Substitution of sand ditch system method on vacuum preloading (study case: Land preparation project in Kalimantan)

. Vacuum Preloading is considered as an efficient method in soil improvement. It is complemented by the implementation of horizontal drain work, namely the Sand Ditch System and the Hand Type Connector. Decision making in the application of these methods must be adjusted to the actual needs and conditions in the project. This research was conducted in soil improvement projects in Kalimantan which was originally planned to use the Sand Ditch System. However, difficulties were found in finding sand as a media for draining water. Consequently, the method substitution was carried out to the Hand Type Connector. The purpose of this study indicate that the application of the Hand Type Connector Method is more effective and efficient in terms of cost and time compared to the Sand Ditch System. The cost and time required by the Sand Ditch System are respectively Rp 56.095,00/m 2 and 194 days. Meanwhile, the costs and time required by the Hand Type Connector are respectively Rp 37.250,00/m 2 and 183 days. Therefore, this project can save cost by 33,59% and time by 5,67%.


Introduction
On August 29, 2019, the Indonesian government announced the relocation of Indonesia's capital which was previously located in Jakarta, to Penajam Paser Utara, East Kalimantan.The relocation of the capital city to East Kalimantan aims to balance the economy in Indonesia and reduce the economic gap between Java Island and other islands outside Java.The relocation of capital city from Jakarta to Kalimantan also had a big impact on industrial development in Kalimantan, especially in Balikpapan and Samarinda City which now become the satellite city for Penajam Paser Utara as the new capital city.
The government's programs regarding nickel downstream in Indonesia have also resulted in the domestic industry, especially the nickel refining industry, becoming very well-developed.This industrial development resulted in high interest from investors to invest in the domestic industry, especially in Kalimantan which becomes the new capital city of Indonesia and has a very strategic location.
Based on statistical data from 2021, East Kalimantan experienced growth in the mining and quarrying business of 45,05% [1].This shows that the demand for industrial estates is increasing, including the growing of nickel industry [2].To fulfil and continue maintaining the development of the nickel industry, it is required to develop a land, one of which is Project A, a nickel smelter project in Kalimantan as the object of this research.
*Corresponding author: j.kevin@wikamail.id In Kalimantan, 41% of the area is highland as high as 200 m above sea level, whereas 25% of the area is hilly lowland with an altitude of 200 m below sea level.The remaining 34% is a wetland which includes swamps, floodplains, peatlands, and many more [3].
At the location of Project A, the land has an area of 26 Ha, where 70% is swampy area and 30% is hills area.Before the construction is carried out, it is necessary to conduct a soil investigation first to determine the soil characteristics of the location in Project A. After conducting a soil investigation, it was found that the soil at the location in Project A is a soft clay type soil.Soft clay type soil has a compressible condition that will cause relatively large land subsidence.It is necessary to do the soil improvements for increasing soil strength and reduce compression that may occur.If the improvements are not done properly, the nickel smelter that will be built later will potentially experience damage before reaching the specified time.
The owner of Project A requires a soil improvement design with the following criteria: 1.The structure load will be supported by the foundation.2. The final elevation is +7,00, with the average existing soil at an evaluation of +2,50 3. Platform or working floor elevation is at an elevation of +4,00 4. The elevation of the ground to be improved is at an elevation below +4,00 5. Loads that will be supported directly by the soil include pavement loads is 15 kPa and traffic loads is 15 kPa.The total operating load for the soil improvement design is 30 kPa.
Based on the results of the soil investigation that had been carried out previously, according to the introduction listed in SNI Geotechnical Design Requirements, the method of soil improvement that is suitable for the condition of Project A can be determined based on Fig. 1 [4].Based on the limited time request from the owner and design criteria above, it is proposed to use the Vacuum Preloading Method with a Sand Ditch System.The implementation of Vacuum Preloading Method generally uses a Sand Ditch, which is considered more effective and efficient because Sand Ditch is the only method that is feasible in time and cost to achieve the desired results compared to other alternative methods [5].In the implementation of the soil investigation in Project A, there were limitations to the soil data.Initially, the area proposed for soil improvement was 4,28 Ha using the Sand Ditch System Method.However, the availability of soil data in areas outside of the 4,28 Ha was very limited so additional soil investigations are necessary.
On the results basis of additional soil investigation, it turns out that the surrounding soil in other areas also required soil improvement work.The area to be improved was 12,9 Ha, which was three times larger than the previous area.Therefore, it is necessary to evaluate the Sand Ditch System in the previous area of 4,28 Ha.
Time and cost have a huge impact on the success or failure of a project.The benchmark for project success is usually a short execution time with minimal effort without sacrificing the quality of work.Systematic project management is necessary to ensure that the projects are delivered by the contract.This ensures that the costs incurred are profitable and that penalties for late project execution are avoided [6].
The aim of this research is to evaluate the effectiveness and efficiency of using the Sand Ditch System Method in the Vacuum Preloading Method on Project A in Kalimantan.In addition, this research aims to find out and evaluate other alternatives to the Sand Ditch System Method in terms of effectiveness and efficiency in terms of cost and time.

Method
Flow chart of the research can be seen on Fig. 2.

