SOIL CONSOLIDATION ANALYSIS AS THE MAIN CAUSE OF LAND SUBSIDENCE IN SEMARANG - INDONESIA

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 10, Issue 02, February 2019, pp , Article ID: IJCIET_10_02_076 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed SOIL CONSOLIDATION ANALYSIS AS THE MAIN CAUSE OF LAND SUBSIDENCE IN SEMARANG - INDONESIA A. Pratikso Civil Engineering Department, Universitas Islam Sultan Agung, Jalan Raya Kaligawe KM. 4, Po Box 1054/SM, Central Java 50112, Indonesia S. Sudarno Civil Engineering Department, Universitas Tidar, Jalan Kapten Suparman No. 39, Magelang Central Java 56116, Indonesia ABSTRACT The aim of this research is to know the specific gravity and depth of soil consolidation. The method of this research was carried out for three years, and in the third stage drilling of soil with a machine at 5 points at 30m depth. Furthermore, the property test and soil mechanical properties in the laboratory. This research produced a map of puddles in Semarang City and shows that the land in the northern part of Semarang city has decreased over time because the land has been consolidated due to the burden of residential buildings, the use of groundwater and motor vehicles. Key words: Consolidation Analysis; Settlement; Silt Content, GIS. Cite this Article: A. Pratikso, S. Sudarno, Soil Consolidation Analysis as the Main Cause of Land Subsidence in Semarang - Indonesia, International Journal of Civil Engineering and Technology (IJCIET) 10(2), 2019, pp INTRODUCTION Semarang city is one of the big cities in Indonesia and becomes the capital of Central Java Province. The administrative area is km2 with a population of 1,481,460 people, Semarang City consists of 16 sub-districts and 117 urban villages with a growth rate of 1.86% every year [9]. Semarang City consists of two morphological units, in the south (upper city) consists of hills, which is the foot of Mount Ungaran which stretches from east to west from Tembalang, Tanah Putih, Tegalsari, Gajah Mungkur, while alluvial coastal plain is located at northern part (lower city) [15]. Among the 16 sub-districts in Semarang City, Nort Semarang and Central Semarang subdistricts are the densely populated areas with a population density of 11,556 and 12,089 inhabitants per km2 [2] editor@iaeme.com

2 Soil Consolidation Analysis as the Main Cause of Land Subsidence in Semarang - Indonesia Changes in the function of land from ponds into residential areas cause environmental problems, among others, to be one cause of puddles in dense settlements with low topography during rain and seawater tides [18]. This puddle occurs due to rainwater and seawater that was previously accommodated in ponds flowing into densely populated areas [ [1] ; [14] ]. The addition of load on the alluvial soil will be the decline of soil due to consolidation resulting in the occurrence of land subsidence in some of Semarang City becomes a serious problem because the area is generally a residential area [28]. Due to the subsidence of settlement become lower, because the location of settlements partially close to the coast, when the tide of seawater overflows into the mainland through the river and drainage subsequent pooled in the settlement [ [11] ; [3] ]. The novelty of this research to previous researchers lies in the methods and tools used. This method is done by drilling the soil with drilling machine at 24 points to 30m depth to get gradually ground samples spread in five sub-districts of Semarang Genuk, East Semarang, Semarang Gayamsari, and Central Semarang. The samples were then subjected to soil properties and soil mechanical tests in Unissula soil mechanics laboratory. From the results of lab tests then analyzed to determine the amount of land decrease due to consolidation. Finally, an analysis of the changes in the extent of the pool caused by the subsidence using GIS program. 2. LITERATURE 2.1. Geotechnical of Semarang Geographically Semarang City is located between '22 "LS '13" LS and ' 21 "BT '36" BT. Semarang City is located on the elevation of +0 to +350 m even there are areas with an elevation below +00 m [22]. Semarang city every rainy season is always hit by flood especially in the low area, even in a low area and its location near the beach although no rainfall due to flooding inundation due to the tide of seawater [13]. City in the northern (lower city) which borders the Java Sea has some problems related to topography [24]. Physical environmental problems arising in the northern coastal environment include abrasion, sedimentation, inundation, seawater intrusion, wind deposition, wind erosion, soil salting and soil contamination [ [6] ; [17] ] Consolidation and Land Subsidence Consolidation is a process of saturation soil content slowly with low permeability due to pore water discharge [29]. The process continues until the excess pore water pressure caused by the increase in total tension has completely disappeared. According to Hancock [30]. Consolidation is the process of discharge of pore water from saturated soil water due to loading. In general, this consolidation will take place in a single course, namely the vertical majors because the layer is exposed to the additional burden that cannot move in the horizontal direction because it is held by the surrounding soil [25]. Under these conditions the drainage of water will run, especially in the vertical direction, this is called one-dimensional consolidation and consolidation calculations are almost always based on one-dimensional consolidation theory. At the time of consolidation, construction on the soil layer decreased [23] editor@iaeme.com

