Structural soil is a mixture of crushed stones for strength and soils for horticultural needs. The stones create a matrix to support pavement. The stone lattice provides stability by stone to stone contact as well as void space for root penetration, aeration, and water infiltration. The soil mixture should be a loam to heavy clay loam. Organic materials are not suitable pavement materials, and are not currently mixed in structural soils. The clay texture is important to increase nutrients and water holding capacity. While the structural soil as a whole is compacted for bearing-load strength, soils in between crushed stones are suspended to allow root growth.

The exact materials and ratio of ingredients vary depending on the products. Some contain hydrogels in addition to the aggregate materials and soils. Some products use lightweight aggregate materials such as expanded shales.

Basics of crushed-stone structural soils:

• Aggregate must be “no-fines,” and all one size. Typically all stones need to be around the same size. CU-soil uses 1-1.5”, Michael Mills uses 3” and removes flat stones with a “cone separator.”
• An angular aggregate creating larger void space and forming a stable matrix must be used.
• Aggregate should be pH neutral or unaffected by leaching. Granite, very hard limestone, or other hard rock is preferable.
• Aggregate size determines size of individual voids. Rule of Thumb: void diameter is 1/5 of the aggregate or sand diameter. Very large voids may encourage separation of soil from the matrix.
• Void size must accommodate root size. Over 80% of average mature trees’ roots are less than 10mm in diameter (3/8”). If larger, usually it is within 3’ of trunk. 2” aggregate creates larger void than 3/8” aggregate.
• Soil must not completely fill voids to allow aeration and drainage. This also keeps soils from compaction and from interfering with the stone matrix. CU recommends filling only 55% of void space with soil. LESS is MORE.
• Soil must stick to stone during the placement. Structural soil must be damp, but protected from rain, during the installation. CU uses hydrogel, Mills uses Soil Stabilizer, a high clay mix to stabilize the soil.
• Soil volume must be adequate for trees. A general rule is to provide 2cf soil volume for every 2sf of area within the mature dripline. This requires approximately 5 times more structural soil than normal soil. The optimum amount is not yet known.
• Use adequate depth to achieve volume and lead roads downwards. UHI recommends a minimum of 24”, preferably 36” depth.
• Extra water holding capacity is usually required. Adding clay, hydrogel, and porous aggregate such as expanded shale and slate will help.