# Assessment of the Structural Integrity of the Roman Bridge of Alcántara (Spain) Using TLS and GPR

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Research Methodology

## 3. Documentary Study for the Assessment of the Bridge of Alcántara

#### 3.1. The Preservation of the Bridge through History

#### 3.2. Update of the Interventions Made on the Bridge

^{3}; V(T)—flowing velocity for flood flow of return period of T years [m/s]; c

_{f}—coefficient of force (or drag) of the section that supports the thrust; A(T)—area of the element projected on a plane perpendicular to the flowing [m

^{2}].

#### 3.3. Description of the Structure and Materials

## 4. Update of the Geometry, Fillers and Materials

#### 4.1. Bridge Geometry Using TLS

#### 4.2. Description of the Extrados Filling of the Bridge Using GPR

#### 4.3. Update of the Description of the Ashlar Masonry

## 5. Structural Analysis

#### 5.1. Analysis of Factory Stonework Structures

#### 5.2. Level I Analysis: Limit Analysis

- S0: original longitudinally sloping bridge. From the data of Bonet, 1991 [23].
- S1: demolition of arch no. 6, modelling arches 1 to 5 and longitudinal slope.
- S2: demolition of arch no. 5, modelling arches 1 to 4 and longitudinal slope.
- S3: entire bridge in the present condition.

- Damage to the stonework dated at more than 500 years ago.
- There is no information as to the location of this damage, what has been repaired nor what damage there may still be.
- The degree of knowledge on the characteristics of the stonework of the bridge does not come from direct testing.
- The aim of this research is to survey its behaviour for more advanced stages of research.

## 6. Discussion

## 7. Conclusions

- It has been demonstrated how non-destructive techniques, such as TLS and GPR, are fundamental to perform the preliminary evaluation of masonry bridges reliably. The low cost of these techniques allows a reliable analysis of the safety of the structure. The real geometry is obtained through TLS and the GPR allows defining the configuration of the fillings. All these data are essential to evaluate the safety of a masonry bridge.
- A considerable degradation in the stonework of the bridge has been confirmed, necessitating further general research into the state of preservation of stone and joints both in accessible and interior areas, since the interventions to date have only been carried out on accessible ones. The degradation of the stonework may lead to a local failure of the structure that cannot be detected using analytical models, neither local nor detailed.
- Research must be made into the state of preservation of the areas of pillars 3 and 4, which are permanently underwater, in order to check the erosive effect of the water.
- It needed a detailed study of the innermost ring of the arches to determine whether it can be taken into account from a structural point of view.
- The reservoir upstream regulates floods. In order to manage the respectful rolling of floods with regard to the bridge, a study must be conducted to find the maximum hydrodynamic action the bridge can withstand without the collapse of or erosive damage to the stonework.
- A detailed assessment should be made of the bridge’s structural safety, a study to describe the factory stonework, paying particular attention to its resistance to slipping in the arches. This research must be carried out on a length in the area of the keystone of the arch, mainly in arch number 6. Moreover, it is important to study the variability of the results and their incidence on the load reduction factor. The description of the resistance to compression can be made with a lower level of study.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 2.**Flowchart of the assessment of historical structures [5].

**Figure 3.**Photograph from downstream during the reconstruction work of the triumphal arch in 1857 after having reconstructed the fifth arch, which had been destroyed [16].

**Figure 5.**Print showing the variation in height between the ends of the bridge and the triumphal arch [23].

**Figure 6.**Flood of 1965. The flood level from the historical prints is marked by a red line [26].

**Figure 7.**Location and joining-up of the 12 scans to measure the Roman bridge of Alcántara [38].

**Figure 10.**Radargrams Prof 8 to Prof 12 superimposed on the longitudinal elevation of the bridge from the TLS survey.

**Figure 11.**Radargram Prof 16 on transect L1, length 100.3 m and maximum depth 3.5 m, obtained by the 500 MHz antenna.

**Figure 12.**Radargram Prof 17 on the transect L1, length 92.9 m and maximum depth 1.5 m, obtained by the 500 MHz antenna.

**Figure 13.**Radargrams Prof 18—left—and Prof 19—right—collected by the 500 MHz antenna on transect T1.

Structural Element | Sanchez Taramás | Fernando Rodriguez | Fernandez Casado | Jesús Liz | Present Study |
---|---|---|---|---|---|

1769 | 1797 | 1980 | 1988 | 2017 | |

Arch no. 1 | 13.67 | 14.00 | 13.63 | 14.00 | |

Arch no. 2 | 23.30 | 23.22 | 22.29 | 22.74 | |

Arch no. 3 | 29.94 | 27.67 | 27.40 | 27.35 | 27.55 |

Arch no. 4 | 27.67 | 27.80 | 28.80 | 28.90 | 29.25 |

Arch no. 5 | 22.13 | 23.29 | 21.90 | 23.58 | |

Arch no. 6 | 13.67 | 14.15 | 13.80 | 13.99 | |

Pillar no. 1 | - | 5.57 | 5.98 | 6.17 | |

Pillar no. 2 | 5.79 | 6.62 | 6.63 | ||

Pillar no. 3 | 6.00 | 8.30 | 8.30 | ||

Pillar no. 4 | 5.72 | 8.10 | |||

Pillar no. 5 | 5.57 | 6.84 |

Radargram (Transect) | Antena (MHz) | Interval (m) | Observations |
---|---|---|---|