3 A. Pratikso, S. Sudarno Figure 1 Location of sampling Figure 2 Point of sample drilling Figure 3 Changes in puddles at research sites editor@iaeme.com

4 Soil Consolidation Analysis as the Main Cause of Land Subsidence in Semarang - Indonesia 2.3. Large Primary Consolidation One Dimension Consolidation is influenced by the balance of soil elements, traction and mineral pressure and pore water flow [ [8] ; [19]; [10] ]. Primary consolidation is the soil consolidation that occurs when the pore water pressure on the soil is more than zero. Soil consolidation can be calculated from the amount of decline caused by primary consolidation with the assumption that the consolidation is the one-dimensional consolidation that can be formulated as follows [ [32] ; [16] ]. e S = H 1 eo For the consolidated clay normally then: e = Cc {log (po p ) - log po} In this case: Cc = The compression index so the equation becomes CcH po p S = log (2) 1 eo po For a thick layer of clay, the soil layer is divided into several sub-layers and the calculation of the reduction is done separately for each sublayer. So the total decrease for all layers is: CcHi Po( i) p( i) S = { log( P )} (3) 1 eo po( i) Where: S = Large primary consolidation Hi = Thickness sub-layer (i) Po (i) = Effective overburden pressure for sublayer (i) p(i) = Addition vertical pressure to sub-layer (i) eo = initial void ratio Cc = Compression index e = Change void ratio H = Thickness of the soil layer 2.4. Consolidation Time Terzaghi (1925) [31], who first introduced the theory of a one-dimensional consolidation speed for a water-saturated clay with the assumption that variations of the average degree of consolidation of the dimensionless time factor (Tv) are given in the table. Tv can be Cv t calculated Tv. Price time factor and degree of consolidation (U ) (Das 1988) can be 2 d expressed as follows: 2 For U = 0 sampai 52,6%, Tv = U % (4) For U = > 52,6%, Tv = ,933 log (100-U%) (5) Cv t50 On the circumstances U = 50% of pore water flows in one direction T 50 (6) 2 Hdr Where: (1) editor@iaeme.com

5 A. Pratikso, S. Sudarno T50 = Consolidation duration 50% Cv = Coefficient of consolidation t50 = For consolidation 50% (table) Hdr = The length of pore water flow during the consolidation process 3. RESULTS AND DISCUSSION The study was conducted on the alluvial plain in Semarang city below, including, central and East Semarang subdistrict. The study area that is experiencing subsidence generally consists of young alluvial rocks. He is only hundreds of years old and continues to experience compression. Soil subsidence is a problem because at the location of research that is experiencing subsidence there are settlements in addition to the location near the beach is also passed by several rivers as a result of frequent flooding due to rainwater and tide of seawater [ [7] ; [4] ]. Samples in the form of drilled soil from the study site were then tested in Unissula soil mechanics laboratory Tools For Sampling and Soil Laboratory Test 1) The instrument used for sampling a set of mechanical drill comprises a complete drill machine with a prime mover, ground bits, tripod to help insert and remove the drill rod. The soil samples were taken every 100 cm depth, while the drill depth was up to 30m. 2) The instrument used for laboratory tests is the soil mechanics laboratory to obtain soil and mechanical soil properties for each soil sample Analysis of Soil Sampling To investigate the use of land subsidence lab analysis as follows: Based on the research objectives to be achieved, as well as the expected availability of data required then there are two ways of analyzing the sample [27]. For soil subsidence research used laboratory analysis as follows: 1) Laboratory analysis of Soil Mechanics results is used to calculate/ predict the magnitude and duration of soil subsidence. From the analysis and calculation, we get LL, Ø, c, Cc, Cv, and e0, after calculated by the physical model can be known big and duration of degradation due to load above ground and soil mechanical properties. Soil sample in the form of drilling data at 5 points spread in 2 Subdistricts, further analysis is determined by 5 drill points with consideration of the amount is enough to represent the research area as a whole [21]. 2) Computer lab analysis is used to know the distribution and changes in the area of inundation. The data used include alluvial plain topographic maps and soil subsidence prediction maps. To know the distribution and extent of the puddle, then map overlayed (overlay) so that known changes in the area of the pool based on subsidence. The process of overlapping using Geographic Information System (GIS) in the form of raster-format digital data [5]. 3) Software Analysis To test the analysis of the alluvial sediment subsidence was obtained by laboratory calculation based on sample and primary data. Calculations for predicting subsidence using GEOSTUDIO Office software resulted in predictions of large and long subsidence with varying loads and different soil mechanical conditions. Then to determine the change of the inundation area by using the GIS (Geographic Information System) software, the analysis of the change of puddle area is obtained by giving the categories of values for the four variables namely the load of soil, the decrease of the groundwater surface, the thickness of alluvial deposits and the carrying capacity of the soil. Furthermore, the four variables are overlaid and editor@iaeme.com

