Gabion walls are retaining walls made by filling wire mesh boxes with rocks. The wire mesh cages provide structure and stability, while the rocks inside provide weight and mass to hold back soil and water pressure. Gabions have been used for centuries in civil engineering projects due to their durability, permeability, and aesthetic appeal.
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Basalt
Basalt is one of the most commonly used rocks for gabion walls. Basalt is an igneous volcanic rock that is formed when lava cools rapidly on the earth’s surface. It has a fine-grained texture and is composed of the minerals plagioclase, pyroxene, and olivine. Basalt is an ideal choice for gabion walls for the following reasons:
- Density – Basalt has a high density which gives it substantial weight and strength for retaining wall applications.
- Durability – Basalt is very resistant to weathering and abrasion, allowing it to withstand harsh environmental conditions.
- Angular shape – The irregular, angular shape of basalt pieces allows them to interlock well when stacked in gabion baskets.
- Availability – Vast quantities of basalt stone are widely available from quarries in many areas.
Basalt is often used for large civil engineering projects like retaining walls, shoreline reinforcement, and riverbank stabilization. Its density, durability, and angularity make it well-suited to provide the ballast and structural integrity needed in gabion walls.
Granite
Granite is another igneous rock commonly used for gabion walls. Granite forms deep in the earth’s crust when magma slowly cools and solidifies. It is composed primarily of quartz, feldspar, and smaller amounts of mica and other minerals. Key properties that make granite a good choice for gabion walls are:
- Hardness – Granite has a Mohs hardness rating of 6 to 7, making it very resistant to abrasion.
- Strength – Granite has high compressive strength, allowing it to withstand heavy loads in retaining walls.
- Weather Resistance – Its mineral composition gives granite excellent resistance to weathering and water erosion over time.
- Angular Shape – Granite breaks into jagged, angular fragments ideal for interlocking in gabions.
Granite gabions are commonly used for retaining walls, shoreline structures, streambank stabilization, and other applications requiring a strong, durable fill material. Its hardness gives granite gabions excellent long-term durability even in harsh environments.
Limestone
Limestone is a sedimentary rock composed mainly of calcium carbonate fossils and shell fragments. It has the following attributes that make it suitable for use in gabion retaining walls:
- Availability – Limestone is abundant and widely quarried in many regions.
- Angular Shape – Limestone fractures into jagged, angular pieces with rough surfaces that interlock well.
- Permeability – The gaps between limestone pieces allow for good drainage and permeability.
- Affordability – Limestone is often cheaper than other rock types like granite and basalt.
The permeable and porous nature of limestone allows water to drain through gabion walls, reducing hydrostatic pressure buildup behind the wall. Limestone’s resistance to weathering makes it suitable for long-term use despite its relatively lower strength compared to granite and basalt. It offers a more affordable filling option for large gabion structures.
Rhyolite
Rhyolite is an extrusive volcanic rock with a composition similar to granite. Its glassy texture provides these beneficial properties for gabion walls:
- Durability – The high silica content of rhyolite gives it very good resistance to weathering and breakdown.
- Angularity – Rhyolite fractures into very rough, highly angular rock fragments.
- Shear Strength – Rhyolite has relatively high shear strength for stable wall construction.
- Aesthetics – Rhyolite’s colors and textured surface provide visual interest.
Rhyolite is sufficiently strong and durable for use in gabion retaining walls, shoreline structures, and other projects requiring stacked rock fill. The angular rock pieces grip together firmly to create a solid gabion structure. Rhyolite’s aesthetic qualities also make it desirable for highly visible gabion walls.
Sandstone
Sandstone is composed of cemented sand grains and has some beneficial properties for gabion walls:
- Availability – Sandstone is a common sedimentary rock type quarried in many areas.
- Workability – Sandstone can be cut into large blocks or rough slabs relatively easily.
- Permeability – The gaps between sand grains allow water drainage.
- Affordability – Sandstone is often cheaper and more available than other rocks locally.
