2 CHOICE OF GROUND IMPROVEMENT TECHNIQUES
D. Pascayulinda
7. CONCLUSIONS
The result of slope stability analysis based on the limit equilibrium method indicates that the existing slopes at the three different location are in safe condition in the absence of an earthquake with the safe factor value is
>1.5. However, after the adding of earthquake loads of 0,75g, the safety factor value decreased <1, hence, the slope condition became unsafe and critical. The results also shows that for slope conditions with sloping variation model, it was obtained that location 1, 2 and 3 are in safe condition up to sloping degrees of 24o, 23o, and 31o if no earthquake occurred. However, after the
adding earthquake loads of 0,75g, the slope condition became unsafe and critical up to sloping degrees of 16o, 15o, and 23o. Based on the findings, it is necessary for activities of excavation and cutting at Poboya gold mine to redesign slopes to be slighter in order to reduce landslides of slopes in case of earthquake.
ACKNOWLEDGMENTS
The authors highly appreciate the Ministry of Research and Technology and Higher Education for funding this research in the form of postgraduate scholarship. Many thanks are also given to the research team for their
significant contribution during data collection in the research field. We also thank the head and the technicians of the Soil Mechanics Laboratory as well as the Structure Laboratory of Gadjah Mada University for their assistance during the laboratory study.
REFERENCES
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Metamorphic Hosted Low Sulphidation Epithermal Gold System. Majalah Geologi Indonesia, 27 No. 2, 131-141.
Deterioration Depth of Cement Treated Clay under Sulfate Exposure
T. Pradita, L. Handoko*, S. Gunawan, and J. T. Hatmoko
Department of Civil Engineering, Universitas Atma Jaya Yogyakarta, Yogyakarta, INDONESIA
*Corresponding author: [email protected]
ABSTRACT
One of a major problem in geotechnical field is soft soil because of its low strength. In order to be able to build a structure above soft soil, soil improvement has to be done. Deep mixing is one of soil improvement implementation. Deep mixing is done by forming soil pile of cement treated clay with in-situ mixing. However, when soft soil is located in marine area that is considered as an extreme environment, the seawater may cause corrosion to the soil pile. Sulfate is one of the chemical content in seawater that is corrosive. Thus, the objective of the paper is to know about how severe the deterioration of soil pile with variation of cement content (cc), water content (wc), and curing time (t) after the pile exposed to sulfate. Magnesium sulfate is used to represent sulfate with the content of 10% which is uniform for all piles. The exposure is represented by the immersing of soil pile onto soil-sulfate mixture for 7, 14, 28, and 56 days. The deterioration of the pile is investigated using penetration test. The output of penetration test is in the form of cone penetration resistance (R) vs penetration depth (d) graph. The result of the test shows that deterioration depth decreases as the increase of cement content, and the decrease of water content. Furthermore, the on the undeteriorated zone, the resistance (Rreff) increases along with the increase of cc. In addition, Ca2+ and Mg2+ ion investigation is also done in order to know the content of ion from the surface of the pile up to 1.5 cm below. It shows that the deeper, the more Ca2+ exists. In contrary to that, Mg2+ decreases as the depth increases. This indicates deterioration at the surface area which exposed to seawater.
Keywords
: Sulfate Exposure, Cone Penetration Resistance, Deterioration Depth, Magnesium Sulfate.1 INTRODUCTION
Soft soil is one of a common concern in civil engineering area. In Indonesia, soft soil can be found in Java. For example, in North Java such as Jakarta, Semarang, and Surabaya mainly consist of soft clay with a very little content of sand (Adi Tirta et al., 2018).
Soft soil especially in the form of clay can be found easily in the marine area. The natural water content of marine clay that is generally higher than the liquid limit creates large void in the soil and thus making marine clay weak (Rao et al., 2011).
Marine area or deep ocean is considered to be one of an extreme environment where hydrostatic pressure occurs and mineral solute exists. Additionally, marine area is considered as an extreme environment because the seawater is having the possibility to cause corrosion due to harmful chemical contained in it. One of the most aggressive chemicals contained in seawater that affect the long term durability of cemented material is sulfate. Sulfate has 7.7% part out of the whole composition of seawater, and magnesium takes 3.65%
portion. Sulfate attack has been reported to be a cause of damage to concrete for over a century, and considered as one of the major deteriorative problems (Ganjian & Pouya, 2005).
When a structure is built above weak marine soft soil, the soil will not be able to support the upper structure.
Thus, a soil improvement is needed to strengthen the ground. Soil-cement mixed piles are widely used to improve soft subsoil (Cui et al., 2016). Soil pile can be erected by deep-mixing the ground. The stabilized soil will have a higher strength, lower permeability and lower compressibility than the native soil. However, in regions with high-salinity groundwater, the soil- cement is susceptible to deterioration (Cui et al., 2016).
