Analysis of Collapsible Soils in Applied Geotechnics Report

This report delves into the critical geotechnical challenges associated with collapsible soils, which pose significant risks to construction projects. It begins with an introduction highlighting the sudden volume reduction these soils experience upon wetting, followed by a comprehensive literature review exploring the characteristics, formation, and global distribution of collapsible soils, including their presence in arid and semi-arid regions. The report then presents a detailed case study from the Sao Paulo State in Brazil, examining the soil stratigraphy, the impact of collapsible soils on the construction of a hydroelectric system, and the methodologies employed to assess and mitigate the risks. The methodology section outlines the extensive laboratory experiments and field programs conducted, including SPTs and cone penetration tests. The results and discussion section presents findings on soil compression curves, collapse potential, and the influence of soil suction on soil behavior. The report concludes by summarizing key findings and their implications for geotechnical engineering practices. This report provides valuable insights into the behavior of collapsible soils and strategies for managing the associated risks in construction projects.

APPLIED GEOTECHNICS
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Contents
INTRODUCTION...........................................................................................................................................2
LITERATURE REVIEW....................................................................................................................................2
CASE STUDY.................................................................................................................................................4
METHODOLOGY...........................................................................................................................................7
RESULTS AND DISCUSSION..........................................................................................................................8
CONCLUSION.............................................................................................................................................12
REFERENCES..............................................................................................................................................13

INTRODUCTION
The risk of the construction of a structure on the collapsible soils usually presents serious
challenges to the engineers of the geotechnical. This is as a result of the sudden reduction in their
volume immediately after wetting. Collapsible soil basically refers to the unsaturated soils which
are capable of withstanding relatively high pressure without necessarily showing certain changes
in their volume but upon wetting they become susceptible to sudden and large volume reduction.
LITERATURE REVIEW
The collapsible soils are almost found in the entire world within the deposits of soil which are
manmade fills, mudflow, and colluvial, alluvial, eolian, loessial and subaerial. Collapsible soils
refer to any soil that is unsaturated and goes through a radical rearrangement of the particle and
may also have a great reduction in volume during wetting or may have an additional load or
both. These soils are found mostly in arid and semiarid areas and may consist of loose structure
of the soil such as having a huge void ratio and content of water far less than that of saturation.
In most cases, there structure consists of low-unit weight. Sediments are not consolidated but
have coarse particles which are just bonded only at the point of contact by very fine silt or a clay

fraction or both, and even by water surface tension within the water interfaces (Ayadat and
Hanna 2013).
The water addition mainly explains what trigger the collapse of the soil. In some cases, the
collapse may occur due to the application of load or wetting or may be both. Therefore, collapse
may happen either through the increment of the stress beyond that of soil or when the soil
strength is lower than that of the stress. When the consideration is given to the physical
properties or is given the physical basis of the bond of the strength, then in most cases all he
collapsible soil become weak when water is added to them. The reduced strength can be more
immediate especially where the grains are linked by capillarity suction, slower when cemented
with chemical and slower in clay buttresses. Therefore, it may take a long duration, at least a
year for the total collapse to occur. The structure of typical collapsible soil is shown below.
Figure 1: The mechanism of the typical collapsing soil that hold bulky and loose grain together.

The presence of these types of the soils in the world as well as their building difficulties has been
recognized. Most researchers have neglected to study these soils, though it is very
understandable since these soils are presented or are common within semi-arid as arid area where
there is little economic developments. Whereas due to increasing projects of irrigations in the
areas, a huge amount of water is currently being put on the land, in places where there had been
no farming and similarly where there had been no cases of the soil collapse.
The collapsing of soils because wetting which is commonly refered to as (hydrocompaction)
may lead to a very big damage to dams, canals, pumping plants, roads, pipeline, field as well
miscellaneous structure connected to irrigation projects. Therefore, it had been taken to the one
of the most form of land destructive subsidence. The following movements of collapse are
therefore not related to the ground. The delineation in between the stable region and the
potentially collapsible soils region is sometimes tending to be very difficult task facing the
engineers of geotechnical.
Figure 2: Impacts of the collapsible soil on the structures (Masoud and Aal 2019)

