Forensic Engineering and Geotechnical Failure Assessment

Forensic Engineering: Assessment of Infrastructures and Geotechnical Failures

• Presenter: Assoc. Prof. Ir. Dr. Ismacahyadi Bagus Mohamed Jais.

• Position: Head/Geotechnical Advisor of Geoforensig, School of Civil Engineering, Universiti Teknologi Mara (UiTM), and Geocon (M) Sdn Bhd.

• Core Focus: The assessment of infrastructure and geotechnical failures through forensic investigation, emphasizing the "Artistic Scientific Geotechnical Interpretation."

Overview of Geotechnical Problems in Malaysia

• Failure of Roads and Fill Embankments: Specifically occurring due to soft ground conditions.

• Tension Cracks: Visible on existing roads, typically signaling the early stages of embankment failure.

• Collapse Settlement: Ground failure leading to the collapse of road segments.

• Structural Distress: Ground settlement causing significant distress to bridge abutments.

• Differential Settlement: Differences in settlement rates affecting approach bridges.

• Apron Splitting: Splitting of the apron structures due to underlying ground settlement.

Geoforensic Investigation: The Artistic Scientific Geotechnical Interpretation

Geoforensic investigation is a multidisciplinary process involving the following components:

• Geological and Geomorphological Interpretation: Understanding the history and formation of the site.

• Land Use Analysis: Investigating historical and present land use patterns.

• Environmental Considerations: Analyzing topographical and hydrogeological factors.

• Fundamental Soil Mechanics: Applying geotechnical interpretation and core principles.

• Visual Inspection: Site evaluation visits and visual assessments.

• Exploration Data: Site investigation (SI) and geophysical interpretation.

• On-site Measurements: Deformation surveys and other field measurements.

• Advanced Simulation: Utilizing sophisticated simulation models and back analyses.

• Problem Dissection: Identifying and categorizing the ultimate causes of failures.

Objectives and Principles of Site Investigation (SI)

Site investigation involves exploring surface and subsurface conditions to inform development. The purpose of subsurface exploration includes:

1. Foundation Selection: Choosing the suitable type and presumed depth of foundation for structures.

2. Bearing Capacity: Evaluating the load-bearing capacity for proposed foundations.

3. Settlement Estimation: Estimating potential settlement for the development.

4. Geotechnical Problem Identification: Determining potential issues before they cause failure.

5. Lateral Earth Pressure: Predicting pressures and movements for retaining walls and excavations.

6. Construction Methodology: Proposing suitable methods based on variable subsoil conditions.

Geology and Geomorphology in Forensic Context

• General Information: Geological data forms the basis for understanding superficial deposits derived from parent material.

• SI Planning: Informs the planning of site investigation works.

• Geological Maps: Record observed features from above, though many are concealed by vegetation or drift. Key takeaways from maps include:

  • Exposures and outcrops.

  • Thickness of strata and the stratigraphical column.

  • Contour lines.

  • Structural contours.

Case Study I: Settlement of Expressway

• Geology: Consists of Ordovician-Silurian formations including schist, phyllite, slate, limestone, minor sandstone, and volcanics.

• Terrain: Hilly; construction platform primarily in cut areas.

• Structural Features: Minor fault lines cutting North-South and West-East.

• Soil Profile: Consists of partly sandy, silty, and clayey material derived from the parent metamorphic and sedimentary rocks.

Case Study II: Settlement of Bridge/Transition Approach

• Location: Teluk Panglima Garang and Jenjarom areas.

• Geology: Dominated by phyllite, slate, shale, sandstone, peat, humic clay, and silt. Argillaceous rocks are mostly carbonaceous.

• Phenomenon: These materials exhibit soft soil phenomena due to high water content and a high water table in lowland areas.

In Situ Test Methods and Exploration Techniques

Common methods for subsurface exploration include:

• JKR/Mackintosh Probe (Dynamic Probes):

  • Used to determine density and indirect strength resistance.

  • Applicable for preliminary embankment design.

  • Restricted to a depth of approximately $15\,m$.

  • Records the number of blows per foot ($blows/ft$).

  • Correlation is based on Terzaghi and Peck ($1967$) for undrained shear strength and relative density.

• Hand Augering (HA):

  • Used in soft to stiff cohesive soils or sandy soils above the water table.

  • Depth limit is approximately $5\,m$.

  • Often used to verify the existence of utilities.

• Deep Boring (DB):

  • Advanced via power rotary drilling (open hole or casing advancement).

  • flushing mediums include clear water, mud water, or bentonite (specifically for coarse sand/fine gravel).

• Rotary Wash Boring:

  • Advanced by a cutting bit and hydraulic thrust.

  • Soil cuttings are transported upward by drilling water.

  • High quality for sampling and testing; allows observation of water losses and penetration rates.

• Standard Penetration Test (SPT):

  • Conducted per ASTM D 1586.

  • Uses a $63.5\,kg$ drop hammer falling $0.76\,m$.

  • The $N$-value is the total number of blows to drive the sampler the last $300\,mm$ (excluding the seating drive of $0.15\,m$).

