Quality of Rocks for Coastal Protection Works

Rock is a natural and commonly used construction material in the hydraulic environments such as revetments for shore protection works and construction of breakwaters etc. The quality control procedures, quality assurance and requirements are unfortunately not well documented in our country. Rock materials required to be durable, sound and of sufficient hardness to resist weathering and shall be free of flaws, rifts, laminations, cracks, clay components, seams, and other blemishes which may impair their strength, durability or function. Use of porous and soft rock shall not be used in any type of the coastal construction works. Quality of rocks are generally tested for the following

1. Grading and Mass Distribution

2. Shape

3. Mass density

4. Water absorption

5. Compressive strength or Point load strength

6. Los angles Abrasion test

7. Block integrity test

8. MgSO4 soundness

The qualities of rock materials for the construction generally required to comply with the CIRIA Rock manual. CIRIA recommendations are based on practical application of Eurocode EN13383 and also formulated from wide experience gained through various rock projects from Europe and around the world. All the above tests except the grading and mass distributions are straight forward and laboratory related testing.

Mass distribution of armour stone grading is based on setting limit values with an associated percentage passing by mass. Nominal limits correspond to the target size of the armour stones and a set of extreme limits corresponds to tolerances in the lower and upper ranges. Nominal lower limit Nll is set as no more than 10% and limited up to 5%, material below which considered as in the Extreme lower limit Ell. Similarly Nominal upper limit Nul is set as no less than 70% and limited up to 97% , material above which considered as in the Extreme upper limit Eul. (See Figure-1 below)

The above criteria ensure a minimum of 60% and up to a maximum of 92% material between the nominal limits, which implies 32% tolerances to carry out different attempts to achieve the required optimal grading. Therefore it is possible to generate number of grading curves by adjusting Nll (10%-5%) and Nul (70%-97%), then identify the most appropriate gradation curve for the available material at the site subjected to the condition that the M50 (median weight or 50% passing) value relating to the adopted gradation should not be less than the design requirement. Alternatively idealized standard grading curves can be generated by setting Nll=6% and Nul=90% which will give the highest reassurance that the M50 plotted lies near (Nll+Nul)/2

LIMITS OF STANDARD GRADINGS (Figure)

Consider the non standard grading of 1-2 t armor rocks as an example, Nll=1.0 t, Nul=2.0 t and the target M50> 1.50 t = (Nll+Nul)/2 Standard grading curve can be generated by setting percentage passing to 6% for 1.0t rocks , percentage passing to 90% for 2.0t rocks in the Rossin Rammler grading equation resulting into M50=1589 kg , Nll can vary between (5-10% ) ie 5,6,7,8,9,10 and Nul to be between (70-97%) say 70,71,72,73,74 and upto 97, which produces 186 combinations resulting into 186 grading curves and M50’s. However different gradation curves with the above combinations with most appropriate for the available material at quarry can also be identified and adopted, subjected to the condition that the M50 value relating to the adopted uniform gradation should not be less than the design requirement weight.