Fragmentation Evaluation of Some Selected Mudrock Indiced by Physical and Mineralogical Properties

Not many mudrock in Indonesia have been evaluated for slaking characteristics. Mudrock disintegrates rapidly when interacting with the atmosphere and hydrosphere. This research investigates the fragmentation of slaked claystone from the Kerek Formation. These formations are known for causing road, slope, and land-strengthening engineering problems along Jalan Semarang-Bawen, Middle Java, Indonesia. In this research, the method includes quantitative analysis of rock slaking level, which collides with the physical and mineralogical properties of the rocks. The slaking test with natural exposure for four cycles was carried out on as many as five samples. It is found the durability parameter value for all samples generally shows a slight difference. The durability index and Fragment Size Distribution Ratio in the 4-month cycle may be considered to evaluate the degree of fragmentation of material mudrock. RGL.0708.10 have an average DIR of 11.81%, and RGI.0908.1 is 2.18%. The value of FSDRatio for RGI.0908.2, RGL.0908.1, RGI.0908.1, and RGI.0708.3 is 1.00, whereas RGI.0708.10 is 0.9997. The indices in this research can be projected and considered to be used in another formation sample with engineering problems. The mineralogical composition of smectite minerals does not show a significant relationship to affect the fragmentation of mudrock with R2 .0.268 in month-4 for the DIR and 0.1322 for the FSDRatio . On the microscopic texture of the sample, there is the presence of calcite minerals, and a modest relationship exists between calcite content and its effect on the fragmentation of mudrock with R2 for DR and FSDRatio , respectively 0.6797 and 0.4732. In addition, physical properties such as natural water content highly influence mudrock’s durability with R2 for DIR and FSDRatio , respectively 0.8321 and 0.9023.


Introduction
Mudrock is a sedimentary rock exposed to atmospheric interactions; the mudrock becomes smaller fragments with fragment size variations-the mudrock classification in this research is based on [14].The disintegration of the rock resulted in numerous construction problems, such as slope stability, foundations, and underground excavation problems [1].For example, [4] found that the Surco flysch in Spain has many slopes problem and vast outcroppings with material variety due to the deterioration of clay-bearing rock.The degree of disintegration can vary depending on the type of mudrock.Mudrocks disintegrate rapidly due to interaction between the atmosphere and the water environment [5].
This research uses the method from [6] to assess mudrock durability.Durability is a measure to evaluate the degree of mudrock resistance to weathering or disintegration phenomena affected by water content changes.This assessment includes the wetting-drying process.Wetting and drying cycle tests usually assess the disintegration behavior of mudrocks in slake-durability tests [7].Therefore, wetting and drying tests without mechanical interference on slake durability were performed among the disintegration index methods [3].After the sample was wet-dried, the mudrocks slaked into smaller fragments.According to [8], the slake durability test is unsuitable for concluding the long durability period.However, according to [9], it conducted the slake-durability test with modification and proposed a new decay index that considers the duration time of the test and disintegrated fragment size.The slakedurability test is overestimated to the force and stress that work on samples, whereas the weathering condition in the field is more complex than in a laboratory.According to [5], the high durability of mudrock correlates to the induration factor indicated by dry density and void ratio.Contrarily, absorption and specific gravity in high value will affect the durability in a lower value.
Based on [10], it conducted a study and field survey that some clay shales in West Java, Indonesia, push some soldier piles and cause a wall collapse.This phenomenon triggers the wall at the toe part and landslide the road, and the culvert breaks, severed by seepage and indicating that the water influences durability.According to [11], it found some foundation failures in the Hambalang area, West Java, due to weathering rock of the Jatiluhur Formation.Based on [12], it tested the samples from this area and shows Jatiluhur Formation has more durable characteristics than the Kerek Formation from Semarang-Bawen, Middle Java.
Not many mud rocks in Indonesia have been evaluated for slaking characteristics so are Kerek Formation.These formations are known for causing road, slope, and land-strengthening engineering problems along the Semarang-Bawen toll road.The climate condition of this area has high rainfall and intense weathering levels that allow the natural agitation of rocks to continue rapidly.However, when it takes a field survey, many disintegrations that took place intensively led to severe slope issues and foundations for toll roads.The problem is also present in slope stability, such as surface erosion and landslide on the many slopes.Based on [13], this formation consists of an alternation of claystone, marl tuffaceous, sandstone, conglomerate, volcanic breccia, and limestone.Description for claystone is light grey-dark, calcareous, partly interlayered with siltstone or sandstone; locally contains forams, mollusca, and coral colonies.The conglomerate layer occurs in claystone at Kali Kripik; and in sandstone.Limestone is commonly bedded, crystalline, and sandy, with a total thickness of more than 400 m.This formation formed in the Middle Miocene age.In delivering the research problem, this research investigates the mudstones from the Kerek Formation and then evaluates their fragmentation resulting from slaking.The sampling is taken from 5 outcrops, as shown in figure 1.Some new parameter is proposed in this research to evaluate the durability of mudrock.

