How to dimension the attack and reception shafts: admissible thrust, hoisting and site logistics?

Q: How to dimension the attack and reception shafts: admissible thrust, hoisting and site logistics?

Well sizing aims to accommodate the equipment, safely transmit thrust and operate with smooth logistics. The design thrust (Ed) integrates face pressure, perimeter friction and bending/rolling losses; the well structure (annulus, screens and slab) must be verified with an adequate safety coefficient. In hoisting, combined loads (TBM, carriages and pipe sections), certified points and clearance height are considered. Logistics defines accesses, sludge plant, energy, ventilation and lighting, with segregation of areas and HSE plans. Guiding rules: usable width ≈ TBM + 1.0-1.5 m per side in attack; in reception ≈ DN + 0.6-1.0 m; usable frame length 6-10 m plus assembly area; flotation/siphoning verification in high NF and minimum documentation: thrust calculation, lifting plan, logistic layout, HSE plan and as-built.

The well is sized for to house the equipment, to reliably transmit the thrust y operate smoothly (assembly, lifting, detritus/sludge management and access). In projects of pipe jacking (microtunneling), The design of wells conditions the viability and performance from the first meter.

1) Allowable thrust (attack shaft)

Design thrust (Ed):
Ed ≈ F_front (excavation pressure × area) + F_friction (τ perimetral × contact surface) + losses (curvature, joints, local friction). As an early guide, τ can be estimated between 5-20 kPa according to soil and lubrication.
Well resistance: thrust rings/crown, screens y bottom slab must resist Ed × γ (γ 1.3-1.5).
Useful geometry: width = TBM + 1.0-1.5 m per side; length = push frame (6-10 m) + mounting area (≥ 6 m).
Hydraulic stability: check flotation, siphoning y punching with high NF.

For the construction detail of the pit and shoring, you can enlarge at Vertical wells.

2) Lifting and maneuvering

Lifting load (Q): TBM + trolleys + pipe sections + tools (consider sludge/water).
Certified equipment and points: beams, eyebolts, slings (coef. ≥ 7) and clear height sufficient for vertical descents without interfering with crowning.
Wellhead: hoisting light major components y pipe sections repetitive with safe guidance.

3) Site logistics (flow and safety)

Access: turning radii, truck bed and functional separation (sludge plant, energy, pipe stockpiling).
Sludge/detritus management: decanting, recirculation or transport; watertight wall bushings; bypass emergency.
Ancillary services: installed power (peaks of pushing and pumping), process water, ventilation and lighting.
HSE: handrails, lifelines, confined atmospheres, plan, plan for vertical rescue and of hydraulic emergency.

4) Quick sizing rules (indicative; validate in engineering)

Variable	Attack well (MT/DP)	Reception well
Useful width/diameter	TBM + 1.0-1.5 m per side	DN + 0.6-1.0 m
Useful length	Push frame 6-10 m + reinforcement zone ≥ 6 m	Safe receiving area + protective screen
Bottom level	≥ drive axle elevation - clearances	≥ axle height with receiving berm
Permissible thrust	≥ Ed × γ (γ 1.3-1.5)	Impact resistance/reception
Clearance height	Hook travel for Q for lifting	Removal of elements and safety

The values depend on DN, L, Rmin, geotechnics, NF, The following information, method (microtunnel/Direct Pipe), equipment and specifications are available.

5) Three reference scenarios

Urban, DN 1200, L 180 m, saturated sands: Rectangular well shored with anti-flange slab; Ed high (front pressure + friction); dedicated sludge plant.
Crossing under highway, DN 600, L 90 m, silt with gravel: prefabricated circular shaft; moderate thrust; priority to maneuverability e hoisting frequent use of tubes.
Large DN 2200, L 240 m, competent clays: wide and short well for assembly; reinforced lifting points and crowning suitable for heavy cranes.

6) Minimum documentation required

Thrust calculation (front + friction + losses) and structural verification of rings/screens/slab.
Lifting plan (load matrix, radios, crane, certified accessories).
Logistics Layout (access, sludge plant, energy, ventilation, lighting).
HSE Plan (confinement, rescue, siphoning, falls to different levels).
As-built of wells and certificates of tests/lifting elements.

For normative criteria and European best practices for driving and related techniques, it is useful to consult the guide DWA-A 125E (Pipe Jacking and Related Techniques)

If you prefer a joint revision of your design, our team of Technical assistance and engineering can propose the dimensioning of the well and the layout if you already have a baseline data, you can request a quotation with the layout, geotechnics and DN/L foreseen.