  • Requires energy efficiency correction to $60\%$ (ASTM D 4633).

• Deep Sounding (DS) / Cone Penetration Test (CPT):

  • Static Dutch Cone Penetrometer with $100/200\,kN$ capacity.

  • Supplements Deep Boring in fluvial or soft formations.

  • Not suitable for soil with abundant gravel.

Bearing Resistance and Soil Property Correlations

Cohesive Soil (Clay)

• Consistency Scale based on SPT ($N$):

  • N < 2: Very soft; Unconfined Compressive Strength ($q_u$) $0-25\,kPa$.

  • $N = 2 - 4$: Soft; $q_u = 25 - 50\,kPa$.

  • $N = 4 - 8$: Medium (firm); $q_u = 50 - 100\,kPa$.

  • $N = 8 - 15$: Stiff; $q_u = 100 - 200\,kPa$.

  • $N = 15 - 30$: Very stiff; $q_u = 200 - 400\,kPa$.

  • N > 30: Hard; q_u > 400\,kPa.

Cohesionless Soil (Sand)

• Relative Density and Pressure Scale based on SPT ($N$):

  • $N = 0 - 4$: Very loose; Not suitable for direct bearing.

  • $N = 4 - 10$: Loose; Allowable pressure $0 - 80\,kPa$.

  • $N = 10 - 30$: Medium; Allowable pressure $80 - 280\,kPa$.

  • $N = 30 - 50$: Dense; Allowable pressure $280 - 470\,kPa$.

  • N > 50: Very dense; Allowable pressure > 470\,kPa.

Empirical Values for Soil Modulus ($E$)

• Cohesive Soil:

  • Very soft to soft: $E = 100S_u$

  • Firm/Medium Stiff: $E = 250S_u$

  • Stiff: $E = 350S_u$

  • Very Stiff: $E = 450S_u$

  • Hard: $E = 500S_u$

• Cohesionless Soil: $E = 500(N + 15)$

Geophysical Methods for Ground Investigation

These methods determine conditions over a distance by measuring physical properties, involving correlations rather than direct property measurement. Methods include:

• Seismic Refraction: Used for sediments over bedrock or determining rippability.

• Electrical Resistivity: Identifies saturated zones, peat, and cavities (low resistivity may indicate weathering or clay).

• Ground-Penetrating Radar (GPR): Used for locating pipes, cables, and shallow archaeology.

• Gravity Method: Captures large fault systems or sinkholes.

• MASW and SASW: Multi Analysis and Surface Analysis Spectral Waves.

Proposed Scope for Geoforensic Investigation (SI)

The scope is based on construction type and specific problematic occurrences:

• Soft Ground/Reclaimed Land:

  • Building: Min 3 BH; MP 4 Nos/BH; 5 BSM/block; 1 trial pit; parallel seismic.

  • Roads: 1 BH/500m; MP 4 Nos/BH; Geophysics 3 lines.

  • Airports: Min 6 lines geophysics; 3 BH; MP 4 Nos/BH.

• Slopes:

  • 1 BH at crest, 1 BH at toe.

  • Min 2 inclinometers per $50\,m$ slope length.

• Bridges/Culverts:

  • Abutment: 2 BH; Pier: 2 BH.

  • Min 4 MP per BH.

  • Monitoring: Minimum 3 months (1 reading/week) or maximum 6 months (2 readings/week).

Laboratory Testing and Soil Parameters

Testing Categories

• Physical Properties: Gradation, Moisture Content ($w_c$), Specific Gravity ($G_s$), Atterberg Limits (LL, PL), Unit Weight ($\gamma$).

• Mechanical Properties: Permeability, Compressibility ($C_c, C_{\alpha}, m_v$), Shear Strength.

Key Formulas and Equations

• Compression Index ($C_c$):

  • Inorganic, cohesive soil: $C_c = 0.007(LL - 7)$

  • General clay: $C_c = 1.15(e - 0.35)$

  • Organic soils: $C_c = 1.15 \times 10^{-2} w_c$

• Consolidation Parameters:

  • Coefficient of Volume Compressibility ($m_v$): $m_v = \frac{1}{f2 \times N}$

  • Coefficient of Vertical Consolidation ($C_v$): Correlation with Liquid Limit ($w_L$) by Z.C. Moh et al. ($1997$): $C_v = 0.033 \times 10^{(-0.025 w_L)}$

  • Coefficient of Horizontal Consolidation ($C_h$): typically $C_h = 2C_v$

Typical Values for Secondary Compressibility ($C_{\alpha}$)

• Very Low: < 0.002

• Medium: $0.008$

• Extremely High: $0.064$

Documentation and Standards

• Testing Standards: BS1377:1990, BS5930:2015, ASTM, MS2038:2006, MS1056:2005.

• Borelogs: Must include reduced levels, drilling methods, soil descriptions (colour, consistency, grain size), SPT plot, and recovery percentages.

• Summary Tables: Comprehensive results including chemical tests (pH, Sulphite, Organic matter), sieve analysis, and strength tests (UU/CU Triaxial).