Retos geotécnicos en la arcilla de la Ciudad de México" Dr. Gabriel Auvinet Guichard.

Introduction to Geotechnical Challenges in Mexico City

Overview of the Webinar

• The webinar is presented by the Mexican Society of Geotechnical Engineering, marking the first of many expected sessions.

• The title focuses on "Geotechnical Challenges in the Clay of Mexico City," indicating a specialized topic within geotechnics.

Speaker Introduction: Dr. Gabriel Ovin

• Dr. Gabriel Ovin Guichard graduated as a civil engineer from École Spéciale des Travaux Publics in Paris and earned his doctorate at UNAM in 1986.

• He has extensive teaching experience since 1968 and has directed numerous theses across various academic levels, showcasing his expertise.

• Dr. Ovin has authored around 330 articles and held significant positions within professional societies, including president of the Mexican Soil Mechanics Society.

Historical Context of Mexico City's Geology

Geological Background

• The Valley of Mexico is situated within a volcanic axis that runs across the country, with its geological context being entirely volcanic.

• Originally an open valley draining southward, it was closed off about one million years ago due to volcanic eruptions forming natural barriers.

Urban Development and Water Management

• Tenochtitlan, founded on an island in Lake Texcoco during the 14th century, became New Spain's capital after Spanish conquest.

• Historical flooding issues led to significant drainage projects like the construction of deep trenches to manage lake levels effectively.

Properties and Composition of Valley Clays

Characteristics of Local Clays

• The urban area built over ancient lake sediments consists primarily of soft clays that are crucial for understanding local geotechnical challenges.

• These clays are described as heterogeneous volcanic clay mixtures containing various minerals, organic components, and microorganisms.

Stratigraphy Insights

• The stratigraphy reveals a dry crust layer with anthropogenic fill above a substantial formation known as Narcos Superior, which contains lenses of silt and sand interspersed with clay.

Geotechnical Properties of Soils in the Valley of Mexico

Soil Profile and Water Content

• The soil profile in the Lake area shows a typical structure with a clay formation at depths of 30 meters, containing water slightly less than shallower layers. Deeper layers are categorized as deep deposits, which are less relevant for foundation engineering.

• The cone penetration test (CPT) indicates low resistance in the upper clay formation, with nearly zero blow counts in standard penetration tests. High compressibility and low undrained shear strength characterize these soils.

Index Properties and Mechanical Behavior

• Typical void ratios range from 5 to 10, corresponding to porosities between 0.83 and higher values. Water content is generally high, ranging from 220% to 420%, but can exceed 800%.

• Liquid limit values typically fall between 110% and 458%, while plastic limits range from 37% to 116%. The plasticity index varies significantly from 73% to 342%.

• The permeability of this material is notably low (around 1 times 10^-7 cm/s), indicating high compressibility with compressibility indices typically between 3 and 8.

Shear Strength Characteristics

• Short-term undrained shear modulus ranges between 4000 kPa and 7000 kPa (400 -700 tons/m²). Undrained shear strength is relatively low, typically between 15 kPa and up to about 35 kPa.

• Sensitivity of the clay indicates that remolded strength is significantly lower than intact strength; typical sensitivity values are around eight. Research by Jesús Alberro suggests internal friction angles surprisingly range from 34^circ to 41^circ.

Dynamic Properties and Modeling Approaches

• Low shear wave velocity (<100 m/s) reflects the soft nature of these clays. Results indicate internal friction angles obtained through triaxial tests align with previous findings.

• Various models have been proposed for the behavior of Valley clays; elastoplastic models like Willer’s model account for material anisotropy post-earthquake studies conducted after the macrosism of1985.

Spatial Variability in Clay Properties

• Extensive field testing has measured wave transmission velocities using various methods such as downhole tests during construction projects like Mexico City airport expansion.

• A significant database comprising approximately ten thousand boreholes allows for spatial variability analysis across different construction sites within Mexico City.

