Montgomery sits on the Selma Chalk formation, a weak limestone that weathers into a stiff, fissured clay locally called 'rotten limestone.' The water table across the city often sits within 15 feet of the surface. For a soft soil tunnel, these two facts change everything. The transition zones between the weathered chalk and the overlying alluvial clays create mixed-face conditions that can stall a TBM for days. Before any alignment is set, we map these transitions using a combination of laboratory strength testing and field stratigraphy. The work feeds directly into the geotechnical baseline report. The 32.3715N, 86.2702W coordinates place most of the metro area within a moderate seismic hazard zone per IBC. That means liquefaction assessment of the loose sands in the alluvial deposits becomes mandatory for any bored tunnel in Montgomery.
In Montgomery, the weathered Selma Chalk loses up to 70% of its intact strength upon contact with water—designing without undrained triaxial data is designing for failure.
How we work
Local considerations
IBC Section 1810 and the FHWA Technical Manual for Design and Construction of Road Tunnels govern the investigation scope for Montgomery. The biggest risk is not the soft ground itself. It is the misinterpretation of the residual strength of the weathered chalk. A designer who uses peak strength values from intact rock cores will under-predict convergence by a factor of two or three. In Montgomery, the weathered chalk behaves like a stiff soil, not a rock. Treating it as rock leads to insufficient support pressure and catastrophic face instability in SEM tunneling. A second risk is sulfate attack on the tunnel lining. The Selma Chalk contains pyrite. Oxidation produces sulfuric acid. Without grain size and chemical analysis, the concrete mix design is a guess. We quantify the sulfate content and specify Type V cement when the environment demands it.
Relevant standards
ASCE 7-22 (Minimum Design Loads for Buildings and Other Structures), IBC 2021 (International Building Code), ASTM D1586 (Standard Test Method for Standard Penetration Test), ASTM D2487 (Standard Practice for Classification of Soils for Engineering Purposes), ASTM D4767 (Triaxial Compression Test for Cohesive Soils), FHWA-NHI-10-034 (Technical Manual for Design and Construction of Road Tunnels)
Associated technical services
Tunnel Alignment Geophysics
Seismic refraction surveys to map the top of the Selma Chalk and identify paleochannels filled with compressible alluvium that would affect TBM performance.
Advanced Laboratory Testing
CIU and CAU triaxial tests on weathered chalk specimens. Consolidation and swell tests on the stiff clays to predict long-term settlement and heave.
Groundwater and Chemical Analysis
Long-term pumping tests combined with water chemistry profiling to assess sulfate and chloride aggressiveness toward concrete and steel linings.
Typical parameters
Quick answers
What is the typical cost range for a geotechnical investigation for a soft soil tunnel in Montgomery?
The investigation cost for a soft soil tunnel in Montgomery typically ranges from US$3,800 for a very preliminary desk study and limited field sampling to US$14,490 for a comprehensive program including deep borings, triaxial testing, and a full Geotechnical Baseline Report.
How deep does the boring need to go for a tunnel under Montgomery?
Borings should extend a minimum of 1.5 tunnel diameters below the proposed invert. In Montgomery, this often means drilling through the entire weathered chalk zone into the more competent limestone, typically reaching a total depth of 80 to 120 feet depending on the cover.
Is the Selma Chalk a soil or a rock for tunneling purposes?
In its weathered state, which dominates the upper 30 to 50 feet in Montgomery, the Selma Chalk behaves as a soft rock or a very stiff soil. It loses significant strength when disturbed or wetted. Our lab program specifically measures the transition from peak to residual strength to classify it correctly for TBM cutterhead design.
What happens if you hit a sand lens in the alluvium during tunneling?
A sand lens in the Montgomery alluvium can hold water under artesian pressure. Encountering one unanticipated can lead to a sudden inflow of water and sand, causing face instability and surface settlement. Our CPT and SPT sampling grid is designed specifically to locate these lenses before the TBM reaches them.