Excess pore pressure generation due to pseudostatic tests in saturated sand Marta Auleda Catal`a May 26, 2005 Abstract Pile foundations are widely used, mainly to transmit structural load to an underlying stiffer soil or rock. This limit state load a certain pile can sustain without failure is known as pile ultimate bearing capacity. During design stage load-tests are performed in-situ on test piles to determine, among others, the value of the bearing capacity. Commonly static tests are performed as they provide the most reliable data. Dynamic tests are much more cost-effective but have a series of shortcomings, mainly the fact that they introduce stress-waves on the pile and that require calibration with the static values. To overcome both nature-kind problems, a new type of test in-between the previous ones, i.e. the pseudostatic test, has been developed. It is still a dynamic test but the loading pulse lasts longer (70-150ms), 20 times the dynamic pulse, emphasizing the static component. Hence, it is both an economical and reliable option as requires no calibration with the static load-displacement curves. Therefore, it is interesting to get more insight on it. Two main factors can influence the bearing capacity of a pile measured on the in- situ tests, namely, loading rate and excess pore pressures. In cases like The Netherlands, where end-bearing piles are driven into saturated sand, these two concepts may play an important role. A previous study had been carried out in dry sand and did not find a remarkable loading rate effect. However, for the case of saturated sand the soil response remains unknown. This research investigates the topic, the objective is to get more insight on the excess pore pressure generation and dissipation, evaluate the static-pseudostatic correlation and investigate the possibility of providing effective predictive tools. The research has been structured in three parts. First a series of experimental scaled tests have been carried out for three loading rates: a CPT (20 mm/s), a static test (1 mm/s) and a pseudostatic test (up to 250 mm/s). The sample consisted in saturated sand that was prepared by means of a fluidizaton-vibration system. Standard sounding rots with a piezocone acted as the pile; five values were recorded: force on the pile head, shaft friction, tip resistance, displacement and acceleration. Later on, the performed scaled tests have been modeled analytically and numerically. An analytical model based on the cone model of Wolf has been developed. Only the soil underneath the pile tip is considered and it is modeled as an elastoplastic material under static fully undrained loading followed by consolidation. PLAXIS is the program used for the numerical model. The soil is represented by means of the Hardening Soil model and, although large deformations as the CPT are not allowed, the installation effects are accounted for. Static and low-frequency dynamic calculations have been performed, under fully drained and undrained loading conditions. Finally conclusions can be drawn, if not form a quantitative point of view, from a qualitative one: There is a loading-rate effect on the generation of excess pore pressures, increasing • loading velocities generate larger excess pore pressures, from an average of 0.003MPa for the static test to 0.03MPa for the pseudostatic one. Experimentally, though, the larger excess pore pressures do not affect values of • tip and shaft resistance, thus, do not affect pile bearing capacity. Analytical and numerical models have been able to explain this fact by showing that the loading process is not fully undrained as first thought but partial drainage occurs instead. Static results and models fit appropriately the pseudostatic ones and vice versa, • pointing towards the suitability of the use of pseudostatic tests even in saturated granular soils. Evaluating the strong and weak points of the results and the robustness and limitations of the methodology used also some recommendations on further research have been proposed. 1 Acknowledgments This thesis is not an isolated research but much more, it is the closure for 6 years of journey throughout the university. 6 years of struggle, studying and hard work but also, and much more important, 6 years of growth, discovery and development of myself as a person. The largest heritage of these years is the experience on the whole, all the people I have met, all the situations I have dealt with, all the moments we shared, all the things I learned by their side. An experience far too overwhelming to describe in a few lines and I do not have words to just write down here how thankful I am to have been able to live it and be part of it. Of this experience, these two years in Delft have been one of the most important parts. I would not be the same Marta I am without this lovely baggage. This thesis has been carried out in the department of Geotechniek of the Faculty of Civil Engineering and Geosciences, Delft University of Technology. I would like to thank first of all my supervisors who have been always there helping me out and without whom this thesis would have not been possible: Prof.ir.A.F.van Tol, ing.H.J.Everts, Dr.ir.P.H¨olscher, Dr.ir.R.Brinkgreve and Prof.Dr.Ing.E.Alonso P´erez de Agreda.