While not as hard and durable as granite or basalt, sandstone can make an affordable, permeable gabion fill material. Reinforced wire mesh gabion boxes can compensate for sandstone’s lower strength. Sandstone works well for shorter gabion walls in low load conditions where drainage is important.
Factors in Choosing Rock Fill
Selecting the best rock type for a gabion wall depends on several factors:
- Strength and hardness – Higher values allow heavier loads and greater durability.
- Density – Denser rocks provide greater mass and resistance to displacement.
- Angularity – Jagged, angular rocks interlock better and drain well.
- Availability and cost – Abundant local rock is most affordable.
- Permeability – Drainage capacity reduces hydrostatic pressure.
- Environment – Resistance to water, weathering, and abrasion.
- Appearance – Color, texture and natural beauty.
Granite and basalt tend to provide the best all-around properties, but sedimentary rocks like limestone and sandstone can also be suitable depending on wall requirements, local availability, and budget.
Sourcing Rock for Gabion Walls
There are several options for sourcing rock material to fill gabion baskets:
- Quarries – Local quarries are the primary source for freshly crushed rock fill like granite, basalt, limestone. Quarries allow selecting rock type and size.
- Construction sites – Blasted rock from excavations and demolished structures can provide reusable fill at low cost.
- River rock – Smooth river rock is not ideal but may be used in some cases where other sources are limited.
- Onsite rock – Rock harvested directly from the gabion wall construction site reduces transport costs.
- Recycled fill – Recycled concrete or other demolished building materials can make very low-cost fill, but may not be as aesthetically appealing.
It is recommended to source rock fill locally when possible to minimize transport expenses. Onsite or nearby quarries typically offer the most cost-effective options with the highest quality rock.
Rock Fill Size
Gabion rock fill should be sized and graded appropriately to enable proper basket filling. Some guidelines for rock fill dimensions include:
- Minimum size – Around 4 inches or 100mm. Smaller rocks may migrate out of mesh openings.
- Maximum size – Up to 8 inches or 200mm for smaller baskets. Larger rocks are acceptable for bigger baskets.
- Gradation – A range of rock sizes is ideal to fill voids efficiently.
- Angularity – Irregular, blocky rock pieces interlock better than rounded river rock.
Oversize rock pieces may be split or hammered down to the proper size range. A gravel base layer may be used beneath the gabion stone to improve leveling and fill voids.
Filling Gabion Baskets
Proper filling and compaction of the rock-filled gabion baskets is critical to building a strong retaining wall. Some guidelines include:
- Fill in layers – Add rock in 12 inch layers, compacting between additions.
- Full compaction – Stomping and tamping ensures maximum density and strength.
- Overfill slightly – Allow extra rock to extend slightly above the top edge.
- Align baskets – Keep basket sides aligned and edges flush.
- Wrapped edges – Ensure mesh edges are fully wrapped and secured.
- Interlocked layers – Bond upper baskets into lower ones for strength.
Taking care during basket assembly and filling is essential to prevent future wall bulges, deformations, and collapses. Properly filled strong, dense and interlocked gabion structures can last for decades or longer.
Advantages of Gabion Walls
There are several reasons gabion retaining walls are a popular choice:
- Strength – Gabions can support very heavy loads when filled properly.
- Permeability – Gabion rock fill drains well, reducing hydrostatic pressure.
- Flexibility – Gabions can adapt to ground movement and settlement.
- Affordability – Gabions are relatively inexpensive to construct.
- Durability – Properly built gabion walls can last 50 years or more.
- Aesthetics – Gabions offer a natural, rustic stone wall appearance.
Gabion retaining walls are a high performance and cost-effective soil retention solution suitable for many applications and environments. Their modular nature also allows easy future height extensions if needed.
Conclusion
Hard, durable rock types like granite, basalt, rhyolite and limestone make the best fill material for gabion retaining walls. Their density, angularity, strength and aesthetic appeal provide long-lasting stability and performance. Locally sourced quarry rock is often the most cost-effective solution. With proper assembly and rock fill compaction and grading, gabion walls can provide affordable, environmentally sound and low-maintenance retaining structures for decades of use.