Examples

Abstract

Seven examples encountered on different sites provides a better understanding of the method.

From 2019 to 2021, the Rincent BTP Services branches in Recife and Sao Paulo, Brazil, carried out more than 2,000 tests on tie-rods.

One of the two major projects involved 19 retaining walls representing a total of 1,811 tie rods. On the largest embankment, 590 out of 741 tie rods were tested using dynamic tests.

The static tests help to exploit the dynamic tests and provide information on the behavior of the tie-rods with a view to re-tensioning (Fig. 19.1).

Fig. 19.1

Source Rincent BTP Recife

Wall of 590 tie rods.

In this study, more than 20 static tests were carried out with the following objectives:

• Determine the tension force in the tie rods to calibrate the dynamic tests

• Analyze their behavior under tensile stress

• Simultaneously analyze the dynamic stiffnesses measured at each load level.

Tie rod 102

The V/F curves as a function of frequency, on which static and dynamic tests were carried out. For 590 elements tested, 4720 curves of this type were analyzed (Fig. 19.2).

Fig. 19.2

V/F function of the frequency

The curve for tie rod 102 gives:

• a stiffness of 1.28 E9 N/m,

• a total length of 23.75 m with a wave velocity in the tie rod of 4500 m/s.

• the free length is 14,4m for the same hypothesis.

The admittance of 1.1 E-6 m/sN leads to a calculation of the equivalent diameter of 0.33m, strands reinforcement plus grout.

Tie rods 138 – 261 – 3127 – 3132 8 strands

138 – 450 – 451 – 5125 – 5138 10 strands

The table of the appendix 5 shows the average stiffness values and the relative incertitude of these measurements, enabling us to draw up the following summary table. Appendix 6 shows the location of tie rods.

The physical significance of high relative incertitude values is that the tie-rod is sensitive to the compression wave generated by the hammer, or more simply that it is brittle. However, a completely relaxed tie-rod will give results with quite variable dynamic stiffnesses.

These tie rods are subjected to static tests, with dynamic tests carried out at each stage during loading and unloading.

The results of these simultaneous tests refine the calculation of a and b, and enable us to establish the formulas which, for the case presented, were:

• Y = 1750x + 7500 pour 8 strands

• Y = 1750x + 11000 pour 10 strands

See Figures 19.3 and 19.4.

Fig. 19.3

Results table

Fig. 19.4

Example of a concrete structure near the tie rod, foot of the wall

Tie rod 709 Bis

The aim of the tests carried out on this tie was to reach the breaking force to validate the possibility of re-tensioning part of the tie.

The dynamic stiffnesses measured on this tie were homogeneous, leading to a calculation of the tension force of 9.38 tons using the formula for 8 strands (Fig. 19.5).

Fig. 19.5

Dynamic stiffnesses N/m table

The static test showed that the distribution plate separated from the head of the tie-rod at a force of 9.6 tons. The tensile test was carried out up to 42,5 tons without failure.

Electrical insulation measurement tests were carried out in accordance with SIA 267/1 (2013), which states:

10.7.4.2 A tie rod, once injected and tensioned, must have an electrical resistance RI 0.1 MΩ (Mega Ohm) (Fig. 19.6).

Fig. 19.6

Electrical isolation measurement

The values of 6 MΩ are well above the required value (Fig. 19.7).

Fig. 19.7

Static test 709bis

It appears that the relative inc7rtitude of stiffness measurements of less than 10% and compliance with the electrical insulation criterion are two criteria that must be met before the tie rods can be re-tensioned.

Tie rods 450 and 451 (Fig. 19.8).

Fig. 19.8

Results table

These two tie rods, whose internal tension is close to zero or null, have both been tensioned up to 45 tons (Fig. 19.9).

Fig. 19.9

Re-tension curve F tons—deformation mm

Tie rods 518 and 5134

The non-destructive and static tests carried out on tie rods 418 and 5134 confirm the preliminary approach to determining which tie rods can be re-tensioned (Fig. 19.10).

Fig. 19.10

Dynamic stiffnesses table

The static test showed that the distribution plate separated from the head of the tie-rod at a force of 9.6 tons. The tensile test was carried out up to 53 tons without failure.

Electrical insulation measurement tests were carried out in accordance with SIA 267/1 (2013), which states:

10.7.4.2 A tie rod, once injected and tensioned, must have an electrical resistance RI 0.1 MΩ (Mega Ohm).

The values of 6 MΩ are well above the required value.

It appears that the relative incertitude of stiffness measurements of less than 10% and compliance with the electrical insulation criterion are two criteria that must be met before the tie rods can be re-tensioned.

The non-destructive and static tests carried out on tie rods 418 and 5134 confirm the preliminary approach to determining which tie rods can be re-tensioned.

The calculated forces in these ties are 9.7t for tie 418 and 13.1t for 5134. The major difference between the two tie rods is that tie rod 418 is sensitive to non-destructive testing, since the relative incertitude is well over 10% (Figs. 19.11 and 19.12).

Fig. 19.11

Static tests

Fig. 19.12

Dynamic stiffnesses under different loads

Dynamic tests show the new functioning of the tie-rod, which is loaded at a force value higher than its internal tension force.

T 308

This tie-rod, located at the foot of a motorway retaining wall, was subjected to a static tensile test and simultaneous dynamic tests at each loading level.

Initial analysis of the test curve revealed:

• a permanent deformation of 13.09 mm

• an elastic deformation of 4.8 mm measured on unloading after having reached 31 tons in tension (Fig. 19.13).

  Fig. 19.13

  

  Load and unload of a static test

In practice, this means that the 31tons load resulted in permanent deformation of the grout cylinder and deformation under load, corresponding to a much lower load than normal on a ground anchor of this type.

The unloading curve is virtually linear, corresponding to the elastic behavior of the loaded part of the ground anchor armature.

A calculation of the tie-rod length mobilized from the elastic deformation yields a length of 4.3 m.

Dynamic tests recorded under a 31-tons load show a vibratory response of 4.4 m for an assumed wave velocity of 4,000 m/s (Fig. 19.14).

Fig. 19.14

Response

The total length of the tie rod is 15 m

This example was chosen to show that static testing can produce erroneous results, such as the internal tension of the tie-rod being greater than 31 tons, whereas the test only stressed a part of the tie-rod located at the rear of the wall. Further analysis of the dynamic tests reveals the presence of a grout bulb immediately behind the retaining wall.

Tie rods 412 – 504 – 567 – 580.

These 8-strands tie rods were re-tensioned after electrical isolation measurements in compliance with SIA 267/1 (Figs. 19.15 and 19.16).

Fig. 19.15

Results table

Fig. 19.16

Re-tensioning operation

The first re-tensioning operations were successfully carried out with limited effort. A key challenge is to design a tensioning device that can be attached to the head of the tie rod. The fact of working at height must also be taken into account.

The test campaign presented here enables us to map the internal force of the tie rods at a given point in time, based on a limited number of static tests and dynamic tests on a large number of tie rods. All these tests, together with electrical isolation measurements, enable us to select the tie rods that can be re-tensioned. This type of investigation leads to a pre-costing of the work to be carried out, optimizing it.