A mechanically stabilized earth (MSE) wall is needed to retain a
10-m-high earth fill. Geogrid is used as reinforcement. Lab testing
shows the ultimate tensile strength of the geogrid is 80 kN/m, and
the external friction angle between the backfill and the geogrid is 30°.
The earth fill material will be compacted at 95% of its maximum dry
unit weight (dmax = 21 kN∕m³) at the optimum water content. Under
typical field conditions, the moisture content of the backfill is 5%.
Direct shear tests indicate the effective friction angle of the backfill is
30 degrees, and the effective cohesion is zero. No surcharge is on the
backfill. Use reduction factors of 1.5, 2.5, 1.2 for installation damage,
creep, and chemical/biological degradation. The foundation soil
beneath the MSE wall is silty clay; its moist unit weight in its natural
condition is 19 kN∕m³, the internal friction angle is 10 degrees, and
the cohesion is 25 kN∕m². Between the geogrid and the foundation
soil, the external friction angle is 9 degrees, and the adhesion is
22 kN∕m².
Determine:
(1) How many layers of geogrid are needed and the vertical spacing
of each layer.
(2) Geotextile embedment length of each layer.
(3) Geotextile overlap length of each layer.
(4) The external stability in terms of overturning, sliding, and bearing
capacity of the MSE wall you designed in (1)–(3).