Strutted Excavation in Soft Soil Incorporating a Jet-Grout Base Slab: Analysis Considering the Consolidation Effect
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ORIGINAL PAPER
Strutted Excavation in Soft Soil Incorporating a Jet-Grout Base Slab: Analysis Considering the Consolidation Effect Jose´ Leita˜o Borges • Ricardo Gomes Pinto
Received: 1 February 2010 / Accepted: 8 January 2013 / Published online: 19 January 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract The influence of the consolidation on a strutted excavation in soft soil is analysed using a computer code based on the finite element method. A base jet-grout slab is considered in order to improve stability against bottom heave failure and minimize wall displacements. The numerical model incorporates the Biot consolidation theory (coupled formulation of the flow and equilibrium equations) with soil constitutive relations simulated by the p–q–h critical state model. Special emphasis is given to the analysis, during and after the construction period, of the pore pressures, shear stresses, stress levels and displacements in the ground, as well as strut compression loads, wall displacements and bending moments, earth pressures on the wall faces and compression loads and bending moments on the jetgrout slab. The safety factor against bottom heave is also evaluated from the finite element analysis considering formulations of the critical state soil mechanics, and also compared to values obtained with traditional methods that use limit equilibrium approach and bearing capacity fundamentals. Keywords Strutted excavation Soft soil Base slab Consolidation Critical state model Coupled analysis
J. L. Borges (&) R. G. Pinto Department of Civil Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal e-mail: [email protected]
1 Introduction Theoretically, when a strutted excavation is undertaken in a saturated soft clay and the wall is not extended below the excavation base, bottom heave failure occurs if the initial total vertical stress on the base level is higher than bearing capacity of the subjacent soil. The two most frequently quoted methods for analysing bottom heave stability has traditionally been the limit equilibrium approach based on Terzaghi (1943) and Bjerrum and Eide (1956). The latter was calibrated against observed bottom heave failures. In order to improve stability against bottom heave failure, several practical solutions can be considered. One solution is to extend the wall below the excavation base, usually down into a stronger stratum (hard stratum). In this case, in simple terms, earth pressure on the wall, below the excavation bottom, is transferred to the hard stratum and to the lower strut levels. However, in practical terms, this solution may not be practicable if the hard stratum lies deeply. If this is the case, one possible solution consists of extending the wall a few meters below the excavation base and, before excavation, constructing a jet-grout slab to support the wall below the excavation level. Another solution consists of using diaphragm walls to act as cross-walls below the excavation base (Fig. 1). This concept
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