Rock sampling
The material for laboratory examination is collected from the fresh condition rock.Rock's description and name classification are based on [14].This field rock shows the layering; when interacting with water, it is easy to form the smaller fragment.The procedure for a treat the sample in the field is based on [15].The sample is in the condition of undisturbed material to prevent the release of stress that makes the sample not natural again.If the release stress is allowed, the sample will cause rapid disintegration.The dimension of samples is determined by cubic shape with 18 cm x 18 cm x 18 cm.Next, the sample is appropriately protected by wrapping plastic and aluminum foil-this protects to prevent the change of water content and temperature of the sample when it takes to the laboratory.Finally, the box-shaped samples are placed into multiplex slabs with bubble wrap filling to prevent mechanical shocks during transportation.

Figure 1.
Sampling location of the research area overlying the geological map that source from [13].

Natural Expose Slaking Test
Sample preparation in 10 fragments with each mass of 45-60 g.The sample is prepared by drying under sun exposure until constant weight before continuing to be naturally exposed on the mesh size 2 mm for a 4-month cycle of natural agitation (figure 2).The sample will experience wetting and drying from the natural environment in natural exposure.After one month and in dry condition, the sample is sieved by a series of mesh sizes of 25.4 mm, 19 mm, 12.7 mm, 6.3 mm, 4 mm, and the last 2 mm.After natural exposure slaking, the slaked material will be fragmented into distributed sizes.Each sample retained in the sieve is weighed.Each weight is then plotted on the fragment size distribution curve.Some parameters are considered to evaluate the material's durability to disintegration.Dry weight after each cycle (  ) and initial condition ( 0 ) is calculated for the Durability Index of retained material (  ) following equation (1).
Fragment Size Distribution Ratio (  ) is proposed to measure the material's durability degree in more detail, considering the retained material in each mesh size.

Physical-Mineralogical Properties
The physical properties test is based on [16] procedure of water content, absorption, density, and porosity.The pore-related parameter is immersed by ethylene glycol against the effect of water on the disintegration of mudrock.The density parameter is measured by dividing the samples' dry mass and volume, whereas the porosity is calculated by dividing pore volume by sample volume.Water content is related to the mass of water in natural conditions compared to the dry mass of rock, whereas absorption uses the mass of water in every pore in rock.X-Ray Diffraction (XRD) is used to obtain the material's composition and identify the presence of smectite group minerals based on semiquantitative analysis in Match software.The semiquantitative analysis considers the mineral's peak intensity value and the difference between minimum and maximum Bragg 2θ in each crystal phase.The database of minerals used in XRD is from JCPDS International Centre for Diffraction Data.The smectite group in the samples can be identified from this card database as vermiculite and montmorillonite.The observation found that the sample contains calcareous; the calcite content is determined to identify the effect on durability.For the microscopic observation, the Scanning Electron Microscope (SEM) is conducted to overview the textural condition of samples.

Rock Description and Mineralogical Examination
The outcrop condition in the field has a great variety of weathering degrees.The lithology that was found is equivalent to Kerek Formation.The fresh condition of mudrock samples lay on road slopes and river slopes (figure 3).Some outcrops show slight weathering with some intense cracks formed by natural processes.The megascopic description of mudrock consists of determining the lithology of claystone.Claystone has brownish grey with clay grain size.The lithology has calcareous, low compactness, and conchoidal structure.When the rock is dripped with chloride acid, the reaction occurs, shown by a bubble reaction on the surface.To obtain more details of the mineralogical composition, it conducts X-Ray Diffraction with the result shown in table 1. Smectite group minerals consist of vermiculite and montmorillonite; calcite content represents the calcareous condition in samples.

