IN SITU TESTING IN GEOMECHANICS Fernando Schnaid Universidade Federal do Rio Grande do Sul
Patologia das Fundações
Geotechnical investigation British Practice (Weltman & Head,1981): Sufficient finance should be allocated for a through investigation to facilitate economic and safe geotechnical design and to reduce the possibility of unexpected ground conditions being encountered during the construction of the works which frequently lead to costly delays in a contract. Such delay can cost many times more than would a properly conducted ground investigation.
Geotechnical investigation U.S Army Corps of Engineers (2001) Insufficient geotechnical investigations and faulty interpretation of results may contribute to inappropriate designs, delays in construction schedules, costly construction modifications, use of substantial borrow material, environmental damage to the site, post-construction remedial work, and even failure of a structure and subsequent litigation.
Polônia
Fotos
Geotechnical investigation French statistic (Logeais, 1982) indicates that from 2000 case studies 80% of problems are a consequence of poor site characterisation. Brazilian Practice: no statistic avaliable, tends to confirm international experience: Aspects related to poor site characterisation are the most frequent source of unsuccessful design in foundation engineering.
Geotechnical investigation French statistics (Logeais, 1982) shows that in 2000 cases in reported in France, 80% of faulty response is due to poor site investigation. Eurocode 7. States that site investigation is a more critical step in avoiding faulty response than the actual theoretical approach adopted in geotechnical design.
Objectives of site characterisation acquire topographical, hydro-geological, geotechnical and geo-environmental information; produce detailed and representative soil profiles; provide suitable geotechnical data to support design,: assessment of the initial geostatic stresses and the stress history; prediction of the stress-strain-time and strength characteristics; assess environmental changes of groundwater and drainage conditions of the site and the surrounding ground and structures. (Jamiolkowski et al, 90s)
Category Test Common Applications Nondestructive or semidestructive Invasive penetration tests Combined tests (Invasive + Nondestructive) Geophysical Tests: Seismic Refraction Surface Waves Crosshole Test Downhole Test Pressuremeter Test Pre-bored Self-boring Plate loading test Cone penetration Electric Piezocone SPT (energy control) Flat Dilatometer Test Vane Shear Test Cone pressuremeter Seismic cone Resistivity cone Seismic dilatometer Ground characterisation Small strain stiffness, G o Shear modulus, G Shear strength In situ horizontal stress Consolidation properties Stiffness and strength Soil profiling Shear strength Relative density Consolidation properties Soil profiling Internal friction angle, φ Stiffness Shear strength Undrained shear strength, s u New interpretation methods Available in situ testing techniques recommendation: correlate results from different test
Onshore & OffshoreTechniques
Onshore & OffshoreTechniques
Onshore & OffshoreTechniques
PIEZOCONE V s 2 G o = ρ Vs Soil Soil profile profile Shear Shear strength Compressibility Stress Stress history q t u 2 u 1
PIEZOCONE
Pressuremeter and cone-pressumeter
Dilatometer
Dynamic penetration tests
Interpretation Methods Class I: Rigorous analytical methods Class II: Numerical solutions (close approximation) Interpretation methods Class III: Approximate analytical solutions Class IV: Empirical approaches Calibration chamber tests Centrifuge tests Structure performance Laboratory tests
Câmaras de calibração after Huang & Hsu (2004) after Huang & Hsu (2004)
Centrífuga Cortesy: UWA UWA Offshore Foundation Centre Centre
Codes of practice Peck (1969) stated that investigation methods form 3 groups: a) Method I: carry out limited investigation and adopt an excessive factor of safety during the design b) Method II: carry out limited investigation and make design assumptions in accordance to general average experience b) Method III: carry out very detailed investigation. Now incorporated into Codes of Practice such as the Eurocode.
Eurocode 7 a) Category I: small and relatively simple structures - the fundamental requirements will be satisfied on the basis of experience and qualitative geotechnical investigation. Routine. b) Category II: conventional types of structures and foundations with no abnormal risks or unusual or exceptionally difficult ground or loading conditions. c) Category III: structures or part of structures which do not fall within the limits of Categories I and II, including very large or unusual structure, structures involving abnormal risks or exceptionally difficult ground or loading conditions and highly seismic areas.
Eurocode 7 3.3.10. Geotechnical parameters from field tests Cone Penetration test Standard Penetration and dynamic probing test Vane test Pressuremeter test Dilatometer test (Geophysics) Review of field and laboratory work Review of derived values of geotechnical parameters Preliminary and complementary investigation
International Practices: extent of the field and laboratory investigation Country Tests Observation Netherlands Piezocone Predominance soft soils UK SPT & triaxial Other tests often performed France Pressuremeter Variety of soils USA SPT Other tests often performed Brazil SPT Other tests in large projects Italy
OBJECTIVES Review currently adopted techniques and interpretation methods Discuss research, new contributions and future trends Present large number of case studies