From the preface of “The Basics of Design and Construction of the D-Box Method
ーGround Development and Strengthening, Liquefaction, Vibration and Seismic Motion Reduction” (Morikita Publishing, Japan)
Proposals for the future – “Nurturing” soil
The soil that covers the surface of our earth is essentially a three-phase mixture consisting of
- soil particles (solid)
- water (liquid)
- air (gas)
Even saturated soil is a two-phase mixture consisting of “soil particles (solid) and water (liquid)”.
Multi-phase mixtures consisting of completely different substances are troublesome for humans to handle, but since they are natural products, they can be said to be “dream materials”.
If we focus on the essential composition of soil above, we notice the following.
Unlike steel or concrete, soil can be “nurtured”.
This is done by “narrowing the gaps between soil particles to make them hard and strong”, just like “nurturing” a child to become an adult.
At this time, care must be taken not to apply too much force too suddenly and destroy the soil.
This is the same as not letting a child suddenly carry a heavy backpack.
These things have long been known as “compression” or “consolidation”.
Why have humans not made more use of these wonderful properties of soil?
Perhaps it is because many people imagine “consolidation” as a troublesome phenomenon in which the soil continues to sink.
To “strengthen” soil in a short time
As will be described in detail in the main text, it has been analyzed and confirmed by model experiments that by gradually applying load to cause “local consolidation” of a certain loading width, the soil can be “strengthened” to about three times the original bearing capacity in a short time.
It has also been analyzed and confirmed by model experiments that by narrowing the loading width inside the “strengthened” area and gradually applying load to cause further “local consolidation”, the soil can be “strengthened” to more than eight times the original bearing capacity in a short time.
Although it is now possible to carry out such calculations and experiments, the soil bag method (D-Box method) has been used in soft ground such as rice fields and swamps for a long time.
Theoretical calculations and model experiments are, so to speak, retroactive.
To put it simply, for example, soft ground with N value of 1 can be “raised” to N value of 3 or 4 as needed.
From now on, there is no need to think that a ground with N value of 1 will always have N value of 1.
As a result, the ground can be “raised and strengthened.”
This idea does not seem to exist in the current field of geotechnical engineering.
It does not make sense to use something that “grows” without nurturing it.
Throughout this book, we propose nurturing the soil, that is, nurturing and strengthening the foundation, as a “proposal for the future”.
Effects of soil bags (including D-Box)
Next, we will discuss the effects of soil bags*1 (D-Box*2), including what has already been clarified.
*1 This refers to high-standard sandbags with clear uses and performance based on the research results of Matsuoka et al.
*2 This product was developed by the Metry Technical Research Institute based on the theory, effects, and results of the soil bag method developed by Matsuoka et al. The D-Box method is still being developed through the cooperation of both parties.
~Quoted from Chapter 1, p. 3 footnote of this book~
- The incredible strength of the soil bag (D-Box) itself
- Increased bearing capacity of the groundーlocal consolidation of soft ground
- Liquidation reduction effect
- Traffic vibration and seismic motion reduction effect
- Frost heave prevention effect
The incredible strength of the soil bag (D-Box) itself
The most effective and ultimate reinforcement method for soil held by interparticle friction is to “completely encase and restrain the soil”.
This is the principle of partition restraint of granular materials.
Soil bags (D-Box) are a typical example of this application.
The mechanical mechanism by which the soil bag itself exerts its incredible resistance has been elucidated, and two-dimensional and three-dimensional expressions of the pressure resistance have been derived.
By simply wrapping and restraining the soil particles, the friction between the soil particles can be increased and the cohesion force C can be reliably obtained without the use of adhesives (cement).
This is the essential reason why soil bags are so strong.
One soil bag can withstand a load of 20 to 40 tf (the weight of one or two train cars).
Increased bearing capacity of the groundーlocal consolidation of soft ground
The mesh of the soil bag containing crushed stone acts like a filter, allowing water to pass through but not soil particles.
As a result, even in the case of a soft clayey ground that is in a waterlogged sludge state, the soil bag absorbs water (water pressure) well, and locally consolidates the soil directly below the soil bag in the shape of a pressure bulb, rapidly strengthening the ground directly below, increasing the bearing capacity of the ground and reducing the amount of settlement.
This is the local consolidation and strengthening effect of the soil bag.
Liquefaction reduction effect
It allows water to pass through easily, releasing water pressure, but does not allow soil particles to pass through, so it also serves as a liquefaction countermeasure.
Traffic vibration and earthquake motion reduction effect
The soil bag has a slight flexibility.
This allows the energy of traffic vibration and earthquake motion to be dissipated as frictional heat energy between the particles of the filling soil and between the soil bag through the invisible microscopic expansion and contraction of the bag.
Frost heave prevention effect
Soil bags filled with crushed stone also have a frost heave prevention effect.
The crushed stone particles in the filling material are large, which means that the gaps between the particles are also large, so water does not rise due to capillary action.
Therefore, there is no water supply and no frost heave occurs.
The soil bag method (D-Box method) is cost-effective
As mentioned above, the cost-effectiveness of the soil bag method (D-Box method), which has five effects with one method:
Measures against soft ground, liquefaction, traffic vibration, earthquakes, and frost heave, is noteworthy.
- soft ground
- liquefaction
- traffic vibration
- earthquakes
- frost heave
It is truly a “Kill five birds with one stone” method.