Physical Properties Test Results
Table 1 summarizes the physical properties of absorption, dry density, water content, and void ratio.Natural water content ranges from 6.27% to 15.76% for all samples.RGI.0908.2 has the highest water content value, and RGI.0708.10 has the lowest.From the dry density result, RGI.0908.1 has the highest value of 1.96 g/cm3, and RGI.0908.2 and RGI.0708.10 has the lowest value with a similar result of 1.80 g/cm 3 .The void ratio ranges between 0.40 and 1.82 for all samples.Therefore, the lowest and the highest, respectively, are RGL-0908.1 and RGI.0708.3.Those parameters indicate the compactness and induration level.The absorption value ranges from 15.79 to 33.38%.Therefore, the absorption value of RGI.0708.3 has the highest value, and the lowest is RGL.0908.1.When the void ratio is high, the absorption is also a high value.The value variation for the smectite group of minerals is shown for all samples except RGL-0908.1,which has no smectite minerals.RGI.0708.3 has the highest percentage of

Natural Expose Observation
After going through the sun drying to obtain initial dry weight, samples start with a hairline crack.The samples were then exposed to natural conditions during the first month.The cycle is repeated until the fourth month.After one month, the sample is sieved with various mesh sizes.The conditioned sample per month is shown in figure 4. Based on the figure, after the first month, all samples had experienced complete disintegration, with fragmented material still retained in notable appearance.In month 2, all samples begin to be retained less fragmented material and continue in month 3. Finally, in the last month, all samples have no retained material; even so, the sample remains fragments with almost no amount.

Durability Index of Retained Mudrock
After the sample is exposed to natural slaking, the sample is sieved by various mesh sizes.The various fragmented material sizes are plotted in each month's fragment size distribution curve.Each sample has its distribution curve, as shown in figure 5.Each sample represents the various characteristics of the percentage of material retained in a series of sieve sizes.The mesh has a size of 2 mm, 4 mm, 6.3 mm, 6 12.7 mm, 19 mm, and 25.4 mm.The quantitative analysis is applied to this curve as represented by the Durability Index of Retained Fragment (  ).This index refers to the amount of retained material in a cumulative 2 mm sieve that is dried after a specific cycle and then divided by the initial dry condition weight.All samples break into smaller fragments at the end of the cycle.In month-1, all samples break into smaller fragments, as samples experience complete disintegration with various mesh sizes.In month-2, only RGI.0708.10 had fragments retained in 2 mm mesh.No fragments were retained in another mesh size.In month-3 and month-4, all samples have no retained material in all mesh.This condition shows that the claystone sample in the research area has low durability.

Sample Initial Condition
After Month-   a lowest   value of 2.18%.It should be noted that lower the   value than lower the durability of the sample to disintegration.When the durability index is used for evaluating the material's degree of resistance to weathering in only 2 mm size, it is proposed to obtain the different indices that reveal how fragment size represents a characteristic of durability material.The proposed parameters, the Fragment Size Distribution Ratio (  ), are modifications taken from Erguler and Shakoor [17].In this modification, the area above the curve divided by the total area encompassing the distribution curve is used as equation (2).
For example, as shown in Figure 6, the RGI.0708.10 have an area above curve 2291.57, and the total area coverage is 2340, the   value in month 1 is 0.9793.The value of the   ranges from 0 to 1. Value approached 1 shows a high level of disintegration; in contrast with close to 0, the samples experience low disintegration.

𝐹𝑆𝐷 𝑅𝑎𝑡𝑖𝑜 =
()    Figure 6.The proposed method to calculate the   using an example Month 1 graph.
Table 2 shows the recapitulation of the value   .The tabulation is made for month-1 until month-4.Most samples have the   range of 0.9793 -1 in the first month and reach the maximum value of 1 in month-4.All samples show low durability to disintegration, indicated by the high value of the   .For the rate of disintegration, it is plotted the cycle month and   (figure 7).All samples reached a maximum of 1 of the   in month 4. The rate disintegration value ranges from 0.0013 to 0.0062, with the unit   / month.This rate is obtained from the gradient of the line equation.The regression analysis method is used to show the trend of the two variables [18, [19].How strongly the independent variable on the dependent variable is indicated by deterministic coefficients (R 2 ).The x-axis in the correlation graph is the independent variable, whereas the y-axis is the dependent variable.
For the correlation between   and   , the independent variables are the durability index, while the dependent variable is the Fragment Size Distribution Ratio.In figure 8, there is a significant correlation between the   value and   following satisfies the deterministic coefficient (R 2 ) with a linear trend.This significant influence occurred in each of the cycle months.

