Phases of Work

Abstract

Carrying out tests to calculate the tension force of tie rods is an operation involving tie rods several decades old. The methodology for on-site intervention is presented, from numbering to recording the information needed for an effective diagnosis, which can then be used for maintenance operations.

2.1 Preliminary Observations

The examples presented in this book are based on projects carried out in Brazil. The heads of the tie rods are protected by concrete blocks which cover the distribution plate in contact with the wall. The geophone is generally glued to this metal plate, and hammer blows are made on the head of the tie rod itself. It is therefore necessary to destroy the concrete protection in order to carry out the tests.

The measured stiffness is the sum of the tie rod inertia of the wall and the tensile force of the tie.

The quality of the dynamic tests is linked to the quality of the mounting of the sensor that measures the velocity. Several solutions are possible, for example, gluing a metal plate that allows the geophone to be fixed by bolting, the vibration response is that of the tie rod, not the sensor.

The quality of the bond must withstand 8 consecutive tests on the head of the tie rod, over which the dynamic stiffness value is calculated, the maximum and minimum values are eliminated, the average is calculated over the remaining 6.

The geophone fastening is the most important point in this work, a poorly performed test is an unusable result.

2.2 Accessibilities

See Fig. 2.1.

Fig. 2.1

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Access to the wall and clearing.

2.3 Preparation of Tie Rods for Testing

In addition to cleaning the heads of the tie rods, the essential operation is the numbering of the tie rods or their identification on an existing document. It is also the opportunity to photograph everything that will help in the interpretation of the tests (Figs. 2.2 and 2.3).

Fig. 2.2

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Numbering the tie-rods.

Fig. 2.3

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Destruction of the concrete heads.

2.4 Characteristics of the Tie Rods

The first characteristic measured is the diameter of the strands, the bars and the dimensions of the load distribution plate (Fig. 2.4).

Fig. 2.4

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Steel diameters.

Note the slope of the base plate and add descriptions such as water ingress and corrosion. Photographs are essential (Figs. 2.5, 2.6 and 2.7).

Fig. 2.5

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Measurement of the inclination of the support plates.

Fig. 2.6

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Water entering.

Fig. 2.7

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The embankment closes a talweg.

2.5 Non-destructive Testing

The preparation of the tie rod head and the quality of the 3D geophone mounting are essential operations to obtain quality results (Fig. 2.8).

Fig. 2.8

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Securing the geophone support.

The next step is to clean the metal surfaces to which the geophone will be attached. There are a number of techniques for fixing the geophone to ensure the quality of the signals recorded. By way of example, the figures below show the response of a velocity sensor correctly attached and directly attached to the same element where surface oxidation has not been removed (Figs. 2.9 and 2.10).

Fig. 2.9

Geophone fixed correctly

Fig. 2.10

Not fixed correctly

The amplitude of the signal is halved, and the response includes numerous parasitic vibrations.

On one dam, the disturbance was caused by the vibrations generated by the flow of water, particularly during floods, as can be seen from the curves obtained near the spillway (Figs. 2.11 and 2.12).

Fig. 2.11

Acquisition in calm period

Fig. 2.12

During flood periods

This particular aspect of the geophone's mounting is essential for obtaining usable dynamic stiffness values.

Two types of acquisition were carried out on the same dam. The first procedure consists in placing the geophone on the head of the tie-rod, with the strike performed on the head of the tie-rod. The results were as follows (Fig. 2.13).

Fig. 2.13

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First configuration.

This is the response for the tie-rod with grout cement alone, and the average stiffness of 1.83E8N/m corresponds to a weight of around 2 tons, using the formula established from tests on this site. The second configuration consists of attaching the geophone to the metal distribution plate. The stiffness results are (Fig. 2.14).

Fig. 2.14

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Second configuration.

The average dynamic stiffness is 4.19 E9 N/m, which allows us to calculate the internal force at 44.5 tons.

Often the performance of non-destructive testing requires persons certified to carry out work at height (Fig. 2.15).

Fig. 2.15

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Non-destructive testing.

The performance of static and dynamic tests simultaneously allows you to calibrate the two methods and define the internal tensile force of the tie. The number of static tests is limited, but improves the reliability 2.14 the results. These simultaneous tests require the design and manufacture of a specific test device (Fig. 2.16).

Fig. 2.16

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Simultaneous static and dynamic tests.

Resistivity measurement is also a non-destructive test necessary to identify the corrosion phenomenon when it exists (Fig. 2.17).

Fig. 2.17

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Resistivity measurement.