Tunnel Support Pressure
Calculate the support pressure a tunnel lining must resist, pv = γ·Hp, from the rock mass unit weight γ (kN/m³) and the rock load height Hp (m) — typically from Terzaghi's method or geomechanical classifications (RMR, Q-system). Support pressure is the vertical stress the loosened rock zone exerts on the support (shotcrete, steel sets, final lining), and it drives the structural design of the lining. In shallow tunnels the load may be the full overburden; in deep tunnels, arching reduces it to a fraction. Estimating it correctly is decisive: underestimating leads to collapse, overestimating raises cost. Enter the unit weight and the rock load height.
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Tunnel support pressure
The support pressure is the stress a tunnel lining has to resist, pv = γ·Hp, the product of the unit weight of the rock mass γ and the rock load height Hp. That height Hp — the thickness of the loosened rock zone bearing down on the support — comes from Terzaghi's method or from rock mass classifications such as RMR and the Q-system, which estimate support pressure directly from quality indices of the rock mass (rock strength, spacing and condition of the discontinuities, presence of groundwater). Support pressure is the design load of the lining: it sets the thickness of the shotcrete, the section of the steel ribs, the number of rock bolts and the sizing of the permanent lining. There is a subtle piece of physics here: in shallow tunnels, with too little cover to develop an arch, the load may amount to the full weight of the column above; in deep tunnels, arching within the rock mass limits the load to a fraction of it (the height Hp), which is what makes support feasible. Getting this estimate right is decisive and delicate at once: underestimating the pressure leads to failure of the lining and collapse of the tunnel; overestimating it needlessly inflates a job that is expensive by nature. That is why design combines calculation, rock mass classification and, above all, continuous observation of convergence while the excavation advances. Enter the unit weight and the rock load height.
Related Tools
Terzaghi Rock Load Height
Estimate the rock load height over a tunnel crown by Terzaghi's classic method, Hp = Cf·(B + Ht), from the rock load factor Cf (depending on mass quality — ~0 for intact rock to >2 for heavily fractured or swelling rock), the width B and the height Ht of the excavation. Hp represents the loosened rock zone above the tunnel that effectively loads the support — Terzaghi proposed that, due to arching in the mass, only a fraction of the total overburden acts on the lining. This loosening-load model is the historic basis for rock tunnel support design. Multiplying Hp by the unit weight gives the support pressure. Enter the load factor, width and height.
Tunnel Face Pressure (EPB/Slurry)
Estimate the face support pressure needed to stabilize the excavation front of a mechanized tunnel, p = K·γ·H, from the earth pressure coefficient K (at rest K₀ ≈ 1−sinφ, or active), the soil unit weight γ (kN/m³) and the axis depth H (m). In closed-face TBMs (EPB or slurry), the pressurized chamber must balance the earth and water pressure at the front, avoiding both collapse (insufficient pressure) and blow-out (excessive pressure). Face pressure is the most critical operational parameter of a TBM, adjusted in real time per cover, water table and soil type. This gives the earth component; total pressure adds hydrostatic water pressure and a safety margin. Enter the earth pressure coefficient, unit weight and depth.
Tunnel Convergence
Calculate a tunnel's convergence — the relative radial deformation of the excavation, ε = (u ÷ r)·100 — from the radial displacement u (the inward movement of the walls toward the center, measured by extensometers or total station) and the excavation radius r, in the same unit. Convergence is the primary monitoring indicator in NATM (New Austrian Tunnelling Method): it measures how much the rock mass deforms after excavation, reflecting stress mobilization and support effectiveness. Low, stabilized convergence indicates a stable mass; high, growing or accelerating convergence signals squeezing, instability or insufficient support, requiring immediate reinforcement. Enter the radial displacement and the tunnel radius.
Pile Tip Resistance
Calculate a pile's tip resistance, Q_p = q_p·A_p, from the tip stress (bearing capacity) q_p (kPa) and the tip cross-sectional area A_p (m²). Tip resistance is the share of pile capacity from the BEARING of its base on a strong soil or rock layer — the pile acts as a column compressing the soil under its tip, mobilizing that soil's bearing capacity (like a shallow foundation, but at depth). The tip stress q_p is the soil's unit bearing capacity at the tip elevation, estimated by bearing-capacity theories (Terzaghi, Meyerhof, Vesic for piles), SPT correlations (q_p = K·N, with K depending on soil and pile type) or the CPT (cone) test. Times the tip area, it gives the force the tip supports. Tip resistance dominates in piles reaching a firm layer (end-bearing piles), and then the pile is very stiff (settles little). Large-diameter piles (caissons) have large tip areas and mobilize high tip resistance. This share adds to the side resistance for the total capacity. Enter the tip stress and the tip area.
Pile Allowable Load
Calculate a pile's allowable (working) load, Q_adm = Q_ult ÷ FS, from the ultimate bearing capacity Q_ult (kN) and the global safety factor FS. The allowable load is the maximum load that can be applied to the pile in service with adequate safety — obtained by dividing the ultimate capacity (the load that would cause FAILURE of the pile-soil system) by a safety factor covering uncertainties. The pile-foundation safety factor is typically HIGH (FS = 2.0-2.5 for ultimate capacity, higher if based only on theoretical formulas without a load test), reflecting the great uncertainty in determining soil capacity (unseen, heterogeneous and poorly known) and the severity of a foundation failure (which can collapse the whole structure). Codes often require different partial factors for tip and friction (which have different uncertainties), or limit-state methods. The allowable load sets how many piles are needed for the column loads: number of piles = column load ÷ allowable load. Load tests (measuring real field capacity) allow reducing the safety factor and optimizing design. Enter the ultimate capacity and the safety factor.
Mold Cavity Pressure
Estimate the pressure that actually reaches the mold cavity by multiplying the injection pressure (at the screw tip) by the pressure transmission factor, which accounts for pressure losses along the runners, nozzle and gates to the cavity. Typically only 40–60% of the machine pressure reaches the part. It is the cavity pressure that defines the clamping force and fill quality. Enter the injection pressure and the transmission factor.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.