High-salinity is one of marine area indication.
Moreover, marine soil at the seabed is mixed with the seawater containing sulfate above it, resulting in the deterioration of the soil pile. Figure 1 shows the cross section of soil pile placement. Seawater above will be infused into the soil below causing the sulfate attack inside the soil occurs radially from every directions.
Figure 1. Pile Placement Cross-Section
Sulfate attack leads to the redundant of the compressive strength of cement blended material (Makhloufi et al., 2016). Furthermore, the deterioration by sulfate attack is characterized by a swelling of material due to the formation of expansive products, which lead at a long term to decohesion of this material and consequently a degradation of its mechanical properties (Al-Dulaijan, 2007). Also, the deterioration decreased the effective diameter of the soil-cement pile. The smaller the pile diameter, the more obvious the difference in bearing capacity between the deteriorated and undeteriorated piles (Cui et al., 2016). Thus, soils containing sulfate should not be used for soil-cement stabilization (Sherwood, 1957).
Salt-rich soft soils have not only general characteristics of common soft soils, but also contain high contents of Mg2+, Cl−, and SO42− (Xing et al., 2009). Hara et al.
(2014) identified that the most deteriorative content of seawater is magnesium sulfate. However, magnesium sulfate does affect unconfined compression strength of specimens. UCS value reaches the peak value when the MgSO4 content is 4.5g/kg (Han et al., 2015).
Otherwise, as the content gets higher than 4.5g/kg, the UCS value decreases. Therefore, to know the severity,
MgSO4 is used as the deteriorative substances and the
effect is studied in this paper. In order to tell about deterioration severity, penetration test is done. The output of the test is in the form of Tip or Cone Resistance (R) vs Penetration Depth (d).
2 METHOD AND MATERIALS 2.1 Materials
Sidoarjo Mud or Lumpur Sidoarjo (Lusi) is a result of mud volcano eruption in Sidoarjo, East Java. Lusi is very soft clay with liquid limit of 58.44%%, plastic limit of 30.77%, and shrinkage limit of 22.27%. Lusi has 2.71 specific gravity and contains coarse grain about 15.53%, silt 54.47%, and clay 30%. Since Lusi has some similar properties with marine clay, Lusi is used in the research.Lusi that is used as the soft soil has
60% of liquid limit. Thus, in order to investigate about the strength behavior of high-water-content soil, the targeted water content is both 60% and twice liquid limit that is 120%. In the specimen making, water-soil mixture has to be homogenous confirmed.
Magnesium sulfate (MgSO4) was used to represent extreem environment. The content of MgSO4 is 10 %, meaning in every 1 kg of water there is 10 gram MgSO4
powder. It is mixed directly with the soft soil because later the mixture will be used both for the specimen mixture and the soaking solution. The immersion or curing times are 7, 14, 28, and 56 days.
Ordinary Portland cement is used as a binder for the specimen. The content variations of OPC are 12%, 14%, 20%, 22%, and 24%. The variations refer to active zone (Zhang et al., 2013) which is suitable to be used for deep mixing is approximately more than 115 kg/m3.
2.2 Specimen Preparation
Figure 2 shows the molding method of specimens. 8 cm Ø x 8 cm height cast is purposed to ease penetration test implementation because by an adequate surface area, the penetration will be more accurate since soil has more area to distribute tension from the cone tip. 20 cylinders are prepared in total in order to make specimens by variation in Table. 1. Water content (wc) is set into 60% and 120%. Where 120% is set to be Table 1. Specimen Mix Design
Soil Water Content,
wc (%)
Cement Content,
cc (%)
Cement Content, C
(kg/m3)
Curing Time, t (days) Penetration Test
7 14 28 56
60 12 115.86 √ √ √ √ √
14 134.28 √ √ √ √ √
120
20 120 √ √ √ √ √
22 131.47 √ √ √ √ √
24 142.84 √ √ √ √ √
Figure 2. Specimen Cast Dimension
twice the liquid limit of 60%. The variations of cement content (cc) are 12% and 14% for the 60% wc
specimens and 20%, 22%, and 24% for 120% wc
specimens. Cement content is weighed based on the determined percentage towards soil weight (Ws). The focus is to compare several identical specimens that are cured at different periods. Thus, curing times (t) are varied into 7, 14, 28, and 56 days. Since the laboratory specimens represent the real soil piles in marine area, magnesium sulfate (MgSO4) is added into the mixture to indicate seawater on it. MgSO4 is used in the form of powder as much as 10% of weight of water (Ww). Ww
can be obtained by using Equation (1).
c s
w W w
W = . (1)
While Ws can be obtained by:
( )
+
+
=
s c c
c s
w s s
G G w
G c V W G
. .
1
. .
.