The soils that collapse always exist up to a considerable depth that is about 30 m to 200 m,
though in areas having a deeper ground water. Both the rate and the mount of collapsing are
being influenced by the mineralogy of present material that: the first void ratio, the history of the
material stress, the grain bulky shape, as well as their distribution size. Other areas are the in
place content of water, the sizes and shapes of pores, some cementing agents, the soil layer
thickness as well as the amount of the load added on structural or hydrostatic.
CASE STUDY
The area which was subjected to the study was located to the northwest region of the Sao Paulo
State in Brazil. The typical stratigraphy of the soil was as indicated below: Such arrangement
was such that the layer on the top was made up of the colluvium with unique characteristics
known to be having spacious structure clay and sand which is fine. Such kinds of the soil have
width or thickness which varies between 5m to 10m. The soil has been under the laterization
influence. This particular process is usually responsible for taking away the silica and bases
hence resulting into a soil which is made up of the minerals which is more resistant like quartz
which constitutes kaolinite and oxides of iron as part of the clay fraction

Figure 3: State of the Sao Paulo in Brazil (Vandanapu, Omer, and Attom 2017).
The layer of the colluvium rests on the sandstone soil which is residual. It is also resting on fine
sand with traces of clay with the index of the density. In the midst of residual sandstone soil as
well as colluvium layers there is a concretions soil layer which is few centimeters thick. The soil
log representative with the standard tests of the penetration is as indicated in the figure or table
below.
Figure 4: Typical profile of the soil in Pereira Barreto, Brazil (Haeri 2016).
In this area there was a complex hydroelectric system. This particular system was made up of
three large power plants which were constructed to assist in the provision of the electric energy
for the Sao Paulo State as well as the whole region at the Centre- south of the Brazil. In this
particular complex, the largest power plant of hydroelectric system is the Treˆs Irma˜os Dam.

The dam whose height was 50m with length of 3640m was constructed over River Tiete with a
reservoir of 785 km2. The construction of the Pereira Barreto Canal which has served as
interconnection between Ilha Solteira dam and Treˆs Irma˜os reservoirs and the Treˆs Irma˜os
Dam alongside the position in terms geographical-geomorphological of the site or the place led
to the location of the city (Thompson and Mechling 2016).
Considering there was already some expectation that such present cases would result into the
introduction of the main changes not only to the landscape but also to the economy of the place,
the hydroelectric power plant project company developed several studies to assist in the
assessment of the impacts thereby generating the preventive as well as corrective measures.
Some of the concerns were the region or the areas which are found in Pereira Barreto which is
the city itself. This is because they were known to be susceptible to the collapse of the soil as a
result of the gradual filling of the reservoir of Treˆs Irma˜os. There were some significant efforts
which were put in place to assist in the identification of filling reservoir to potentially control it
through the zoning of the risk places.
METHODOLOGY
Extensive laboratory experiments were carried out including field programs which are under
facilitation by the company which has shown responsibility of power from the water points/falls
that assist in the characterization of the soil collapse. Other than the exploratory boring which is
carried out by the use of SPTs there were other tests including tests of penetrating cone alongside
evaluation of the buildings which had been approximated to be 20 in the sites. (Kalantari 2013).
The data from the evaluated structures were recorded including the rise in the level of the
groundwater as well as the subsequent the three marks settlements with time. The water level