´ I am especially indebted to Dr.ir.P.H¨olscher for his determining help and coaching with all the analytical developments and to Dr.ir.R.Brinkgreve for his valuable help with all the questions and problems with PLAXIS. I am also greately thankful to Dr.ir.W.Broere for his very useful suggestions about the numerical model and to ir.J.Dijkstra for his gen- erous support and advice. I need to extend my thanks to everyone in Geotechniek department for welcoming me and making me feel so comfortable among them. Of course thousand thanks to my room- mates and fellows of headaches, stress, coffee, football discussions and some sweet laughs too: Joana Pragosa, Remco Kleinugtenbelt and Eelco Veenstra, thank you guys for your companionship. A big hug as well to Chris Sevink and Stefan Segers. The best part of my Erasmus-free mover stay in The Netherlands, the most enriching one is the intercultural exchange and the possibility to meet such great people with so different backgrounds. Some very special people in the melting pot: thanks to Jordina Boada, ’tiet’ Oriol Lloberas, ’tieta’ Anna Berenguer, to Jordi Figueres and his beer-PhD researchers Amer and Yann, to Oriol Serra, to Giulio Sovran and Diego Quadrelli my good inseparable italian friends, to Masako Matsuoka and Amalia Guanter for being the funniest housemates ever and Elena Las Heras my favourite neighbour, to Joao Fernandes and Sofia Freitas a really nice portuguese couple, to Gaspar Gonz´alez and Pepe Pont and to the spanish crowd, Danielino and company, Stijn van Dam, and my favourite aussie Vanessa Ratard. Ook hartelijk bedankt aan mijn Koornmarkt huisgenotjes van de familie Walter, dank je voor mij te adopteeren en een meer thuis laten zijn, jullie hebben de nederlandse-ervaring echt gemaakt! Moltes gr`acies als meus amics d’EG, en especial a la Laura, la Pilar, en Jordi, l’ Abra- ham, la Clara, en David, la Vane...i molts d’altres, gr`acies per deixar-me compartir amb vosaltres tots aquests anys! Gracies a tota la colla de l’Ipsi, S`onia, Enric, Alex,` Lloren¸ci Marc, i la meva Gemmota i al Mas tan xulos sempre! Last but not least...my last word of thanks goes to my whole family, natuurlijk! Gr`acies per estar sempre al meu costat i per tot el vostre afecte. I gr`acies sobretot als meus pares, Jaume i Margarita per recolzar-me tant, per ajudar-me tant, en definitiva, per estimar-me tant. E anche a te, Stefano, grazie, grazie, grazie, gr`acies per tots els moments plegats, per fer-me tan feli¸c,per ser sempre aqu´ıben aprop, no importa la dist`ancia. Moltes gr`acies a tots, grazie, thank you, dank je wel! Delft, May 2005 Marta Auleda i Catal`a 2 Contents 1 Introduction 9 1.1 Outlineofthereport............................... 10 2 Problem analysis and objectives 12 2.1 Problemanalysis ................................. 12 2.1.1 Introduction of the subject . 12 2.1.2 Definition of the problem . 16 2.2 Objectives..................................... 17 2.3 Scopeoftheresearch............................... 18 2.4 Limitations .................................... 20 2.4.1 Experimental testing limitations . 20 2.4.2 Analytical modeling limitations . 20 2.4.3 Numerical modeling limitations . 20 3 Literature research 21 3.1 General considerations . 21 3.1.1 Loading-rate effects . 21 3.1.2 Excess pore pressure effects . 22 3.2 Experimentaltesting............................... 25 3.2.1 Calibration chamber testing . 25 3.2.2 Pore pressure measurement . 26 3.3 Analyticalmodeling ............................... 27 3.3.1 Introduction ............................... 27 3.3.2 Soil-pile interaction models . 28 3.3.3 Pore pressure and consolidation models . 40 3.3.4 Conclusion ................................ 43 I Experimental testing 46 4 Test set-up and regime 47 4.1 Introduction.................................... 47 4.2 Thecalibrationchamber............................. 47 4.3 Thesand ..................................... 48 4.3.1 Sandproperties.............................. 48 4.3.2 Preparationofthesoil . .. 49 4.4 Overview of the test series . 50 4.4.1 Testregime ................................ 50 4.4.2 Equipment ................................ 51 4.4.3 Testlocation ............................... 52 3 5 Experimental results and evaluation 53 5.1 Introduction.................................... 53 5.1.1 Verification of testing procedure . 53 5.1.2 Failurecriteria .............................. 53 5.2 Resultspresentation ............................... 54 5.2.1 CPTresults................................ 54 5.2.2 Staticresults ............................... 55 5.2.3 Pseudostaticresults ........................... 55 5.3 Resultsanalysis.................................. 59 5.3.1 CPTandStatictests..........................