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Geotechnical Engineering: Types and Methods of Deep Ground Modification

Chris is a registered professional civil engineer and has worked on a more than 300 public and private projects over the last decade.

types-and-methods-of-deep-ground-modification

In our modern world, there is an ever increasing pressure to build bigger and better structures to meet the demands of government, business, and an urbanized population. In order to stand tall and stable, bigger structures require bigger foundations that in turn depend on a strong, dense, and stable ground. In many parts of the world, the ground is simply not suitable in it's natural condition for the placement of massive structures. Thus, it has become necessary to employ a variety of methods to modify and improve the ground so that a high performing foundation can be built on top of it.

Today, geotechnical engineers have invented a number techniques that can successfully improve the structural integrity of a variety of soil types at significant depths in an effort to meet the demand for new construction. Each of these innovative methods has varying ranges of applicability, costs, and suitability for a given set of site conditions. In this article, I will discuss nine different methods that geotechnical engineers use to prepare the ground for heavy foundations, tall buildings, and other infrastructure that requires a strong and stable base.

Deep Dynamic Compaction (DDC)

This method of ground improvement uses cranes to drop heavy weights (10 to 170 tons) from a height of up to 85ft onto the ground. The weights are dropped in a grid pattern typically spaced anywhere from 6 to 40ft apart. The impact of the weight on the ground creates low frequency energy waves which move and shake the soil causing compression and densification. The maximum depth of improvement is typically around 70ft to 100ft with the maximum densification occurring at around 1/2 of the effective depth.

DDC is most suitable for saturated sands and silty sands though improvements can still be made on certain fine grained soils if they are located above the groundwater table. DDC can also help with collapsible soils or soil strata that have large void spaces (like Karst). Liquefaction potential of soils is also reduced when deep dynamic compaction is employed.

This method has several advantages in that it is a low cost way to improve soil properties. The method also does not necessarily require specially trained workers to do. Some disadvantages include the relatively shallow effective depths of improvement (typically 30 to 35ft) and the potential to damage nearby buildings and infrastructure with the large ground vibrations.

The following video contains some great information regarding deep dynamic compaction:

Vibrocompaction/Vibroflotation

This method of ground improvement uses a large crane/truck to lower vibrating probes into the ground. The probes vibrate with a cyclical action in order to cause granular soils to rearrange themselves into a more dense configuration. Vibrocompaction cannot be used on silts or clays and is generally more expensive then deep dynamic compaction. Some advantages to vibrocompaction are that it is easier to use than many other compaction methods and that you can more easily achieve a uniform and densified soil surface. Vibrations felt on the ground are often significantly less than those caused by deep dynamic compaction or blasting. The depth of improvement is really only limited to the soil strata, the project budget, and the availability of compaction equipment.

The following video shows an animations of the vibrocompaction process:

Penetration/Pressure Grouting

With this method of ground improvement a very fluid, cementicious grout is pumped into the ground under high pressure. The high pressure forces the grout to fill the void spaces in granular materials resulting in higher density soils, improved strength and stiffness and a lower hydraulic conductivity.

Typical application pressures are on the order of 1psi per foot of depth. Usually only coarse grain soils can be treated however if micro-fine cement grout is used, fine sands may also be able to be treated. This grouting method requires boring several holes (most often in a triangular pattern) spaced 3 to 10ft apart across the site. The treatment process can be tedious and costly but can yield significant improvements to bearing capacity when done correctly. Penetration/Pressure grouting is a widely available construction technique and is commonly used to repair damaged foundations.

Below is a video showing the pressure grouting method being used to repair a foundation:

Compaction Grouting

Compaction grouting injects stiffer grout into the ground at a specified depth using medium to high pressure. The grout injection process creates and expands a grout cavity (e.g. a grout bulb) near the bottom of the column which presses against the surrounding soil increasing its density. This compacts or even consolidates the soil (if injected below the ground water table). It is important to monitor the ground surface when employing this method. If not done properly the ground can lift up or "heave." Because of this, compaction grouting cannot be used at shallow depths.

