ISC 5 5 9 SEPT 2016 SELF-BORING PRESSUREMETER TESTS AT THE NATIONAL FIELD TESTING FACILITY, BALLINA Fillippo Gaone James Doherty Susan Gourvenec Centre for Offshore Foundation Systems, UWA School of Civil, Environmental and Mining Engineering, UWA Centre for Offshore Foundation Systems, UWA
National Field Testing Facility, Ballina Site description Located north of Ballina, New South Wales A1 Site location Ballina
National Field Testing Facility, Ballina Site description Located north of Ballina, New South Wales Managed by CGSE to develop and demonstrate new and existing site investigation tools. In situ testing involves: CPT/CPTu SDMT Shear vane HPT (Hydrostatik Profile Tool) Piezoball T-bar SBPM
National Field Testing Facility, Ballina Classification of the ground conditions Normalised Cone Resistance, Q tn 1000 100 10 1. Sensitive Fine Grained 7 2. Clay - Organic Soil 3. Clays - Clay to Silty Clay 4. Silt mixtures: clayey silt & silty clay 5. Sand mixtures: silty sand to sandy silt 6. Sands: clean sands to silty sands 7. Dense sand to gravelly sand 8. Stiff sand to clayey sand (OC or cemented) 9. Stiff fine-grained (OC or cemented) 0m to 1.5m 0m 1.5m 1.5m to 15.5m 1.5m 12m 12m 15.5m 18m to 18m 18m to 20.4m 1 2 1 0.1 1 10 Normalised Friction Ratio, F R (%) Robertson et al. (1986) Q tn -R r chart 6 5 8 4 3 9 0.50m AHD ALLUVIAL CLAYEY SILTY SANT -1.00m AHD ESTUARINE SILTY CLAY -12.50m AHD SILT / STIFF PLEISTOCENE CLAY -0.50m AHD Water Level F r [%] 0 200 4000 10600 200 30 40 10 20 0 0 2 4 6 8 10 12 14 q net [kpa] 2 4 6 8 10 12 14 S u [kpa]
The Self-Boring Pressuremeter What is a pressuremeter? Cylindrical, insitu testing device to measure: Inflating pressure Radial expansion Introduced by Menard (1956) in France Placed into a pre-drilled bore hole Soil disturbance: Borehole walls remoulding Stress relief Pressuremeter prototype 1956
The Self-Boring Pressuremeter Developments in pressuremeter equipment 1. Installation into a pre-bored hole 2. Installation by pushing 3. Installation by self-boring mechanism
The Self-Boring Pressuremeter Self-boring mechanism Almost undisturbed soil basic soil parameters Consists of a cutting shoe and a rotating cutter Installation steps: Conventional drilling of a bore hole to 1m above the target depth Gently pushing the pressuremeter into the ground by simultaneously cutting and hoisting the borehole cuttings Rest period to allow excess pore pressure to dissipate Source: www.cambridge-insitu.com
Data Interpretation Interpretation of the basic soil parameters σ h, s u and G Total horizontal stress, σ h : Inspection method Lunne & Lacasse (1982) Undrained shear strength, s u : Gibson and Anderson (1961) Shear modulus, G UR : Palmer (1972) But more is possible
Total Pressure, p [MPa] Data Interpretation Inspection method (Lunne & Lacasse, 1982): : 0.12 0.11 0.1 0.09 0.12 0.11 0.1 0.09 0.08 0.08 Total pressure, p [M Pa] 0.07 0.06 0.05 0.04 0.03 σ h Lift off 0.07 0.06 0.05 0.04 0.03 0.02 0.02 0.01 0.01 0 0 2 4 6 8 10 12 Average Cavity Strain ε [%] 0 0.00 0.00 0.01 0.10 1.00 10.00 ln ε [%]
Total Pressure, p [MPa] Data Interpretation Gibson and Anderson (1961) Based on an elastic-perfectly plastic soil model, Gibson and Anderson derived the following equation to determine the undrained shear strength s u. s u = p ln V V 2 1 V V 1 + v p 0 E 0.11 0.1 Based on the Menard pressuremeter 0.09 S u,uncorrected For the SBPM the equation can be reduced to: s u = p ln V V 0.08 0.07 0.06 1 1 S u,corrected 0.05 2 4 8 16 Ln ε [%]
Total Pressure, p [MPa] G UR / p' [-] Data Interpretation Palmer (1972) The shear modulus may either be determined from the initial part of the stress-strain curve or from an unload-reload loop carried out at a nominated cavity strain 90 Depends on the strain range 0.14 80 70 60 0.12 0.10 0.08 0.06 G = 1 dp 2 dε 50 40 30 0.04 0.02 0.00 0 2 4 6 8 10 Average Cavity Strain, ε [%] 20 10 0 BH 1 BH 5 BH 6 0 0.5 1 1.5 2 Strain range, ε r [%]
Depth, z [m] Depth, z [m] Results Validation against other field data from the Ballina site: Total Stress [kpa] Undrained shear strength, s u [kpa] 0 0 30 60 90 120 150 180 0 0 5 10 15 20 25 Lift off pressure 1 1 2 2 3 3 4 4 σ h (k 0 = 1.5) 5 5 6 σ v 6 7 8 σ h (k0 = 0.7) 7 8 3 Arm interpretation CPT Shear vane
Results More is possible σ h, s u Initial Parameters Model Simulation The new inverse analysis tool developed at the University of Western Australia allows the identification of a representative MCC parameter set. Model Data Compare Model Data with Test Data and evaluate objective function Optimisation routine determines new set of parameters Is objective function < Tolerance No Yes κ, λ, R, M Terminate with optimal parameters
Conclusions The self-boring pressuremeter is a repeatable and reliable method of determining the in situ undrained shear strength, shear modulus and in situ horizontal stress It is an economical alternative to laboratory testing The quality of the data relies mainly on the quality of the self-drilling process and on the applied interpretation method
JamesDoherty2 SusanGourvenec @SusanGourvenec This exercise is a research project, funded by the Australian Research Council, for the purpose of advancing geotechnical knowledge and practice for the benefit of all.