When constructing a gabion wall, the type of rock used can significantly impact the structural integrity and longevity of the wall. Selecting the right rock is crucial to ensure the gabions can properly support, reinforce and retain the soil behind them. The best rocks for gabion walls are hard, dense, angular rocks that interlock well. The shape and size of the rock can also influence the quality of the gabion fill. Let’s explore the key factors to consider when choosing rocks for gabion baskets.
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Rock Hardness
The hardness or strength of the rock is one of the most important properties. Harder, denser rocks will better withstand weathering and erosion forces over time. Sedimentary rocks like limestone and sandstone tend to be too soft and porous for quality gabion fill. The best options are igneous or metamorphic rocks such as granite, basalt, gneiss and durable sandstone. These rocks have compressive strengths over 10,000 psi, allowing them to bear heavy loads without fracturing or deteriorating.
Soft, weak rocks are more prone to cracking, flaking and breaking down. This leads to smaller pieces working through the gabion wire mesh over time. Using only hard, competent rocks ensures the gabion structure will maintain its cohesion and stability for decades. Specifications often require rock with Los Angeles abrasion test values of 40 or less.
Impact Resistance
In addition to compressive strength, the rock must be able to resist impacts without shattering or cleaving. Igneous rocks like granite and basalt are exceptional in this regard due to their crystalline structure. The rock should be tested to survive impacts of up to 1.5 meters onto a steel plate without fracturing. Weak or brittle rocks will chip, split and pulverize too easily under impacts and loading conditions.
Weather Resistance
The rock must also withstand weathering through wet-dry and freeze-thaw cycles. Water can degrade weaker sedimentary rocks by dissolving cements between grains or expanding in cracks during freeze events. Over time, this leads to breakdown of the rock into smaller fragments that compromise the gabion structure. Hard igneous and metamorphic rocks resist these weathering forces the best.
Rock Shape
The shape of the rock material is also very important for proper gabion fill. Rounded, cubical and rectangular rocks with some angular facets are preferred. River rock and quarry stone are common sources that yield rock with suitable shape. The rocks should allow tight packing and interlocking within the gabion baskets.
Angularity
Angular rocks with some flat surfaces are ideal because they fit together snugly like puzzle pieces. This friction and interlock between the rocks enhances shear strength. Completely rounded stones have less interlocking potential. Cubical rocks are preferred over thin, slab-like rocks which could align and create voids. The rock fill should be tightly packed with minimal empty space to act as a cohesive, solid mass.
Rock Size
Rock size is also critical. The ideal average size is around 4-8 inches. The rock shouldn’t be oversized and jammed into the gabions. Using rock around 1/3 to 1/2 the gabion basket thickness allows filling the baskets securely without empty space. Oversized rocks leave gaps while undersized rocks fall out through the mesh. A range of rock sizes that complement each other when packed is ideal.
Other Gabion Rock Considerations
Local Availability
Using rock sourced locally near the project site is recommended. This minimizes transportation costs for the heavy rock materials. Often, suitable rock can be quarried nearby or recycled from local building site rubble. However, the structural requirements noted above for strength, shape and size shouldn’t be compromised based on availability.
Weight and Density
Higher density rocks around 165-170 lbs/ft3 are best for stability. Heavier rocks enhanced friction, shear resistance and anchoring. Lighter, porous rocks under 150 lbs/ft3 should generally be avoided.
Durability Testing
Inspect rock samples and conduct testing as needed to ensure durability. Properties to evaluate include water absorption, abrasion resistance, chloride content, freeze-thaw resistance and more. Established test standards are published by organizations like ASTM International.
Angular rock fill
Sharp, angular rock fragments typically make the best fill for gabion retaining walls and other structures. The irregular shapes allow tight packing and interlock. Cubical rocks with some flat surfaces are preferred over thin, slab-like shapes.
Heavy and dense
Heavy, dense rocks around 165-170 lbs/ft3 are recommended. Greater density enhances friction and gravity loads to improve stability. Lighter rocks are more prone to shifting and erosion.
Hard and durable
Hard, competent rocks resist fracturing, weathering and deterioration over time. Weaker rocks will eventually break down into smaller pieces that can wash through the gabion baskets.
Proper rock size
Rocks ranging from roughly 4-8 inches in diameter allow optimal filling of the baskets without gaps or empty space when properly packed and interlocked.
Suitable Rock Types
There are a number of specific rock types that meet the strength, durability, density and shape criteria outlined above. Some of the best options include:
- Granite – Excellent hardness and durability. Various color variations. Can be angular or rounded.
- Basalt – Very hard volcanic rock. Angular shape provides stability.
- Gneiss – Hard metamorphic rock with granular composition.
- Quartzite – Extremely durable metamorphic rock.
- Durable sandstone – Harder sedimentary rock resistant to weathering.
Unsuitable Rock Types
The following types of rock generally have properties unsuited for gabion walls:
- Limestone – Too soft and porous. Prone to deterioration.
- Shale – Poor hardness and competency. Easily breaks down.
- Schist – Has foliated layers susceptible to flaking.
- Slate – Potential for splitting along cleavage planes.
- Flint – Very hard but fractures conchoidally.
Gabion Rock Suppliers
Quarries and aggregate suppliers that stock rocks suitable for gabion baskets may be located regionally. Typical product names include gabion stone, crusher run, ballast stone, and riprap stone. Waste and recycled concrete rubble also has potential for use as gabion fill if properly sized and shaped.
When sourcing gabion rock, work directly with suppliers to obtain samples and confirm the product meets the necessary specifications. Request test data on critical factors like L.A. abrasion values, density, weather resistance and compressive strength.
Ideally, personally inspect the rock at the quarry or have an engineering representative qualified to evaluate suitability. Carefully vet sources to ensure the highest quality product will be delivered.
Installation Best Practices
Proper installation is just as crucial as the rock selected. Here are some key construction practices for optimal gabion filling:
- Tightly pack rock fill in compacted layers around 12 inches
- Stack angular rock fragments to maximize interlock
- Orient elongated rocks perpendicular to the wall face
- Use rock with a range of sizes to fill voids efficiently
- Avoid segregation or size gradation within baskets
- Fill to a rounded, domed surface to shed water
- Machine tamping improves compaction
Consistent, professional packing is necessary to prevent future settling and deformation of the gabion baskets. Follow manufacturer guidance and specifications throughout the installation process.
Conclusion
Hard, dense, angular igneous and metamorphic rocks between 4-8 inches across provide optimal fill for gabion retaining walls. Cubical or rectangular rocks with some flat faces allow tight packing and interlocking. Softer, weaker sedimentary rocks as well as excessively large, rounded or flaky rocks should be avoided. Carefully source durable gabion stone from local qualified quarries to meet the project specifications. Proper installation and compaction of the rock fill is also key to ensure lasting structural integrity of the gabion baskets.