In order to have a more efficient usage of construction areas in congested urban areas a vertical development of buildings becomes necessary. Currently we more often face situations where urban buildings need as many parking spaces, so, due to lack of space, that requires the development of several underground floors.
The design and execution of deep excavations in congested urban areas is quite a challenge especially in terms of geotechnical engineering and it requires a good knowledge of the soil mechanics and soil interaction with the retaining walls of the excavation.
The analysis of excavation retaining walls can be performed using two calculation methods (sizing methods): Limit Equilibrium Method (LEM) and a numerical method – Stiffness Ratio Method (FEM/SRM).
Limit equilibrium method
Is based on the colaps asummed conditions, when the entire soil shear strenght is uniformly mobilized around the supporting wall. Steady state limit calculations are based on the consideration of linear, simple distributions of soil lateral efforts.
The method is widely used and provides acceptable results for specific structural forms (eg. console walls), but it is not indicated for walls supported on many layers (multilayer bracing systems). There can be pointed out certain situations where this method is applicable with certain specific features according to the particular situation. From this point of view, the following static schemes are possible:
a) Wall embedded in ground on the depth f and free (unrestrained) on the excavation depth (console wall)
b) Wall supported in ground on the depth f and anchored (braced) at the top
c) Wall embedded in ground on the depth f and anchored (braced) at the top
Stiffness Ratio Method (FEM/SRM)
Considers the soil on the wall’s depth f as an elastic medium (Winkler type) characterized by the modulus of subgrade reaction to horizontal displacement, kh, and modulus of subgrade reaction to vertical displacement, kv.
Physically, the Winkler medium is composed of elastic springs disposed between the element and a rigid base. The coefficient kh increases linearly with depth according to the relation kh=mz, where m is the coefficient of proportionality. If a beam is placed vertically in this medium and is loaded horizontally, at a certain depth z it will occur a horizontal displacement y(z), and between soil and beam will be mobilized a horizontal pressure σh(z).
According to Winkler hypothesis, this pressure is proportional to the displacement with respect to relation: σh(z) = khy(z). On the depth f, the element is divided in segments of length a, and the continuous supporting between soil and retaining element is replaced with a punctual supporting realized in the centers of these segments. When the element deforms under the action of earth lateral pressure, in the springs occurs elastic reaction , where b is the width of the wall element.
Further is presented the calculation of secant piles retaining wall for an 14 m depth excavation using the software SPW (Sheet Pile Walls).
The site is located in the center of Cluj-Napoca, Romania. The project’s objective was the construction of a services and office building with height of 2B + G + 12.
The height of Basement 1 is 5.06 m, while the height for Basement 2 is 2.98 m, and the overall depth of the excavation goes to -10.50 m.
To support the soil during the excavation of the basement and to keep under control the groundwater level, there was a 4 sides tight enclosure of secant piles having a diameter of 60 cm, spacing distance of 45 cm and lengths between 17,15 m and 19,65 m with a double role – strength and sealing of the enclosure.
Secant piling solution was chosen because groundwater was encountered at a depth of 3 m. As the excavation goes forward, the retaining walls are supported by three layers of horizontal metal struts having a diameter of 80 cm, located at the following depths: -0,50 m, -4,50 m and -9,0 m.
The secant sheet pile retaining wall definition formed by 60 cm diameter sheet piles disposed at 90 cm spacing. Primary sheet piles are made of C8/10 concrete containing added bentonite (waterproofing role) and the secondary (strength) sheet piles are made of C25/30 concrete, reinforced vertically by 8 independent 25 mm diameter bars and a 10 mm diameter spiral reinforcement.
This article comes from geostru edit released