Natural materials

Natural materials – silt

In this blog post we will characterize the natural material silt. We will try and answer some of the most important questions in relation to silts such as; What is silt? What is silt used for? What is the difference between silt, sand and clay? Does silt pollute water? What is siltation?

Definition of silt

Silt is a characteristic soil whose grain size lies between that of clay and sand with sizes ranging between 0.002 mm and 0.063 mm.

Related read: Soil characteristics – sand, Natural materials – clay

Silt is a detritus material meaning that it stems from weathered rock material and you can actually feel the individual particles between your fingers if rubbed gently. The physical properties of silt is hard to make general characterizations about as the individual properties depend on its ability to adhere to water and drain. An example of possible explanatory properties is the Atterberg limits.

Atterberg limits

The silty soils adherence to water makes is hard to accurately characterize. This means that the defining characteristic when it comes to soil dynamics is the ability to describe the Atterberg limits of silty soil. Atterberg limits are used to describe cohesive materials for their strength and water limit, see source.

The Atterberg limits are a simple way of describing the bearing capacities of wetted silt and clayey soils. These Atterberg limits are some of the defining limit states behind the soil characteristics and are used to accurately describe the water content and corresponding soil behavior in a simple manner.

Individual soil states

The Atterberg limits are described in four different manners called states. These states are given from silty and clayey soils and includes; Solid, Semi solid, Plastic and Liquid.

The definition of each of the subzones varies based on the type of clay and silt being investigated. Here the defining property is the ability to adhere and keep water content within the soil. Generally speaking, as the silty soils water content increases, it transforms its characteristics from that of a solid towards a liquid.

Atterberg limits visualized

The key defining characteristic behind the definitive behavior of silts and clays is their ability to keep water within the substrate. This is best illustrated from the resulting stress-strain curves usually employed when defining solid state materials bearing strengths. An illustration of the different characteristic states of the material alongside the bearing capacity of each state is outlined in Figure 1.

Illustration of the Atterberg limits where the individual soil characteristics are outlined this includes differences within the Solid Limit, Plastic Limit and Liquid Limit state space of the watery soils
Figure 1: Illustration of Atterberg limits with different strength characteristics shown versus different states of the material. This includes the stress-strain curves of the material which describe the material properties through the SL, PL and LL states

With this initial explanation of the Atterberg limits, it quickly becomes evident the importance of defining each of these limits in an accurate manner.

Thought process behind silty\clayey soils bearing capacity

To set the stage for the upcoming explanation of why these limits are important, lets try and imagine a scenario where a large infrastructure project is trying to be built.

Firstly one may investigate the initial area visually ensuring that there are no immediate obstacles needing to be removed before geotechnical investigations might be performed.

Second, a set of initial tests have examined the cross-section of the earth and found out that the initial state of the underground consists of a mixture of clay and silty soil.

Third, now one may wonder what the initial strength of such soils might be and if at all the soil exhibits sufficient capacity for bearing the load of the structure proposed for the site in question.

In conclusion, it becomes evident that the bearing capacity of silty clayey soils must be understood properly for the construction of larger infrastructures to be feasible.

As we now understand, it is important to estimate the limiting behavior of soils when subjected to loads and therefore we need to calculate the Atterberg limits. In the following sections we dive deeper into the calculation of the different types of Atterberg limits. Starting of with the liquid limit, LL.

The Casagrande cup experiment

The Casagrande cup experiment is used for the determination of the soil water content based on a standardized number of blows until a small valley created through use of specialized instruments collapses. See Figure 2 for an illustration of the equipment and resulting collapse of the valley.

The Casagrande test experiment illustration of the equipment utilized alongside with an explanatory figure of a silty soil sample.
Figure 2: Casagrande cup filled with substrate. Top left: valley and filled cup before number of blows. Bottom left: Collapsed valley after x number of blows. Right: Illustration describing the specialized equipment used for creating the valley and material used as shock absorption.

When the valley starts to fall together the number of blows is numbered on a piece of paper. The soil sample is then weighed and dried with the resulting weight before and after measured carefully. This way you can determine the number of blows before failure mechanisms of the substrate are activated as a function of the water content of the sample.

A relationship between the number of blows and the water content can then be drawn and theoretical distributions for the soil strength can be inferred through careful regression techniques. An example of a given relationship between water content and number of blows is outlined in Figure 3. The predetermined liquid limit is theoretically defined for 25 number of blows to the sample.

Describing the water content and corresponding number of blows in the Casagrande test for identification of the liquified limit of the soil sample.
Figure 3: Water content versus number of blows within the Casagrande cup experiment before the collapse of the shear walls. The liquid limit of the Casagrande experiment corresponds to the straight line interception at 25 blows.

The liquid limit described by this test allows for an easy and fast interpretation of the liquid limit of silty clayey soils.

Now in order to try and estimate the plastic limit of the soil we need another test measure technique called the rolling test.

Rolling tests determining plasticity level

In order to determine the plastic limit of the soil, the limit where constant stress cause constant strain, we need to perform a rolling test on a sample of the substrate.