6 Soil Consolidation Analysis as the Main Cause of Land Subsidence in Semarang - Indonesia the result is a potential land subsidence map. The potential land subsidence map overlaid with land subsidence maps (secondary data) results is an evaluation of land degradation map and changes in puddle area [ [26]; [12]; [20] ] Physical Condition and Soil Mechanics In this research the physical and mechanical properties of soil at the location of the research in the form of primary data as much as 5 drill points spread an alluvial plain with the depth varied between m. The location of the drill point as shown in Figure 2. For the needs of large predictive calculations and the length of subsidence of soil properties and soil properties are analyzed further. The result of soil physical and mechanical test at the research location was obtained from 5 point drill a sample and the result of laboratory test can be seen in Table Table 1 Condition of Soil Mechanism at Five Research Sites (Laboratory Results and Advanced Analysis) Jalan Gajah Mada Jalan. Dr. Cipto Jl. MT Haryono 0 to -1m Dump -1m to -2m Groundwater level -2m -2m to -25m -25m to Silt very soft Soft Clay Silt & clay hard to hard -1 to -2m Dump Groundwater level -2m -1m to -25m -25m to clay very softs to soft Silt & clay hard to hard 0 to -1m Dump Ground water level -3m -1m to -25m -25m to Silt Very soft to soft Silt & clay hard to hard In the form of a mixture of silt material which is sandy and pebbly with a specific gravity of land ( ) 1.65 t / m3. Form a silt layer with a variation of shell fragment mixture, sand, gray and grayish gray. The N-SPT values range from 2 to 7 with a very soft to mildly soft consistency. LL between 60-81%, PI between 30-50%, () 5 o - 11 o, (c) kg / cm2. Cc = 0,55, eo = 1,136 It is a clay layer with a mixture of shell fragments, sand, gray and grayish gray. The N-SPT values range from 2 to 7 with a very soft to mildly soft consistency. LL between 60-81%, PI between 30-50%, () 5 o - 11 o, (c) kg / cm2. Cc = , eo = Not consolidate In the form of silt material, there is a sandy and pebble drug rug land with a specific gravity of soil ( ) 1.69 t / m3. It is a clay layer with a mixture of shell fragments, sand, gray and grayish gray. The N-SPT values range from 2 to 7 with a very soft to mildly soft consistency. LL between 60-81%, PI between 30-50%, () = 11 o - 18 o, (c) = kg / cm2. Cc = eo = ,138 Not consolidate In the form of silt material, there is a sandy and pebble drug rug land with a specific gravity of soil ( ) 1.63 t / m3. Form a silt layer with a variation of mixture of shell fragments, sand, gray and gray-blackish gray. N-SPT values between 2-5 and CPT values ranging from 5 kg / cm2, with very soft to soft consistency. LL between 60-81%, PI between 30-50%, () = 11 o - 18 o, and (c) = kg / cm2. Cc = 0.5. eo = 1, 209-1,303 Not consolidate 4. JL. Piere 0 to -1m Dump In the form of layers of silt material, there is a sandy and editor@iaeme.com

