Settlement assessment of gypseous sand after time-based soaking

Abstract

This paper investigates the effect of the soaking process on gypseous sand soils. The recent research investigates the time-based soaking method on high gypsum contents (29%) of soil samples. The softening of gypsum materials upon the wetting process leads to break the bonds of the soil particles and the settlement of structures will occur. The settlement value and rate are the basic problems for the geotechnical engineers. The samples of this investigation have been collected from the selected site in Al-Najaf city in Iraq, then these samples are remoulded to the density of 85% of the maximum dry density from Proctor test and moisture content of 4% in the cell of the computerized Oedometer device and under different stress levels (1.11, 2.23 and 4.47 kg/cm 2). The soil specimen is soaked under three stress levels (1.11, 2.23, and 4.47 kg/cm 2) separately. The results revealed that with increasing the soaking periods and stress levels, there is increasing in the collapse potential of such soils. The collapse potential was increased from <1% after half-hour soaking to about 8% after two-week of soaking.

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Settlement assessment of gypseous sand after time-based soaking
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IOP Conf. Series: Materials Science and Engineering 737 (2020) 012080
IOP Publishing
doi:10.1088/1757-899X/737/1/012080
1
Settlement assessment of gypseous sand after time-based
soaking
Mohammed Sh. Mahmood1,*, Ali Akhtarpour2, Rusul Almahmodi3 and Mustafa M. Abdal Husain 4
1Asst. Prof., Ph.D., University of Kufa, Faculty of Engineering, Civil Dept., Al-Najaf/Iraq
2Asst. Prof., Ph.D., Ferdowsi University of Mashhad, Faculty of Engineering, Civil Dept., Mashahd/Iran
3Formerly M.Sc. Student, Ferdowsi Universtiy of Mashhad, Faculty of Engineering, Civil Dept.,
Mashahd/Iran
4Ph.D. Student, Universtiy of Mashhad, Faculty of Engineering, Civil Dept., Mashahd/Iran
Abstract. This paper investigates the effect of the soaking process on gypseous sand soils. The
recent research investigates the time-based soaking method on high gypsum contents (29%) of
soil samples. The softening of gypsum materials upon wetting process leads to break the bonds
of the soil particles and the settlement of structures will be occurred. The settlement value and
rate are the basic problems for the geotechnical engineers. The samples of this investigation
have been collected from the selected site in Al-Najaf city in Iraq, then these samples are
remoulded to the density of 85% of the maximum dry density from Proctor test and moisture
content of 4% in the cell of the computerized Oedometer device and under different stress
levels (1.11, 2.23 and 4.47 kg/cm2). The soil specimen is soaked under three stress levels (1.11,
2.23, and 4.47 kg/cm2) separately. The results revealed that with increasing of soaking periods
and stress level, there is increasing in collapse potential of such soils. The collapse potential
was increased from <1% after half-hour soaking to about 8% after two-weeks of soaking.
1. Introduction
Estimation of the soil geotechnical properties is not easy to establish, as in the estimation of tolerable
and differential settlement [1]. The settlement value and rate are the basic problems for geotechnical
engineers [2]. Collapsible soil is considered as a problematic soil because it is sensitive to moisture
and with increasing moisture content, volume reduction occurs. It is considered as a non-elastic
deformation [3,4]. The collapsibility of the soil is re-arrangement of the grains of the unsaturation soil
state due to the wetting process [4]. Several types of research have been made on the behavior of
infiltration of unsaturated soils. Al-Saoudi and Al-Shakerchy, 2010, concluded from field testing that
the rate of infiltration illustrated a decreasing trend versus cumulative time [5]. Similarly, Al-Saoudi et
al., 2013 and Al-Saoudi et al., 2014 revealed the same condition of infiltration of water into the soil
using, also, field testing [6,7]. Al-Shakerchy, 2009, depended on laboratory model, stated that the
infiltration and rate of infiltration are changed with respect to time due to the wetting progress of the
soil [8].
The study of gypsum soils from the eighties has attracted the attention of many scholars and scientists.
In general, the recognition of problematic soils in terms of engineering geology and geotechnics is of
great importance [3]. The gypsum distributes in Iraq according to Barzanji, 1973 [4]. Many previous
researchers have investigated the effect of the gypsum content on the different soil properties. From
different sites in Iraq, Al-Khuzaie, 1985 [9], Nashaat, 1990 [10], Al-Mufti, 1997 [4], Salman, 2011
[11] and Mahmood, 2017 [12] investigate the effect of gypsum content on the shear strength of the soil

