Earthquake Geotechnical Engineering and Soil Dynamics¶

Session Summary

Session 3 of GeoVadis (GAIC 2025) presents breakthrough developments in earthquake engineering, cyclic soil plasticity, and vibration mitigation. Focus areas include sustainable biopolymer treatment against cyclic liquefaction, packing index quantification in granular soils, sand-rubber mixture (SRM) wave barriers, seismic isolation using disconnected piled rafts and geocells, and field monitoring of blast-induced ground vibrations.

1. Biopolymer Mitigation of Silty Sand Liquefaction (N.V. Dasari, M.D. Tellam, K.K. Gonavaram)¶

Green Soil Stabilization Alternative¶

Chemical grouting and cement stabilization incur substantial carbon footprints and risk groundwater contamination. Dasari et al. investigate natural chitosan (a biopolymer derived from crustacean chitin shells) as an eco-friendly soil stabilizer to mitigate liquefaction in loose silty sands ($FC = 15\% - 25\%$): - Hydrogel Bridging Mechanism: Upon mixing and curing, chitosan forms viscous biopolymer hydrogel filaments that coat granular quartz surfaces and bridge inter-particle pore throats. - Cyclic Resistance Ratio ($CRR_{15}$): Strain-controlled cyclic triaxial testing reveals that treating loose silty sand with $1.0\%$ chitosan increases the cyclic resistance ratio ($CRR$) by up to $180\%$. - Pore Water Pressure Retardation: The rate of excess pore water pressure generation ($\Delta u / \sigma'_0$) slows dramatically, preventing catastrophic loss of effective confinement during earthquake cycles.

       Untreated Loose Silty Sand               Chitosan Biopolymer Treated Sand
       ┌───────────────────────────┐            ┌───────────────────────────┐
       │   ○       ○       ○       │            │   ○───────○═══════○       │
       │     ○       ○       ○     │   ─────►   │   │  Biopolymer   │       │
       │   ○       ○       ○       │            │   ○═══Hydrogel════○       │
       │ (Loose grain contacts)    │            │ (Interlocking bonded webs)│
       └─────────────┬─────────────┘            └─────────────┬─────────────┘
                     │                                        │
             Earthquake Shaking                       Earthquake Shaking
                     ▼                                        ▼
       ┌───────────────────────────┐            ┌───────────────────────────┐
       │ Rapid Δu spike (ru ──► 1) │            │ Damped Δu (ru < 0.40)     │
       │ Shear liquefaction failure│            │ Elastic-plastic integrity │
       └───────────────────────────┘            └───────────────────────────┘

2. Packing Index for Liquefaction Resistance (M.U. Rehman, R.K. Kandasami, S. Banerjee)¶

Beyond Relative Density ($D_r$)¶

Relative density ($D_r$) often correlates poorly with the liquefaction resistance of sands with varying grain angularity and broad particle size distributions. Rehman et al. propose a micro-mechanically informed Packing Index ($I_p$):

\[I_p = \frac{e_{max} - e}{e_{max} - e_{min}} \cdot \left( \frac{d_{50}}{d_{10}} \right)^{\alpha} \cdot \Phi_s\]

where $\Phi_s$ is particle sphericity and $\alpha$ is a gradation coefficient. - Micro-CT Verification: 3D X-ray tomography demonstrates that sands with identical $D_r$ but lower sphericity form higher coordination numbers ($Z_c$) and stiffer contact force arches. - Correlation with $CRR$: The packing index achieves a unified correlation with cyclic resistance ratio across both uniform clean sands and well-graded silty sands ($R^2 = 0.94$).

3. Vibration Screening via Sand-Rubber Mixture (SRM) Trenches (A. Boominathan, J.S. Dhanya, et al.)¶

Dynamic Wave Barrier Mechanics¶

Ground-borne vibrations induced by impact pile driving, high-speed rail lines, and heavy machinery induce structural resonance and acoustic fatigue in adjacent urban structures. Boominathan et al. deploy open vs. infilled wave-screening trenches using Sand-Rubber Mixtures (SRM): - Impedance Mismatch: Shredded scrap tyre crumbs mixed with coarse sand ($30/70$ by weight) create an acoustic impedance mismatch ratio ($\rho v_s$) of less than $0.35$ relative to virgin subsoil. - Rayleigh Wave Attenuation: Numerical 3D finite element and experimental field trials reveal that an SRM trench of normalized depth $H / \lambda_R \ge 0.75$ attenuates Rayleigh wave amplitude by up to $68\%$ in the passive screening zone. - Geotechnical Stability: Unlike open trenches that require shoring or collapse in high groundwater tables, SRM trenches maintain self-supporting vertical trench walls while filtering vibration.

4. Seismic Performance of Disconnected Piled Raft Foundations (A.K. Suman, J.S. Rajeswari)¶

Cushion Layer Seismic Isolation¶

In high-seismicity regions, connecting rigid piles directly to the foundation raft concentrates immense inertial shear forces and bending moments at the pile heads: - Disconnected Cushion Design: A compacted gravel-geogrid cushion layer ($0.5\,\text{m} - 1.0\,\text{m}$ thickness) is inserted between the raft underside and the pile heads. - Inertial Decoupling: Dynamic centrifuge tests and finite element simulations confirm that the cushion layer slips plastically during peak spectral accelerations, bounding the maximum shear transfer into the piles. - Moment Reduction: Pile head bending moments are reduced by $50\% - 75\%$ compared to rigidly connected piled rafts, eliminating brittle shear failures at the raft-pile junction while providing adequate settlement control under static gravity loads.

5. Blast-Induced Vibration Monitoring (A. Anil, T. Naskar, A. Boominathan, A. Joseph)¶

Demolition Dynamic Signatures¶

During controlled blast demolition of heavy industrial chimneys and coal towers, explosive energy generates transient shock waves that propagate through complex geological strata: - Triaxial Velocity Sensors: Orthogonal geophones (transverse, vertical, radial) record Peak Particle Velocity ($PPV$) and frequency spectra. - USBM Attenuation Law: Field data fit the scaled distance equation: $$PPV = K \cdot \left( \frac{R}{\sqrt{Q}} \right)^{-B}$$ where $Q$ is explosive charge per delay (kg) and $R$ is standoff distance (m). - Vibration Control: Real-time array telemetry verifies that vibration frequencies remain above natural structural frequencies ($> 20\,\text{Hz}$), avoiding structural resonance in adjacent plant buildings.

6. Soil Dynamics Instrumentation Matrix¶

Dynamic Phenomenon	Primary Monitoring Sensor	Complementary Instrument	Output Parameter
Seismic Liquefaction Triggering	High-Frequency Vibrating Wire Piezometer	Downhole Accelerometer Pair	Dynamic $\Delta u$, cyclic shear strain $\gamma_{cyc}$
Vibration Barrier Verification	Triaxial Surface Geophones ($4.5\,\text{Hz}$)	Digital Seismograph	Peak Particle Velocity ($PPV$), FFT frequency
Disconnected Piled Raft	Dynamic Earth Pressure Cells	Pile-Toe Sister Bar Strain Gauges	Raft contact stress, pile axial load transfer
Impact Pile Driving Attenuation	Piezoelectric Accelerometers	Laser Doppler Vibrometer	Ground vibration peak acceleration ($a_{max}$)
Subsurface Shear Wave Velocity	Downhole Geophone / Cross-hole Probe	P-S Suspension Logger	Shear wave velocity $V_s$, small-strain modulus $G_{max}$