For the project of pile foundations with points in solid or semi-solid rocks the Czechoslovak Standard gives the values of derived standard stress under the point of the pile. Several structures which are being realized have required to drive a considerable part of the piles into the rock massif, owing to the very big concentration of 10ads.In order to utilize also the bearing capacity on the skin of the pile in the rock massif, special tests were performed during the engineering- geological research. The purpose of these tests was to determine the adhesion between the rock massif and the concrete testing bodies. The obtained values were used for the computation of the bearing capacity on the skin of the piles which formed the foundation system.
Pour le projet de fondations sur pieux la Norme tchecoslovaque ne donne, en vue de la determination de la capacite portante dans les roches solides ou demi-solides, que les valeurs de la contrainte normale derivee, au-dessous de l'extremite inferieure du pieu. Dans le cas de plusieurs construction realisees à l'heure actuelle, il a ete necessaire, etant donne une concentration de charge extrêment elevee, d'encastrer les pieux, pour une partie considerable, dans un massif rocheux. Anfin d utiliser egalement la capacite portante de la surface d'un pieu encastre dans un massif rocheux, on a procede, dans le cadre de la prospection geologique, à des essais permettant de determiner la cohesion entre la roche et le beton d'un eprouvette. Les valeurs ainsi obtenues ont servi de base pour calculer la capacite portante de la surface des pieux d'un système de fondations.
Fuer den Entwurf der Tiefenfundamente fuehrt die tschechoslowakische Staatsnorm ČSN 73 1002 an fuer die Bestimmung der Tragfahigkeit in Felsen- und Halbfelsengebirgen nur die Werte der abgeleiteten Normbeanspruchung unter dem unteren Ende des Pfahles. Bei einigen in Gegenwart ausgefuehrten Bauten es nötig war, im Hinblick auf die konzentrierte Belastung von hohem Werte, die Tiefenfundamente auf einem erheblichen Teil ihrer Lange in den Felsenmassiv einzuspannen. Um auch die TragFahigkeit auf dem pfahlmantel in dem Felsenmedium ausnuetzen zu können, wurden im Rahmen einer Ingenieur-geologischen Untersuchung Pruefungen vorgenomen, deren Zweck die Bestimmung der Kohasion zwischen dem Gesteine und dem Beton des Pruefkörpers war. Die erworbenen Werte bildeten die Unterlage fuer die Bestimmung der Tragfahigkeit auf dem Mantel des Tiefenfundamentes.
In the projects of very high buildings, bridges and similar engineering structures, the designer very often meets with a concentration of high loads; for this reason deep foundations reaching into partly weathered or unweathered hard rock subgrade are considered. The skin resistance of deep foundations in the horizon of the overburden formations cannot be exploited, because the space formed by a relatively easy excavation of soil of various thickness is used for the construction of two or three underground floors. In another method, the soil is excavated up to the hard rock surface and from there the deep foundations are executed up to a depth of several metres into the hard rock, because of the concentrated load and the unsatisfactory quality of the rock surface, In the both cases the designer needs the value of the skin resistance qs. In the Czechoslovak Standard ČSN 73 1002 this value is given only from class 6 of rock, because it is assumed that the footing bottom will be placed mostly on the surface of the hard rock subgrade, or, that the deep foundation will be fixed at a small depth(three times the diameter) in relation to the pile length, which is determined by the bearing capacity of the foundation structure and the structural modulus. It has been ascertained also in many static load tests of piles in the field that, until ultimate strength is exceeded, the load bearing element takes over approximately 90 pc. of the load by its skin, while the area under the bottom end of the pile contributes merely by a small ratio. This relation has been found above all in geological conditions represented approximately up to a half of the depth of deep foundation by an overburden consisting of a mixture of cohesive and non-cohesive soils, and at the remaining half by a hard-rock subgrade with various degree of weathering and tectonic faults. Considering that the fixing depth of a deep foundation into a hard-rock subgrade is often five times the diameter and more, it is obvious that the bearing capacity of this part of the deep foundation is higher than, the bearing capacity qo for the maximum fixing depth corresponding to the Standard (The fixing depth is determined by ratio o/d, where D is the bedding depth of the element in the respective stratum - usually which has the highest bearing capacity - of the hard rock, and d is the diameter of the element - pile.)