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IOP Conf. Series: Materials Science and Engineering 737 (2020) 012080
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for different soil samples, While Moula and Al-Saoudi, 2010 [13], Razouki and Al-Azawi, [14],
Abbas and Muarik, [15], Fattah, et al, 2017 [16] and Mahmood, 2018 [17] have studied the
deformation of the soil due to different processes. All these researchers have stated that with
increasing of the gypsum content, there were a decreasing in the shear strength and increasing in the
settlement.
Soil wetting fallouts in the change in volume, shear strength reduction and stiffness and type and
amount of this change depending on several factors, such as soil structure, the degree of wetting and
stress state [18]. The collapse percentage (CP) was defined by Jennings and Knight, 1975, as in Eq. 1,
where ≤ 1% is in no problem [18].
CP= e/(1+e) (1)
Many buildings have been damaged because of gypsum content within a case of rising of the water
table, the softening of gypsum materials that are between the particles of the soil is occurring [19,20].
Al-Najaf city soil is mainly sand with different percentage of the gypsum after Al-Shakerchy, 2007
[21] and Al-Saoudi and Al-Shakerchy, 2010 [22]. Gypsum soils have long been the cause of problems
in the city, such as settlement, cracks in the buildings and the destruction of landscapes. The recent
paper investigates the settlement of the city sand soil due to short term process at different stress level
and different soil relative density and gypsum contents using Oedometer cell. The purpose of this
paper is to give a clear and accurate effect of the certain process and gypsum content on the estimation
of the settlement value and rate in the Al-Najaf city. Two-weeks of soaking process increased the
maximum dry density (MDD) from standard Proctor compaction test corresponding to the initial
condition, such as water content. The soil is, according to the increase in MDD, experienced an
additional settlement upon the soaking [23]. There were no collapse percentage (CP) in the low
gypseous sand soils (<5 %) using Oedometer test after soaking durations according to Eq. 1, but there
were increasing in the settlement due to soaking process [24].
The recent paper investigate the effect of soaking process within half-hour, one-week and two-weeks
on the settlement value of the sand soils with 29% of gypsum in Al-Najaf city southern east of Iraq
using Oedometer test. The settlement is assessed under different levels of normal stress.
2. Methods and materials
The soil samples are taken from one of the northern districts (Al-Jameah) in the province of Al-Najaf
city in Iraq. The soil sample is named SW of the highly gypsum contents (29%). Table 1 summarizes
the properties of the soil sample. The soil is mainly sand and the max. dry density is obtained from
standard Proctor test.
Table 1.Soil Properties of the Tested Samples.
Soil Properties
Value
Sand, %
67.8
Soil Classification, USCS
SW
Natural Water Content, %
3
Gypsum Content, %
29
Max. Dry Density, gm/cm3
1.825
Optimum Moisture Content (O.M.C.), %
15
56 Oedometer tests are made to estimate the settlement of the soil sample with respect to normal
stresses and time. These tests are performed using a computerized multi-cell Oedometer, as in Fig. 1,
under six stress levels (0.27, 0.55, 1.11, 2.23, 4.47 and 8.95 kg/cm2). These tests included soaking
process with different periods at three certain stress levels (1.11, 2.23, and 4.47 kg/cm2). All
specimens are investigated with initial density as a percent of the maximum dry density (85%). The
tests are started with initial water content (unsaturated), then the soil is wetted to the saturated