(2)
Where Ww is weight of water, Ws is weight of soil, and wc is water content, Gs is soil specific gravity, V is total volume, ρw is water density (1000 kg/m3), cc is cement content, and Gc is cement specific gravity. The mixing duration is kept for 10 minutes uniformly for every specimens so that the mixture is mixed homogenously.
Before pouring the mixture into the mold, the bottom part of the mold has to be sealed so that the liquid mixture does not leak. The surface molded mixture has to be flat in order to ease the penetration test afterwards.
After the surface flatten using spatula, specimen’s surface is covered with porous paper in order not to mix the liquid mixture with the immersing solution. The porous paper has to be sealed at the folded section so that the paper does not release. Specimen is then put inside a PVC that acts as an immersing container. The immersing solution is the mixture of soil, water, and magnesium sulfate. MgSO4 content in this solution is in accordance with the content in specimen mixture which is 10% Ww. Solution is then poured inside PVC until it reaches 10 cm height above the surface of the specimen. The opening of PVC is then covered by plastic to prevent evaporation and to make wc always in the stable content. The specimens are then cured for 7, 14, 28, and 56 days. Penetration test is performed right after the curing time. Furthermore, chemical analysis is also done in order to know about the content of ion calcium and magnesium from the surface of the specimen, which is directly exposed to sulfate solution, up to certain depth.
3 CEMENT REACTIONS
Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients. There are some components of OPC that have their own role. Tricalcium aluminate, C3A releases a lot of heat during the early stages of hydration, but has little strength involvement. Gypsum slows down the hydration rate of C3A. Cement low in C3A is sulfate resistant cement
(SRC). Tricalcium silicate, C3S hydrates and hardens fast. It is largely responsible for OPC’s initial set and early strength gain. Dicalcium silicate, C2S hydrates and hardens slowly. It is mostly responsible for strength gain after one week. Ferrite, C4AF is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000° F to 2,600° F). It hydrates fast, but does not contribute much to the strength of the cement paste (Hekal et al., 2002).
3.1.1 Water-Cement Reaction
When cement is mixed with water, hardening reaction will happen. There are 2 types of reactions occur, hydration and pozzolanic reaction. Hydration occurs only between cement and water, while pozzolanic reaction occurs among cement and soil particles such as silica and alumina.
C3S hydration:
2 C
3S + 6H→ C
3S
2H
3+3 Ca(OH)
2(3) C2S hydration
2 C
2S + 4H→ C
3S
2H
3+ Ca(OH)
2(4)
C3S and C2S are the most important components because they are responsible for strength. They require approximately the same amount of water from hydration, but C3S produces more than twice of Ca(OH)2 that C2S hydration produces. From the hydration process, the product Ca(OH)2 will be triggered to react with soil’s substances as what shown in Equation (5) to Equation (8). The reactions are included in pozzolanic reaction unity.CaO + H2O → Ca(OH)2 (5)
Ca(OH)2 → Ca++ + 2(OH)– (6)
Ca(OH)2 + SiO2 → CSH (7)
Ca(OH)2 + Al2O3 → CAH (8) Equation (5) is the lime hydration that occurs for the first time. Cement has abundant amount of calcium and therefore will react once it is mixed with water. Cation exchange on Equation (6) occurs after lime hydration.
Equation (7) and Equation (8) shows the first stage of pozzolanic reaction. Calcium hydroxide together with silicon dioxide that is originated from the soil, will form C-S-H or calcium silicate hydrate. Moreover, calcium hydroxide will also produce C-A-H or calcium aluminate hydrate in its reaction with aluminum oxide that is also one of the composition of soil.
Ca2+ + 2(OH) + SiO2 + Al2O3 → C-A-S-H (9) Equation (9) is the second stage of pozzolanic reaction.
The product is calcium aluminate silicate hydrate or C- A-S-H that has higher strength compared to the previous reactions.
3.2 Deterioration Caused by Seawater
Deterioration is the process of a cemented material to become progressively weaker or worse. The severity of deterioration can be caused of the reaction between MgSO4 with cement hydrates, and the repetitive crystallization cycles of MgSO4.nH2O by drying– immersion of the hardened pastes that can produce internal stresses in pores leading to the formation of cracks. The reaction of MgSO4 with cement hydrates is shown in Equation (10) and Equation (11).
MgSO4 (aq) + Ca(OH)2 → CaSO4 . 2H2O + Mg(OH)2
(10) MgSO4 (aq) + C-S-H → CaSO4 . 2H2O + M-S-H
(11) Magnesium sulfate will replace the existence of calcium and form gypsum, which is a low in hardness and therefore reducing the strength of cemented-soil.
