IN SITU TESTING IN GEOMECHANICS

Fernando SchnaidUniversidade 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.

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)

Objectives of site characterisation

Category Test Common Applications

Non-destructive or semi-destructive

Geophysical Tests: Seismic

Refraction Surface WavesCrosshole Test Downhole Test

Ground characterisationSmall strain stiffness, Go

Pressuremeter TestPre-bored Self-boring

Shear modulus, GShear strengthIn situ horizontal stressConsolidation properties

Plate loading test Stiffness and strength

Invasive penetration tests

Cone penetration ElectricPiezocone

Soil profilingShear strengthRelative densityConsolidation properties

SPT (energy control) Soil profilingInternal friction angle, φ´

Flat Dilatometer Test StiffnessShear strength

Vane Shear Test Undrained shear strength, su

Combined tests (Invasive + Non-destructive)

Cone pressuremeter New interpretation methods

Seismic cone

Resistivity cone

Seismic dilatometer

Available in situ testing techniques

Available in situ testing techniques

recommendation: correlate results from different test

recommendation: correlate results from different test

Onshore & OffshoreTechniques

Onshore & OffshoreTechniques

Onshore & OffshoreTechniques

PIEZOCONE

qt

u2u1

2VsGo ⋅= ρ

Vs

Soil profileShear strength CompressibilityStress history

Soil profileShear strength CompressibilityStress history

PIEZOCONE

Pressuremeter and cone-pressumeter

Dilatometer

Dynamic penetration tests

Interpretation

methods

Class I: Rigorous analytical methods

Class IV: Empirical approaches

Class III: Approximate analytical solutions

Class II: Numerical solutions (close approximation)

Structure performance

Laboratory tests

Calibration chamber tests

Centrifuge tests

Interpretation Methods

Câmaras de calibração

after Huang & Hsu (2004)after Huang & Hsu (2004)

Centrífuga

Cortesy: UWA Offshore Foundation CentreCortesy: UWA Offshore Foundation 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 designb) Method II: carry out limited investigation and make design assumptions in accordance to general average experienceb) 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 testsCone Penetration testStandard Penetration and dynamic probing testVane testPressuremeter testDilatometer test(Geophysics)

Review of field and laboratory workReview of derived values of geotechnical parametersPreliminary 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