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TECHNICAL NOTE

Cone pressuremeter testing for wind farm foundation design

Summary

Using conventional cone penetration Testing (CPT) technology, the use of standard cone friction (CF) and piezocones (CPTU) cones can be used to determine the insitu engineering properties of soil materials.

For some design applications the information obtained from standard CPTs and CPTUs must be supplemented, usually through conventional site investigation techniques and laboratory testing methods.

This note introduces the use of the cone pressuremeter as specialist instrument for use in providing supplementary data on the insitu properties of the soil. Three recent case studies are described, where CPT was used to enhance the foundation design for three proposed wind farm fields in Cambridgeshire.

Introduction

For many engineering design applications (basement construction, retaining walls and dynamic laterally loaded pile foundations), the determination soil performance necessitates the consideration of insitu lateral stress (s9 h0 ) or coefficient of earth pressure at rest (k 0).The standard 'friction' cone penetrometer test measures real-time cone end (q c) and sleeve resistance (fs), while the piezocone (CPTU) measures the additional parameter of porewater pressure (u). Stiffness is assessed using empirical correlations.

Shear modulus together with shear strength can be determined using pressuremeter instruments.

Wind turbine foundations are subject to strenuous vibrations (dynamic) lateral loads in addition to relatively nominal vertical dynamic loads. It is therefore critical that the minimum dynamic rotational and horizontal stiffness is not exceeded.

Wind farm case studies

Client McNicholas Construction Services wanted to investigate three sites for proposed wind farms:

Red House wind farm, east of Holbeach, Lincolnshire

Glassmoore wind farm, south east of Whittlesey, Cambridgeshire

Deeping St Nicholas wind farm, south east of Spalding, Lincolnshire.

Works were carried out between January and March 2005, with each investigation taking about five days.

Consultant for the design was Jason McFarlane of Gifford & Partners.

At the time of investigation works, six turbines were proposed for the Redhouse site and eight turbines at both the Glassmoore and Deeping St Nicholas sites. The expected height of the turbines was 60m above ground level.

Objectives

The proposed foundation method was driven square section concrete piles. The foundations would be driven under relatively high impact conditions.

Gifford & Partners considered it most likely that load shear modulus should be as near as possible representative of the ground conditions after driving. Thus, rather than deploying very low strain instrumentation/methods (ie seismic), an intrusive method was chosen which would determine G9 at higher strains.

This would allow use of formula to obtain a more realistic value of G9 for the driven foundation solution.

Additionally, the design specification required Gifford to demonstrate that a minimum rotational and horizontal stiffness of the soil would not be underestimated, so determination of a low to intermediate value (G o to G) rather than G max would be more appropriate.

Previous ground investigation information consisted of cable tool percussive borehole logs and was inadequate in terms of appropriate data and both depth and lateral extent of the data.

As a result of the design information requirements, a piezocone pressuremeter (CPM) was specified at each turbine location in conjunction with CPT driven Mostap sampling.

Scope of works

The investigations comprised:

CPTUs with CPM to depths up to 20m below ground level

CPM tests at depths between 2m ad 14m below ground level

Mostap sampling from ground level to 5.6m

A programme of laboratory testing

A factual parameters report.

Geology

Redhouse Wind Farm

The ground conditions encountered were fairly uniform across the site.

Essentially units of loose very silty sand and sandy silt were observed to depths to 4m below ground level. These were underlain by a main strata group consisting of medium dense to very dense, locally gravely sand units to a maximum depth of 25m. These units were locally interbedded with units of very stiff sandy silt, which varied in thickness between 1.5m and 5.5m.

Glassmoor and Deeping St Nicholas Wind Farms The geology observed at both the Glassmoor and Deeping St Nicholas Wind Farm site, as expected, was very similar.

Very soft alluvial clay units were encountered from ground level to a maximum of 6m depth. Where encountered, these were underlain by a unit of dense to very dense gravely sand, which varied in depth and thickness across the site (up to 8m depth and between 1m and 3m thick). Underlying these units was the Jurassic Ampthill Clay unit. This consisted of very stiff clay and silt units to the maximum investigation depth of 18m.The granular stratum was only observed to any great extent at the Deeping St Nicholas site, although thin lenses or beds (less than 0.5m thick) were observed locally at the Glassmoor site.Groundwater level was not established by the CPTs.

