Home GeoForum Forums Geologia & Professione Geotecnica Il thread sulle fondazioni compensate

Adesso, ho sottoposto a Grock il quesito, quanto tempo ci vuole per un fondo scavo in limo sabbioso permeabile a rigonfiarsi? Il responso è 'NIENTE'. Il modello fisico, in mancanza di fenomeni di consolidazione in funzione del tempo, appare essere breve in relazione ai tempi di costruzione. Per cui, non sarebbe corretto utilizzare Qnet come carico di progetto...

In permeable silty-sand soils (e.g., classified as SM in the Unified Soil Classification System, with typical permeability k ranging from 10^{-3} to 10^{-5} cm/s), heaving after excavation is primarily an immediate elastic rebound due to unloading, with any time-dependent component tied to the dissipation of negative excess pore-water pressures generated during rapid excavation. Complete heaving typically occurs within minutes to hours, as the high permeability allows for quick drainage and pore pressure equalization, making the process essentially instantaneous for practical geotechnical design purposes in most cases. This contrasts with low-permeability cohesive soils, where dissipation can take days to months.

Key Factors Influencing Time

• Elastic vs. Time-Dependent Heave: The initial heave is elastic and occurs immediately upon stress relief from excavation. Any delayed component arises from negative excess pore pressures (suction) dissipating as water flows into the unloaded zone, similar to an inverted consolidation process. In permeable silty-sand, this dissipation is rapid due to high hydraulic conductivity.
• Coefficient of Consolidation (c_v): Time estimates are based on Terzaghi's one-dimensional consolidation theory, where time t for 90% dissipation (U=90%, time factor T_v ≈ 0.848) is given by:
  
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  t = \frac{T_v \cdot d^2}{c_v}
  

  Here, d is the drainage path length (typically 0.5 to 5 m for shallow excavations of a few meters depth, assuming double drainage to the excavated surface and underlying permeable layers), and c_v is the coefficient of consolidation.
  • Typical c_v for silty-sand: 10^{-5} to 10^{-3} m²/s (or approximately 0.001 to 0.1 cm²/s), based on empirical ranges for silty to fine sandy soils.
  • Example Calculation: For d = 2 m (e.g., half-thickness of a 4 m influenced layer) and c_v = 5 × 10^{-4} m²/s (mid-range for silty-sand), t ≈ (0.848 × 4) / 5 × 10^{-4} ≈ 6,784 seconds ≈ 1.9 hours.
  • For lower c_v (10^{-5} m²/s, toward siltier end), t ≈ 94 hours (≈4 days); for higher (10^{-3} m²/s), t ≈ 0.9 hours.
• Excavation Rate and Conditions: If excavation is rapid, undrained conditions may prevail initially, generating negative pore pressures that dissipate post-excavation. Slower excavation allows partial drainage during the process, reducing post-excavation time. Groundwater presence below the excavation can extend times slightly due to seepage, but in permeable silty-sand, this is still short (hours).
• Site-Specific Influences: Factors like anisotropy, small-strain stiffness, and excavation geometry can affect timing, with numerical models (e.g., PLAXIS using Soft Soil Creep) showing sensitivity to permeability—higher k (as in silty-sand) accelerates completion. Field observations indicate that for slopes or cuts in similar soils, full dissipation can match the time to failure (hours to days), but for stable shallow excavations, it's faster.

In geotechnical practice, for permeable soils like silty-sand, heaving is often assumed complete immediately after excavation, as delays are negligible compared to construction timelines. Site-specific CPT dissipation tests or lab oedometer tests are recommended to refine estimates.