This guideline offers detailed instructions on constructing dry stone retaining walls tailored for hilly terrains. It covers aspects such as selecting suitable stones, construction methods, ensuring wall stability, drainage provisions, and maintenance practices to safeguard slopes against landslides and erosion.
Overview
This guideline offers detailed instructions on constructing dry stone retaining walls tailored for hilly terrains. It covers aspects such as selecting suitable stones, construction methods, ensuring wall stability, drainage provisions, and maintenance practices to safeguard slopes against landslides and erosion.
Audience
Contents
Structure
The standard covers the design and building of retaining walls in mountainous regions, focusing on soil retention on slopes with considerations for seismic activity and rainfall typical to these areas. It includes types of walls such as gravity, cantilever, counterfort, and anchored variants, addressing earth pressure calculations, seismic forces as per IS 1893, drainage to alleviate hydrostatic buildup, and materials including concrete and masonry.
This section outlines design essentials for retaining walls including RCC cantilever and reinforced earth walls. It emphasizes using Rankine or Coulomb theories for earth pressure, considering water pressures, and ensuring factors of safety of at least 1.5 for sliding and overturning. Key calculations for earth pressure, overturning moments, and sliding forces are explained along with typical soil property ranges.
Details on selecting stones following IS 1123 guidelines are provided, specifying stone sizes from 225x100x75 mm up to 600x200x300 mm, preferring rough and flat shapes for enhanced friction and interlocking. The inward slope of the wall face should range between 1 vertical to 6 horizontal and 1 vertical to 3 horizontal, with a perpendicular base to improve seismic resistance. Filling materials should be coarse, angular, and voids must be filled and compacted properly.
Instructions on construction emphasize precise stone selection and placement, maintaining the inward batter between 1:6 and 1:3, and positioning the wall base perpendicular to the wall face. These measures enhance keying, reduce pressure on the wall toe, increase resistance to sliding, and improve seismic performance.
Backfill must be composed of non-cohesive, free-draining materials like broken stone dust, stone chips, or gravelly soils, avoiding fine soils or river shingle to maintain friction. Hand packing rather than dumping ensures maximum internal friction. The backfill width should be no less than 500 mm with the upper 300-500 mm layer rendered impervious to limit water penetration.
Drainage is essential to avoid hydrostatic pressure buildup behind the wall. The standard recommends installing weep holes or pipes spaced roughly between 1.5 to 3 meters, using filter layers such as gravel or coarse sand, and maintaining a drainage layer thickness of at least 300 mm. Backfill slopes should direct water to drainage outlets, and clay soils near the wall should be avoided.
Toe protection measures include ensuring the wall base width and foundation depth distribute loads safely, verifying resisting moments counteract overturning moments, checking shear stresses remain within allowable limits, and maintaining bearing pressures below soil capacity. Use of rectangular stones with adequate overlap and ensuring frictional resistance at the base are critical for stability.
Though detailed clauses are not explicitly provided for bonding elements in dry stone walls, typical practices involve using high tensile steel strips or geosynthetics with proper spacing and embedment to resist pullout. Bond lengths and safety factors are determined based on tensile loads and soil pressures, ensuring adequate anchorage in compacted backfill.
Quality control measures include ensuring drainage paths are not blocked, toe protection for walls taller than 3 meters, correct installation of RCC bonding elements at 1 m intervals with specified cross-sections, and thorough packing of stones to avoid sliding. Regular inspections verify proper placement of components and erosion resistance.
Maintenance involves routine inspections for cracks, bulging, seepage, and settlement. Minor cracks can be treated with epoxy or grout, while major damages may require section replacement. Drainage systems must be kept clear, exposed reinforcements protected from corrosion, and backfill recompacted as needed. Stability factors for sliding and overturning guide repair assessments.
The committee consists of experts from academic institutions, public works departments, central government agencies, research organizations, development boards, and environmental bodies, chaired by Dr. Gopal Ranjan. It ensures multidisciplinary contributions encompassing structural, geotechnical, environmental, and material science expertise for developing guidelines relevant to hill area retaining structure design.
Frequently Asked
Dry stone retaining walls require rough, flat stones for enhanced friction and interlocking. Stones must not be smaller than 225 mm × 100 mm × 75 mm (approximately 5 kg) and can be as large as 600 mm × 200 mm × 300 mm (around 45 kg). The stones should be oriented so that their largest dimension lies across the wall length to maximize stability. Voids between stones are filled with coarse angular materials like broken stone dust or stone chips, avoiding fine soils or smooth river shingle. Additionally, the wall face should have a minimum inward slope of 1 vertical to 6 horizontal to ensure stability.
Backfill placement should utilize non-cohesive, free-draining materials such as gravelly or sandy soils, broken stone dust, and stone chips, while avoiding fine-grained soils and smooth river pebbles that reduce friction. The backfill must be hand-packed rather than dumped to maximize internal friction. A minimum width of 500 mm is required, with the top 300 to 500 mm layer made impervious to limit water infiltration. Moistening the filler material and ramming it after each stone layer enhances load distribution and increases wall strength.
Effective drainage is key to preventing water pressure buildup behind retaining walls. The use of weep holes or drainage pipes spaced between 1.5 and 3 meters apart is recommended. Behind the wall, a drainage layer of at least 300 mm thickness composed of well-graded gravel or coarse sand should be provided. The backfill must be sloped to direct water toward drainage outlets, and clayey soils near the wall should be avoided to reduce water retention. Filling voids with coarse angular materials and ensuring proper moisture management further enhance drainage effectiveness.
Bonding elements such as mortar bands or reinforcements are generally not mandatory for dry stone retaining walls if constructed properly with skilled labor and quality control. However, if bonding is incorporated, it should be done using well-mixed and cured mortar along with proper interlocking of stones. Coarse angular fillers like stone dust or chips should be used to fill voids and improve load transfer. In environments lacking skilled supervision or adequate water, dry stone packing without bonding is preferred to maintain flexibility and ease of repair.
Dry stone masonry, when properly executed, performs similarly to mortar masonry in terms of compressive strength and sliding resistance but offers greater flexibility, which is advantageous in seismic conditions. Mortar quality in hill areas is often compromised due to unskilled labor and water scarcity. Dry stone walls are easier to repair, do not require curing, and can exceed the compressive strength of hill foundation soils. Both types require skilled labor and supervision, but dry stone walls provide a cost-effective, durable alternative up to approximately 4 meters in height.
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