What Is Ground Improvement?
Ground improvement is typically a method or a procedure used for enhancement of ground which is in a poor condition or a disturbed state. Various geotechnical techniques are used to re-engineer the existing soil to improve its characteristics as well. Soil improvement is usually done to meet the requirements of the type of structure that is to be constructed on that particular piece of land.
Why Is Ground Improvement Needed?
Improvement of ground is done for the following reasons:
1. To improve the load-bearing capacity of soil.
2. To increase the density of soil.
3. To control ground settlements and permeability of soil.
4. To reduce the liquefaction of soil.
5. To increase the stability of slope.
What Are The Benefits Of Ground Improvement?
The methods of ground improvement enhance the weak soils as well as the unsuitable and contaminated soils. Soil improvement has various benefits such as:
- Less consumption of time as the design and implementation is relatively fast.
- When the techniques are applied for ground improvement, it hardly produces any waste. So no disposal costs are engaged in it.
- Easy design and construction of substructure.
- Ground improvement is applicable and also eﬀective on various types of soils.
Diﬀerent Methods For Ground Improvement
These are various techniques available for ground improvement. The methods of ground improvement are categorized based on the types of soil. They are as follows:
a. For Cohesive Soil
- Sand drains
- Wick drains
- Stone columns
b. For Cohesion-less Soil
- Terra probe
- Compaction piles
General Techniques For Ground Improvement
- Removal and replacement of soil
- Deep mixing of soil
Ground Improvement Methods For Cohesive Soils
Pre-compression is a method of ground improvement used for soils like clay and silt. It is also known as preloading or surcharging.
In this method, the dedicated land area for construction is covered with a surcharge, temporarily before construction. This surcharge is filled upto a suitable height to cause pre-compression of the soil below. This is kept on the soil for a longer duration for consolidation of the soil below.
The height of the surface generally varies from 3m to 10m respectively. The surcharge is removed just before the commencement of construction of the structure. This method is economical but also time consuming, on the other hand. It delays the construction so it is not preferred much.
2. Sand Drains
This method is also used to enhance the properties of ground with cohesive soils like clay and silt. Sand drains are sand columns constructed at a required depth, according to the structure.
These columns are used to provide strength and to reinforce poor soils. Sand drains are mainly used for stiﬀening compressible foundation soils and to increase the time rate of settlement to control the post-construction settlements. The major drawback of sand drains is that it is a very expensive method.
3. Wick Drains
Wick drains are used as an alternative to sand drains, as the construction of sand drains is quite expensive. Wick drain is a recently developed ground improvement technique by geotechnical engineers.
Wick drains are prefabricated vertical drains that comprise corrugated plastic ribbons covered by geosynthetic membrane.
A header drain is provided at the top of wick drains, to discharge the pore water. Wick drains are strip shaped drains of size 100mm wide and approximately 5mm thick. Wick drains have almost replaced sand drains as it is a less expensive method as compared to sand drains.
The wick drains are inserted into the soil using tubes. Once the wick drains are pushed into the soil, the test tubes are gradually withdrawn, leaving the wick drains into the ground. Recently, some machines are also developed for easy installation of the drains.
4. Stone Columns
Soft clays can also be treated using the stone column, as a method of ground improvement. Stone columns are constructed by making holes upto the desired depth in the ground & filling the hole with gravel or small stones respectively. The size of stones may vary from 6mm to 40mm.
The hole for stone columns is excavated using equipment called vibroflot. It consists of a long tube of about 2m to 3m long and 200mm to 500mm wide. It is filled with jets at the top & bottom. A mass rotating the vibroflot helps in the excavation of the soil. Once the excavation is done, the hole is filled with stones or gravel.
The diameter of the stone column varies from 0.5m to 0.7m and the spacing between two adjacent columns is usually 1.5m to 3m.
Ground Improvement Methods For Cohesionless Soils
As discussed in stone columns, vibroflot is a cylindrical tube shaped equipment consisting of a vibrator and jets at the top part and the bottom part of the tube. This method is generally used for the compaction of loose soils. It is also used to reduce the possibility of diﬀerential settlement.
The vibroflot is sunk into the ground with the help of a water jet or air, till it reaches the weak soil. Once the equipment reaches the loose and weak soil, the water jet is stopped and due to the vibration of equipment, the soil gets compacted, expelling the extra air and water, filling up the voids. The vibroflot is then withdrawn in stages of 12 inches gradually. Some backfilling is to be done as the vibroflot is withdrawn.
Vibroflotation is eﬀective upto a depth of 20m. The required density is achieved in 70 percent of cases.
2. Terra Probe
Like vibroflotation, this method is also a technique of Vibro-compaction of cohesionless soils. The terra probe, having an open ended pipe of 75cm diameter, is provided with a vibratory pile at its bottom.
The terra probe is penetrated into the ground using the vibratory pile driver inside the terra probe. Once the terra probe is reached at the desired depth, it is slowly withdrawn while the vibratory driver continues to operate.
The soil around the probe is densified. This procedure is repeated at various locations on site. The hole’s spacing is about 1.5m. Terra probe can be used upto a depth of 20m. It is an ideal method of ground improvement for the compaction of saturated sand deposits.
3. Compaction Piles
Cohensionless soils can be treated using compaction piles. To construct a compaction pile, a close-ended, hollow pipe is driven into the ground where the soil is relatively weak.
The soil is compacted by the vibration when the pipe is driven into the soil. The pipes are then withdrawn and the holes are filled up with sand to form a compaction pile. The radius of influence can be upto 3 to 4 times the diameter of the pile.
General Methods For Ground Improvement
1. Removal & Replacement Of Soil
When the thickness of the layer of weak soil is relatively less, it is possible to remove the poor soil and replace it with a new layer of good quality soil. This method is easy and costs eﬀective. Good quality soil should be available in the near area, to avoid unnecessary transportation costs. For excavation and backfilling of soil, labours or conventional earthmoving equipment can be used.
2. Deep Mixing
Enhancement of ground can be done by mixing either cement or lime with the soil deep inside the ground. For this operation a special type of auger with hollow drill is used. The auger is penetrated by rotating it inside the ground deeply, upto a required depth. It is provided with a mixing tool for mixing cement or lime with the soil, at the bottom.
The auger is then withdrawn and the cement or lime slurry is passed through the rod to the mixing tool. The mixing tool mixes the slurry thoroughly with the soil, forming vertical columns of stabilized soil.
Grouting is one of the methods used for the improvement of ground. This method is eﬀective sand, silt, and rocks as well. There are two types of grout which are cement grout and chemical grout. The grout is injected using an injector or a piece of suitable equipment, into the ground where there is a presence of weak soil.
Cement grouts are less expensive as compared to chemical grouts. So, they are widely used. The chemical grouts are mostly used where the cement grouts are not much eﬀective, such as in fine sand or silt deposits.
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