Homework 2

Project 2 (due 04-17-00)
A 25-ft deep excavation is to be made in a deposit comprised of 18 feet of loose-medium
sand (N=12 blows/ft) overlying medium-dense sand (N=30 blows/ft.) The total unit
weights of the upper and lower sand strata are 118 and 124 pcf, respectively. The
groundwater table is at a depth of 4 ft. Assume a dewatering system will be installed
such that the groundwater level in the excavation is at the base of the excavation. The
dimensions of the excavation in plan are 60 by 80 ft. Both strata are classified as fine
poorly graded to silty sands (SP-SM.)
Prepare preliminary designs for the following steel sheetpile wall alternatives:
A cantilever wall.
An anchor and tieback supported wall.
Each design should specify:
 The recommended steel sheetpile section – use a z-section (see the sheetpile manual
at www.pilebuck.com/charts/pilechts/zpile.html for available z-sections.) For the sheetpiles,
use steel grade ASTM A 328, having a yield stress of 38,500 psi. Base the design on
a working stress equal to 0.6 of the yield stress.
 The depth of wall embedment to provide adequate stability against overturning (in the
case of a cantilever wall) and adequate resistance against piping and heave due to
 The orientation, depth, and spacing of the tiebacks.
 The dimensions of the tieback anchors.
 The total seepage inflow into the excavation which must be handled by the
dewatering system.
Clearly state any assumptions made in the analysis and selected factors of safety.