This book explains in detail the four effects other than frost heave.
It is important to avoid exposing the soil bag (D-Box) to sunlight (ultraviolet rays).
Although it contains ultraviolet protection agents, the bags will deteriorate over time.
However, if buried in the ground, the bags will last semi-permanently because they are made of polyethylene and polypropylene.
How the soil bag (D-Box) method was discovered
Here, I would like to touch on my personal experience.
Looking back, the story of how I discovered the soil bag (D-Box) method is very interesting.
The path to discovery
Since I started researching the “soil bag method” in the early 1990s, I have hardly thought of developing this method as a measure against soft ground, liquefaction, vibration, or earthquakes.
As I conducted model experiments based on the principles of soil properties, I realized that the best reinforcement method for granular materials such as soil is to completely encase and restrain them.
Then I remembered that “sandbags” have been around for a long time.
I also realized that a single sandbag has an incredible load-bearing capacity (20 to 40 tf) and discovered that it can easily control soft ground such as flooded rice fields.
In addition, we received comments from the site such as,
“You no longer feel the vibrations from traffic or heavy machinery,”
“The earthquake motion felt like a whoosh, not a rattle, so it wasn’t scary,”
“The kokeshi dolls in the house didn’t fall over,”
“The gravestones held up extremely well, even though the surrounding buildings were collapsing,”
and
“There were no cracks in the mortar joints of the gravestones.”
Then, we investigated and measured them, and found that this was indeed the case.
The reasons and mechanisms behind these phenomena were reasonably plausible.
In other words, this method was not developed with intention from the beginning, but it has been proven effective in experiments and on-site as a result.
It was a very lucky method, and it has been revealed that it kills five birds with one stone.
It was not an intentional discovery, but rather the result of pursuing principles and rules
There are limits to human ability and wisdom.
Even when you start thinking enthusiastically about discovering something new, it often doesn’t produce any significant results.
When we look into how the key discoveries in Nobel Prize research were discovered,
we often hear stories like, “We found something by chance through a series of coincidences,” or “It wasn’t what we had originally intended.”
That is where the “true interest” lies, and what we call “true discoveries” that go beyond human ability.
In other words, human wisdom and predictions are limited.
In that sense, this method has developed greatly as if naturally guided by something while simply pursuing the principles and rules of soil properties.
It has only just dawned on me that this method is a very “lucky” one.
Why we recommend the D-Box method
Looking at the fields of Nobel Prizes in Medicine and Physiology,
we have heard comments like, “We discovered that a drug originally developed for livestock also works well for humans,” and “It is an extremely significant achievement that has helped many patients around the world.”
Just like the human body, the ground is a natural object, and there are many complex and mysterious aspects to its phenomena, especially vibration and seismic motion problems, so I don’t think we need to wait until the whole picture is clarified.
Looking at over 5,000 construction cases to date, it has been proven to be nearly 100% effective unless there are obvious construction errors.
The principle has been clarified, and there is already ample evidence of its effectiveness.
Use it as much as you can, and if necessary, check its effectiveness with measuring instruments.
Some designers say that they will not adopt a method unless there is an analysis or calculation method that can predict the results in advance.
However, if the analysis and calculation methods used in the world were perfect, there should be no ground-related accidents.
If this method, which simply wraps the soil in a permeable bag, restrains it, and tightens it, is truly good and based on the essence of the soil, I am sure that a clear analysis method will surely be developed in the future.
What is difficult to understand?
When explaining the D-Box method, there is one point that many engineers have difficulty understanding.
That is, with the D-Box, for example, the upper embankment load is reduced in the depth direction in the shape of a “pressure bulb” (in other words, it is efficiently distributed), and it has almost no effect on the soft ground layer deep in the ground.
Many engineers have a “one-dimensional consolidation” (full-surface loading) thinking pattern due to the methods that have been used so far ー
cement improvement method, crushed stone replacement method, sand drain method, paper drain method, etc.
ーand it seems that they cannot erase the fear of land subsidence that will continue indefinitely in the future.
I often explain it as follows:
“It is best to construct D-Boxes by rolling them one by one in order” (local consolidation effect).
“If you lift about 100 D-Boxes and set them all at once, it will be close to one-dimensional consolidation (full load) and will sink significantly.”
It is important to understand this difference.
Local consolidation and strengthening effect directly below the D-Box
The local consolidation and strengthening effect directly below the D-Box can be imagined as a hard part like the lower half of the “Marimo” at Lake Akan in Hokkaido, Japan that appears directly below the D-Box and gradually fades out into the surrounding soft ground.
And as the embankment load gradually increases, the lower half of the “Marimo” directly below the D-Box will also become as hard and large as necessary.
This “bottom half” of the soil is hard and strong (naturally occurring in the soft ground due to localized consolidation), and supports the increase in the load of the upper embankment (in other words, distributes the stress naturally), and does not cause the soft ground below to be consolidated.
Soil Bag (D-Box) Construction Method: Nature-Friendly
This is a method that does not go against nature, but makes use of it.
Until now, perhaps humanity has been confronting nature more than anything else.
Now that the deterioration of the global environment has reached its limit, I would be grateful if nature-friendly construction methods that coexist with nature, are assimilated into nature, and are actively promoted all over the world.
June 2020
Lead author: Hajime MATSUOKA
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