Compaction grouting is best used on loose granular soils or collapsible soils though it has been used with some success for certain fine grained soils. The improvement in soil characteristics is related to the type of soil being treated and the spacing and pattern of the compaction grouted columns. The cost of this method can be moderate to high depend on equipment availability and application techniques.

The video below shows an animation for how compaction grouting works:

Jet Grouting

When jet grouting is used a special drilling rig with a jet nozzle is used to drill a hole into the earth to a specified depth. The nozzle ejects water and/or air to erode the soil at depth creating a cavity that can be filled with grout. This grouting technique creates soil-cement columns of any height with diameters ranging from 2-3ft wide on up to 16ft wide depending on the soil type and jet grouting equipment being used.

Jet grouting can be used in most soil types, although it works best in soils that are easily eroded like sands and gravels. Cohesive soils, especially highly plastic clays, can be difficult to erode and may require lengthy drilling times to create the soil cavity. Improvement results are also less noticeable for cohesive soils. Jet grouting requires specialized equipment and training and can be very expensive to use. Even so, some advantages include the ability to treat only specific soil layers or even the ability to treat soils under buildings (even from inside the building) and other infrastructure.

The video below explains the jet grouting process and shows it being used to improve ground conditions:

Deep Soil Mixing

When deep soil mixing is used, a drill rig with one or multiple counter rotating augers drills down into the ground to mix the soil with additives. Typically grout, lime, flyash, or even some other additives such as montmorillinite clay are added to the soil during the mixing process to improve strength and stiffness. The soil's compressibility as well as its hydraulic conductivity are reduced during the process. The drilling augers can be very wide and can potentially treat soils resulting in a column material as wide as 10-12ft though typical columns range from 2 to 4ft. The augers can essentially treat any depth of material however most available equipment cannot exceed 80 to 100ft of depth improvement.

This techniques is used to improve soils for foundations, spill containment, and even to lines of soil-cement columns that can act as a temporary earth retaining wall. Some advantages of deep soil mixing include low noise problems, high production rates, avoidance of dewatering. Some disadvantages include the moderate to high costs of the equipment and the potentially long lead teams required to see improvements in soil strength and stiffness.

The video below shows an animation of the deep soil mixing process:

Blasting

Blasting is the use of explosives to compact/consolidate soils. This technique works well for gravels and mild sands but is not effective on silts or clays. Blasting is best used to densify hydraulic or dredged fill materials. Blasting typically consists of drilling several holes to depth below the groundwater table, placing explosives at the bottom of the hole, backfilling and tamping and then blowing it up. The type and amount of explosives will dictate the hole spacing and the depth of improvement that can be achieved. Some of the disadvantages of this method include the potential dangers associated with using explosives, the high cost, and the fact that there is a limited range of soils where this technique will be effective. Specialized training and licenses are also needed to use this method and it cannot be used near existing buildings.

The videos below shows the use of explosives to compact and improve ground conditions:

Soil Replacement/Chemical Treatment

Soil replacement is a technique that can be used to simply remove the poor quality soil and replace it with good or engineered soil. The advantage with this technique is that it is easy to do, requires no specialized equipment, and most general contractors can do this work. However, this method does have a few major disadvantages. Primarily, deep excavations can be economically nonviable and may also require a lengthy time to complete as application of new soil layers can only be done in relatively small lifts. In areas with a high ground water table, dewatering of the site may also been needed to achieve the desired result. Most soil replacement/treatment projects are done at shallow depths of less than 10ft.

Soil replacement means excavating the poor soil, disposing of it, and bringing in new soil to establish a strong base for a foundation.

Soil replacement means excavating the poor soil, disposing of it, and bringing in new soil to establish a strong base for a foundation.