Furthermore, the formed gypsum from the above two reactions reacts with calcium aluminate hydrate (C4AH13) and calcium monosulfoaluminate hydrate (C4ASH12) to form ettringite as mentioned below:
C4AH13 + 3CaSO4. 2H2O + 14H2O → C3A. 3CaSO4.
3H2O + Ca (OH)2 (12)
C3A. CaSO4. 12H2O + 2CaSO4. 2H2O + 16H2O →
C3A. 3CaSO4. 32H2O (13)
The formed products are M-S-H as a result of decalcification of C-S-H, gypsum, and ettringite. All of these lead to a decrease of compressive strength and the last two products can cause crack formation (Hekal et al., 2002). Similar with gypsum, ettringite also has weak characteristic. The difference is that ettringite gives swelling effect to the cemented material.
4 RESULT AND DISCUSSION 4.1 Penetration Test
Figure. 3 shows a penetration curve of 60% wc, 14 cc specimen that was cured for 56 days. The dots symbolize the data that is gotten from laboratory research while the solid line represents the fitting line.
In this research, fitting line is proposed using Equation (14).
( )
(
1 reff)
reff
R R R
d
= −
+ (14)
Where R is tip cone resistance, Rreff is reference cone resistance, d is penetration depth (mm), and α,β,γ are fitting constants. dn is the deterioration depth. All contants are obtained from Matlab program.
The closest the gap between fitting line and laboratory data, the more accurate the result. It indicates that the experiment fulfills the initial hypothesis which was made using Equation (14). At some point, the resistance becomes constant. It indicates that the specimen has reached its maximum resistance. Figure.
4 shows the results of penetration test of 120% wc that is cured for 7, 14, 28, and 56 days with variation of cement content of 20%, 22%, and 24%. The longer the immersing period, the more strength gained. It is proved by the increase of cone resistance along with the rise of curing term. Figure. 5 shows the penetration test Figure 3. Penetration Test Specimen 60% wc, 14 cc for 56- days Curing Time
0 5 10 15 20 25 30
0 100 200 300
P en et ra ti o n D ep th ( m m )
Cone Resistance (N)
Laboratory Data
Fitting Line (Equation 14) Rreff
dn
result of 60% samples. The increment of immersing period should indicate the increase of cone resistance.
However, there is a peculiarity in Figure. 5 (a) that the resistance of 28 days-cured specimen is that the resistance of 28 days-cured specimen is lower than 14 days-cured one. It is also clear that the transition deterioration depth of 14 days-cured sample is more sudden compared to the 7 days specimen. Transition deterioration depth is a condition when fully deteriorated zone proceed to switch into the undeteriorated zone by some slight increase of resistance. In contrary to this condition, Figure. 5 (b) shows the reasonable result where the resistance increases along with the immersing period. Even so, the 14 days-cured samples still shows a significant transition from fully deteriorated zone into the undeteriorated one.
Due to some peculiarities occur on the test results, there is a possibility that water content is the one in charge of the error. Table 2 and 3 show the water content of every specimen theoritically and practically after the curing time. Theoritical water content can be calculated by Equation (15).
w ct
c s
w W
W W
= +
(15)
Where 𝜔𝑐𝑡 is theoretical water content, Ww is weight of water, Wc is weight of cement, and Ws is weight of soil.
However, although the specimens are set in such way to fulfill the planned contents, they may not have the exact value. For instance, for 120% wc specimen, the water content that is added between one and another is not necessarily the same (exact 120%) because specimens are mixed in different time. The same applies with cement content. All the uncertainties are caused by human error during mixing, and thus some cannot be compared one and another. The imprecise data can be seen in Table 2, there is some unsynchronized data where the pattern of water content is random as the curing time increases. It is irregular that one specimen has scattered data and the other has linear data. For example for specimen 120% wc 20 cc, water content in 14 days curing time is significantly higher compared to the 7 days curing time. The pattern should have been in the form of wc decrease as the curing time increases. Furthermore, there is an odd value contained on specimen 120% wc 22 cc 7 days- cured where the bottom part of specimen which is not directly exposed by soaking mixture contains higher water content compared to the surface area and the theoretical one. Similar condition is also found in Table 3, where the bottom-part wc in specimen 60% wc 14 cc
28 days-cured is higher than the surface area. By all the deviate data, it cannot be assured that the dose of each material measured is precise. Thus, in order to gather the conclusion, some specimens that are not feasible need to be excluded. The excluded specimens are 60%
wc 14 cc 28 days and 60% wc 12 cc 14 days.
(a) 0
5
10
15
20
25
30
0 50 100
Penetration Depth, d(mm)
Cone Resistance, R (N) 7 days 14 days 28 days 56 days Fitting Line
0 0.5
1
1.5 2
2.5
3 3.5
4
0 5 10 15
Penetration Depth, d(mm)
Cone Resistance, R (N) 7 days 14 days 28 days 56 days