1. Master of Science Thesis
Konstantinos Kaltekis
27-11-2014
Design considerations regarding underwater concrete floors in circular building pits

2. 1
DELFT UNIVERSITY OF TECHNOLOGY
MASTER THESIS
Design considerations regarding underwater concrete floors in circular building pits
Author:
Konstantinos Kaltekis
Thesis committee:
Prof. Ir. A.F. (Frits) van Tol
TU Delft (Section Geo-engineering)
Ing. H.J. (Bert) Everts
TU Delft (Section Geo-engineering)
Dr. Ir. C. (Cor) van der Veen
TU Delft (Section Structural and Building Engineering)
Ir. Jan van Dalen
Senior Geotechnical advisor (Strukton)
Delft University of Technology
Faculty of Civil Engineering and Geosciences
Master Geotechnical Engineering
November 27, 2014

3. Abstract
In regions such as the Netherlands where the underground phreatic water level is high, constructing a dry deep building pit can be a challenging operation. A secure and frequently applied solution is the construction of an underwater concrete floor which is enclosed between retaining walls and is anchored by tension elements that assist in the handling of the large upward water pressures.
For the successful realization of a deep excavation in urban environments, limiting the settlements in the adjacent area is one of the fundamental difficulties, which can be surpassed by the formation of a circular building pit consisting of diaphragm wall panels. These panels create a stiff ring which will hardly displace horizontally. Although this feature is advantageous for the settlements behind the wall, it creates complications regarding the strutting function of the underwater concrete floor because the floor’s concrete shrinkage can be larger than the inward displacement of the diaphragm wall at the location of the floor, something that increases the risk of the occurrence of a gap in the wall-floor interface. The occurrence or not of this gap significantly modifies the design considerations of the underwater concrete floor which are strongly related to the transfer or not of shear and normal pressure force from the retaining wall to the floor.
The aim of this thesis is initially to investigate the possibility of a gap in the contact area between the diaphragm wall and the underwater concrete floor. The results of the inward diaphragm wall displacements taken from the numerical analysis performed with the FEM software Plaxis were compared against the magnitude of the concrete shrinkage which was determined by approximation formulas from the literature and concrete regulations. The comparison indicated that the risk of a gap occurrence is relatively high.
Following this acknowledgment, it can be chosen to contemplate preventive or mitigating measures in order to ascertain the type of connection between the diaphragm wall and the underwater concrete floor. There are three different design cases that can be established concerning the nature of the wall-floor interface state which can comprise a typical shear and normal force connection, a shear connection and no connection. Therefore, next goal of this thesis is to provide design considerations and optimizations relating to the particular characteristics of each design case.
Should a typical connection exists, the underwater concrete floor can be dimensioned according to the principles suggested within the CUR-Recommendation 77:2014. This thesis focuses on investigating design alterations such as different anchor configuration patterns, concrete qualities or addition of steel fibers and their influence on the corresponding assessments of the floor’s dimensioning process.
The second design case includes considerations of the underwater concrete floor design for the situation where only shear transfer is occurring. In that case, the absence of an eccentric normal pressure force makes the application of reinforcement imperative. Various design modifications (i.e. adjusted anchor configuration patterns , different anchor embedment depths, different anchor dimensions) and their influence on the dimensioning are also considered for this design case.
Finally, for the third design case the underwater concrete floor is dimensioned independently since it is assumed that it is not in contact with the diaphragm wall. The application of reinforcement is required also for this design case but the acting bending moments on the floor are much smaller than in the previous design cases. On the other hand, the leakage risk is far greater in this case and the interface must be made water-tight without the wall being in contact with the floor, an operation that can prove really hard to implement in practice.