• Geographic Information Systems (GIS)-based data management enables efficient querying for site-specific exploration campaigns based on nearby borehole results.

• Visualization tools help assess subsurface conditions including surface crust thicknesses, upper clay formations, and underlying materials essential for interpolation among boreholes.

Understanding Sedimentation and Soil Properties

The Impact of Sedimentation on Soil Properties

• The sedimentation process in lakes leads to horizontal layering, affecting the vertical structure of soil formations.

• Data processing from boreholes must consider that soil properties change over time due to consolidation processes, particularly in clay caused by deep pumping in aquifers.

Variations in Soil Properties Over Time

• Significant differences in soil properties were observed between 1952 and 1986, especially at lower strata which consolidate more due to pumping effects.

• Past borehole data becomes increasingly obsolete for current projects as soil conditions evolve.

Estimating Spatial Variations Using Geostatistics

• Techniques inspired by geostatistics are employed to estimate spatial variations of soil properties at untested points using nearby borehole data, acknowledging inherent uncertainties.

• Conditional simulations can generate possible configurations of properties compatible with existing borehole data rather than just expected values.

Application of Kriging Methodology

• An application of kriging was demonstrated for estimating the depth of a hard layer based on available borehole data, allowing for contour mapping across the area.

• Geostatistical methods help define contours representing spatial variations, such as increasing depth towards the east and south regions.

Identifying Anomalies Affecting Soil Depth

• Notable anomalies affecting hard layer depth include urban developments like Tenochtitlán's historical fill areas causing significant settlement issues.

• These anomalies often correlate with historical construction activities that have altered natural sedimentation patterns and contributed to subsidence.

Utilizing Geographic Information Systems (GIS)

• GIS databases facilitate studies on lacustrine zone anomalies, including historical islands and constructed dikes impacting current land use.

• Maps have been developed identifying both artificial structures like canals and natural features such as alluvial fans within the study area.

Understanding Urban Development Impacts

• Historical reconstructions reveal how urban development has transformed areas once rich in water bodies into densely populated zones with significant fill materials.

• Evidence remains visible beneath urban structures indicating past geological conditions that have been drastically altered through human activity.

Advancements Through Statistical Modeling

• Geo-statistical methodologies allow for defining equal thickness contours of surface fills based on extensive sampling throughout urban centers.

• Three-dimensional models illustrate maximum fill thicknesses, enhancing understanding of material variation within city centers.

Virtual Boreholes and Water Content Analysis

• Virtual boreholes can be generated at intervals where physical samples are absent, providing insights into spatial property variations across different locations.

• Observations indicate that the upper sandy formation exhibits non-uniform water content distribution among its substrata.

Geotechnical Models and Their Applications in Large Projects

Importance of Geotechnical Models

• Geotechnical models are essential for large-scale projects, such as drainage systems and the current airport project at Lake Texcoco, where studies have been conducted to map the thickness of geological layers.

• The data collected allows for the creation of three-dimensional models that are highly beneficial in engineering projects, exemplified by over 350 boreholes drilled for the new international airport in Mexico City.

Stratigraphic Modeling

• With extensive borehole data, stratigraphic models can be developed along runways and beneath terminals, enhancing understanding of subsurface conditions.

• The geotechnical zoning proposed by Professor Mar includes three main zones: Zone 1 (Lomas), Zone 2 (transitional), and Zone 3 (custre). These zones help define construction regulations based on existing borehole data.

Precision in Zoning

• As more information becomes available, boundaries between these zones can be refined into sub-zones for better regulatory compliance while maintaining simplicity in zoning classifications.

• Seismic zoning is supported by geotechnical zoning and geophysical tests; however, the custre zone has been subdivided due to varying characteristics of deep deposits.

Challenges with Data Variability

• The depth contours of deep deposits have been defined using geostatistics; however, estimation precision varies significantly across different areas depending on data availability.

