Inizio questo argomento con una ricerca commissionata a Copilot-GPT5 in modalità deep research, che allego in PDF. È un'interessante sintesi dello stato della pratica, ma mi lascia leggermente insoddisfatto per quanto attiene ai dettagli, che comunque approfondiremo a volte ed esageratamente in questo specifico thread. L'aspetto positivo di questo summary è che ricorda che trascurare la decompressione potrebbe essere anticautelativo, specie in presenza di coesivi e falda e altezze di scavo considerevoli (swelling, heave, buoyancy).
In terreni granulari, credo che i citati effetti non governino, ma di nuovo, possiamo scatenarci con tutti i dettagli pensabili ed impensabili in questo thread.
Alla data delle ricerche, 21/09/2025, lo stato della pratica di considerare una Qnet per i cedimenti appare legittimo, ma bisognerebbe valutare più nel dettaglio aspetti quali la variazione del modulo elastico in funzione del disturbo del terreno, della decompressione e della ricompressione. Inoltre, aggiungo io, qualche autore suggerisce di non utilizzare la Qnet anche in presenza di scavo, avendo osservato l'andamento dei cedimenti reali dopo la costruzione (a proposito però di cedimenti edometrici, con la variabile tempo non azzerata). Altro aspetto importante: si presume che il tempo di costruzione della struttura sia (relativamente) breve, altrimenti avremo un rigonfiamento (heave) che configurerà un nuovo piano di posa più elevato e il cedimento non potrà essere più basato sul Qnet.
Ultima modifica di mccoy; 21/09/202519:11.
"Data speak for themselves" -Reverend Thomas Bayes 1702-1761 P(Ai|E)=(P(E|Ai)P(Ai))/P(E)
Continuiamo il discorso. Io sto interrogando le AI su questo argomento.
In essenza, personalmente mi oppongo alla prassi generalistica dei libri di testo, i quali affermano che, sul fondo di uno scavo, ai fini del calcolo dei cedimenti, bisogna adottare Qnet, ossia una pressione dovuta alla costruzione, però depurata dalla pressione esercitata dalla colonna di terreno rimosso.
Nel caso di cedimenti immediati, e quindi praticamente sempre in terreni granulari e roccia, abbiamo che modellando il terreno stesso come una molla elastica monodimensionale:
- Dopo l'esecuzione dello scavo la molla si decomprime, spostando il fondo scavo verso l'alto (cedimeno negativo= rigonfiamento) - Dopo la ricostruzione completa della struttura, la molla si ricomprime, però partiamo sempre dal livello zero, che in questo caso è il livello di fondo scavo. - La pressione che agisce sulla molla dopo la ricostruzione è sempre quella lorda dell'edificio, senza depurazione dell'effetto dello scavo. - Il cedimento deve essere calcolato a partire dal nuovo livello zero, ossia dal fondo dello scavo. - Pertanto, non avviene alcuna compensazione dei cedimenti in questo modello reale. Il carico che agisce effettivamente sul fondo scavo è il carico effettivo, non quello netto o depurato del terreno rimosso.
Adesso, bisogna ovviamente perfezionare il modello e vedere cosa succede con la ricompressione, in teoria. Con la decompressione la molla perde rigidezza, ma poi la riacquiasta. A questo effetto si sovrappone la maggiore rigidezza del ciclo di ricompressione (abbiamo un effetto locale di sovraconsolidazione). L'effetto totale qual è?
Su questo punto ho effettuato delle interrogazioni iterative a GPT5, che conserva la memoria delle precedenti domande e simulazioni.
Siamo alfine pervenuti a questo modello. Qualsiasi obiezione, dubbio e considerazione è benvenuta.
Certamente, utilizzare un Qnet è pura fantascienza. LA costruzione comunque mantiene la sua forza peso, non si alleggerisce in virtù della rimozione del carico litostatico. L'utilizzo di Qnet è un artificio ceh compensa altri meccanismi, ma un artificio che può diventare pericolosamente otimistico.
Ultima modifica di mccoy; 21/09/202511:02.
"Data speak for themselves" -Reverend Thomas Bayes 1702-1761 P(Ai|E)=(P(E|Ai)P(Ai))/P(E)
Single vertical spring represents the compressible subsoil beneath the raft. Eref = 15 MPa (pre‑excavation secant modulus). pref = pre‑excavation overburden at z = hexc + 0.5 m (i.e., γ′·(hexc + 0.5)). Excavation unloads the spring to p1, construction applies the gross foundation pressure Q so p2 = p1 + Q. Stress law E(p) = Eref·(p/pref)n with n = 0.5. Recompression multiplier Kr models stiffer reload branch. Disturbance factor φ reduces baseline stiffness where appropriate.
