Prof. Dr.-Ing. Martin Achmus Institute of Soil Mechanics, Foundation Engineering and Waterpower Engineering. Offshore subsoil investigations

Transcription

1 Prof. Dr.-Ing. Martin Achmus Institute of Soil Mechanics, Foundation Engineering and Waterpower Engineering Offshore subsoil investigations Addis Ababa, September 2010

2 Offshore subsoil investigations Presentation structure: Introduction Geophysical investigations Geotechnical investigation Requirements 2

3 Foundations for offshore wind energy towers 3

4 Geophysical Survey: (Hydroacoustic methods) Echo sounding Seismic Side scan sonar

5 Echo sounding Bathymetry (greek bathos = depth and metro = measure) Depth, inclination and roughness of the sea bottom surface; morphological changes Sending of an ultrasonic signal ( khz), Runtime measurement Single beam / multi-beam, fan width 2 to 4 times the water depth Bathymetry of River Elbe estuary

6 Side Scan Sonar (SSS) Two-dimensional presentation of sea bottom Determination of obstacles and gas bubbles Detailled presentation of the sea bottom, comparable with aerial photo Colours are artificial; rough and uneven surface appears darker For identification of sediment type ground-truthing necessary

7 Seismic survey Reflection and refraction seismics Aim: Identification of type and location of geological units Reachable survey depth: (60)m

8 Seismic survey Transmitter Direct wave Receiver Layer 1 Thickness H1 Wave velocity v1 1. Signal: Velocity in water 2. Signal: Water depth 3. Signal: Bottom of upper layer 4. Signal: etc. Layer 2 Thickness H2 Wave velocity v2 Usage of several hydrophones High resoluted informations about the soil layers Data evaluation with special interpretation systems Marking of reflection horizons in 2- and 3D seismic plots

9 Interpretation of a Seismic survey Geological pre-report (Lesny et al. 2009) Geological report (after calibration)

10 Result of a geophysical survey Geophysics is a necessary and very helpful tool in offshore soil investigations! 10

11 Geotechnical investigations: (Field and laboratory tests)

12 Field investigation methods direct methods indirect methods Test pits & Borings Dynamic probing & Cone penetration Reachability of the location to be explored High costs for vessels and crew Possible hindrance by wheather impacts (wind and waves) Necessarity of special offshore boring and sounding methods

13 Cone penetration test according to DIN Cone area: 10cm² [blue] Surface area of friction sleeve: 150cm² [red] Velocity of penetration: 2cm/s Counter weight load: kn Penetration depth limited by counter weight load Recording of cone resistance (q c ) and skin friction (f s ), also pore pressure measurement possible (CPT-U)

14 CPT-results (onshore) Skin friction Cone resistance Boring log Friction coefficient

15 Offshore-CPTs Usage of ballast block from jack-up barge or from a vessel in deep water

16 Systems for continuous CPTs (Lesny et al. 2009) Limited depth, e.g. in dense sands

17 Discontinuous CPT (down hole) Alternating sounding and boring, also change between sampling and sounding possible Required depths of up to 70 m reachable

18 Interpretation of CPT tests in sand Relative density D r Baldi et al. (1986): Cone resistance D r 1 = C 2 ln C 0 q c ( σ ) C 1 C 0, C 1, C 1 σ' σ v0 σ m q c - (empirical) soil constants [-] - effective stress [kpa] - effective vertical stress[kpa] - average effective stress = (σ v0 + 2 σ h0 )/3 [kpa] - measured cone resistance [kpa] Average effective stress Sand, NC and OC

