Code of practice for the structural safety of buildings on shallow foundations on rocks
1991 Edition

IS 13063 recommends safety factors to ensure foundation stability on rock surfaces. For sliding resistance, a Factor of Safety (FOS) of at least 1.5 is required when accounting for dead, live, earth, water pressures, uplift, and wind or seismic forces; if wind or seismic loads are excluded, the FOS should be at least 1.75. For overturning, a minimum FOS of 1.5 is generally mandated. When foundations are on moderately inclined rock slopes (less than 15°), embedment of at least 30 cm is advised to maintain a sliding FOS not less than 1.10, even without supporting downslope bearing.

According to IS 13063, vertical joints occupying more than 20% of the foundation area should be addressed by deepening excavation or cleaning and grouting with a 1:1 cement-sand mixture. Open horizontal joints require removal of overlying rock or grout filling. Solution cavities covering less than 20% of the foundation base necessitate a reduction in safe bearing pressure as per IS 12070. For soluble rocks such as dolomite on limestone with water-charged joints, grouting to the effective foundation width using cement-sand mortar is specified to ensure stability.

IS 13063 specifies several field tests to assess rock mass properties: the Point Load Strength Index Test provides a rapid estimate of rock strength; the Uniaxial Jacking Test determines the deformation modulus of rock masses; and the In Situ Shear Test evaluates shear strength, particularly in moderately jointed rocks. These tests are conducted per respective IS codes and provide critical data for foundation design.

The code advises avoiding foundations on steeply dipping rock surfaces (>15°). Where unavoidable, foundations should be embedded at least 60 cm into rock with appropriate sliding stability checks and possible use of anchors. For moderate slopes (<15°), embedding of at least 30 cm with a minimum sliding safety factor of 1.10 is recommended. Weak bedding planes should be avoided at the foundation base by shifting footing or increasing depth. For undulating rock surfaces with mixed hard and soft rock, settlement joints must be provided, and soft rock depth beneath footing should be at least one-third of the footing width.

Permissible settlement limits per IS 13063 depend on structure type and rock conditions. Maximum settlements for steel and reinforced concrete structures are typically 12 mm, with differential settlements expressed as fractions of length L1 (distance between columns or length of wall). For example, steel structures allow differential settlement up to 0.0033 × L1 and angular distortion of 1/300. Plain brick walls permit lower differential settlements depending on L1/H ratio. In highly weathered or weak rock, foundations should be treated as soil per IS 1904, with adjusted settlement considerations. Mixed rock conditions require provision of settlement joints to accommodate differential movements.