Correlation of Physical-Mineralogical Properties to the Durability Parameter
The various geological and engineering parameters express the disintegration variety of mudrock [5], [20].In this section, the correlation is obtained by finding the influence parameter on the durability of mudrock.The x-axis in this correlation is a void ratio, absorption, dry density, water content, smectite group minerals content, and calcite content, while the   and   is plotted as the y-axis.Table 3 and 4 summarizes the correlation between the physical and mineralogical properties and durability indices.The durability parameter in month 4 shows a high value of R 2 .Therefore, the   and   in month 4 are considered the durability evaluator to slaking.
The R 2 value between   to void ratio, water content, absorption, dry density, smectite content, and calcite content are 0.5748, 0.8321, 0.5209, 0.0765, 0.2680, and 0.6797.From this value, the regression analysis between the   and the physical properties of mudrock indicate a significant influence on the water content and modest for the calcite content, void ratio, and absorption.The identic correlation occurred in bivariate analysis between the   and physical properties.The R 2 value between   to void ratio, water content, absorption, dry density, smectite content, and calcite content are 0.5725, 0.9023, 0.3839, 0.1694, 0.1322, and 0.4732.This value shows a significant influence of water content on durability and a modest influence on the calcite content and void ratio to fragmentation.On the other hand, Smectite group minerals that consist of montmorillonite and vermiculite do not significantly affect slaking material.Same with the dry density that does not significantly influence durability.The result is that all correlations show a high correlation in the parameter natural water content; for the void ratio and calcite content, the level is modest.According to [5], the high claystone absorption value correlates to larger pore space.The pore water pressure in the rock makes the grain lose cohesion and produces a crack that disintegrates the mudrock.Natural water content in this research has a high value of R 2 that significantly affect the disintegration.According to [9], the percent of water absorption is the most critical parameter controlling mudrocks' durability.However, the properties that may significantly affect the disintegration of mudrock are not always the parameters that will significantly affect the disintegration.Refer to [22]; it revealed no significant relationship between slake durability in the second cycle with chemical composition and mineralogy.This research found that calcite content moderately influences slaking, and the absorption is a low correlation, although the void ratio is moderate.To investigate this, the Scanning Electron Microscope is conducted to evaluate the microstructure of samples.From the table, the correlation between calcite content and   , and   shows in month-4 a modest correlation.It has a trend of higher calcite content, the durability higher to disintegration.From the graph, calcite content from fossils fills the claystone pore and may prevent water from entering the pore and make pore water pressure cause the slaking.

Conclusion
The mudrock of the Kerek Formation may indicate various degrees of disintegration based on quantitative parameters.The natural exposure slaking test simulates the weathering condition where the rock is exposed.Most samples disintegrate into smaller fragments even during natural exposure's first month.The   and   show each other that all samples in the research area show low durability to disintegration in the range of value 2.18-11.81%.After month-4, all samples have a value of   1.00.This value expresses complete disintegration and no retained fragment on the mesh.Evaluating the mudrock for an extended period of exposure is essential.The samples have no retained material for the case of claystone in this research area in month-4.The proposed method to calculate the durability parameter can be applied to other mudrock samples in Indonesia.Mudrock with low durability and rapid disintegration can affect engineering activities, such as causing slope failure or foundations in the road where highway construction in the research area takes place.Some parameters, such as calcite and natural content, have considerable influence on the durability of mudrock with R 2 respectively 0.6797 and 0.8321 for the   whereas for the   is respectively 0.4732 and 0.9023.Further (c) (d) investigation through microfabric observation is considered to identify the characteristics in more detail of the internal structure of the samples.

Figure 2 .
Figure 2. Natural exposure slaking test on the claystone samples.

Figure 3 .
Figure 3.The typical outcrop of Kerek claystone in the field lie on the river slope showing the bedding planes and conchoidal structure.

Figure 4 .
Figure 4. Condition of samples on 2 mm mesh size from the fragmented sample after the cycle of disintegration for four months.

Figure 5 .
Figure 5. Fragment size distribution curve for (a) Month-1, (b) Month-2, (c) Month-3, (d) Month-4.Based on the average value of   during the entire cycle for all rock samples, RGI.0708.10 are more durable than other rock samples.These samples have an average   of 11.81%, while RGI.0908.1 has

Figure 7 .
Figure 7.The rate of disintegration is calculated from the line equation of the graph between the   and cycle.

Figure 8 .
Figure 8.The significant correlation between   and   .

Table 1 .
Physical properties of mudrock samples.

Table 2 .
value for each month.

Table 3 .
Correlation between   and physical and mineralogical properties.