elevation which started with the rising in the residual soil never led to the introduction of the
settlements that resulted from developing due to the approach of the development base by water.
There is gradual rise of the moisture to a high elevation when the base of the collapsible soil
settlement is approached by the water level. In this regard the settlement continues to increase
and finally reaches 100mm. This represents a mark of settlement as S-5.
RESULTS AND DISCUSSION
Figure below give the results of the test to impose soil collapse that were done using a gradual
reduction of soil suction, wetting as well as drying cycles. The identification of collapsing is
done by the discontinuity within the compression curves, of which the additional strain arises
upon wetting, though having no increase load. Therefore, the strain is computed by the equation
of Jennings and Knight (Shin and Byrne2013.).
CP = collapse potential
∆ec = the wetting variation of the void ratio
E0=the initial ratio of void.
∆HC= the height variation of the sample after wetting.
H= the initial height of the sample
The figure below shows plot of CP collapse potential against the net stress toward vertical (av –
ua). the potential of collapse goes up to a certain maximum and then decreased with the
increment of the stress. The vertical stress in which the soil gives a greater potential of

collapsing that is about 100 kappa in the suctions used in the test are 60 kPa as well as 200 kPa.
In the same way, the result obtained both natural and compact sample (KALPAKCI 2017).
However, the variation of collapse having stress that maintained the similar trend for the two
test, while the initial soil suction in most cases influence the collapse strain magnitude since a
very great strain took place within the samples having an initial soil suction of 200kPa,
Table 1: A table fitting parameter of the equation of van Genuchten (Haeri 2016)
Figure 5: The soil compression curves having different suction va, pore air pressure av and
vertical stress

Figure 6: Soil suction influence on the preconsolidated stress of the soil (Haeri 2016).
Figure 7: The soil suction effect on the compression index ( Cc) of the soil (Haeri 2016).

Figure 8: Specimen compression curve under the initial suction of 60 kPa as well collapse strains
(Alonso 2015).
Therefore, the following behavior can be connected by a very large rigidity attained by the soil
with a bigger suction which compresses less as compared to the similar soil having lower
suction. However, there is a big room for deformation in the time when there is a reduction in
suction; therefore, soil reaches a new equilibrium condition. In the above, the CPs of the test that
used specimen having 200 kPa of the suction initial is plotted as the suction’s function. In the
same figure, the soil suction progressive reduction, the drying and wetting cycle as well as the
corresponding development of collapse are indicated. Any of these curves corresponds to the
loaded specimen to stresses that varied between 50 kPa to 400 kPa. The initial peculiarity of the
result tested showed that the potential of collapse at point zero suction reaches the maximum
value, which reduces as the stress rises (Houston and Nelson 2013).
The suction gradually reduced leading to the development of the collapse which was basically
dependent on the upon the average value of the vertical stress. From the study it was evident that

the larger the value of the stress, the more the distribution of the collapse strain with the
reduction of the suction. In the cases of the low stress, the important collapse strain began to
grow or started its development only at the low section (Abbasi, Bahramloo and Movahedan
2015).
This was actually below value of 10 kPa. It was at zero suction that the largest strain took place
regardless of the value of the average vertical stress. The suctions cycling did not result into the
introduction of the additional important strain in the specimen. This implied that after the
equilibrium was achieved under the same suction and stress, there will be occurrence of
additional collapse in case the suction is reduced below the value of the equilibrium.
CONCLUSION
The behaviour of the collapsible soil which was unsaturated and obtained from the region of
Brazil was under the influence of the gradual rise of the water table of the ground. This
behaviour was studied by the use of the suction-controlled tests. The results of the tests indicated
that the collapse strain depend on the suction as well as net vertical stress. The maximum
collapse strain of the soil was found to be around 100 kPa of stress. Beyond this value, there was
a decrease as the stress increased. Similarly, the initial suction of the soil traces which had direct
earing of the collapse as of the soil regard to the strain, suction under the vertical stresses
through such larger induced suction led to the larger values of strain. The end result acted upon
wetting of the soil or just reduction in the suction.

REFERENCES
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Masoud, A.A. and Aal, A.K.A., 2019. Three-dimensional geotechnical modeling of the soils in
Riyadh city, KSA. Bulletin of Engineering Geology and the Environment, 78(1), pp.1-17.

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