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Ground Investigation: Depth of knowledge

Innovative insitu testing has helped deliver detailed understanding of challenging ground conditions for a new ship lift in Germany

Construction work for a new high-tech ship lift on the Havel to Oder Canal at Niederfinow in Germany is set to be completed later this year. Engineers are quietly confident of the ground engineering elements, after working with Fugro to apply innovative ground investigation techniques to help overcome some unusual challenges.

Although the site is home to Germany’s oldest working ship lift, designers were concerned that there was a risk of ground deformation due to the presence of lignite. Specialist testing was critical to ensure the geotechnical risks were fully understood and mitigated through the design to safeguard the success of the overall project.

fig3 niederfinow overhead

The new boat lift is being built alongside the existing structure

The existing 60m high counterbalance principle lift was built in the 1930s to replace four locks which connected the River Oder with the canal that links Berlin and Szczecin in Poland.

Despite its heritage, the structure is still in daily use but its 80m long vessel capacity means that it must be replaced to allow the canal to carry larger ships and reduce delays.

The new lift will have capacity for 110m long vessels and offer a faster cycle time, but it is expected the old lift will remain in service until 2025.

An Arge consortium – formed by Implenia Construction, DSD Brückenbau, Johann Bunte

Bauunternehmung and Siemag Tecberg – is working on the replacement project for Berlin waterways authority Wasserstraßen-Neubauamt Berlin.

While the trough and vertical structural elements are made of reinforced concrete, the cable-pulley carrier and the channel bridge are steel structures. The complex mechanics of the lift can only tolerate very small deviations with respect to verticality.

The site investigation at feasibility stage showed the presence of layers containing lignite or silt-lignite mixtures in the area of the planned foundations. The lignite was predominantly cohesive, formed in bands a few hundred millimetres thick. It was observed during the investigations, mostly as an interwoven structure similar to a marbling effect. The thickness of the lignite-containing layers amounted in some places to several metres, which could have significantly affected the deformation behaviour of the structure.

To further assess the mechanical behaviour of the lignite-containing layers, Arge worked with Fugro to undertake insitu testing in coordination with the client’s geotechnical consultant, federal waterways research group Bundesanstalt für Wasserbau.

A key advantage of testing strength and deformation parameters insitu compared to laboratory based testing is that it avoids the disruptive effects of sample extraction, transportation and storage.

Conventional sampling techniques would have led to disturbance from drilling or driving, and adverse effects on the results of geotechnical laboratory tests. For this reason, cone penetration testing (CPT) was preferred for the Niederfinow scheme for the identification of the silt-lignite mixtures and the lignite.

fig6 niederfinow img 1771

Insitu testing provided more detailed information for the ground model

Due to the silts and lignite being overlain by dense materials, penetration to the target depth by means of a conventional 20t CPT truck or crawler was not possible. For this reason the CPTs were carried out by a mobile unit, pre-drilling the dense materials then installing the CPT casing. The thrust for the CPT was provided by anchoring the mobile unit to the drilling rig.

Pressure meter tests were carried out at the target depth locations using a Fugro-developed pressure meter integrated into a conventional cone penetrometer. The pressure meter itself is located above the cone; testing is performed by full displacement of the surrounding soil and the expansion of a cylindrical membrane.

The processing and interpretation of the test data, and correlation with available laboratory test results, was carried out by Implenia.

An axisymmetrical finite element method (Fem) model was established to simulate the cavity expansion by means of hypoplastic and viscohypoplastic constitutive equations.

State-independent parameters, initial void ratio and degree of over-consolidation have been derived from oedometric and drained triaxial tests on high quality samples. Due to the numerical back-calculation of the pressure meter tests using the axisymmetrical Fem model, it was possible to calibrate and refine the input parameters.

The pressure meter tests meant it was possible to obtain a reliable assessment of the mechanical properties of the lignite-containing layers as the basis of the calibration of the numerical model.

Deformations of the structure during construction were predicted by means of a 3D Fem model of the soil-structure interaction. Input parameters of the hypoplastic constitutive equations were derived from laboratory tests and later verified by back-calculation of the pressure meter tests.

