Under ASCE 7-22 and IBC 2021, tunnel projects in San Bernardino must classify ground conditions beyond standard site class. The city sits atop the San Bernardino Valley aquifer system, where soft alluvial soils and interbedded clay layers create unique deformation challenges for deep excavations. We perform geotechnical analysis for soft soil tunnels combining borings with SPT and CPT soundings to define stratigraphy down to 60 m, then run triaxial CU tests to extract undrained shear strength parameters for numerical modeling. For shallow cover sections we complement with corte directo to evaluate interface friction along the tunnel crown.
In San Bernardino, thin sand lenses within clay sequences can trigger sudden pore pressure drops during tunnel excavation — dissipation testing catches that.
Approach and scope
One pattern we see regularly in San Bernardino is the presence of thin sand lenses within thick clay sequences near the Santa Ana River corridor. These lenses behave as drainage paths during construction but can trigger sudden pore pressure drops. Our approach integrates dissipation tests during CPTu soundings to estimate consolidation coefficients in real time. We also run resonant column tests on undisturbed samples to capture shear modulus degradation curves — critical for seismic tunnel lining design. The analysis package includes:
Stratigraphic profiling with SPT/CPTu
Triaxial CU and CK0U cycles
Consolidation parameters for long-term settlement
Seismic site response (Vs30 via MASW)
This data feeds directly into PLAXIS 2D or FLAC models for sequential excavation analysis.
Technical reference image — San Bernardino
Site-specific factors
A 1.5 km stormwater tunnel near the San Bernardino Metrolink station encountered a 4 m thick organic clay layer at 12 m depth. The contractor had designed for a uniform clay profile, but the organic zone had Su below 20 kPa and high compressibility. Without pre-investigation with our geotechnical analysis for soft soil tunnels, the TBM would have experienced excessive face loss and surface settlement exceeding 50 mm. We now recommend shear wave velocity profiling along the entire alignment to map these soft pockets before tender.
What is the typical cost range for a geotechnical analysis for soft soil tunnels in San Bernardino?
For a 1.5 km tunnel alignment with 6 boreholes and full lab testing, the cost ranges between US$4,530 and US$15,980 depending on required depth, number of CPTu soundings, and numerical modeling scope. Budget about US$8,500 for a standard package.
Which soil types pose the highest risk for tunneling in San Bernardino?
Organic clays and highly plastic alluvial clays (CH) with undrained shear strength below 30 kPa are the highest risk. They cause excessive face pressure and long-term consolidation settlements. Interbedded sand lenses also create localized water inflow during excavation.
What is the difference between SPT N-value and undrained shear strength for tunnel design?
SPT N-value is a relative density measure for cohesionless soils, while undrained shear strength (Su) is the actual resistance of cohesive soils under undrained conditions. For soft clays in San Bernardino, we rely on Su from triaxial CU tests because SPT in soft clays can yield unreliable N-values due to disturbance.
Do you provide instrumentation for monitoring during tunnel construction?
Yes, we design and specify surface settlement points, inclinometers, and vibrating wire piezometers along the tunnel alignment. The monitoring data feeds back into observational method adjustments, especially in the soft clay zones near the Santa Ana River.
