Geologic time scales are on the order of tens of thousands of years. Rocks are known to be hard and durable. It is therefore tempting to think that a hydraulic structure made of rock should have no problem lasting an engineering service life of say, 50 years.
It turns out that achieving engineering service life is no easy task and depends heavily on strict quality control during construction, quarry selection and sourcing. Rocks were formed under high heat and pressure. Once they are exposed and quarried near the Earth’s surface, they are subject to low pressure, temperature fluctuations, oxidation and various forms of weathering.
Conditions in a hydraulic structure are even more severe. It is hard to imagine an environment more efficient at breaking down a rock structure than a coastal wave environment like the one in the image below. In a coastal hydraulic structure, rocks degrade progressively within an engineering service life. Over time rock disintegrates into smaller and smaller fragments.
Consider the tools engineers use to design durability into a rock structure. The value, M50, is the median mass of the rock population and describes the typical size of the rocks to be used. This “size of rock” measure is an important design feature for the stability of the structure. The environment will decrease the M50 over time. This can be addressed by demand-based design or by supply led design. For demand-based design engineers must design the rocks to be strong enough to sufficiently limit M50 reduction. For supply led design, they must design other compensations in the structure to account for the rate of M50 degradation.
With demand-based design, engineers are limited by the availability of high-quality rocks. This is dictated largely by the geology of the area. It is uneconomical to freight large rocks over long distances. Heavy transport of rock also damages local roads and infrastructure.
A supply led design may require a larger M50 in anticipation of the degradation rate. This can create other logistic and supply challenges as it forces transport of increasingly large and cumbersome materials. This practice can also be a public nuisance and even hazardous as introduced rock fractures go unaccounted for in the surrounding area.
The chart below is an example of the Armorstone Quality Degradation (ADQ) model to predict M50 degradation taken from The Rock Manual: The use of rock in hydraulic engineering. Over a 50 year service life, the fraction of mass remaining can be less than 80% of the original for rocks with a “good” quality rating (ADQ = 3). Also note that this model considers wear and not major breakages which occur during placement as well as during the service life. Not only does the structure lose total mass over time, the individual rocks reduce in size and become more vulnerable to wave energy.
Models like the ADQ model can be used to incorporate weathering and rock breakage into the design but these rely on extensive testing and quality assurance monitoring of the rock source. It is very difficult and expensive to reject rock that has arrived on site. Likewise, it is difficult and expensive to employ someone to continuously manage quality control at the quarry source.
Geosynthetics offer a proven, tested alternative to rock armor in the form of Geosynthetic Sand Containers (GSCs).
Because GSCs are a manufactured product, quality control and quality assurance of the materials can be achieved through ISO accredited production procedures and lab-based quality control monitoring.
Filling and closure procedures should also be implemented to ensure the final filled unit meets the engineering requirements. This means filling each unit identically and as full as possible with a free draining silica sand. The fill material - sand, can generally be sourced locally which is an advantage over the transport cost and other challenges associated with rock armor.
The service life of the GSC structure depends on the environment it is exposed to and the quality of construction. Engineers are currently publishing an estimated service life of greater than 40 years. Actual service life in many cases is likely greater. The technology was developed and implemented in the 20th century. With each passing year, the in-service performance of current structures informs a greater expected service life for new structures.