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IOP Conf. Series: Materials Science and Engineering 737 (2020) 012080
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doi:10.1088/1757-899X/737/1/012080
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condition at the certain stress level and be left for the different soaking periods of half-hour, one-week
and two-weeks, then the stress level is re-applied to estimate the collapsibility potential. Figure 2
illustrates the tests program.
Figure 1. Multi-Cell Computerize Oedometer.
Figure 2. Program of Oedometer Tests.
3. Tests results
Figures 3 to 5 provide the settlement-time relationship under different normal stresses and half-hour
soaking process at the three specific stress levels (1.11, 2.23, and 4.47 kg/cm2) for comparing the
collapsibility of the soil specimens. Generally, with increasing the normal stress there are increasing in
the settlement due to the rearrangement of the soil particles, as shown in the figures. There are small
increases in the settlement, in each stress level, due to the short time (half-hour) soaking process.
Table 2 summarizes the settlement values before and after the soaking process and percentage changes
at certain stress levels. As can be seen from Table 2, with increasing in the stresses, the percentage
change ((SAFTER-SBEFORE)/SBEFORE*100) is decreased (from about 3% to 2%). This situation of
settlement may be attributed significantly by the stress more than the short time soaking process.

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IOP Conf. Series: Materials Science and Engineering 737 (2020) 012080
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Figure 3. Settlement vs Time under Different Stress Levels with Half-Hour Soaking at 1.11kg/cm2.
Figure 4. Settlement vs Time under Different Stress Levels with Half-Hour Soaking at 2.23kg/cm2.
Figure 5. Settlement vs Time under Different Stress Levels with Half-Hour Soaking at 4.47kg/cm2.

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IOP Conf. Series: Materials Science and Engineering 737 (2020) 012080
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To investigate the effect of soaking periods, same above tests procedure is made on soil specimens
with a soaking period of one-week and for the same soil characteristics and loading. Figures 6 to 8
present the settlement versus time under gradually loaded with different normal stresses and soaking
process at the same stress levels in previous tests. Compared to half-hour soaking, there is a clear
increase in the settlement after the soaking process for one-week under the same specific stress levels
(1.11, 2.23, and 4.47 kg/cm2). Table 3 illustrates the values of settlement corresponding to the soaking
process and percentage settlement changes ((SAFTER-SBEFORE)/SBEFORE*100). It can be seen from the
data in Table 3 that there is a significant increase in a settlement related to the soaking effect. Vice
versa from the case of half-hour soaking, the percentage changes in the settlement are increased with
increasing the stress level (from about 20% to 36%) and this condition may be due to the effect of one-
week soaking is more than the stress effect.
Table 2. Summary of the Specimen Settlement before and after Half-
Hour Soaking Process.
Stress Level,
kg/cm2
Settlement, mm
Before Soaking
After Soaking
Change, %
1.11
-0.7
-0.72
+2.857
2.23
-0.85
-0.87
+2.353
4.47
-1.44
-1.47
+2.083
4. Analysis and discussion
Several reports have shown that there is a negative effect on the different gypseous soils, as mentioned
in the literature review, such as, increasing in the deformation and decreasing in the shear strength.
The present study was designed to estimate the effect of the time-based soaking process on the
collapsibility of high gypsum (29%) sand soil in Al-Najaf city in Iraq. The soaking periods were half-
hour, 1-week and 2-weeks within certain normal stresses of 1.11, 2.23, and 4.47 kg/cm2 using
Oedometer cell. The specimens were remoulded in the Oedometer cell with the density of 85% of the
laboratory maximum density depending on standard Proctor test.
The current study found that with increasing the soaking period and the normal stress level there are
increasing in percentage changes ((SAFTER-SBEFORE)/SBEFORE*100) in the settlement. In this study,
soaking had a significant effect in the long period. A possible explanation for this might be that the
softening of the gypsum materials is increased with increasing of the soaking period. This finding was
also reported by Mohammed et al. 2018 [24] for low gypsum sand soil under different soaking periods
before starting the loading.
Further analysis is made with reproducing of the data. Figure 9 provides the void ratio versus normal
stress corresponding to the starting of the half-hour soaking. As stated above, there are no significant
changes in soil behavior due to the specific soaking. While with increasing the soaking period up to
one-week, the change in void ratio is more obvious and this change is increased with increasing the
stress level of soaking, as shown in Figure 10.