The methodology of performing a rolling test is in short to roll out a specific sample of the soil under assumptions of constant stress and cross section until the water content start to crack. When cracking is initiated the sample is quickly weighed before being dried out in the lab heaters for 24 hours.

Soil sample roll speed

When rolling the soil sample out you are actually drying out the sample therefore it is important to realize that the speed at which you are rolling out the sample influences the drying rate and correspondingly can result in errors as the sample is considered to contain a constant water content.

Faster speeds and pressure on the silty soil sample will cause inaccurate measurements of the water contents due to the pressure distribution on the soil pushing out water from underneath the soil sample.

A characterization technique for the natural material silt.
Figure 4: Example of a rolled silt soil experiment. The silty soil is observed through use of the cracked and jagged sample

However, rolling out the soil sample slowly is neither a good option as the contents of the water sample is likewise going to dry out before being able to accurately predict the water contents.

A perfect roll out speed is necessary for the plastic limit of the soil to be accurately described. This speed depends and varies individually from the different soil samples and thus expert engineering judgement is needed for determination of the plastic limits.

Differentiating clay from silt

When trying to investigate soil properties and before determining and characterizing the difference qualitative parameters of the soil sample it is important to consider the distinctive behavior’s separating silt from clay. The distinction is easily made through use of two different test techniques. Firstly the ‘Elephant’ test technique is easily utilized

Elephant test technique

The elephant test technique for distinguishing soil behavior’s is quickly and accurately utilized even without expensive field laboratory equipment. The distinctive test is performed by creating an elephant like trunk shape of a small soil sample which is approximately 3 mm thick in shape.

The shape of the constructed elephant trunk is then examined and seen whether or not it is able to withstand the force underneath its own gravity. If the trunk easily upheld its own shape under its own weight, then the soil sample is considered to be clay.

However if the soil sample cannot easily withstand the force of gravity with deformed shapes and cracks on the surface then the soil sample is considered silt. This is an easy and inexpensive way of making sure that the soil sample in question is actually silt and not clay and visa versa.

Another commonly used methodology is to create a bended U-shaped sausage which is then utilized to check for cracks in a similar manner as the elephant. These inexpensive ways of determining whether or not a soil sample is consisting of clay or silt is invaluable.

With this being said, there also exist other methodologies to easily investigate whether a soil sample is clay or silt one of these is called a sedimentation test.

Sedimentation testing

The sedimentation test is a technique similar to that of the elephant test in that this specific version is likewise meant as an example to try and differentiate the soil sample into clay and silt respectively. The clay particles when settling in a tube will create a mixture of solid particles and liquids called a suspension while a silt sample will create a coarser sample.

Suspension definition and behavior

A suspension is particular interesting when trying to investigate the hydrodynamic properties of a fluid and when trying to investigate eco-systemic impacts of large infrastructure projects.

The ecosystem can easily be affected by suspensions in the sense that endangered species are affected by floating clay particles where gills of fish are clotted and marine fauna such as eelgrass are threatened by sedimentation layers stealing available sunlight.

The problems also arrives when considering the time spent on the settling particle velocities which for a given suspension can be extremely small, allowing individual particles to easily stay in the suspended water column for extended periods of time before it will be able to settle on the ground surface.

For clay the particles can stay in suspension for hours even days without settling in still water due to the water undulations within the water column causing the fine material to keep sculpting around.

Settling behavior of silt

The difference in settling between clay and silt is easily measured when conducting sedimentation experiments. As explained, the clay settling velocities are small allowing for the suspension to keep particles in the water column for extended periods of time corresponding to days.

For silt the suspension will fall out of the water column much faster than clays and thus when investigating soil samples and characterizing the sample into clay and silt, measuring the settling time and velocities would allow you to easily distinguish the soil sample into clay and silt.

The silts settling velocity is much larger than clay due to mainly two phenomena, the particle sizes of silt are an order of magnitude larger and flocculation effects are often quite significant for silty materials. The flocculation is when particles of different sizes clump together during settling resulting in increasing settling velocity of the entire joint lump of particles compared to the individual ones.

This particulate behavior leads to a common phenomena called siltation.


Siltation is a phenomena common in river -streams and -mouths as a result of excavating, extreme rainfall events or pollution. The siltation of river streams poses environmental dangers to ecosystems as the consequent sedimentation strangles bottom feeding and living creatures, while simultaneously damaging eggs and larvae exposed to silted environments.

Some common example of siltation events occur during marine dredging where silted material is transported upwards in the water column. The upward momentum allows whirling of particles up into the undisturbed water column causing the individual particles to be transported large distances and spread out over potentially endangered areas.

Another example of siltation is during extreme rainfall events where soil banks are eroded away due to increased water volumes causing silt and clay rich banks to be eroded and washed into the water column.

When transported downstream and before settling down into more calmer waters this siltation of water streams can cause suffocation of fish and degradation of wildlife. It is therefore considered an environmental disaster when rivers are exposed to siltation events.

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