Prof 8 (L1) | 200 | 0–85.9 | Beginning of transect L1 with 200 MHz antenna in right buttress |

Prof 9 (L1) | 200 | 82.0–110.0 | Partially overlapping Prof 8 |

Prof 10 (L1) | 200 | 110–134.15 | - |

Prof 11 (L1) | 200 | 134.15–173.3 | |

Prof 12 (L1) | 200 | 173.3–190 | End of transect L1 with 200 MHz antenna |

Prof 16 (L1) | 500 | 0–100.3 | Depth parameters up to approx. 3 m |

Prof 17 (L1) | 500 | 0–92.9 | Depth parameters up to approx. 1.5 m |

Prof 13 (T1) | 200 | 0–5.5 | Adjacent to the triumphal arch. Left according to the water flow |

Prof 14 (T1) | 200 | 0–5.5 | Centre of the opening of the triumphal arch. Right |

Prof 15 (T1) | 200 | 0–5.5 | Pillar no. 5 |

Prof 18 (T1) | 500 | 0–6.5 | Idem Prof 13 right of the triumphal arch. Depth up to approx. 2 m |

Prof 19 (T1) | 500 | 0–6.5 | Idem Prof 18. Depth up to approx. 4 m |

**Table 3.**Resistant characteristics of the granite of the bridge of Alcántara according to [47].

Density kg/m^{3} | Module of Deformation (MPa) | Compression Resistance (MPa) | Traction Resistance (MPa) | Poisson Coefficient |
---|---|---|---|---|

2645 | 11,028 (12.0) ^{1} to 59,993 (5.2) ^{1} | 26.0 (7.1) ^{1} to 159.8 (2.5) ^{1} | 1.56 (11.3) ^{1} to 8.08 (11.4) ^{1} | 0.2 to 0.3 |

^{1}The variation depends on the direction of the numbering according to the direction of effort. In brackets, the value of the coefficient of variation of the tests.

Material | |||
---|---|---|---|

Resistant Parameter | 1 | 2 | 3 |

Resistance to compression (MPa) | 7.18 | 40 | 73 |

Angle of friction (°) | 0.33 (18) | 0.49 (26) | 0.65 (33) |

**Table 5.**Summary of the assessment of the bridge of Alcántara for different materials and for each circumstance the bridge has undergone.

Material | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

Circumstance | 1 | 2 | 3 | |||||||||

a ^{1} | b ^{2} | c ^{3} | d ^{4} | a ^{1} | b ^{2} | c ^{3} | d ^{4} | a ^{1} | b ^{2} | c ^{3} | d ^{4} | |

S0.1 (Cart) | D | 29.0 | A6 | 2.3/1.9 | D | 116.0 | A6 | >3.0/2.7 | D | 257.0 | A6 | >3.0/>3.0 |

S0.2 (IAP11) | D | 4.8 | A6 | 2.2/1.8 | D | 17.7 | A6 | >3.0/2.6 | D | 38.9 | A6 | >3.0/>3.0 |

S1 | C | 6.7 | A5 | 2.2/1.7 | C | 61.8 | A5 | >3.0/2.4 | C | 114.0 | A5 | >3.0/>3.0 |

S2 | Unstable | D | 46.8 | A4 | >3.0/2.5 | C | 53.4 | A4 | >3.0/>3.0 | |||

S3 | D | 4.8 | A6 | 2.5/1.8 | D | 17.7 | A6 | >3.0/2.6 | D | 38.5 | A6 | >3.0/>3.0 |

^{1}D: slipping, C: mechanism of collapse.

^{2}Value of the load coefficient.

^{3}Number of the arch with the lowest load coefficient.

^{4}Load reduction factor of the strength to compression or friction, which causes the collapse.

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**MDPI and ACS Style**

Pérez, J.P.C.; De Sanjosé Blasco, J.J.; Atkinson, A.D.J.; Del Río Pérez, L.M.
Assessment of the Structural Integrity of the Roman Bridge of Alcántara (Spain) Using TLS and GPR. *Remote Sens.* **2018**, *10*, 387.
https://doi.org/10.3390/rs10030387

**AMA Style**

Pérez JPC, De Sanjosé Blasco JJ, Atkinson ADJ, Del Río Pérez LM.
Assessment of the Structural Integrity of the Roman Bridge of Alcántara (Spain) Using TLS and GPR. *Remote Sensing*. 2018; 10(3):387.
https://doi.org/10.3390/rs10030387

**Chicago/Turabian Style**

Pérez, Juan Pedro Cortés, José Juan De Sanjosé Blasco, Alan D. J. Atkinson, and Luis Mariano Del Río Pérez.
2018. "Assessment of the Structural Integrity of the Roman Bridge of Alcántara (Spain) Using TLS and GPR" *Remote Sensing* 10, no. 3: 387.
https://doi.org/10.3390/rs10030387