Subsidence Issues in Mexico City

Historical Context of Subsidence

• Initial beliefs attributed city subsidence to building weight; however, Dr. Navor Carrillo's work from 1947 clarified that groundwater pumping from deep aquifers is a primary cause.

Mechanisms Behind Subsidence

• Pumping induces declines in piezometric levels within clay layers, increasing effective stress and leading to compression and surface subsidence. Other factors like construction weight also contribute but are secondary.

Monitoring Subsidence Effects

• Numerous wells exist throughout Mexico Valley contributing both to subsidence issues and potable water supply challenges; thus complete cessation of pumping is not feasible due to water scarcity concerns.

Evidence of Subsidence Over Time

Documented Settlements

• Monitoring since the late 19th century shows significant settlements exceeding 10 meters over 110 years at notable structures like Palacio de Minería and Catedral Metropolitana due to variable pumping policies.

Visual Evidence

• Animation illustrating hydrostatic dissolution effects demonstrates how pressure reduction leads to increased effective stress and resultant ground settlement patterns observed historically.

Structural Impacts Due to Ground Movement

Formation of Depressions

• In Mexico Valley's custre zone, a depression has formed due to ongoing subsidence. This phenomenon is visually represented through exaggerated depth contours highlighting affected areas.

Infrastructure Responses

• Historical records indicate that after artesian wells lost efficiency in the 19th century, mechanical pumps were installed which now show signs of emerging above ground level over time—evidencing ongoing land movement.

Monitoring Ground Subsidence in Mexico City

Overview of Ground Subsidence

• The discussion begins with the positioning of lumbreras (light wells), which are generally placed above the hard layer of soil, supporting structures while the rest of the city experiences subsidence.

Development of Monitoring Systems

• A new system called "Simo" has been developed to monitor ground subsidence, utilizing data from various institutions like Conagua and local water commissions to create regional subsidence maps.

Historical Data on Subsidence

• Historical records indicate that since 1862, certain areas have experienced significant subsidence, with some locations sinking between 13m to 14.5m, particularly near Cerro del Marqués and Lake Chalco.

Speed and Measurement Techniques

• Current measurements show varying rates of subsidence; some areas experience up to 40 cm per year while others have much lower rates.

• Traditional leveling networks are used for measuring subsidence, but newer indirect methods like interferometry are emerging as promising alternatives despite limitations in certain areas like Lake Texcoco due to water interference.

Impacts of Soil Composition on Seismic Response

• The clay composition in Mexico Valley significantly affects seismic responses; during the 1985 earthquake, different soil types led to varied horizontal accelerations.

Amplification Effects During Earthquakes

• Research indicates that soft soils amplify seismic waves more than firm ground, leading to increased displacement during earthquakes. This phenomenon was noted prior to the devastating effects observed in 1985.

Future Predictions for Seismic Design Standards

• As soil conditions evolve due to ongoing subsidence and changes in water content, future seismic design standards will need adjustments. Predictions extend as far as 2047 regarding these necessary changes.

Ground Fracturing Observations

• The presentation highlights issues related to ground fracturing and cracks attributed mainly to hydraulic fracturing during rainy seasons.

Interaction Between Different Soil Types

• Not all cracks result from hydraulic fracturing; some arise from stratigraphic anomalies where fragile layers overlie more stable materials.

Consequences of Surface Tension Due to Subsidence

• As subsidence occurs, it creates tension at the surface level leading to destructive transitional cracks that can severely impact construction integrity.

This structured summary provides a comprehensive overview of key discussions surrounding ground subsidence monitoring efforts in Mexico City based on the provided transcript.

Erosion and Cracking Issues in Mexico City

Monitoring Crack Activity

• The Institute of Engineering has developed a monitoring system for cracks, capturing data on their locations to create a Geographic Information System (GIS).

Systematic Surveys of Cracks

• Systematic surveys have been conducted, particularly in areas like Iztapalapa and Texcoco, allowing for precise mapping of crack locations and their geometric characteristics.

