Guidelines for retaining wall for hill area, Part 2: Design of retaining/breast walls
1997 Edition

According to IS 14458 Part 2 (1997) for retaining structures in hilly terrain, the factor of safety against sliding at the slip surface beneath the foundation should exceed 1.5 under static conditions and 1.0 under seismic loading. Design must adhere to IS 1080 and IS 1904 standards for foundation construction. Wall dimensions should be selected such that the soil's allowable bearing capacity surpasses the foundation pressure. The backfill surface is to be kept horizontal with a surcharge load of 1.5 t/m². Additionally, a 300 mm thick layer of silty soil mixed with boulders is recommended to prevent rainwater infiltration, and stone pitching should be provided at the toe to mitigate erosion. The slope gradient in the design should generally range from 1:5 to 1:3. These measures collectively ensure stability against both sliding and seismic forces typical in steep hill environments.

IS 14458 Part 2 mandates the inclusion of seismic forces in the design process for retaining walls alongside other loads such as self-weight, live loads, earth pressure, and water pressure. Seismic load calculations should follow the criteria outlined in IS 1893 for earthquake-resistant design. For low traffic volume roads, seismic forces may be disregarded. The factor of safety against sliding at the foundation base must be at least 1.5 under static conditions and no less than 1.0 during seismic events. Water pressure is conservatively taken as at least 30% of the full hydrostatic pressure even when drainage is provided. Foundation design must conform to IS 1080 and IS 1904. These provisions ensure that retaining walls maintain structural integrity and safety during seismic activity.

IS 14458 Part 2 addresses a range of soil classifications with corresponding safe bearing capacities essential for foundation design of retaining walls. For very compact well-graded mixtures (GW, GC, SC), a bearing capacity of 100 kN/m² is recommended. Very compact gravel and gravel-sand mixes (GW, GP) are assigned 80 kN/m², while medium to compact gravels (SW, SP) have 60 kN/m², decreasing to 40 kN/m² for loose gravels. Coarse to medium sands range from 40 kN/m² (very compact) to 30 kN/m² (medium to loose). Fine to medium sands and silty or clayey soils have capacities between 15 and 30 kN/m² depending on compactness. Homogeneous inorganic clays (CL, CH) vary from 5 kN/m² in soft conditions to 40 kN/m² when very stiff or hard. Inorganic silts (ML, MH) range similarly from 5 to 30 kN/m². These values guide preliminary design in absence of detailed soil testing.

IS 14458 Part 2 prescribes several drainage strategies to reduce water pressure behind retaining structures. An inverted filter layer should be installed immediately behind the wall to facilitate efficient drainage of groundwater and rainwater seepage, thereby preventing hydrostatic pressure buildup. The use of weep holes is recommended to allow water escape, but catch water drains should not be placed near the top of breast walls to avoid seepage into cut slopes; instead, they should be positioned at the wall toe to collect runoff and water from the weep holes. A 300 mm thick layer of silty soil mixed with boulders is advised to prevent rainwater ingress. Pitching with stone masonry at the base protects the toe from erosion. Slope gradients for drainage paths and terraces should be maintained between 1:5 and 1:3 for effective runoff management. Additionally, retaining wall foundations must extend at least 500 mm below side drains into firm ground to assure stability.

To minimize erosion at the toe of retaining walls in hilly terrain, IS 14458 Part 2 and sound engineering practices suggest diverting natural gullies and surface runoff away from the wall base to prevent concentrated flows that can erode soil. Vegetative cover such as grass turfing on adjacent slopes helps stabilize soil and reduce surface erosion. For rock slopes, stepped foundation designs reduce soil displacement and enhance stability. Where water flow is anticipated, energy dissipators or riprap should be installed at the toe to lower flow velocity and prevent scouring. The use of self-draining backfill materials is crucial to avoid buildup of hydrostatic pressures that exacerbate erosion. These combined methods effectively protect the retaining wall toe and improve overall durability.