Since the response of the hypoplastic constitutive equations depends on the stress and deformation history, all relevant construction stages, including excavation, construction of the underwater concrete, dewatering, as well as the phases of the ship lift construction, had to be considered.

fig9 3 d fem model

Finite element analysis was used to simulate the cavity expansion by means of hypoplastic and viscohypoplastic constitutive equations

During construction, the observational method was used to compare calculated deformations to monitoring data. The results provided the basis for the definition of coordinates for the climbing formwork to fulfill the strict accuracy tolerances.

According to Fugro, the pressure meter tests on the lignite-containing layers were readily justified by the results despite the relatively high cost procedure. For the designers, the results proved extremely valuable in deriving and verifying the strength and stiffness behaviour in combination with classical laboratory testing on undisturbed lignite samples taken from boreholes.

Fugro and the client believe that the testing undertaken at Niederfinow demonstrates the versatility of CPT to address the challenges of soil property evaluation in sensitive ground conditions, while reducing programme, causing less soil disturbance and mobilising less equipment.

The ability to carry out pressure meter tests insitu with the CPT also provided a time-efficient and robust approach to the acquisition of geotechnical data to support ground engineering design.

For the Niederfinow project, this detailed design data has proved successful so far in managing the risks during the construction phase.

  • This article was written by Robert Quaas and Ed Poulding of Fugro and Christian Schwab of Geolink Geotechnik.

Hole in one

Innovative adaptation of its cone penetration testing (CPT) equipment has enabled Fugro to configure the downhole assembly with interchangeable or supplementary probes to increase the range of soil interrogation within a single probe position.

The company developed this new solution in response to demand from clients for quicker delivery of new infrastructure, combined with constraints on budget, access and time for site investigation becoming ever more challenging. Despite these challenges, robust ground data remains critical to assessing and managing the cradle-to-grave risks of the more resilient, longer-lasting assets which are the goal of a sustainable 21st century society.

Fugro’s latest development helps to address these challenges by allowing for the use of a cone pressure meter (CPM) to be placed behind the cone to carry out pressure meter testing, which can be transported and operated using a single truck, or jack-up barge for marine work.

In contrast, conventional pressure meter testing requires the drilling of a dedicated borehole, creating the problem of additional soil disturbance and increased costs, or relies on the use of carefully applied self-boring techniques.

By combining CPT with pressure meter testing, soil information can be derived in a much more time- and cost-effective way.

The concept required intensive engineering development of the probe to ensure it would work effectively within the CPT apparatus and perform to the required parameters for pressuremeter data collection. Theoretical analysis and test data indicate that reliable soil parameters can be obtained, despite the effect of initial disturbance.

In advance of the fieldwork, the CPM system is calibrated to establish the membrane resistance and system compliance. The probe is advanced to the desired test level through full displacement of the ground using a CPT rig.

CPM tests are performed by inflating the membrane to a maximum of twice the original diameter in a load- or strain-controlled manner. Alternate cycles of unloading/reloading are performed at various points of inflation and/or deflation.

The test is undertaken with all measurements recorded digitally for quick and easy processing. Specially developed data processing and reporting software is available to ensure rapid turnaround of results. The inclusion of a piezocone penetrometer in the CPM also enables clear identification of the soil for subsequent CPM testing.

The CPM tool can be installed using routine CPT equipment and associated personnel, replacing the need for a separate rig with specialised operators.

Installation of the CPM probe and execution of the test are fully operator independent, ensuring repeatable test results.

With fewer people and less kit on the ground, site operations and construction risk management are simplified.

Applications are not confined to use on dry land. The CPM probe can also be mobilised for nearshore marine work using a standard land drilling rig operated from a jack-up barge.

Fugro says investment in this level of soil and ground behaviour understanding will yield value throughout a project by enhancing the derived data on soil mechanics and, therefore, ground risk. This in-depth knowledge is essential to avoid under or over engineering structures and prevent problems due to unforeseen ground conditions that could affect the construction and life-long integrity of the asset.

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