The presence of lateral pressure of backfilling sidesoils induces bending
moments in the opposite direction from those produced by vertical loads and bedding support reaction. Such bending moment reduces the flexure in pipe wall and as a result this causes an equivalent increase in supporting strength of the pipe.

The lateral pressure of backfilling soils on drains is affected by the deflection of drains. With no occurrence of deflection, lateral pressure induced is in the form of active pressure. If pipe deflection occurs, the drain increases its horizontal dimension so that passive pressure is developed.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

Whenever possible box culverts should be designed for free flow at design flow rate. A box culvert with surcharged flow is similar to inverted siphon which creates maintenance problem. There is a risk of blockage by silt and debris and the inspection and maintenance of submerged culvert is difficult when compared with free flow condition. For long box culverts (more than 20m) it is even more important to design for free flow to decrease the risk of blockage which results in an inaccessible path.

From hydraulic point of view, the change from free flow to submerged
condition would cause an increase in head loss leading to a rise in upstream water level.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

Rock used for riprap should be blocky and angular, with sharp edges and flat faces. Angular stones proved to be effective to withstand external forces. Rounded stones have a high tendency to roll and inadequately protect the channel bed and bank. The ratio of length to thickness of angular stones should be less than 2.

If rounded stones have to be used, they should not be placed not steep embankments. Moreover, the size of rounded stones shall be increased (say 25%) with the corresponding increase in thickness of riprap layer.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

Riprap by dumping involves the dumping of graded stone by dragline or crane in such a way that segregation would not take place. Dumped riprap is a layer of loose stone so that individual stones independently adjust to shift in or out of the riprap. The dumped riprap is very flexible and would not be damaged or weakened by minor movement of the bank caused by settlement. Moreover, local damage or soil loss can be readily repaired by placement of more rock.

Riprap by hand-placing involves laying of stones by hand and by following a pattern with the voids between the large stones filled with smaller stones and the finished surface is kept even. The interlocking riprap produces a tidy appearance and decreases flow turbulence. Also, owing to the interlocking nature of riprap it allows the formation of riprap on steeper bank slopes. The thickness of riprap can usually be reduced when compared with dumped riprap. However, it requires much labour for installation of riprap. Another drawback is that the interlocking of individual rocks produces a less flexible revetment so that a small movement in the base material of the bank could cause failure of large portions of the revetment.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

Riprap is an erosion-resistant ground cover made up of large, angular and loose stones (rock, concrete or other material) with geotextile or granular layer underneath. Riprap is commonly used in drainage channel to provide a stable lining to resist erosion by channel water. It is also used in channels where infiltration is intended but the velocity of flow is too large for vegetation.

A layer of geotextile is normally provided under riprap to perform separation from underlying soils. This prevents the migration of fined-grained soils from sub-grade into riprap and results in settlement and
loss of ground.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.