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IOP Conf. Series: Materials Science and Engineering 737 (2020) 012080
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Figure 6. Settlement vs Time under Different Stress Levels with One-Week Soaking at 1.11kg/cm2.
Figure 7. Settlement vs Time under Different Stress Levels with One-Week Soaking at 2.23kg/cm2.
Figure 8. Settlement vs Time under Different Stress Levels with One-Week Soaking at 4.47kg/cm2.

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IOP Conf. Series: Materials Science and Engineering 737 (2020) 012080
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doi:10.1088/1757-899X/737/1/012080
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Table 3. Summary of the Specimen Settlement before and after One-
Week Soaking Process.
Stress Level,
kg/cm2
Settlement, mm
Before Soaking
After Soaking
Change, %
1.11
-0.53
-0.64
20.755
2.23
-0.87
-1.03
18.391
4.47
-1.35
-1.84
36.296
(a)
(b)
(c)
Figure 9.Void Ratio vs Stress with Half-hour Soaking at (a) 1.11kg/cm2, (b) 2.23kg/cm2 and (c)
4.47kg/cm2.
(a)
(b)
(c)
Figure 10.Void Ratio vs Stress with One-week Soaking at (a) 1.11kg/cm2, (b) 2.23kg/cm2 and (c)
4.47kg/cm2.
Additional tests were made to verify the recent results. Two-week of a soaking process is adopted for
two stress levels (2.23, and 4.47 kg/cm2). Figure 11 presents the results of void ratio against stress for
the two stress levels of soaking. The changes are larger and more effective.
Table 4 provides the collapsibility potential (CP) as was defined by Jennings and Knight, 1975, Eq. 1
[18]. The results of collapsibility potential can be categorized into three groups of soil collapse
severity as defined by Jennings and Knight, 1975; no problem (CP>1), moderate trouble (1<CP<5)
Start of
Soaking
Start of
Soaking
Start of
Soaking

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and trouble (5<CP<10). The troubles are occurred in the condition of long term soaking process under
high normal stress.
Overall, these results indicate that the soaking process and soaking periods are the main reason for the
alteration of gypseous sand soil in Al-Najaf city. It can thus be suggested that take cautions in
selection and analysis of soil properties of such soils.
Further work is required to establish the viability of the laboratory tests through in-situ tests under
natural conditions of the soils.
(a)
(b)
Figure 11. Void Ratio vs Stress with Two-Week Soaking at (a) 2.23kg/cm2 and (b) 4.47kg/cm2.
Table 4. Collapse Potential for the Different Stress Levels and Soaking
Periods.
Stress Level,
kg/cm2
Collapse Potential, %
After Half-hour
Soaking
After 1-Week
Soaking
After 2-Weeks
Soaking
1.11
-0.19*
-0.60*
-
2.23
-0.20*
-0.80*
-7.20***
4.47
-0.34*
-2.60**
-8.20***
* No Problem, ** Moderate Trouble, *** Trouble
5. Conclusions
In this investigation, the aim was to assess the gypseous sand soil in Al-Najaf city under different
soaking periods and stress levels using Oedometer tests. These tests confirmed that with increasing of
soaking periods and stress levels, there are increasing in collapse potential of such soils. The collapse
potential was increased from <1% after half-hour soaking to about 8% after two-weeks of soaking.
These findings have significant implications for the understanding of how gypsum softening affects
soil behavior. An issue that was not addressed in this study was whether soil sensitivity is low.
Acknowledgment
Special thanks to the staff of soil laboratory in the civil engineering department, faculty of
engineering, University of Kufa for their cooperation in accomplishing this paper.

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