Geogrid Calculator for Reinforced Soil Walls

A mechanically stabilized earth wall resists earth pressure not by mass but by transferring it into horizontal layers of tensile reinforcement. Internal design starts from the lateral stress at each layer, sigma_h = Ka (gamma z + q), where Ka is the Rankine active coefficient, gamma z the overburden at depth z and q any uniform surcharge. The tension in a layer is that stress times its tributary vertical spacing, Ti = sigma_h x Sv, so spacing and required strength trade off directly. Spacing is capped near 0.3 to 0.6 m because each layer must sit between compaction lifts, and traffic surcharge is commonly taken as 10 to 20 kPa. Allowable strength is ultimate tensile strength divided by a combined reduction factor covering creep, installation damage and durability, typically between 2 and 7; uniaxial wall geogrids run about 20 to 200 kN/m ultimate. Each layer must also anchor. Total length is the width of the active Rankine wedge, roughly (H - z) tan(45 - phi/2), plus the embedment beyond the failure plane needed to mobilize pullout friction on both faces. Practice sets floors: FHWA and AASHTO guidance sets a minimum reinforcement length near 0.7 times wall height and minimum embedment near 1 m. Backfill quality is a design item: granular fill of at least 34 degrees friction angle, fines below 15 percent, particle size under 75 mm. External stability is checked separately, conventionally near 1.5 for sliding, 2.0 for overturning, 2.5 for bearing and 1.3 to 1.5 for global slip. The analysis is static and omits seismic loading and groundwater.

Reinforced soil as an engineered system dates to the work of Henri Vidal, a French engineer who in the early 1960s developed and patented the technique he called Terre Armee, or Reinforced Earth, using galvanized steel strips laid in compacted granular fill and attached to precast concrete facing panels. The concept spread quickly for highway embankments and abutments because it built tall walls with modest foundations and simple repetitive labour, and the first United States installations followed on California highway projects in the early 1970s. Polymer reinforcement changed the economics again. Work by Brian Mercer and colleagues at Netlon in England in the late 1970s produced the punched-and-drawn polymer grid, in which a sheet of polyolefin is perforated and then stretched so the ribs orient and the apertures open, giving a stiff planar product that interlocks with aggregate instead of merely rubbing against it. Commercial geogrids followed in the 1980s and displaced steel in many applications because they do not corrode. Design practice was then consolidated by public agencies and standards bodies: the FHWA National Highway Institute manuals for MSE walls and reinforced slopes, Section 11 of the AASHTO LRFD bridge specifications, and BS 8006 in the United Kingdom, first issued in 1995 and revised in 2010. Standardized index and performance tests, including wide-width tensile and soil-geogrid pullout methods, gave the reduction factors their empirical basis.