Geological response to Mexico, 1985 Bruce Ashcroft And Amanda Chapman

Introduction First earthquake on 19 th September 1985 at 0719, 8.1 on Richter scale Second earthquake on 20 th September 1985 at 1940, 7.5 on Richter scale 9500 people perished injured More than left homeless

Introduction (cont.) Soil Structure Resonance in old lake bed was a major factor All of the buildings that were damaged and completely collapsed were newer constructions with an average age of 40 years 400 buildings collapsed in Old Lake Bed Zone of Mexico City

Damage to Hospitals Damage to Hospitals

Mexico City’s geology Built on soft, poorly consolidated lake sediments from the former Lake Texcoco Young, porous, highly compressible clays are largely montmorillonite; water content around 300% Thick sequences of soft sediments amplified seismic waves and caused the ground to shake much more vigorously than the surrounding bedrock Buildings over ten storeys amplified resonance in clays Clays resonated about six times more violently than the hills surrounding the city

Mexico City’s geology

Liquefaction Ground failure in which water saturated sediment turns from a solid to a liquid as a result of shaking Occurs below the water table at shallow depths The pore water pressure increases causing a temporary loss of effective stress during period of earthquake vibration: Effective Stress = Total Stress – Pore Water Pressure

Liquefaction (cont.) High pore water pressure leading to low contact forces and reduced effective stress

Damage to Railroad System Damage to Railroad System

Engineering Approaches to cope with Liquefaction and Seismic Waves Option 1 – Zoning –Characterise soil susceptibility –Avoid construction Option 2 – Selection of Structural System –Failures: Concrete frame cracking Flat plate shear failure Cross-bracing buckling Shallow foundation settlement Top floors due to amplification of oscillations

Option 2 continued…… –Stable Structures: 5 storey garages with substantial columns and beams in both directions Pre-stressed cross-bracing Large panel pre-cast structures Suitable connection detailing Buoyant foundation

Engineering Approaches to cope with Liquefaction and Seismic Waves Option 3 – Improve the Soil –Vibrocompaction Crane-supported vibrating proker Granular, non-cohesive soil Grain structure collapses leading to densification –Vibroreplacement / Stone Columns Gravel backfill Grid pattern –Dynamic Consolidation Dropping a heavy weight ft 5-10m grid

Option 3 continued… –Compaction Grouting Slow flowing grout injected under pressure Granular soil Surrounding soil displaced and densifies –Sand Drains Accelerates water expulsion Excess pore water pressure reduced Accelerates consolidation –Geotextiles Highly permeable Increase shear strength Only installed in placed ground