A case study of large screw pile groups behaviour

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5 juni 2015

A case study of large screw pile

groups behaviour

Alice Manzotti

Content of the presentation

• Introduction

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• Introduction

• General soil conditions at the site

• Foundation design - Instrumented pile load test • Initial settlement estimation

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3 oil tanks (each of 33000 m³ , D=48.8m & H=19m ) on very soft deposits

Introduction

–

The site and location of the tanks at Ostend

Introduction

–

Allowable settlements for steel tanks

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Reference Total average

settlement Differential settlement (mm) Tilt w settlement Δave(mm) settlement (mm) w API 653 (1995) - 0.031R

-Klepikov (1989) 180 (large tank) 110 (small tank) 0.004D (large tank) 0.008D (small tank) 0.004H (visible) 0.007H (ultimate) USACE (1990) - 0.008R

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-l y lenses

General soil conditions at the site

heterogeneous fil l clay ey mat e rial w it h sand

Typical CPT profile on the site and CPT soil type classification

dense sand (about 4m thick)

silty clay (20m thick)

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Soil resistance variability differential settlements !!!

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The foundation design

foundation on a large group of ~ 420 end bearing

di l t il

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

Ultimate unit pile tip resistance of

di l t il f 460

TANK 01 8

The foundation design

-Belgian practice

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 - 1986 )

preferential pile tip level

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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

Initial settlement estimation

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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)

q_c(MPa) E (MPa) M (MPa)

0.66 4.60 34 40

3 0.66 4 5

10 10.07 77 97

14.5 5.28 63 66

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Initial settlement estimation

• initial prediction very much depending on the • 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 na oidabl also to settlement tro gh unavoidably also to settlement trough

settlement monitoring of the tanks was deemed to be essential

Monitoring tank settlement

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Each tank is being monitored on 16 points along its perimeter, equally divided at a center angle of 22.5° from each other

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Tank settlement during hydro-test

settlement of foundation = 21mm under a load of 180 kPa (combination settlement of foundation + compression asphalt layer)

Tank settlement during hydro-test

• As each tank was tested separately and for a

h t i d th i t f th l d i

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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

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Tank settlement during operation

first measurement (January 2014) = residual average deformation at the end of hydro-test second measurement (September 2014)

Tank settlement during operation

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both tank 1 and 2 exhibit nearly perfectly planar tilt of 12-13 mm (0.00026 m/m)

tank 3 tilts almost

directly north for about 20 mm (0.0004 m/m)

!!!! local subsoil heterogeneities below tank 3

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Tank settlement during operation

• additional average settlement at this point • additional average settlement at this point

has reached values of 34 to 40mm

• higher value of the tilt (compared to hydro-test) as expected due to interaction of different loads

different loads

• values of average settlement, tilt and distortion still far below critical values

Analysis of the time settlement

behaviour

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Based on the compressibility parameters from CPT + hydro-testing , a single value of cvfor the silty clay was found to get the

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Analysis of the time settlement

behaviour

- Predicted vs measured average settlement Fitting done on the basis of c /d² the basis of c_v/d (also the drainage path length is indeed unknown!!!)

This leads so far to a value of the to a value of the time factor c_v/d² = 0.0022 month-1

Conclusions

• Due to the large scale of the construction

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• 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