Most of the grains in a soil cannot be seen with the naked eye, and still engineering manages to describe them. The episode shows how a stack of sieves turns a sample into a grain size distribution curve, what the shape of that curve reveals about the ground and which numbers sum it up.
Prefer reading? The full transcript is just below. The timestamps jump to the exact moment in the video. Narration and subtitles in English.
In three points
The grain size curve is the soil's document. Each sieve becomes a point on the graph: how much is gravel, how much is sand, silt and clay.
The shape of the curve tells the behaviour. An open curve means a well graded, dense soil, good for carrying load; an upright curve means a uniform soil, full of voids and an excellent drain.
Two numbers sum up the curve. The effective diameter D10 and the coefficient of uniformity, Cu = D60/D10, hold in any laboratory in the world.
Narrated transcript
The episode in text
Full text of the episode narration. Click a timestamp to watch that passage.
The soil's document is a graph
Every identity card has a photo, a name and numbers.
The soil's card is a graph. And whoever learns to read that curve discovers the character of a site before setting foot on it. The soil's identity card.
The stack of sieves
Most of the grains in a soil cannot be seen with the naked eye. Even so, engineering manages to describe them with the grain size distribution curve.
In the laboratory, the dry sample goes through a stack of sieves, from the coarsest mesh to the finest. What is retained on each one is weighed.
How much passed through each sieve becomes a point on a graph, and the points draw the grain size curve: how much of the soil is gravel, how much is sand, silt, clay.
That is the document. And the shape of that curve tells how the soil will behave.
An open curve: the market bag
A stretched, open curve means grains of every size: a well graded soil. For carrying load, that is excellent.
It is the logic of the market bag: between the oranges, lemons fit. Between the lemons, beans fit. Between the beans, there is still room for flour. The volume of the bag did not change, and the amount of things inside only grew.
Soil works the same way: the medium grains live in the voids of the coarse ones, the fines in the voids of the medium ones. Little void is left, and it is the void that gives way. Less void, more contact between the grains: a denser, firmer soil.
An upright curve: dune sand
A grain size curve standing upright, closed, means grains of similar sizes, like dune sand: a uniform soil.
An arrangement full of voids, with no smaller grain to fill them. But this kind of soil has its use too: water flows through it easily, and that makes it an excellent drain.
The No. 200 sieve and sedimentation
A curious detail: the last sieve, the finest one, No. 200, has an opening of 0.075 mm, about the thickness of a human hair. Fine. Like a silk stocking.
Below that, sieving stops working. The fines stick to the wires of the sieve and to each other, and they do not pass, even though they are smaller than the hole.
It is the electrochemical forces from episode 2, once again. To measure the fines, the laboratory swaps the sieve for sedimentation: in water, the bigger grain sinks first, and the time it takes to fall reveals its size.
Visual and tactile description in the field
But before all of this, still in the field, the soil has already been given a name. In the borehole, the sample comes out of the sampler and runs through the fingers: sand is gritty, silt is silky, clay is sticky and mouldable.
That is the visual and tactile description, and the result goes into the boring log: brown silty clayey sand.
The name is valuable, it is the first portrait of the ground. But it is a fingertip reading: it lists the fractions and does not say how much there is of each. How much, only the curve can tell.
D10 and the coefficient of uniformity
That is why engineering sums up the grain size curve in indices. The effective diameter, the D10, is the size below which 10% of the soil lies, by weight. In sands, it has the greatest influence on permeability, because it is the fines that narrow the path of the water.
The coefficient of uniformity divides the D60 by the D10. A high number, an open curve: the market bag. A low number, an upright curve: the dune.
Two numbers that hold in any laboratory in the world. And any geotechnical engineer on the other side of the planet already has a good idea of which soil we are talking about. That is the power of soil mechanics: turning soil into numbers, and numbers into decisions.
The hook for the next one
From here on, the numbers take over for good. Weight, voids, moisture, saturation. All measured. All calculated. All written down to the decimal point.
There is just one problem. None of them is true.
Find out in the next episode.
Questions from the episode
What gets answered
What is the grain size distribution curve?
It is the graph that shows how grain sizes are distributed in a soil. In the laboratory, the dry sample goes through a stack of sieves, from the coarsest mesh to the finest; what is retained on each one is weighed, and the percentage that passed through each sieve becomes a point. The curve tells how much of the soil is gravel, sand, silt and clay.
What does the shape of the curve say about the soil?
An open, stretched curve means grains of every size: a well graded soil, in which the medium grains fill the voids of the coarse ones and the fines fill the voids of the medium ones. Little void is left, and the soil is denser and firmer. An upright curve means grains of similar sizes, like dune sand: a uniform soil, full of voids, through which water flows easily, which makes it a good drain.
Why does sieving stop at the No. 200 sieve?
The No. 200 sieve has an opening of 0.075 mm. Below that size, the fines stick to the wires of the sieve and to each other, through electrochemical forces, and do not pass even though they are smaller than the hole. To measure silt and clay, the laboratory uses sedimentation: in water, the bigger grain sinks first, and the time it takes to fall reveals its size.
What is the visual and tactile description?
It is the first identification of the soil, made in the field during drilling. The sample comes out of the sampler and runs through the fingers: sand is gritty, silt is silky, clay is sticky and mouldable. The name goes into the boring log, for example brown silty clayey sand. It lists the fractions but does not say how much there is of each; only the grain size curve can tell that.
What are the D10 and the coefficient of uniformity (Cu)?
The D10, or effective diameter, is the size below which 10% of the soil lies, by weight; in sands, it has the greatest influence on permeability. The coefficient of uniformity is Cu = D60/D10. A high number means an open curve and a well graded soil; a low number means an upright curve and a uniform soil.
From the episode to the field
Where this story becomes work
Naming the soil in the field and measuring its grain size curve in the laboratory is everyday work. These are the Damasco Penna services that carry out that identification.