7 A. Pratikso, S. Sudarno 5. Tendean pebble rug land with a specific gravity of soil ( ) 1.79 t / Jl. Patimura Jurnatan Groundwater level -3m -1m to -25m -25m to Silt Very soft to soft Silty clay hard to hard 0 to -2m Dump -2m to -3m Groundwater level -3m Silt very soft to soft -3m to -6m Sand -6m to -25m -25m to Silt Silt very soft to soft Silty clay hard s/d hard m3. Form a silt layer with the varying mixture of shell fragments, sand, grayish and gray bitter. The N-SPT values range from 2 to 7 with a very soft to mild soft consistency, LL between 60-81%, PIs between 30-50%. This soil layer can be classified as inorganic clay of high plasticity, () = 7 o - 11 o, and (c) = kg / cm2. Cc = 0,500. eo = 0, 714-1,055. Not consolidate In the form of layers of silt material, there is a sandy and pebble rug land with a specific gravity of soil ( ) 1.74 t / m3. Form a silt layer with a mixture of shell fragments variations, sand, grayish and grayish blackish. The N-SPT values range from 2 to 5 and the CPT value ranges from 5 kg / cm2 with very soft to the soft consistency, LL is between 60-81%, PI is between 30-50%. ()= 11 o - 18 o, and (c) = kg / cm2. Cc = eo = 1,106. Consists of a layer of gray sand, sand silt, there are loose shells and shells with soft to a solid consistency. N-SPT value ranges from 2 to 50. Not consolidate Form a silt layer with a mixture of shell fragments variations, sand, grayish and grayish blackish. The N-SPT values range from 2 to 5 and the CPT value ranges from 5 kg / cm2 with very soft to the soft consistency, LL is between 60-81%, PI is between 30-50%. () = 11 o - 18 o, and (c) = kg / cm2. Cc = eo = 0, 871. Not consolidate No. Bor Point No. Bor Point Table 2 Big Prediction and Length of Decline Achieve T90% Using Terzaghi Analysis and Geostudio Software Location Large Decrease Per. Terzaghi Large Decrease Per. Geostudio Decrease Length (Years) 1 Jalan Gajah Mada 46,4 50,0 16,934 2 Jalan Dr. Cipto 45,9 49,03 17,828 3 Jalan MT Haryono 42,4 45,6 17,147 4 Jalan Piere Tendean 50,5 54,5 17,649 5 Jalan Pattimura Jurnatan 74,3 78,7 19,486 Table 3 Big Prediction and Length of Yearly Decline with GEOSTUDIO Software Analysis Location Large Decrease T90% Decrease Length (Years) Annual Decrease Per Year 1 Jalan Gajah Mada 50,0 16,934 2,95 2,95 5,91 8,86 11,81 14,76 2 Jalan Dr. Cipto 49,03 17,828 2,75 2,75 5,50 8,25 11,00 13,75 3 Jalan MT 45,6 17,147 Haryono 2,66 2,66 5,32 7,98 10,64 13,30 4 Jalan Piere 54,5 17,649 Tendean 3,09 3,09 6,18 9,26 12,35 15,44 5 Jalan Pattimura 78,7 19,486 Jurnatan 4,04 4,04 8,08 12,12 16,16 20,19 Year 2016 Year 2017 Year 2018 Year 2019 Year editor@iaeme.com

8 Soil Consolidation Analysis as the Main Cause of Land Subsidence in Semarang - Indonesia y y y y y Figure 4 Comparison chart Big (St) and Long (t) Land Subsidence in Area Studies Large Decrease Location 4. CONCLUSIONS Figure 5 Great Graphic Decrease According to Software Geostudio Pile soil at a depth of 0-2m is a layer of sandy and sandy silt material. There are also soils of rug having a soil specific gravity ( ) 1.63 t / m3 to 1.74 t / m3. Groundwater elevation at depths between 1 and 2 m below the soil surface Silt coating with a mixture of shell fragments, grit, gray and grayish gray. Consistency is very soft to soft. LL between 58% - 79%, PI between 28% - 48%, () 10 o -19 o, and (c) between 0.06 kg / cm2-0.19kg / cm2. Cc = eo = 1, is at a depth of 2m to 25mAt a depth of more than 25 m of unconsolidated soil. From the analysis, it is seen that the northerly (direction of the Java Sea) decreases the greater, but the decline time is longer. ACKNOWLEDGMENTS The research described in this paper was financially supported by The Indonesian Ministry of Higher Education, Technology and Research editor@iaeme.com

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