Summary

The main characteristics of these nine methods are summarized in the following table:

MethodDescriptionSoil TypesApplication Pattern or SpacingMaximum Improvement DepthMaximum ImprovementsAdvantagesDisadvantagesCosts

Deep Dynamic Compaction (DDC)

Dropping heavy weights onto the ground surface

Saturated sands or silty sands, partially saturated sands.

Grid pattern 6 to 40ft apart

Up to 100ft, effective depth up 30-35ft

Densification: +80%, SPT Blow Count: +25, CPT Cone Resistance +1400-2200psi

Easy to do, low cost, no dewatering of soil needed, Widely Available

Limited Improvements below 30ft, Ground Vibrations could impact adjacent properties

Low

Vibrocompaction/ Vibroflotation

Vibrating rods are pushed into the ground and withdrawn to densify the soil

Sands, silty sands, or gravelly sands with lo fines

Grid pattern 5 to 10ft apart

As far as the rods can be pushed into the ground

Densification: +80%, SPT Blow Count: +25, CPT Cone Resistance +1400-2200psi

More uniform compaction, Ease of use. Less vibration than blasting or DDC

Not effective for shallow depths, requires special equipment

Low to Moderate

Penetration/Pressure Grouting

Injecting flowable high pressure grout into bore holes.

Sands, gravels

Triangular pattern 3 to 10ft apart

None

Filled voids, increase in strength due to solidification

Any depth can be treated, good for spot treatments

High costs, Not effective for silts, clays, or coarser materials with fines

Moderate to High

Compaction Grouting

Injecting stiff grout into bore holes to compact surrounding soils

Almost any compressible soil. Better on granular soils

Grid pattern 3 to 10ft apart

Depends on Equipment

Depends on soil Type

Works in almost any soil, good for spot treatments

High costs. Cannot be used for shallow soils

Moderate to High

Jet Grouting

A drilling rig with a grout jet attachment is pushed into the grount. The jet erodes a cavity which is filled with grout.

Any soil though less effective in high PI clays

Various

Depends on Equipment

Depends on soil Type and grout mix. Increases soil strength through solidification.

Good for spot treatments, can go under existing structures or treating only specific soil layers

High costs

High

Soil Replacement and/or Chemical Treatment

Remove poor quality soils and replace with good, treated, and/or engineered soils

Any for pure replacement. For treatment use Cement for sands and silty sands, Lime for clays

N/A

Typically only 10-20ft however equipment exists to excavate 100+ft

Depends on replacement soil used. Can potentiall get large increases in strength or density

Achieve desired soil properties, easy to do (standard earthwork construction)

Potential High costs, Lengthy time to do the work, requires dewatering, relatively low depths

Low to Very High

Stone Columns/ Vibroreplacement

Create Columns of aggregate in the soil.

Silty or Clayey sands, silts, and clayey silts

Grid pattern 3 to 10ft apart

Depends on Equipment

SPT Blow Count: +25, CPT Cone Resistance +1400-1750psi

Uniformity, proven effectiveness

Special equipment and training needed. Can't be used with cobbly soils. Limited use on gravels.

Moderate to High

Deep Soil Mixing

Uses counter rotating augers to drill and mix additives into the soil

All Soils

Varies

Depends on Equipment, typically the limit is 80 to 100ft

Depends on equipment, spacing, additive design

High strength improvements

Special equipment and training needed.

Moderate to Very High

Blasting

Using Explosives to compact, consolidate and densify soils

Gravels to mild silts

Varies depending on soil type and explosive type

Any

Varies on soil type

Works very well for hydraulic fills

Special Training required, can't be used near existing buildings

Moderate to Very High

© 2018 Christopher Wanamaker

Comments

FlourishAnyway from USA on May 05, 2018:

This was an excellent review. I learned a lot from it. It makes a lot of sense that if you’re building on land that’s not stable enough for a large structure then you need to do some remedial work to ensure a strong foundation.

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