Drainage Challenges in Mexico City

• The drainage issues in Mexico City are significant; historical infrastructure such as the Great Canal has been affected by regional subsidence, complicating water management.

Impact of Subsidence on Infrastructure

• Due to subsidence, the effectiveness of the Great Canal has diminished, necessitating the construction of pumping stations to manage water flow towards tunnels.

Development of Deep Drainage Systems

• A deep drainage system was constructed between 1967 and 1975 to address inefficiencies in existing systems. Advanced machinery was utilized for building collector networks.

Flood Risks Despite Infrastructure Improvements

Ongoing Flood Threats

• Despite improvements, flood risks remain high; potential flooding scenarios illustrate how minor failures could lead to significant inundation events in central areas.

Construction of New Tunnels

• To mitigate flooding risks further, the Eastern Emisor Tunnel is under construction. It will be one of the longest tunnels globally at 62 km with a diameter of 7 m.

Access Points for Maintenance

• Access points called "lumbreras" are being built up to 200 m deep for excavation equipment access and maintenance purposes within the tunnel system.

Historical Context: Settlements and Structural Integrity

Historical Observations by Aztecs

• The Aztecs noted significant settlements when constructing major structures like the Templo Mayor due to underlying soil conditions affecting stability.

Notable Examples of Structural Settlements

• Various heavy constructions during colonial times experienced notable movements; an example includes the Church of La Santísima Trinidad which suffered considerable sinking over time.

Cathedral Metropolitan's Stability Issues

• The Metropolitan Cathedral faced uneven settling due to its weight on pre-Hispanic foundations. Restoration efforts included geometric corrections led by engineer Enrique Santoyo.

Foundation Challenges Summary

• Building foundations can lead to excessive settlements if not designed properly. Superficial foundations often result in instability compared to those anchored deeper into stable layers.

Understanding Foundation Challenges in Mexico City

Issues with Arcosa Formation and Negative Friction

• The arcosa formation presents challenges such as negative friction effects and loss of contact between the slab and ground, leading to the popularity of intermediate solutions like compensation and friction piles.

Case Study: Column Independence

• The famous case of the Column Independence illustrates issues where a slab settled on wooden piles after behavioral problems, resulting in an apparent inversion of the foundation.

Algorithms for Settlement Calculation

• Established algorithms exist for calculating settlements or inversions in foundations, including a model developed by Dr. Resis in 1973 that accounts for both negative and positive friction.

Advanced Modeling Techniques

• Modern tools like finite element methods allow for sophisticated modeling of foundations, considering regional settlement phenomena explicitly at various levels.

Innovative Foundation Solutions

• New foundation techniques involve circular inclusions (unreinforced piles not connected to structures), which effectively transfer loads through a foundation slab to surface layers, reducing settlements to acceptable levels.

Limitations of Current Solutions

• While effective for low-rise buildings, these new solutions may not suffice for taller structures due to inadequate settlement reduction and potential seismic performance issues.

Construction Technique Advancements

• Advances in construction techniques ensure consistent diameter and quality of concrete used in inclusions, enhancing structural integrity.

Transportation Infrastructure Challenges

• The metro line A traverses areas with varying soil conditions (lacustrine zones vs. firm materials), leading to significant differential settlements that pose risks requiring frequent maintenance.

Historical Context of Metro Line Settlements

• Since its construction in the 1980s, metro line A has experienced up to 8 meters of regional settlement compared to negligible movement over firmer soils, causing structural deformation concerns.

Future Maintenance Needs

• Similar challenges are anticipated with new metro line 12 passing through lacustrine zones; ongoing maintenance will be necessary due to expected differential behavior across varying soil types.

Conclusion on Clay Problems in Mexico City

• The presentation concludes by emphasizing the complexity surrounding clay issues in Mexico City’s subsurface conditions while encouraging further discussions among professionals.