3
Performance of Various Structures Along the Lahar Paths

The effects of the lahars on the structural behavior of facilities at various locations along the flow path is presented in this section.

A granary (warehouse I; Site 1A, Table 2.2 ) located on the west side of Armero 1.5 km from the mouth of, and directly in line with, Lagunillas Canyon experienced a strong impact from the lahar. The mudflow reached the top of the building, which is approximately 6-7 m high. The reinforced-concrete frame of the building did not collapse under the impact of the flow. However, some of the reinforced structural members were severely damaged. In addition, the exterior masonry wall fill-in within the structural frame was “blown out” by the lahar.

A close-up of one of the front columns of the granary building is shown in Figure 3.1 . Complete removal of the concrete from the face of the column on the front of the building, exposing the steel bar reinforcement within the column, demonstrates the high abrasive effect of the lahar. Two steel hoppers installed in a front room inside the building were buckled by the force transferred through the wall.

Within much of the southern half of Armero, total destruction of all buildings was observed. Here only the remnants of the foundations of the buildings in the business district remained. The lahar sheared off the upper part of all structures at the foundation level even though some of these structures had steel-reinforced connections between the walls and the foundations. The twisted reinforcing bars, stripped of concrete, attest to the shear forces that impinged on these structures. Very little debris was noted throughout the city center, since most of it had been transported several kilometers to the southeast.

In southeast Armero, in a downstream direction, numerous structures were at least half buried in the lahar, yet most of the walls remained erect.



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3 Performance of Various Structures Along the Lahar Paths The effects of the lahars on the structural behavior of facilities at various locations along the flow path is presented in this section. A granary (warehouse I; Site 1A, Table 2.2 ) located on the west side of Armero 1.5 km from the mouth of, and directly in line with, Lagunillas Canyon experienced a strong impact from the lahar. The mudflow reached the top of the building, which is approximately 6-7 m high. The reinforced-concrete frame of the building did not collapse under the impact of the flow. However, some of the reinforced structural members were severely damaged. In addition, the exterior masonry wall fill-in within the structural frame was “blown out” by the lahar. A close-up of one of the front columns of the granary building is shown in Figure 3.1 . Complete removal of the concrete from the face of the column on the front of the building, exposing the steel bar reinforcement within the column, demonstrates the high abrasive effect of the lahar. Two steel hoppers installed in a front room inside the building were buckled by the force transferred through the wall. Within much of the southern half of Armero, total destruction of all buildings was observed. Here only the remnants of the foundations of the buildings in the business district remained. The lahar sheared off the upper part of all structures at the foundation level even though some of these structures had steel-reinforced connections between the walls and the foundations. The twisted reinforcing bars, stripped of concrete, attest to the shear forces that impinged on these structures. Very little debris was noted throughout the city center, since most of it had been transported several kilometers to the southeast. In southeast Armero, in a downstream direction, numerous structures were at least half buried in the lahar, yet most of the walls remained erect.

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FIGURE 3.1 A corner pillar of the building shown in Figure 2.12 , showing concrete abraded to expose rebar. This degree of abrasion is particularly noteworthy given the short duration of the event. (Photo by R. Updike, USGS.)

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This was particularly true near the flow margins. Most of the structures were constructed of adobe bricks or concrete blocks, with individual structural elements (e.g., walls, ceiling beams) poorly tied to each other. Another type of construction typical in Armero was adobe reinforced with bamboo laths and then faced with stucco. Although the laths offered some degree of flexibility to the wall elements, the stucco and adobe tended to crumble when large deformation occurred. Without the adobe support, the walls tended to crumble to the level of the roofline. Failure of these buildings often began at the corners, since the wall elements at the corners were poorly tied together, and thus prone to fail under flexure. The impact of the mudflow on various structures along the path of the northern lobe was less severe. The main steel-reinforced concrete framework of warehouse II, located approximately 2.0 km north of Armero (Site 12, Table 2.2 ) did not sustain catastrophic damage under the impact of the mud wave. The corner of the building closest to the central axial flow along Santo Domingo Creek had one steel-reinforced concrete column of the warehouse frame, which was dangling by the reinforcing bars. This column was the only structural element that failed in the warehouse. Because this column was located at the corner of the building nearest to high-energy flow, the possibility of one or more large boulders impacting the column was more likely. In fact, a 1-m-diameter boulder was found slightly downstream of the column, inside the warehouse. Although the structural framework of the warehouse generally remained intact, some of the lower concrete block walls filling in between structural framework elements were carried out by the mudflow. The block walls removed were on that side of the building nearest the creek where the level of mudflow was higher and the removed walls were totally submerged in the mudflow. Figure 3.2 shows a detail of the downstream wall of the building where bulging of the walls and columns resulted from the impact of the mudflow. The foreground of the photograph also shows the sheared surface of the concrete blocks from the removed fill-in wall, as well as the entrained debris that accumulated behind the wall. Structures located in the same valley farther downstream, where the mudflow became more fluid and somewhat shallower, sustained moderate damage. For example, at the ranch previously mentioned, parts of the five buildings in the complex remained erect. An adobe block building remained fully intact, sustaining only damage to doors, windows, and roof covering. The lack of large boulders in tractive transport reduced the amount of flow impact damage. An inspection of the structures at the Serpentarium complex, located 3 km downstream from warehouse II, indicated no structural damage to the various facilities. The most serious problem at this locality was dealing

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FIGURE 3.2 Reinforced pillars on downstream wall of the building in Figure 2.15 . The concrete block fill-in has been nearly entirely removed. Pillars are cracked and bent in the downstream direction. (Photo by R. Updike, USGS.)

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with the 30 cm or more of consolidated mud that had flooded most of the facilities. Damage in the town of Honda, along the Gualí River, was quite different than at Armero. As previously indicated, the channel and floodplain of the Gualí River are constricted in a narrow, deep valley within 1 km upstream of the city of Honda. As the lahar entered this stretch of the valley it rose in height (nearly reaching the height of the bridge subdeck), was highly fluid, and moved rapidly. The scour of valley walls generated by the high flow resulted in the removal of sufficient foundation materials at the base of the bridges and buildings along this channel to cause partial collapse and serious structural damage to several buildings ( Figure 3.3 ). The substantial removal of foundation materials at the base of some of the bridges across the channel precluded further use of these bridges until remedial measures are implemented or the bridges are replaced. In summary, the damage to structures in the lahar paths can be attributed to the following factors: forceful impact of the moderately dense, highly fluid mass that ranged up to several meters thick, resulting in normal, shear, and torsional stresses; trajectory impact of large objects (boulders, debris) that were in tractive or suspended transport by the flow; tractive removal of foundations, vertical support members, and roof support framing, as well as nonstructural wall and roof fill-in; rapid abrasion, particularly of masonry construction, by the swiftly flowing clastic material; partial or complete burial by the flows, resulting in preconsolidation lateral stresses, followed by consolidation loading and postconsolidation tension when the material adhered to the structure; accelerated oxidation of metallic elements due to the low pH of the entrained fluids; and erosion of foundation soils, causing ground failure beneath structures.

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FIGURE 3.3 Foundation damage to apartment buildings in Honda along the channel of the Gualí River. (Photo by R. Updike, USGS.)