A land-based structure of any type is only as strong as its foundation. For that reason, soil is a critical element influencing the success of a construction project. Soil is either part of the foundation or one of the raw materials used in the construction process. Understanding the engineering properties of soil is crucial to obtain strength and economic permanence. Soil stabilization is the process of maximizing the suitability of soil for a given construction purpose.
The necessity of improving the engineering properties of soil has been recognized for as long as construction has existed. Many ancient cultures including the Chinese, Romans, and Incas utilized various techniques to improve soil stability, some of which were so effective that many of the buildings and roadways they constructed still exist today. Some are still in use.
In the
Why soil stabilization?
ü To increase strength, bearing capacity and resistance to deteriorative forces of nature and man made environment.
ü To decrease the volume change tendency, settlement and to control permeability.
Traditionally stable sub-grades, sub-bases and/or bases have been constructed by using selected, well-graded aggregates, making it fairly easy to predict the load-bearing capacity of the constructed layers. By using select material, the engineer knows that the foundation will be able to support the design loading.
Gradation is an important soil characteristic to understand. A soil is considered either “well-graded” or “uniformly-graded” (also referred to as “poorly-graded”). This is a reference to the sizes of the particles in the materials. Uniformly-graded materials are made up of individual particles of roughly the same size. Well-graded materials are made up of an optimal range of different sized particles.
It is desirable from an engineering standpoint to build upon a foundation of ideal and consistent density. Thus, the goal of soil stabilization is to provide a solid, stable foundation. “Density” is the measure of weight by volume of a material, and is one of the relied-upon measures of the suitability of a material for construction purposes. The more density a material possesses, the fewer voids are present. Voids are the enemy of road construction; voids provide a place for moisture to go, and make the material less stable by allowing it to shift under changing pressure, temperature and moisture conditions.
Uniformly-graded materials, because of their uniform size, are much less dense than well-graded materials. The high proportion of voids per volume of uniformly-graded material makes it unsuitable for construction purposes. In well-graded materials, smaller particles pack in to the voids between the larger particles, enabling the material to achieve high degrees of density. Therefore, well-graded materials offer higher stability, and are in high demand for construction.
When soil stabilization?
With the increased global demand for energy and increasing local demand for aggregates, it has become expensive from a material cost and energy use standpoint to remove inferior soils and replace them with choice, well-graded aggregates. One way to reduce the amount of select material needed for base construction is to improve the existing soil enough to provide strength and conform to engineering standards. This is where soil stabilization has become a cost-effective alternative.
Defining soil stabilization:
Soil Stabilization is the permanent physical and chemical alteration of soils to enhance their physical properties.
Improving an on-site (in situ) soil’s engineering properties is referred to as either “soil modification” or “soil stabilization.” The term “modification” implies a minor change in the properties of a soil, while stabilization means that the engineering properties of the soil have been changed enough to allow field construction to take place.
Soil stabilization is a collective term for any physical, chemical, or biological method or any combination of such methods employed to improve certain properties of natural soil to make it serve adequately an intended engineering purpose.
Soil is one of nature’s most abundant construction materials. Almost all construction is built with or upon soil. When unsuitable construction conditions are encountered, a contractor has four options:
(1) Find a new construction site
(2) Redesign the structure so it can be constructed on the poor soil
(3) Remove the poor soil and replace it with good soil
(4) Improve the engineering properties of the site soils
In general, Options 1 and 2 tend to be impractical today, while in the past; Option 3 has been the most commonly used method. However, due to improvement in technology coupled with increased transportation costs, Option 4 is being used more often today and is expected to dramatically increase in the future.
Advantages to soil stabilization:
o Stabilized soil functions as a working platform for the project
o Stabilization waterproofs the soil
o Stabilization improves soil strength
o Stabilization helps reduce soil volume change due to temperature or moisture
o Stabilization improves soil workability
o Stabilization reduces dust in work environment
o Stabilization upgrades marginal materials
o Stabilization improves durability
o Stabilization dries wet soils
o Stabilization conserves aggregate materials
o Stabilization reduces cost
o Stabilization conserves energy
Methods of soil stabilization:
Removal & Replacement:
Excavate unsuitable soil and replace compaction fill used when soil is too loose use same soil for fill which has high unit weight which have engineering properties. Removal will be done first soil has excessive organics. It is expensive method because we want to dispose and import the soil. Both are suitable only above ground water table. Earthwork operation is different if soil is wet.
Precompression:
For improving soil we have to cover them with a temporary surcharge fill. Preloading, surcharging. Suitable for soft clayey and silty soils because static weight of fill cause them consolidate thus improves settlement of strength properties after the properties attains, surcharge id removed and construction proceeds surcharge fills 3-
Insitu densification:
It is the method of densifing shallow soil, using heavy vibratory rollers up to 2m.
Vibrocompaction:
There are two methods of vibrocompaction, Terraprobe and vibroflot. Terraprobe consists of vibratory pile hammer attached to steel pipe. Pile is vibrated. A vibroflot contains vibrator and water jet. Depth up to 3- 15m, silt content less than <12-15%.
Dynamic compaction:
Cost effective method of densifing loosen sandy and silty soil. Primary zone of influence typically extends to depth of 5-10m with lesser improvements below these depths. It is used to treat 5-10m with lesser improvements below these depths .It is used to treat liquefaction prone soil, collapsible soil. It is evaluated by performing STP&CPT tests before and after construction.
