At pipe jacking (microtunneling)the chamber pressure must balance the effective earth pressure plus the pore pressure to the front elevation; it is then adjusted on-site with the land balance, the thrust/torque and the auscultation. The choice between EPB y hydro-shield/Slurry is given by permeability, granulometry y water table; The conditioning (foams, bentonite, polymers) is calibrated to ensure that stable support, transportability y settlement control.
1) Starting point for chamber pressure
Initial model: pcaˊmara≈σv,front′+uwp_text{camera} \approx \sigma’_{v,text{front}} + u_w, with corrections for coverage, overloads y overcut; The operating level is around this value with fine margins (i.e. ±0.05-0.20 bar), depending on the response of the terrain and the sensitivity of the environment. To contrast calculation criteria and operational limits, you can rely on the DAUB Recommendations pressure front in soft soils and in the BTS/ICE Specification for Tunnelling (3rd ed.), which include good operating and control practices.
Field validation: search balance ≈ 1, stable pressures and agreement with piezometers/ascimeters/topography.
2) EPB or hydro-shield (quick decision).
EPB → silts/clays and fine-medium sands (low-medium permeability). Stabilizes the front with the own conditioned detritus.
Hydroshield/Slurry → clean sands and gravels (high k) and High NF; maintains the support with pressurized suspension and formation of filter cake. A clear public guideline on ground control in EPB is the document of the Hong Kong Government (CEDD), useful for calculation and operation audits.
3) Conditioning and signs of good performance.
EPB (plastic front and pumpable)
Additives: foams (FIR/FER) + polymers/bentonite to lower torque, seal and give thixotropy.
Indicators OK: stable pressure, pushes/torque without peaks, homogeneous return, balance ≈ 1.
Typical settings: dry material/trees → up dosage; pressure losses → improve sealing y lubrication perimeter.
Hydroshield/Slurry (stable cake and pressure transmission)
Suspension: bentonite (± polymers) with density y viscosity objective; control of level y leakage loss.
Indicators OK: stable pressure transmission, consistent flow rates, low seepage and controlled settlements.
Typical settings: losses → increase pressure/density and optimize cake; unstable return → check circuit, desanding y rheology.
4) Quick table for saturated and mixed soils
| Dominant condition | Preferred method | Chamber pressure keys | Main conditioning |
|---|---|---|---|
| Saturated fine sands (low-medium k) | EPB | p≈σ′+uwp ’p≈σ′+uwp \approx \sigma' + u_w; stability by compaction | Foams + polymers (sealing/plasticity) |
| Clean sands / gravels (high k) | Hydroshield | p≳σ′+uwp \gtrsim \sigma’ + u_w to avoid inflow | Bentonite + density/viscosity control; filter cake |
| Mixed soils (alternating strata) | EPB mixed o Slurry | pp variable by stratum; monitor transitions | Dynamic foam/mud adjustment; mixed modes |
| NF very high / sensitive environment | Hydroshield | Fine control of pressure level/tank | Very controlled density and seepage loss |
(k: soil permeability)
5) Good practices for not inducing settlements
Set-points and thresholds (±Δp) defined before start-up; smooth ramps when varying pressure.
Auscultating and closing the balance sheet (excavated volume ↔ return).
Rheology management (foams/sludge) and lubrication according to the stratum.
Contingencies: pressure loss → stop, seal, inject, resume; overpressure → controlled relief and deformation check.
6) What to send for us to fine-tune your set-up
Stratigraphy with k y NF, DN/L/Rmin, and overloads.
Environmental restrictions (window, turbidity/discharge limits).
Sensitive environment (infrastructure, services, coastline).
If you are interested in a initial operation plan with pressure curve and additive dosing, our equipment for the Technical assistance and engineering can prepare it from your surveys and plotting; to start the valuation, request offer.
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