Governing expressions
pref = γ′ · (hexc + 0.5)
p1 = γ′ · 0.5 (post‑excavation confinement at z = h + 0.5; enforce p′min as needed)
Common: p1 = 10 kPa (γ′·0.5; enforce p′min = 10 kPa). Choose Kr = 1.2. Two disturbance cases: φ = 1.0 (no disturbance) and φ = 0.7 (30% reduction).
hexc (m)
pref (kPa)
p1 (kPa)
p2 = p1+Q (kPa)
Eunloaded (MPa) (φ=1)
Eloaded (MPa) (φ=1)
Erecomp (MPa) (Kr=1.2)
3.0
70
10
110
5.67
18.81
22.57
4.0
90
10
110
5.00
16.58
19.90
5.0
110
10
110
4.52
15.00
18.00
hexc (m)
Eunloaded (MPa) (φ=0.7)
Eloaded (MPa) (φ=0.7)
Erecomp (MPa) (φ=0.7)
3.0
3.97
13.17
15.80
4.0
3.50
11.61
13.93
5.0
3.17
10.50
12.60
Physical interpretation (summation of effects)
Decompression: excavation reduces p → E drops (Eunloaded ≪ Eref).
Gross re‑confinement: applying the gross load Q raises p to p2 and restores stiffness to Eloaded; if p2>pref Eloaded can exceed Eref.
Recompression: real soils often reload on a stiffer branch; Kr models this (Erecomp = Kr·Eloaded).
Disturbance: excavation can reduce Eref (φ<1) and offset recompression benefits.
Overall: choose Edesign from Eunloaded, Eloaded, Erecomp with conservative choices for φ, Kr, and n; compute settlements with layered methods or distributed springs.
Practical implementation steps
Measure Eref in situ at zref = h + 0.5 where possible.
Set pref = γ′·(h + 0.5). Choose φ to reflect expected disturbance (e.g., 1.0, 0.7, 0.5).
Compute p1 (post‑cut) and p2 (post‑cut + gross Q). Enforce p′min (5–20 kPa) to avoid zero stiffness.
Compute Eunloaded, Eloaded, and Erecomp (choose n = 0.4–0.6; Kr ≈ 1.0–1.5).
For settlement use Edesign (best estimate) and run sensitivity cases with reduced φ and Kr.
Where critical, calibrate φ and Kr with plate tests and use staged FEM for final design.
Critical appraisal and cautions
Critical appraisal and self‑reflection
Conceptual strengths
The model captures the primary physics: unloading reduces stiffness; gross loading can re‑constrain and increase stiffness; reloading can be stiffer than virgin loading; disturbance reduces all moduli.
Using pref tied to the pre‑excavation overburden at the key depth makes Eref empirically meaningful and isolates excavation influence.
The approach is simple, transparent, and easy to implement for rapid design iterations and sensitivity studies.
Main limitations and cautions
Scalar stress law ignores stress anisotropy and directional stiffness changes; excavation changes horizontal stresses too, and stiffness is tensorial in reality.
The power‑law scaling and choice of n are empirical; values vary with site, density and testing method — calibrate where possible.
Recompression multiplier Kr is a crude model of hysteresis; real unloading/reloading behaviour can be nonlinear, path dependent, and irreversible. Kr should be calibrated to unload/reload lab tests or plate tests.
Disturbance factor φ is also empirical and spatially variable; treating it as uniform risks masking local weak spots.
Applying gross Q entirely at the shallow evaluation depth is conservative for that depth but may misrepresent vertical load distribution; use influence factors or layered elastic solutions for more realistic load partitioning.
For cohesive soils the undrained immediate stiffness and pore pressure effects dominate, so this effective‑stress based scaling must be adapted for undrained modulus and short‑term behaviour.
Risk mitigation
Always run sensitivity analyses for φ (e.g., 1.0, 0.7, 0.5) and Kr (1.0–1.5) and for alternative n (0.4–0.6).
Use layered models (multiple springs) under the raft to capture depth variation and load spreading; calibrate with one or two plate/load tests after excavation if possible.
Where excavation is large (p1 very small) or q large relative to pref, prefer staged numerical modelling with an unloading/reloading capable constitutive law.
Short recommendations
For preliminary design use the schema above to produce E_unloaded, E_loaded and E_recomp profiles and then perform settlement calculations with conservative and best‑estimate parametric runs.
For final design calibrate φ and Kr with field tests and perform staged FEM analysis to capture anisotropy, arching and stress path effects.
Avoid relying on a single scalar modulus; present a band of predicted settlements with clear assumptions about disturbance and recompression.
Ultima modifica di mccoy; 21/09/202508:58.
"Data speak for themselves" -Reverend Thomas Bayes 1702-1761 P(Ai|E)=(P(E|Ai)P(Ai))/P(E)
Adesso, sono cosciente che la mia sfida è eretica e quasi improponibile perché riguarda i sacri canoni della geotecnica. Però, ho sottoposto la controsfida a Grock, che ha difeso i sacri canoni, e poi ho dato in pasto la risposta di Grock (che ha un limite se free) a GPT5. Il risultato è interessante.
