DAMAGES OBSERVED IN THE 2010 CONCEPCIÓN EARTHQUAKE RELATED TO SOIL PHENOMENA

Transcription

1 DAMAGES OBSERVED IN THE 2010 CONCEPCIÓN EARTHQUAKE RELATED TO SOIL PHENOMENA Felipe Villalobos 1, Efraín Ovando 2, Manuel Mendoza 3 and Paulo Oróstegui 4 1 Lecturer, Dept. of Civil Engineering, Catholic University of Concepción, Chile, avillalobos@ucsc.cl 2 Professor, Instituto de Ingeniería, UNAM, México, eovs@pumas.ii.unam.mx 3 Professor, Instituto de Ingeniería, UNAM, México, mjm@pumas.ii.unam.mx 4 Geotechnical Engineer, Constructora Lancuyen Ltda., Concepción, Chile, orostegui@lancuyen.cl

2 INTRODUCTION Strong earthquake on 27/02/2010, M w = 8.8, 3.34am (local time), 90 s of duration Hypocentre100 km north of Concepción, 30 km deep, extended approx. 450 km long and 150 km wide Slip estimated of around 10 m to 12 m More than 300 aftershocks above 5 Richter and more than 20 above 6 Richter Measured max. accelerations in San Pedro de la Paz between 0.5g and 0.6g Barrientos (2010)

3 San Vicente Bay Hualpén Concepción Bay Bío Bío River Penco Dichato City move after year 1751 Studied area Important metropolitan area of the country Around 1 million inhabitants High seismicity, previous destructive earthquakes (1570, 1657, 1730, 1751, 1835, 1939, 1960) San Pedro de la Paz

4 GEOLOGY AND GEOTECHNICS OF THE GREAT CONCEPCIÓN Presence of a coastal mountain chain, the Ocean Pacific and the Bío Bío River 2 km wide Coastal plain with island hills, lakes, rivers and swamps Complex conditions due to variety of soils and rock units Metamorphic rocks, schist and phyllite (SE) in Andalué Igneous rocks, tonalite, granite and granodiorite (Pzg) in Chiguayante, Caracol hill, Penco, etc. Sedimentary rocks, sandstones and siltstones (Ec) in Lomas de San Andrés, Santa Sabina, Interportuaria motorway, Lenga, etc. Rocks are highly weathered, fine soil deposits are mostly of marine origin and fluvial transported sediments

5 Curves of equal period in Concepción centre Geology units (Gajardo 1981) P La lo aril epe Ch Bayona San Pedro de la Paz Pzg Anda lué San Ignacio school a Lo Peq uén Am or ólv Ca r ac ol (Ramirez & Vivallos 2009) to S anta Jua na

6 COLLAPSED STRUCTURES 1 1 Alto Río apartment building, no observable soil failure. Weak structural line at ground floor failed Silos, 1950 built row did not present damage, 1970 built row had broken bolts connecting steel shells to footing, no soil failure found 3 3 Old Bío Bío Bridge 1940, domino effect, decks falling and destroying piers, possible liquefaction, although foundation damage not observable 4 O Higgings Tower, no soil failure found

7 Ground floor Alto Río axis with few vertical resistant elements Deep foundations, stiffness and continuity of foundations specially when there are in plan and in elevation eccentricities

8 Damage due to liquefaction Presence of fine cohesionless materials in saturated low lands In San Pedro de la Paz liquefaction occurred in originally wetland areas (Los Batros) without appropriate soil improvement or foundations San Ignacio School suffered liquefaction despite the use of geogrids and geotextiles with 3 m compacted sand Singular architecture, offset room with different roof height could not follow the soil and structure deformation due to weak connections Lack of confinement towards the swamp, it is suggested to extend compacted soil fill or build a retaining structure

9 San Ignacio School detaching and rotating of offset part of the building anta Juana Road, restaurant settled due to ground displacement towards the river 2 m 1 m sand found in the bottom of cracks

10 Santa Juana Road lateral spreading Lateral spreading along Santa Juana Road next to the Bío Bío River Reclaimed land made of silt and clay not well compacted used to avoid flooding Big open cracks parallel to the river damaging seriously structures, piping, etc. Reclaimed material slid over liquefied sand towards the river

11 Steel company harbour in San Vicente Bay a) b) c) d) a) Pre-cast piled foundation of the wharf, 1945 b) Portal crane tilted due to derailing c) Several pile caps broken in 1972 wharf extension (driven steel tubes concreted inside, plus annular concrete cover and bars) d) View of 400 m long wharf Advance state of corrosion

12 Foundations scoured by tsunami After the earthquake a series of tsunami flooded coastal towns Dichato was one of the most devastated and only a few houses were standing Standing houses in front of the shore had their foundations exposed Tsunami strong current scoured footings

13 Conclusions and recommendations Failures and damages are not necessarily caused by only one reason There is a lack of available acceleration recordings in the most affected areas It is necessary to study possible amplification effects due to presence of soft soil layers Geology research is needed to determine deeper stratigraphy and the existence of faults Restrained drain conditions jeopardise well performance against excess pore water pressure increase A variety of soil improvement techniques and deeper foundations should be considered as possible solutions to avoid significant damages to structures

14 Thank you for your attention Spot the collapsed building