19 Interpretation of CPT tests in sand Relative density D r D r Baldi et al. (1986): 1 = C 2 ln C 0 q c ( σ ) C 1 Normally consolidated sand K 0 = 0.45 σ σ v0 σ m C Empirical values (Baldi et al. 1986) Normally and overconsolidated sand C C C 0, C 1, C 1 σ' σ v0 σ m q c - (empirical) soil constants [-] - effective stress [kpa] - effective vertical stress[kpa] - average effective stress = (σ v0 + 2 σ h0 )/3 [kpa] - measured cone resistance [kpa] Dimension of stress : kpa Empirical values (Jamiolkowski et al. 2001) Normally consolidated sand K 0 = 1-sin ϕ Normally and overconsolidated sand σ σ v0 σ m C C C Dimension of stress: bars (1 bar = 98,1 kpa)

20 Interpretation of CPT tests in sand Angle of internal friction ϕ ϕ - dependence on relative density D r and effective stress σ Schmertmann 1978: ϕ acc. to Schmertmann and D r acc. to Baldi et al. Angle of internal friction dependent on relative density D r and soil composition (for 150 kpa < σ < 300 kpa) Angle of internal friction for normally consolidated and poorly graded fine sand

21 Interpretation of CPT tests in cohesive soil Undrained shear strength c u From CPT results Exemplary determination of the cone factor N k q σ c v0 cu = Nk From CPTU results q σ t v0 cu = Nkt N k - cone factor from CPT results, 11 N k 19 N kt - cone factor from CPTU results, 15 N kt 20 To be calibrated by UU triaxial tests

22 Interpretation of CPT results: stiffness Non-cohesive soils: Janbu (1963): E s, T = k m p a σ p v0 a 0,4 k m dependent on on D r and q c (Baldi et al. 1981) For cohesive soils: Mayne 2007: E s α ( q σ ) c t v0 α c 5 ca.1 α c ca. 2 α c 20 for soft to stiff clays and NC sands for plastic organic clays for overconsolidated clays

23 Boring from a jack-up platform Boring works in the Baltic Sea (near Rügen); water depth ca. 25 m All onshore methods can be used best quality Normally continuously cored samples D=100mm

24 Sample quality classes Quality classes: Sample parameters: 1 Z, ω, ρ, k, τf, ES (undisturbed) Shear strength 2 Z, ω, ρ, k Stiffness Worse quality (disturbed in strength) 3 Z, ω 4 Z composition 5 totally disturbed (only layering) Higher costs Density Permeability Water content Composition Layers Requirements offshore wind regulations (BSH-Standard): For cohesive soils at least class 2 For non-cohesive soils at least class 4 (better 3 or 2)

25 Boring from a vessel Usually samples D=72mm

26 Subsoil conditions in the German North Sea Typical subsoil in the German North Sea - recent marine sediments < 3,0 m - holocene layers Loose to medium dense silty sands, partly cohesive layers of small thickness - pleistocene layers dense sands, partly gravelly and at some locations boulder clays Geological sections in the German North Sea (acc. to Sindowski 1970) local gutter structures from glacial erosion processes

27 Borings and CPTs Boring log CPT diagram Derived boring log

28 Combined boring / CPT (alternating) Boring log CPT diagram Derived boring log

29 Requirements for offshore wind farms in Germany BSH-Standard

30 BSH-Standard defines minimum requirements on geophysical surveys, geotechnical field investigations and laboratory test programmes Offshore wind farms are classified Geotechnical Category 3 (complex construction) A geotechnical expert has to be commissioned, who is responsible for the geotechnical reports and also supervises the field investigations Geophysical survey has to be carried out, covering all wind tower locations Calibration with respect to borings is required Pre-investigation phase: At minimal 5 or at 10% of the locations a boring and a CPT has to be carried out presupposition for 1st approval Main investigation phase: At each location at least one boring or CPT has to be carried out required depth in accordance with DIN 4020 presupposition for 2nd approval

31 A comprehensive laboratory programme is required Behavior of soils under cyclic loading has to be assessed Liquefaction has to be assessed Cyclic soil tests! Cyclic triaxial test device Behavior of model piles under cyclic lateral loading

32 Thanks for your attention! 32