Citazione
Short verdict Use of net stress (Qnet = Qgross − overburden removed) is the correct starting point for elastic, increment‑based settlement calculations in frictional soils, but it must be applied with care: Qnet alone does not guarantee zero settlement because of stress‑path effects, hysteresis, redistribution and site specifics.
Why Qnet is the correct starting point
Elastic incremental formulations depend on stress increments (Δp), not absolute gross loads; standard methods (Schmertmann, CPT‑E, influence‑factor summation) explicitly use Δp (net pressure) for settlement prediction.
If excavation fully removes the pre‑existing overburden and the structural contact pressure exactly restores the original pre‑excavation effective stress at every point beneath the footing, elastic theory predicts no further elastic compression at those points (no positive Δp).
Why Qnet alone can be misleading in practice
Unloading is not perfectly reversible: large decompression can cause irreversible fabric change and loss of contact stiffness; reloading may produce plastic strains even if incremental Δp = 0.
Redistribution and load spreading: a raft’s gross load spreads with depth; parts of the soil volume will experience non‑zero Δp even if a single shallow point shows Qnet ≈ 0. Settlement is an integral volume response, not a pointwise equality.
Construction and transient loads: temporary cranes, stockpiles, traffic and vibrations introduce additional positive Δp during works even when design Qnet = 0.
Cohesive or partially saturated layers: undrained response, excess pore pressures and consolidation mean net effective stress accounting alone does not capture time‑dependent settlements.
Minimum confinement and arching: shallow post‑cut zones retain some confinement from arching or face support; assuming zero residual confinement is unrealistic and p′min or Emin must be chosen.
Empirical/parameter issues: Δp‑based methods require representative moduli Es or G that depend on strain level and stress history; using small‑strain E without adjustments can mislead.
How to apply Qnet correctly and safely (practical recipe)
Use Qnet as the incremental input in elastic/CPT methods (Schmertmann, CPT‑E, influence‑factor summation), but distribute load with depth using influence factors Iz and layer discretisation rather than applying Qnet at a single shallow depth.
Account for unloading/recompression: reduce reload modulus or adopt a recompression law when excavation produces significant decompression; treat reload stiffness separately from virgin stiffness and calibrate to U–R oedometer or plate tests.
Include a disturbance factor φ and/or minimum confinement p′min (e.g., 5–20 kPa) to prevent unrealistically low stiffness near faces; run sensitivity cases (φ = 1.0, 0.7, 0.5).
Check elastic settlement sums from Δp together with potential plastic/rearrangement components where the stress path crosses yield boundaries (use OCR, CPT or lab data to estimate plastic parts).
For profiles with cohesive horizons or low permeability, complement immediate elastic checks with consolidation analysis and staged pore‑pressure dissipation modelling.
Validate predictions with at least one field test (plate load or instrumented trial) after staged excavation and adjust design moduli accordingly.
Final synthesis Qnet is the rightful elastic driver in standard settlement procedures and is widely used. However, in an excavation → construction sequence you must not treat Qnet = 0 as proof of zero settlement. Excavation alters the soil state and stiffness, introduces redistribution and path‑dependence, and can produce irreversible effects not captured by net‑stress algebra. Use Qnet together with influence‑factor depth distribution, recompression adjustments, disturbance factors and validation testing to obtain safe, realistic settlement predictions.
Reference hint Schmertmann method and influence‑factor settlement formulations, CPT‑E procedures, and standard elastic influence‑factor summations (consult original Schmertmann, Mayne / Poulos literature).
Ultima modifica di mccoy; 21/09/202511:56.
"Data speak for themselves" -Reverend Thomas Bayes 1702-1761 P(Ai|E)=(P(E|Ai)P(Ai))/P(E)
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:
Code
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.
Ultima modifica di mccoy; 22/09/202514:32.
"Data speak for themselves" -Reverend Thomas Bayes 1702-1761 P(Ai|E)=(P(E|Ai)P(Ai))/P(E)
Ho sottoposto una simile domanda a GPT5, il quale mi ha fornito una lunga trattazione, ma le conclusioni sono simili alle precedenti.
Citazione
Practical summary For permeable silty‑sand with metre‑scale drainage to the open excavation, expect most heave and pore‑pressure equilibration at the bottom within minutes to a few hours.
Expect equilibration times to increase rapidly with drainage length squared and with decreasing permeability; delays to many hours or days appear when k ≤ 1·10−6 m/s or L is several metres.
Bottom behaviour is strongly controlled by drainage geometry, presence of low‑permeability lenses, groundwater, temporary slabs/support and the sequence of reloading; always run sensitivity bounds and verify with in‑situ monitoring.
"Data speak for themselves" -Reverend Thomas Bayes 1702-1761 P(Ai|E)=(P(E|Ai)P(Ai))/P(E)
