A case study of large screw pile groups behaviour

Transcription

1 Jongerenforum Geotechniek 5 juni 2015 A case study of large screw pile groups behaviour Alice Manzotti Content of the presentation 2 Introduction General soil conditions at the site Foundation design - Instrumented pile load test Initial settlement estimation Monitoring tank settlement behaviour (during hydro-testing & during operation) Analysis of the time settlement behaviour 8/06/2015 1

2 Introduction The site and location of the tanks at Ostend 3 oil tanks (each of m³, D=48.8m & H=19m ) on very soft deposits 3 Introduction Allowable settlements for steel tanks 4 Reference Total average settlement Differential settlement (mm) Tilt w Δ ave (mm) API 653 (1995) R - Klepikov (1989) 180 (large tank) 0.004D (large tank) 0.004H (visible) 110 (small tank) 0.008D (small tank) 0.007H (ultimate) USACE (1990) R - 2

3 l 8/06/2015 General soil conditions at the site 5 heterogeneous fill clayey material with sand y lenses dense sand (about 4m thick) silty clay (20m thick) Typical CPT profile on the site and CPT soil type classification 6 Tank 03 Soil resistance variability differential settlements!!! 3

4 The foundation design 7 foundation on a large group of ~ 420 end bearing displacement screw piles per tank, based in the dense tertiary dense sand at ~ 22m of depth 460mm diameter Omega pile type designed (at an overall safety factor of >2) to allowable load of 1000 kn/pile Omega screw pile installation The foundation design - Belgian practice Ultimate unit pile tip resistance of a displacement pile of 460mm diameter, as a function of depth, using all CPT results relevant to tank 01 (according to the Van Impe/De Beer method ) TANK 01 8 preferential pile tip level 4

5 8/06/ Instrumented single pile test load Fully instrumented test pile (to be loaded up to a pile base settlement of 10% of pile diameter) Test pile instrumentation 10 Initial settlement estimation An initial settlement estimation was done for a single loaded tank using the method of the equivalent raft and the soil parameters out of CPT Depth (m) qc (MPa) E (MPa) M (MPa) NO data below 35m depth!!! (Robertson,2009) 5

6 Initial settlement estimation 11 initial prediction very much depending on the actual compressibility of the unknown clayey layers (below 35 m depth) interaction of the 3 tanks loading will lead unavoidably also to settlement trough settlement monitoring of the tanks was deemed to be essential Monitoring tank settlement Each tank is being monitored on 16 points along its perimeter, equally divided at a center angle of 22.5 from each other 12 6

7 Tank settlement during hydro-test 13 settlement of foundation = 21mm under a load of 180 kpa (combination settlement of foundation + compression asphalt layer) Tank settlement during hydro-test 14 As each tank was tested separately and for a very short period, the impact of the load is presumably limited to the immediate response of the stiff sand layer and the upper part of the underlying silt clay layer no real interaction between tanks = limited tilt of the tanks (2-3mm) residual average deformation at the end of hydro-test ~ 8mm for all tanks 7

8 Tank settlement during operation 15 = residual average deformation at the end of hydro-test first measurement (January 2014) second measurement (September 2014) Tank settlement during operation 16 both tank 1 and 2 exhibit nearly perfectly planar tilt of mm ( m/m) tank 3 tilts almost directly north for about 20 mm ( m/m)!!!! local subsoil heterogeneities below tank 3 8

9 Tank settlement during operation 17 additional average settlement at this point has reached values of 34 to 40mm higher value of the tilt (compared to hydrotest) as expected due to interaction of different loads values of average settlement, tilt and distortion still far below critical values Analysis of the time settlement behaviour Based on the compressibility parameters from CPT + hydrotesting, a single value of c v for the silty clay was found to get the best fit between predicted and measured average settlements 18 (SteinP 3DT program) 9

10 19 Analysis of the time settlement behaviour - Predicted vs measured average settlement Fitting done on the basis of c v /d² (also the drainage path length is indeed unknown!!!) This leads so far to a value of the time factor c v /d² = month -1 Conclusions 20 Due to the large scale of the construction, the influence depth is considerably larger then the extent of the soil investigation and monitoring was deemed essential Additional meas rements ill allo for Additional measurements will allow for further optimization of the model to better extrapolate the long term behaviour 10