At Varicore, we regularly receive questions about backfill. We sometimes even encounter the misconception that select backfill is a concept linked exclusively to Multi-Flow drainage. Your backfill choice will have no greater and no less effect on the life of a Multi-Flow system than it will on any other drainage system. Multi-Flow systems, and all other drainage systems enjoy longer life when quality backfill is used.
It is a well known fact that French drains frequently block up, sometimes in a remarkably short amount of time. This blockage typically occurs on the trench liner. Small particles of clay or silt are carried by moving water until they are intercepted by the filter, which eventually fills in. The actual life span of a French drain depends on the soil type and the rainfall amounts. This same blockage can occur with round pipe or panel drain wrapped in geotextile. Highway departments and golf course managers have wrestled with this issue for many years.
Very coarse sand
The best solution to this problem is to surround the geotextile filter with very coarse sand. Sand is an excellent filter of clay and silt. As the water containing these contaminants moves through the sand, it slows down and the particulate matter drops out. An inch or more of sand is a very effective filter.
According to the USDA system of classification, very coarse sand has an approximate particle size of between 1.0 and 2.0 mm. Some designers have used this for a sand spec:
When passed over a sieve, very coarse sand will have:
Unfortunately, good quality sand is not uniformly available. The closer installers come to this recommendation, the longer their system will last. Absolute conformity is frequently not practical. On one hand, sand contaminated with clay or silt will impede the movement of water as well as accelerate the blinding of the drainage system. On the other hand, large diameter pea rock and mixed particle size gravel allow for rapid movement of water in the beginning, but are susceptible to infiltration by fines. They might not be aggressive enough in protecting the fabric filter. Buck shot or washed medium sand are better choices, but will not perform as well as clean, very coarse sand.
When the perfect sand cannot be found, look for an alternative following these two criteria and in this order: