How do I define water above ground level?

1. Open the Excavation page of the define box by double clicking the margin beside the main diagram.
2. Enter a negative value for the active or passive water depth.

How is water modelled in cohesive soils?

• Notation

Pressure is calculated as outlined in the British Steel Piling Handbook (7th Edition), Ch. 4 (BSPH) and the Pile Buck Steel Sheet Piling Design Manual. Different active and passive pressure coefficients are used for the Rankine and Coulomb models.

The net pressure is calculated by finding the sum of the active and passive earth and water pressures:

• Earth pressure

Active and passive earth pressures are given by:

• p_a = g k K_a - K_ac C
• p_p = g h K_p + K_pc C

It is recommended that a minimum standard surcharge of 10kN/m&Mac178; be applied to the surface of the retained soil in the design of all retaining walls (Ref: BS 8002: 1994). This is the default in SPW911, but the value may be changed in the Excavation page of the define box. Additional loading due to vehicle movement, foundations, etc can be modelled by increasing the standard surcharge.

• Water pressure

Water pressure at any depth, d, is given by:

p_w = (d - d_w) g_w

In the earth and water pressure equations:

Granular soils (C = 0;f > 0): bulk and submerged densities are used above and below the water table respectively. Water pressure is then calculated within each soil layer.

Cohesive soils (C >0;f = 0): bulk density is used above and below the water table. The limit of water pressure in cohesive strata is assumed to be the depth at which the water pressure equals the earth pressure.

Mixed soils (C > 0;f > 0): bulk and submerged densities are used above and below the water table respectively. Water pressure is applied within each mixed soil layer.

• Minimum equivalent fluid pressure (MEFP)

The calculated net pressure can have a negative value when soils with high cohesions exist in an excavation, and the pressure is then assumed to be zero.

In such cases, allowance should be made for the intrusion of water between the soil and the sheet piles by applying full hydrostatic pressure. Full hydrostatic pressure may be selected in the Soils page of the define box.

Where ground water is unlikely to be present, an alternative substitution for zero net pressure in cohesive soils is to assume a pressure due to an equivalent fluid of density 5kN/m³ (Re

The MEFP is applied after all other pressure sources have been applied. The MEFP can be ignored by setting the minimum fluid density to 0kN/m².fon

• Tension Cracks

Water may penetrate to considerable depths in cohesive soils via tension cracks. These may open in the immediate short term to a depth given by: (2C - Q)/g

Tension cracks in cohesive soils may be defined to a specified depth in the Soils page of the define box. When applied, the net active pressure is the maximum of calculated soil pressure and water pressure within the tension crack , and the maximum of calculated soil pressure and MEFP below the tension crack.

• Passive softening

Passive softening can occur when soft cohesive soils exist at dredge or excavation levels (Ref: BSPH, pg. 5/8). Passive softening can be switched on/off in the Soils page of the define box.

If passive softening is applied, then the cohesion of the passive side is assumed to increase linearly from zero to the full value over the passive softening depth entered. The default value of the passive softening depth is 1m, but this may be varied.

Note: In some cases, the net pressure above excavation level can be negative (eg. when the passive water level is less than the excavation depth). In such instances, the net pressure is assumed to be zero.

On Page 34 of the Pile Buck Steel Sheet Piling Design Manual, Fig. 12b shows the net water pressure on sheet piling with varying water levels as set forth by Terzaghi. Does SPW 911 treat water pressure in this same way?

SPW911 does not exactly follow the method shown on pg. 34, but it can give effectively the same result if appropriate settings are chosen. If anything, the software probably predicts greater pressure which will result in safer designs. More detailed information on this subject can be found in Sheet Pile Design by Pile Buck.

Should minimum fluid density be set to 0 if full hydrostatic  pressure is used?

Set to 0 to ignore min. fluid pressure, set to 9.81kN/m² (62.4 psf) to apply full hydrostatic in cohesive soils.

"The Fine Print" for SPW 911

The information, including technical and engineering data, details, procedures and results, presented in this program have been prepared in accordance with recognized contracting and/or engineering  principles, and are for general information only.  While every effort has been made to insure its accuracy,  this information should not be used or relied upon for any specific application without independent  competent professional examination and verification of its accuracy, suitability and applicability, by a licensed professional engineer.  Anyone making use of this material does so at his/her own risk and assumes any and all liability resulting from such use.  The entire risk as to quality or usability of the data contained within is with the user.  In no event will Pile Buck^® International, Inc. be held liable for any damages including lost profits, lost savings or other incidental or consequential damages arising from the use of or inability to use the information contained within.  By continuing, the user of this program hereby agrees that the user has read the terms and conditions outlined in the attached "End User License Agreement" and "Disclaimer of Warranty and Limited Warranty" and agrees to be bound by the terms and conditions mentioned therein.  If the user is not in agreement with the terms and conditions outlined in the attached "End User License Agreement" and "Disclaimer of Warranty and Limited Warranty" in its entirety please return this program immediately for a complete refund of the original purchase price.

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