Keywords: geotechnical, ground anchorage, numerical modelling, dynamics, frequency response, eigenvalue analysis, damping.

The support of engineering structures such as tunnels, retaining walls, mines, dry docks, dams and pre-stressed structures commonly involves the installation of ground anchorages. The ongoing research programme at Aberdeen University has involved field investigations on active construction sites to assess anchorage response to blast loading concurrent with finite element and laboratory studies and recent work associated with a new non-destructive testing system for ground anchorages called GRANIT® (GRound ANchorage Integrity Testing). The ongoing research was reinforced by the development of a lumped parameter dynamic model. This paper attempts to show the influence of load on the frequency response of ground anchorages installed in the field using the lumped parameter model developed.

The 'complete ground anchorage system' is divided into six subsystems each representing a part of the whole anchorage. The initial lumped parameter dynamic model has been developed in order to investigate how each part of the anchorage system may respond dynamically to potential failure of a ground anchorage. In order to be able to represent the influence of load level, more accurate modelling of the anchorage head has been undertaken, which appears to be the most influential component in determining dynamic response[1].

The previous laboratory tests conducted over the research programme involved applying an impulse load at the free length of a bar embedded in a confined concrete cylinder in order to simulate the rock mass. The axial impulse load was applied by a specially developed and constructed impact device which forms part of the GRANIT® system. The resulting vibrational responses were measured by an accelerometer positioned at the protruding length of the bar. An increase in the main frequency were noted with increasing level of load.

In order to find the natural frequencies of the lumped parameter model and to allow comparison with the results obtained from the experimental rigs, the damped natural frequencies and mode shapes were calculated using the stiffness, damping and mass matrices. This is equivalent to calculating the complex eigenvalues and eigenvectors of the system[2].

Current results indicate a good correlation between the results obtained from the lumped parameter model and previous laboratory simulations. The lumped parameter model shows the increasing trend of fundamental frequency with increasing load level. This leads to the conclusion that the head of the anchorage is the most influential part of the complete ground anchorage system when observing the effect of load on the dynamic anchorage performance[3]. The lumped parameter model also shows the same increasing trend of frequencies with increasing the load level as was found in the results obtained from the GRANIT® test undertaken on laboratory rigs.

The capability of the numerical model to replicate the frequency response of laboratory rock bolt anchorages provides encouragement to extend the work in an attempt to show the influence of load on the frequency response of ground anchorages installed in the field using the lumped parameter model developed for the first time.

As a consequence a series of GRANIT® tests on 25mm diameter bolt anchorage cased in a bore hole of 152 mm in diameter situated at the AMEC site in Adlington have been carried out. The bolt anchorage was installed in the site in mostly sandstone ground. Rapid cement was used as a grout for these anchorages which is different from the anchorage laboratory rigs where resin was used as the grout.

The damping and stiffness coefficients of the model will be changed to reflect the characteristics of the field anchorage. This will show the relationship between frequency and load level of the anchorages which can be compared with the results obtained from the field employment of the GRANIT® system.

1

A. Ivanovic, R.D. Neilson, and A.A. Rodger, "Numerical modelling of single tendon ground anchorage systems", Geotechnical Engineering 149 (2) pp. 103-113, 2001. doi:10.1680/geng.149.2.103.39325

2

D.E. Newland, "Mechanical Vibration Analysis and Computation" First Ed., Longman Scientific & Technical , 1989.

3

A. Ivanovic, R.D. Neilson, and A.A. Rodger, "Influence of Prestress on the Dynamic Response of Ground Anchorages", submitted to ASCE, Journal of Geotechnical and Geoenvironmental Engineering. doi:10.1061/(ASCE)1090-0241(2002)128:3(237)

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