Traglastberechnung von schlanken Verpreßpfählen in weichen bindigen Böden

Ultimate load capacity of slender injection piles in soft soils. Small diameter driven injection piles are applied as a low cost foundation element for many years. They reach extremly high slenderness and tend to buckling behaviour. Some recent damage cases show that the bearing capacity is frequently overestimated. The design codes do not give evidences for the estimation of the ultimate load capacity in detail. The purpose of this paper is to show a practical method of analysis which includes the essential input parameters of apparent cohesion intercept and pile imperfection as well as the elastoplastic properties of pile material and soil.     

Auswirkung der Herstellungsmethode auf den Gebrauchszustand von Schlitzwänden in weichen bindigen Böden

Consequences of the construction process on the service serviceability of diaphragm walls in soft soils. In order to prove the serviceability of retaining structures, the finite element method is frequently used. A conventional calculation neglects the construction process of the retaining wall and adopts the earth pressure at rest as initial condition. However, the presented FE‐calculations show, that this kind of approach underestimates wall deflections, surface ground settlements and strut forces in case of a diaphragm wall in soft clayey ground. The stepwise construction of the panels affects the stress distribution in the adjacent ground and evokes the partial mobilisation of the earth resistance in the upper third of the wall. In order to consider the influence of the diaphragm wall construction, a modified initial stress field depending on the fresh concrete pressure is proposed.     

Entwurfsoptimierung von Verbau und Wasserhaltung tiefer,wasserdichter Baugruben in bindigen Böden

Optimised design of deep, watertight excavations in cohesive soils. In the urban environment deep excavations are often constructed as waterproof, in which case the earth‐ and water‐pressures grow with increasing depth to such an extent, that extensive measures against bending, large stabilising forces and deep embedments become necessary – often at the limit of the technical possibilities, particularly in the case of tied back excavations in clays with orthotropic hydraulic conductivities. An important means of optimising deep watertight excavations is to reduce the magnitude of the hydrostatic and seepage water pressures acting on the retaining system. This may be achieved by means of: (1) a reduction of the watertight embedment depth below excavation level and (2) a local reduction of the pore‐water pressure behind the retaining wall by means of pressure relief drillings. In combination with case histories the results of three‐dimensional numerical analyses are reported herein, simplified load figures presented and recommendations for economically and technically optimised excavation concepts given.     

Knicken von schlanken Pfählen in weichen Böden

Buckling of slender piles in soft soils. Buckling of piles is resisted by the surrounding soil. Therefore, in sufficiently stiff soils there is no risk of buckling of piles with conventional geometry (L/D ratio). National and international design codes require checks of the safety against buckling for slender piles in soils with an undrained shear strength of c_u < 15 kPa or c_u < 10 kPa, respectively. In practice, the codes are taken to imply that buckling failure does not occur at higher undrained shear strengths. However, using simple calculations this assumption has been proven to be invalid for slender piles in very soft soils. Therefore, buckling tests on piles up to 4 m long were carried out at the Zentrum Geotechnik of the Technical University of Munich. Based on these results a concept for calculating buckling loads was derived, which covers all essential aspects observed in the tests. The proposed model accounts for material non‐linearity as well as second order effects.     

Baugrubenverformungen in weichen Böden bei spannungspfadabhängigem Materialverhalten

Es werden die wesentlichen Ergebnisse zur Untersuchung der Baugrubenverformungen in weichen Böden bei spanungspfadabhängigem Materialverhalten anhand von 1g‐Modellversuchen, Elementversuchen, numerischen Berechnungen sowie Validierungen an drei Fallbeispielen vorgestellt. Wesentliche neue wissenschaftliche Erkenntnisse resultieren insbesondere aus spannungspfadabhängigen Triaxialversuchen. Die bodenmechanischen Kenngrößen sind danach eindeutig spannungspfadabhängig. Darüber hinaus wird eine verbesserte, unmittelbar in der Ingenieurpraxis anwendbare Vorgehensweise zur Berücksichtigung der spannungspfadabhängigen Steifigkeit vorgestellt, die für zukünftige Projekte bei vergleichbaren Randbedingungen besser abgesicherte Prognosen der Beanspruchungen und besonders der zu erwartenden Verformungen ermöglichen. Deformations of excavations in soft soils with stress‐path‐dependent material behavior. The substantial results of a research work on deformations of excavations in soft grounds with stress path dependent material behavior on the basis of 1g‐model tests, element tests, numerical analysis as well as validations of three practical case studies are presented. The investigations revealed a pronounced dependency of the stiffness on the stress paths. Therefore, a new approach for numerical analyses is introduced which allows a consideration of the stress path dependent stiffness according to the characteristic stress path zones. Taking into account both temporal and spatial effects in the construction of the three case studies, a good agreement is reached further between the measurements and numeriod results. Thus, the new approach contributes to the optimization of the analysis of excavations in soft soils.     