Blast Densification:
Curious than the above one. It consist of drilling a series of boring & using them to place explosive underground. It is effective in clean sands. Because of vibration of safety issues it is only suitable for remote sites.
Insitu Replacement:
It is intended to provide load bearing members that extend through weak strata. The stone column acts as vertical drain thus helps in accelerate consolidation settlement of mitigate seismic liquefaction problem.
Grouting:
It is the process of injection of special liquid or slurry material into ground. Two types: cementitious and chemical grout. Cement grout made of Portland cement that hydrates after injection forming a solid mass. Chemical grout includes wide range of chemicals that satisfy once they are injected into ground.
Four principal methods:
Intrusion grouting: filling joints in rock by injecting grout through pipe cementitious grout is suitable. It is used to prepare foundation for dams.
Permeation grouting: injection of thin grouts into soil to permeate into voids. Chemical grout is used.
Compaction grouting: It uses a stiff grout that is injected into ground under high pressure through a pipe. It is used to repair structures that have excessive settlement.
Jet grouting: It uses a special pipe equipped with horizontal jets that inject grout into soil at high pressure.
Insitu deep mixing:
It uses rotating mixer shafts, paddles or jet that penetrates into ground while injecting of mixing Portland cement. It includes deep cement mixing, deep jet mixing. The treated soil has greater strength, reduced compressibility than original soil.
Surface mixing:
Upper soil gets ripped, applying the admixture mixing with special equipment and compacting. Once mixture has cured it forms very hard and durable soil. It forms a layer called sub base in highways and airports. It is no more than 200 mm.
Reinforcement:
Tensile reinforcement members improve the soil stability of load carrying capacity. This used in construction of compacted fill slopes of earth retaining structures.
Types of soil stabilization:-
There are mainly two types of stabilization. They are Chemical stabilization and Mechanical stabilization.
1) Chemical soil stabilization:-
One method of improving the engineering properties of soil is by adding chemicals or other materials to improve the existing soil. This technique is generally cost effective: for example, the cost, transportation and processing of a stabilizing agent such as soil cement or lime to treat an in place soil material is probably more economical than importing aggregate for the same thickness of the base course.
Additives can be mechanical that is their load bearing properties bolster the engineering properties of the soil. They can also be chemical that is they react with or change the chemical properties of the soil thereby changing the properties of the soil.
Combining the additives with the soil is done using various machines. The method to be used depends on three factors:
1. What machines are available.
2. The location (urban or rural).
3. The additives that are used.
The following are the machines used to combine the additives with the soil:
1) Rotary Mixer:
The most economic and time efficient method is to use a rotary mixer, a large machine that incorporates additives with the soil by tumbling them in a large mixing chamber that is equipped with a rotor designed to break up and mix the materials. It is capable of introducing the additives and water very uniformly into the soil and hence the rotary mixer is unrivaled in production by other methods.
2) Pugmill:
For some of the applications which require a lot of precision, a pugmill is used. A pugmill is a large mixing chamber that resembles a cement mixer. Measured pre-graded aggregates, additives and usually water are usually mixed in the pugmill and applied in uniform thickness. Pugmills produce high quality soil stabilization but at higher costs and lower production speeds.
3) Motor grader:
Blade mixing is usually done using a motor grader. This type of mixing is not as efficient as the other mentioned systems but is far less complex than the other systems. Essentially the additives are placed in flat windows and the blade of the grader mixes the additives with the soil in a series of turning and tumbling actions. It is very difficult to uniformly control mixing percentages and mixing depths using this technique.
Additives used:
There are many kinds of additives available. Not all kinds of additives work for all soil types and a single additive will perform differently with different soil types. Generally an additive may be used to act as a binder, alter the effect of moisture, increase the soil density or neutralize the harmful effects of a substance in the soil. Following are some of the most widely used additives:
ü Portland cement
ü Quick lime/hydrated lime
ü Flyash
ü Calcium chloride
ü Bitumen
ü Chemical or bio remediation
2) Mechanical soil stabilization:-
This refers to either compaction or the introduction of fibrous and other non- biodegradable reinforcements to the soil. This practice does not require chemical change of the soil. There are several methods used to achieve mechanical stabilization.
(i) Compaction:
Compaction typically employs a heavy weight to increase the soil density by applying pressure from above. Machines such as large soil compactors with vibrating steel drums are often used for this purpose. Here over compaction of the soil should be avoided and given great consideration because in the case of over compaction, the aggregates get crushed and the soil loses its engineering properties.
(ii) Soil Reinforcement:
Soil problems are sometimes remedied by engineered or non engineered mechanical solutions. Geo-textiles and engineered plastic mesh are designed to trap soils and help control erosion, moisture conditions and soil permeability. Larger aggregates such as gravel, stones and boulders are often employed where additional mass and rigidity can prevent soil migration or improve load-bearing properties.
(iii) Addition of graded aggregate materials:
A common method of improving the engineered characteristics of a soil is to add certain aggregates that lend desirable attributes to the soil such as increased strength or decreased plasticity. This method provides material economy, improves support capabilities of the subgrade and furnishes a working platform for the remaining structure.
(iv) Mechanical Remediation:
Traditionally this has been the accepted practice to deal with soil contamination. This is a technique where contaminated soil is physically removed and relocated to a designated hazardous waste facility far from centers of human population. In recent times however, chemical and bioremediation have proven to be a better solution both economically and environmentally.
No comments:
Post a Comment