Code of practice for design and construction of port and harbour structures, Part 6: Block work
1989 Edition

For port and harbour block work, the standard specifies the use of solid or cellular precast concrete blocks designed for gravity-type wharf walls. These blocks must comply with IS 456:1978 for concrete quality. To ensure durability in marine environments, especially in tidal or submerged zones, the concrete should be dense and impermeable, typically achieved with a water-cement ratio near 0.50. Blocks are shaped to resist damage during transport and placement and are installed on a prepared rubble leveling course to form stable structures.

Berthing and mooring loads are treated as lateral forces applied to the wharf wall’s face at the contact height with vessels. These loads combine with earth pressures, water pressures, and other environmental forces. The design must ensure that the wall withstands these combined lateral forces without tension, maintaining safety factors of at least 1.5 against sliding and overturning (with seismic effects considered). The weight of the blocks and backfill, both dry and submerged, must be sufficient to resist overturning and sliding forces, and foundation bearing pressures must remain within safe limits.

Foundation dredging involves removing underwater sediments such as silt, sand, and clay using methods like dragging, suction, or submarine blasting to the specified width and depth along the wharf alignment. Subsequently, a trench is excavated to design dimensions and slopes, clearing loose materials above the bearing stratum. The trench is backfilled with rubble topped by a 200 mm graded metal layer (50 mm size). If rock is exposed, either a concrete layer or thin graded metal is applied. The bed surface is given a lateral inward slope of 1:100 to counteract differential settlement. Alignment and leveling are controlled using steel frames, shore-based surveying instruments, and underwater divers for positioning.

Block handling and launching require heavy-lift cranes equipped with suitable lifting gear. The process begins with placing smaller pre-abutment blocks (~10 tonnes) on land, followed by sequential placement progressing seawards. Blocks are positioned vertically or with a slight tilt, closely contiguous, under the guidance of 2-3 divers to ensure accurate underwater placement. After completing each row, alignment and slope are immediately checked and any deviations corrected promptly to prevent misalignment or structural creep. This careful, staged approach assures the stability and durability of the block work.

To achieve durability in marine conditions, concrete blocks must be produced using dense, impermeable concrete with a water-cement ratio around 0.50, as required by IS 456:1978, to resist chemical attack from seawater. Construction practices include carefully controlling foundation and blockwork levels, slopes, verticality, and alignment throughout erection. After blockwork reaches the high water level, cast-in-situ mass capping concrete is applied with expansion joints for monolithic behavior and to accommodate fittings. Blocks are also designed with appropriate shapes and sizes to facilitate handling and resist marine loads, ensuring long-term structural integrity.