Bewertung von Setzungsmessungen flachgegründeter Gebäude in weichen Böden

Evaluation of settlement measurements of buildings with shallow foundation on soft soils. Available settlement measurements of 10 buildings with shallow foundations on lacustrine clay in Southern Germany represent the basis of a statistical evaluation of the primary‐ as well as the creep‐settlement. Generally, the measurements show that the predicted settlements using the standard one‐dimensional consolidation tests are significantly larger than the actually occurred settlements. According to the evaluation a correction factor to the calculation of settlement in soft soils was derived. By means of back‐analysis of the measured settlements the coefficients of the consolidation and the secondary compression are determined so that the time‐settlement behavior could be also investigated.     

Scherfestigkeit durch Kapillarität in unzementierten ungesättigten bindigen Böden

Capillary strength of uncemented and unsaturated cohesive soils in direct shear. Shear strength of unsaturated soils is larger than in the saturated state due to capillary forces. This affects stability of geotechnical constructions to an often unknown amount. Capillary‐induced stresses may be calculated by the pF‐curve (saturation‐suction‐relation). In this paper, a method for calculation of shear strength is given, which is useful for the whole range of suction. Maximum capillary shear strength may be calculated, which is a minimum value of the whole shear strength and has to be supplemented by the strength of other binding mechanisms. As an example, the method is shown with 6 soils. The critical height of an unsupported vertical slope and the crack‐closing load of a mineral sealing layer is calculated and the possibility of large suction values is discussed.     

Zyklisch viskoelastisch‐viskoplastischer Stoffansatz nichtbindiger Böden und Schotter

Cyclic viscoelastic‐viscoplastic constitutiv approach for granular soils and ballast. In modern track building, in particular railway tracks, grain stability and long‐term behaviour of non‐cohesive soils and ballast have gained importance. Especially the capacity of conventional ballasted tracks are of major interest. The developed cyclic viscoelastic‐viscoplastic constitutive approach presented in the actual work describes the long term plastic and elastic material behaviour. Major dependencies are defined by loading conditions and states of stress. With a numerical model implemented, the modelling of the plastic deformation behaviour of complex three dimensional structures under cyclic loading is made possible too.     

Betonangriff in eisendisulfidhaltigen Böden

Concrete Attack in Iron Disulphidic Soils In various areas of Germany superficial soils contain iron disulphide. Due to aeration, e. g. by excavation of soil during construction measures, this mineral can oxidise and release the reaction products sulphuric acid and sulphate under sufficient humidity. As a result, acidic and sulphatic conditions can be adjusted in the construction ground, which may exercise a combined acid‐sulphate‐attack on adjacent concrete structures. The actual extent of the oxidation of iron disulphide in the construction ground and the resulting impairments in adjacent concrete structures have been investigated within an interdisciplinary research project at the Ruhr‐Universität Bochum. In preliminary results pH‐values down to pH 2 and sulphate concentrations over 20, 000 mg/l were determined in iron disulphidic soils in dependence of various physical and chemical factors. Under these conditions corrosion processes take place in adjacent concrete, which are dominated initially by a solvent attack. After a long‐lasting exposure of about one year or more new formations of sulphate minerals can be observed beyond the (acid) corrosion front in deeper, hitherto undisturbed areas, which may indicate a progressing sulphate attack.     

Das Angriffspotential sulfatsaurer Böden in Norddeutschland

The Potential for Chemical Attack by Acid Sulfate Soils in Northern Germany Combined Acid and Sulfate Attack on Concrete Some soil types in Northern Germany show remarkably high pyrite contents which can release huge quantities of acidity and sulfate during oxidation. These pyrite‐rich peat and marsh soils were generally formed within the last 10.000 years and are often referred to as potential acid sulfate soils. These soils provide a significant risk potential for concrete structures in the ground and are often neglected in current regulations. The patchy occurrence and missing link to pyrite bearing rock types impedes identification and mapping. Upon oxidation the often very low acid neutralizing capacities accompanied by high pyrite contents lead to a combined acid and sulfate attack on potential concrete structures in the ground.     

Spaltzugversuche mit kohäsiven Böden

Performance and evaluation of spitting tensile tests with cohesive soils. The splitting tensile test is the tensile test with smallest laboratory effort. Cohesive soils may permanently carry tensile stresses and e.g. fail in tension mode due to dessication or tensile stresses in slopes. In order to develop a soil‐mechanic model of those phenomena, standardized experimental determination of tensile strength is important. Results of test as well as FE calculations on the basis of different material laws are presented. Dimensional analysis helps in analyzing and formulation of empirical approximation functions for the results of FE‐calculations. As a conclusion, recommendations for processing and evaluation of tensile tests with cohesive soils are given and remaining questions are listed. The method of parametrization with preceding dimensional analysis may be used for any other geotechnical problem, too.