engineering

Structural Soil

Structural Soil is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Structural Soil rather than just read about it. In short: Structural Soil is a medium that can be compacted to pavement design and installation requirements while permitting root growth. It is a mixture of gap-graded gravels (mostly made of crushed stone) and soil (mineral content and organic content).

Key takeaways

Structural Soil belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Structural Soil to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Structural Soil from memory before moving on to harder problems.

Reference excerpt

Structural Soil is a medium that can be compacted to pavement design and installation requirements while permitting root growth. It is a mixture of gap-graded gravels (mostly made of crushed stone) and soil (mineral content and organic content). It provides an integrated, root penetrable, high strength pavement system that shifts design away from individual tree pits.

History of structural soil Amsterdam treesoil is the first structural soil applied to increase the life expectancy of urbantrees. The reason for this development began in 1966 in Amsterdam in the Netherlands because of the high mortality rate among trees. And at the beginning of the 70s Amsterdam treesoil was the solution for the trees surrounded by pavers. In the late 70's it was the city of Groningen, also in the Netherlands, which had a solution for heavy traffic. For the first time a structural soil based on crushed stones was applied. It was called skeleton soil (in Dutch it is called "skeletbodem"). Soon after the first positive results became known, this solution received a lot of follow-up in various countries. Now after many decades of experience, we see various trends in the development of structural soil. One of these is to distinguish between two separate main groups.

SBSS Sand Based Structural Soil GBSS Gravel Based Structural Soil

Problems with typical installations

Previously the main problem facing the establishment of trees in paved areas is the lack of enough volume of soil for tree root growth. Soils under pavements are typically so compacted that it stops roots from growing. Older established trees with their roots under pavement grow poorly and often die. They can also cause pavement failure and displacement. Overall pavement preparation and repairs can shorten the life expectancy of a tree to 7–10 years where we could see them grow for at least 50 more years.

CU-Structural Soil Structural soil was researched and developed in the 1990s by Cornell University’s Urban Horticulture Institute. In 1999, AMEREQ signed a licensing agreement with Cornell University and currently holds the patent rights to Cornell’s CU-Structural Soil Urban Tree Planting Mix. It is marketed as CU-Structural Soil for quality control and is produced by a network of qualified AMEREQ-licensed companies. CU-Structural Soil on average costs $35–$42 per ton. Other companies have formed their own brand of structural soil based on Cornell’s work. For example, STALITE has developed STALITE MATRIX Structural Soil that they claim holds more moisture.

Wallace Structural Soil/Gap Graded Soil

Wallace Laboratories modified work in 1994 which had been done by others to help improve the technology. The concept had been used in Europe in the 1980s. They did not seek a patent on their work but left the technology in the public domain for others to freely use. Their work preceded the filing of the Cornell University patent. Briefly, the aggregate size was increased to about 2 inches (5.1 cm), the soil texture was changed to a clay or clay loam in order to increase the water holding capacity and nutrient capacity, the soil was conditioned with the linear polyacrylamide, and the soil chemical and physical properties were specified. Their procedure has been extensively used worldwide. One municipal installation used about 50,000 cubic yards of the Wallace Labs formulation.

Composition

Structural soil is composed of crushed stone (typically limestone or granite) narrowly graded from ¾-1 ½” highly angular with no fines, clay loam which should conform to the USDA soil classification system. The hydrogel is added in a small amount to prevent the separation of the stone and soil during mixing and installation. Usually a layer of stone is spread, then the dry hydrogel is spread evenly on top and screened moist loam is placed on top. The entire mixture is then turned until a uniform blend is produced. Structural soil is not typically stockpiled; it should be mixed and installed soon after delivery. If a stockpile is required, the soil needs to be protected from the elements so it does not become contaminated. Installation typically calls for two cubic feet of soil is needed for every square foot of crown projected. It is also recommended for irrigated trees to have a low-volume drip irrigation. Cornell also suggests a minimum of 24” to 36” for CU-Structural Soil depth and they have established no minimum for length and width of installation, however, because it is a structural soil it was designed to go under the entire pavement area. Testing has shown that structural soil is safe around utilities and that you can use trees from any production system such as balled-and-bur lapped, bare root, containerized and boxed trees.

Continued development Cornell is continuing its development of CU-Structural Soil, expanding its use as the need for trees and other greenery within highly urbanized areas grows. CU-Structural Soil has been used in over 1000 applications and has been proven a very viable option for construction in cities.

References

Illustrations

Worked examples

Example 1 — a first encounter with Structural Soil

Start with the simplest possible case. Write down what Structural Soil claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Structural Soil before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Structural Soil ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Structural Soil

In research: Structural Soil appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Structural Soil in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Structural Soil is common in secondary-school and first-year university syllabi. It links to neighbouring topics Pavements, Soil-based building materials, so understanding it makes those chapters shorter.
In everyday life: Look for Structural Soil outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Structural Soil in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Structural Soil means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Structural Soil out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Structural Soil in simple terms?

Why does Structural Soil matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Structural Soil?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Structural Soil.