Cone pressuremeter (CPM) Lankelma's sister company SETech has developed a new full-displacement 3.5MPa capacity pressuremeter for installation behind both CF and piezocone penetrometers (Figure 1).

Test procedure Before introducing the piezocone CPM into the ground, calibration checks were performed to ensure the data obtained was corrected for the effects of membrane stiffness and compliance. This was and is generally performed by infl ating the pressuremeter bladder both in air and in a very stiff steel casing.

The piezocone CPM was pushed into the ground using 20t hydraulic rams mounted on a 22t (20t capacity) crawler unit (Figure 2 ). The instrument was pushed into the ground at 2cm/sec (¦5mm), with the piezocone simultaneously recording (in real time) the tip, sleeve and porewater pressure parameters.

As the ground was disturbed by the leading penetrometer, once at the required depth, the pressuremeter test started as soon as practical, to minimise further disturbance incurred by time delay.

The membrane was then inflated until the maximum possible strain was reached. Once the membrane had lifted off the body and expansion started, two unload-reload loops were performed (Figure 3). These loops were performed at approximately 8% and 14% expansion (although a range of 0% to 50% can be achieved), with a maximum displacement capacity of 3.5MPa. The magnitude of these loops were controlled to prevent failure of the soil around the cavity and to enable determination of the soils' elastic properties.

The instrument and test is performed in a stress and strain controlled manner using a strain control unit (SCU). The SCU controls the pressure rate during the initial part of the test, such that an adequate number of data points are obtained to define the initial loading curve. Once plastic yield of the soil around the pressuremeter occurs, the SCU limits the expansion rate to a predetermined value.

Once the test was completed, the instrument was pushed to the next test depth, again simultaneously recording the cone parameters. The test depth recorded was the depth at the centre of the expanding membrane.

Results and interpretation The CPT and CPTU test results were processed and interpreted in the standard manner. Geotechnical parameter profiles for undrained shear strength and equivalent SPTN60 against depth were also provided. Mostap samples were split, described and subsampled in accordance with BS5930 and BS1377 with basic suites of tests being performed.

Piezocone pressuremeter Pressuremeter tests results were used to determine shear modulus (G) and undrained shear strength (Su).

The shear modulus (G) was determined by interpretation of the unload-reload loops (Figure 4).

This is determined by taking the line from the beginning of the reloading curve to the intersection of the unloading and reloading curves, with G being taken as half the gradient of this line.

Shear strength was calculated using the Houlsby and Withers method (1988). This uses total pressure against the natural log of maximum strain, minus current strain (ie ln (emax - ecurrent). A line is then plotted along the linear proportion of the unloading curve, with the undrained shear strength being taken as equal to half the gradient of the curve.

Secant shear modulus A secant shear modulus (E s), was also calculated by determining the point at which reloading commenced and calculating the modulus from the slope of the line to points on the reloading curve. Secant shear modulus values were plotted over a range of equivalent axial strains, with envelopes of the different soils units encountered for each site (Figure 5).

Elastic modulus Although not performed for the case studies detailed here, shear modulus may be correlated with elastic modulus using the following equation:

E = 2 G (1 + n) Where: E = Elastic modulus G = Shear modulus n = Poisson's ratio Conclusion The advantages of using the piezocone pressuremeter can be summarised as follows:

Small displacements can be measured to estimate soil stiffness.

Disturbance is repeatable.

The system is robust so that it can be used in all soils, including glacial tills.

It is easy to maintain so downtime can be kept to a minimum.

Installation using static CPT equipment increases productivity and reduces disturbance of the surrounding soil.

Modular system enables CPTU data to be obtained simultaneously within the same hole.

Final comments The use of CPTUs enables correction of cone tip resistance for the effects of porewater pressure. Therefore, the corrections allow greater accuracy of the CPT results. This correction is specified increasingly by CPT clients in favour of straightforward cone tip resistance.

Where modular instruments such as the pressuremeter are being used, it is suggested that a CPTU is used as a matter of course. Similarly, where stiffness parameters are required, specifi cations should consider using the CPTU, seismic module and pressuremeter as a combined instrument.

By using this very powerful instrument rapid, high quality geotechnical data can be obtained through the CPTU with the advantage of being able to derive G under both low (seismic) and medium to high strain (pressuremeter) conditions.

Acknowledgements Tony Heany, McNicholas Construction Services, for permission to release the site data and details, Jason Mcfarlane, Gifford & Partners, and Jamie Irvine, SETech, for their technical input.

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