Soil Stabilisers - Misunderstood or Misleading

Soil Stabilisation Series – Article 2

There are a wide range of soil stabilisers on the market that are currently being utilised in real-life engineering projects that have not been proven to be successful methods of stabilisation, and/or are being incorrectly applied. This article will be identifying a number of well-known soil “stabilisers” that are quite often misunderstood or frankly, misrepresented as soil stabilisers. In particular, the article will be discussing as to why some stabilisers are deemed effective, and why are others are not, and why they are still utilised extensively throughout the industry. The following stabilisers will be evaluated: lignosulfonates (tree resins), Magnesium Chloride, Calcium Chloride, Liquid Polymers, enzymes, polyacrylamides, and pozzolans all of which have been mentioned quite often when discussing effective stabilisers. 

Lignosulphonates

These chemicals are the by-product of the timber and paper industries and have some application as soil binders or dust suppressants. However, their application is really only appropriate for temporary roads or in arid climates. Lignosulphonates are water-soluble, therefore when they come into contact with water, they break down. As the material breaks down, the lignosulphonates bonds create a slippery layer on the surface of the material increasing risk of harm and liability. This property of Lignosulphonates makes it an effective, but short term option when applied in arid climates, however, a colleague described lignosulphonates as sugar, “harden when dry, and liquified when wet”. 

Salts

Magnesium Chloride and Calcium Chloride are classified as deliquescent chemicals. A Deliquescent is a material that has the ability to absorb water from the atmosphere to help control dust. However, these chemicals do not possess any binding or stabilising properties. These salts are especially vulnerable to water, as both of these chemicals are prone to being dissolve in the presence of free water (runoff or rain). For dust control, they need a mix of no rain and humid air to work as an effective dust control agent. Meaning both Magnesium Chloride and Calcium Chloride are only effective in very specific situations.

Enzymes

Enzymes increase the wetting and bonding capacity of soil particles by assisting soil bacteria in releasing hydrogen ions, which results in a pH drop (leading to a more acidic soil) which leads to the breaking up of the soil structure. This allows for soil materials to easily absorb moisture, and become more densely compacted as a result. This method may have some benefits, however, enzymes are only truly effective on a very narrow range of soils that, naturally, already have a high natural binder content, such as clays. Enzymes have been tested on a number of various soil types, and have found to be most effective in organic soils, but, overall, they still only show minimal strength gains. They are also utilised in the cases as a subgrade stabiliser for economic reasons, as well as for erosion control. 

Polyacrylamides

Polyacrylamides (PAMS) are long-chain polymers can work as flocculants and dust control agents due to their ability to attract and bind fine particles, but cannot be considered a soil stabiliser. They have proven to have no impact on permanent strength, and are counter-productive in the presence of water. This “stabiliser” has been picked up even in Australia in rural, local councils as a “quick fix” method by engineers or foremen with little technical qualifications. The reason it is usually used as a quick fix is due to low cost. In other words, on state and federal projects, it’s more than unlikely that polyacrylamides would be utilised for the purpose of a soil stabiliser, especially as the engineers on-site as qualified professionals with technical expertise in soil stabilisation. Polyacrylamides are usually utilised in controlling erosion as chemically it is utilised often as a flocculant. It forms ionic bonds of small soil particles, thus clumping them together to make larger particles. As a result, this makes the soil more resistant to the erosive forces of dispersion and shear.

Natural or Manufactured Pozzolans

What is known to work across most situations for the sole purpose of road base materials are pozzolanic stabilizers. However, in and of themselves they are not appropriate as stabilisation agents. Some parts of the world have natural deposits of pozzolanic soils (often volcanic in origin) which have been used for thousands of years to provide stable roadways. Ancient Roman engineers developed these in conjunction with lime to produce structures still standing today such as the Pantheon in Rome itself. Flyash (from coal combustion), is a common example of widely used pozzolanic material in modern cement blends to reduce this going into landfill, as well as delivering technical benefits in long term strength gains.

Pozzolanic materials in the presence of water and Calcium Hydroxide produces a cementitious compound. This compound when correctly applied improves the fluidity of flowable fill and grout. This occurs due to the chemically spherical shape and particle distribution of the compound. Pozzolanic stabilisers are an extremely versatile material that can achieve desirable results even when exposed to wet or dry conditions and can be applied on a variety of different soils. Practically, pozzolanic stabilisers can be utilised on flexible or rigid pavement surfaces. It’s packed full of properties that make it a suitable candidate for many soil stabilising situations. It can survive various conditions, across different soil, and various pavements types. It can also be mixed with other compounds to adjust certain properties further to maximise its effectiveness. 

Liquid Polymers

Any stabiliser that is composed of liquid polymers must be completely dry to be able to activate its properties binding strength, and for this reason, it is an impractical stabiliser for the purpose of road projects where time is an important factor, and situationally cannot adapt to different conditions. As the ground never fully dries to a depth of 50mm. In most situations, there is already enough moisture in the ground to disable the polymers ability to bind. GRT has developed a range of polymer-based technologies and practices that will be discussed in Article 3 & 4 of this series.

Stabilisation – the common challenge

Enzymes, lignosulfonates and liquid polymers are an effective soil stabiliser and are able to achieve a reasonable result, however, only in carefully selected applications. In common circumstances, a lack of understanding often leads to misuse of the product. The main problem that has been identified by engineering professionals is that most stabilisers are reasonably sufficient. However, most stabilisers are not versatile, and hence, cannot be used as a quick fix in just any situation. Each soil stabiliser has been engineered for a specific purpose, and when these stabilisers are utilised for the wrong circumstances is when they become useless. 

To the government, infrastructure stabilisation can play an important role, especially in all road construction they can save box depth and base volume, which ultimately saves money. This is particularly effective when the cost of base materials is high and this forces companies to looks for alternatives and the associated costs to be considered. This usually results in the transportation of base materials across long distances, which again increases cost. After such a rich mining history, here in Australia, most mines are now located in remote locations, which are far from quarries, therefore transportation costs are always a contributing factor overall project cost. In this case, bearing capacity on a hardstand yard, or pavement is utilised to save the client money.

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