentrated bentonite suspensions by controlling their rheological properties. The first portion of this research focuses on measuring the rheological properties of the various engineered bentonite suspensions over time. The second point of focus of this research is the parameters affecting the flow of the bentonite suspensions through granular soils, and the final focal area is determining the hydraulic performance of the grouted granular soils. In order to achieve these objectives, an experimental program was developed in this research. First, rheological tests were performed with the bentonite suspensions with and without various concentrations of sodium pyrophosphate (SPP); SPP is an ionic additive that is used to reduce the initial yield stress and viscosity of bentonite suspensions. A stress controlled test with the vane geometry produced rheological parameters with a minimal disturbance. Suspensions were stored in sealed cups and tested at various times to measure the long term thixotropic changes in yield stress and viscosity. Second, the various concentrations of the bentonite suspensions were injected at a constant pressure through clean sands which were prepared at various conditions (relative density, fine contents, and grain size) in order to investigate soil and suspension parameters affecting the flow of the bentonite suspensions. The results from these experimental tests were utilized to develop a groutability criterion of bentonite suspensions for practical purposes. Finally, the saturated hydraulic conductivity of the treated soils was measured using falling and rising head method. The traditional concept of “clay void ratio” was re-examined. The results from this study showed that the modified bentonite suspensions could be used as an alternative grout in permeation grouting to improve hydraulic performance of the permeable granular soils. / text

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Μια μεγάλη κατηγορία μεθόδων βελτίωσης – ενίσχυσης εδαφών είναι αυτή των ενέσεων. Η παρούσα έρευνα αποτελεί μέρος μιας εκτεταμένης προσπάθειας αντικατάστασης των χημικών διαλυμάτων με τα οικονομικότερα και αβλαβή προς το περιβάλλον ενέματα τσιμέντου και κυρίως τα αιωρήματα λεπτόκοκκων τσιμέντων, λόγω της δυνατότητας διείσδυσής τους ακόμα και σε πολύ λεπτόκοκκες άμμους. Στόχος της διατριβής αυτής είναι η συγκριτική αξιολόγηση της βελτίωσης των υδραυλικών και μηχανικών ιδιοτήτων των εμποτισμένων άμμων με χρήση ευσταθών και ασταθών αιωρημάτων , αφού το τελικό ποσοστό εξίδρωσης φάνηκε να συνδυάζει επιμέρους παραμέτρους ( λόγος νερού προς τσιμέντο και μέγεθων κόκκων τσιμέντου) και συνεπώς κρίθηκε καθοριστικός ο ρόλος του στην επιτυχία του εμποτισμού. Για τις ανάγκες της εργαστηριακής διερεύνησης χρησιμοποιήθηκαν τρεις τύποι τσιμέντου , ενώ εμποτίστηκαν τέσσερις καθαρές ασβεστολιθικές άμμοι διαφορετικής κοκκομετρικής διαβάθμισης καθώς και μία χαλαζιακής προέλευσης. Τα εμποτισμένα δοκίμια εξετάστηκαν σε δοκιμές διαπερατότητας και ανεμπόδιστης θλίψης και ακολούθησε παραμετρική διερεύνηση των μεγεθών αυτών για την ποσοτικοποίηση της επιρροής τους από μια σειρά παραμέτρων (λόγος νερού προς τσιμέντο, τελικό ποσοστό εξίδρωσης , κοκκομετρία και σύσταση άμμου, τύπος και κοκκομετρία τσιμέντων).Τελικά, ο λόγος νερού προς τσιμέντο αναδεικνύεται ως η σημαντικότερη παράμετρος επίδρασης των τιμών τόσο του συντελεστή διαπερατότητας όσο και της αντοχής σε ανεμπόδιστη θλίψη των εμποτισμένων άμμων. / Injections are one of the methods used for soil improvement and strengthening. This research is part of an extensive effort in order to replace chemical grouts with cheaper and harmless to the environment cement grouts; especially with microfine cement suspensions, because of their capability to penetrate even very fine sands. The aim of this study is to estimate the improvement of the hydraulic and mechanical properties of grouted sands using stable and unstable suspensions, since the bleed capacity seemed to combine individual parameters (water to cement ratio and cement grain size). Therefore, this factor was considered decisive for the success of grouting. During the laboratory investigation, three types of cement were used and four asbestolithic, clean sands of different grain gradation were grouted, as well as a quartz sand. Permeability and unconfined compression tests were performed on the grouted specimens. Then, a parametric study of the tests results was followed in order to quantify the influence of various parameters (water to cement ratio, bleed capacity, grain size and composition of sand, cement type and particle size). Finally, the water/cement ratio emerges as the most important parameter, which influences the values of both the coefficient of permeability and the unconfined compressive strength of the grouted sands.