Vacuum preloading
In this case, from the results of the soil investigation that has been carried out on Project A, the type of soil improvement method needs to be carried out based on SNI Geotechnical Design Requirements is the Vacuum Preloading Method with PVD.
Vacuum Consolidation is the application of vacuum pressure to a sand blanket installed above a vertical diverter that will increase water flow and accelerate the consolidation process.To reach vacuum conditions, the blanket must be wrapped using a membrane [7].
The Vacuum Preloading Method was originally proposed by Kjellman in 1952.The main concept of the Vacuum Preloading Method is to apply vacuum suction to the isolated soil mass to reduce the pore water pressure and atmospheric pressure in the soil, thus increasing the effective stress without changing the total pressure and resulting in decreased consolidation in the soil.Application of heap load (preloading) causes in resulting pore water pressure that occurs due to the negative pore water pressure plus the heap load that is caused by the suction pressure from the vacuum pump.Therefore, the pore water pressure generated by the previous method will be lower compared to the pore water pressure due to the Preloading Method [8].The schematic of the Vacuum Consolidation system is depicted in Fig. 3.
Prefabricated vertical drain (PVD) is an artificial drainage system, that is placed vertically on soft soil layers.In general, PVD is a synthetic material for drainage.The material used consists of two components, namely geotextile filter fabric which is used as a filter so the water flowing into the soil pores can enter easily, and also the core/drain core (plastic drainage core) which is used as a place for water passage [9].As a result of preloading, water flowing out towards the PVD will move in a radial direction and then the water will be forwarded in the vertical direction towards the surface through the core.The drain water in the lateral direction will be forwarded by a prefabricated horizontal drain (PHD) [10].A schematic of the Prefabricated Vertical Drain (PVD) system is shown in Fig. 4.
The installation of the Vacuum System begins with the application of a layer of coarse sand, which serves as a platform.Following that, when vertical drainage (PVD) is already installed, followed by horizontal drainage and other complementary equipment.PVDs can be installed in equilateral triangle or square arrangement.[10].

Fig. 4. Prefabricated vertical drain (PVD)
The connection between the outlets for horizontal drainage and some of the equipment should be wellconnected to the membrane.The membrane is laid, and anything that may cause leakage of the membrane when the vacuum pump is running should be dealt with first.While the vacuum pump is turned on, the joints between the membrane sheets are checked for leaks.The suction of the vacuum pump acting in the sand layer under the membrane will be channelled down through the vertical pipe.The vacuum pressure applied can be up to 90 kPa [11].

Prefabricated horizontal drain (PHD)
A Prefabricated Horizontal Drain (PHD) is one of the geosynthetics that has function as a drainage channel formed from filter jacket and mechanically strong polypropylene core.The function of the PHD is to accelerate the consolidation process of the soil and improve the area with soft soil.The water that flows horizontally through the PHD is the water that flows through the PVD vertically [10,12].
Due to its low installation cost and reduced space requirements, as well as its ease of handling, PHD is a viable solution to many problems associated with soft clay soil.In the design of a Horizontal Drain System, two factors must be considered.The first factor is the smear effect, which is caused by the driving of the mandrel during installation and results in the formation of a less permeable zone in the immediate vicinity of the drain.The smearing effect does not need to be considered in the case of mixed or remoulded soil or when PHDs are manually placed in layers within the soil.The second factor to consider is long-term drain clogging, also known as the good resistance effect.To account for this effect, the discharge capacity must be properly calculated.When the discharge capacity is quite high, however, well-resistance can be ignored.
PHD with a size of 20 mm x 100 mm as the horizontal drain in the soil improvement.When planning a PHD, it can be adjusted to the pattern and schedule of the PHD [13].

Sand ditch system
A Sand Ditch System is used to flow water from the vertical drains and provide a solid pad for the vertical drain installation.They alter the hydraulic gradient of vertical flow in the vertical drain to speed consolidation when the horizontal drainage rate in the drains is sufficient.Sand Ditch has the main function to maintain the flow capacity from PVD to PHD and keep it flowing optimally.In the Vacuum Preloading Method, PHD still functions as the main horizontal drainage channel, while the sand is can be useful to reduce the risk of water flow from the PVD that is connected to the PHD.For this function, sand without high drainage capacity can also be used as the fill material for the channel [14][15].
In general, the Sand Ditch System is widely used in soil improvement methods because it is considered to be an effective and efficient method compared to other alternative methods.The primary concern with this project is the limited quarry of sand material since it is needed to serve as the drainage layer for the vertical drains system.PHD with a Sand Ditch System is a PHD that requires a sand ditch as media for water drainage.In this system, PHD pipes consist of 2 types, namely main pipe (VCM main wire hose) with a diameter of 50 mm and distribution pipe (VCM distribution wire hose) with a diameter of 25 mm.Main wire hose and distribution wire hose in Horizontal Drain Sand Ditch System on Project A can be seen in Fig. 5.

Hand type connector
Hand Type Connector Method does not use sand as media for water drainage.Hand Type Connector is made of polymer, which will connect the PVD pipe with the PHD pipe so that the PVD and PHD pipes can be connected tightly.The PHD pipe is made of transparent polymer material with spiral wire reinforcement, which has a thicker material when compared to the PHD material in the Sand Ditch System.
Hand Type Connector Method entails using PVDs to apply suction inside the soft soil layer and connecting them to a pump system via airtight collector tubes.The collectors are installed in place of the membrane and horizontal drains, and the PVDs are connected to the collectors.After the system's airtightness has been verified, a conventional fill can be carefully constructed above the connectors and collector tubes [16].
The connection of PVD to PHD requires human to insert the connector to be tied and tightened, so it has lower productivity than the Sand Ditch System.However, because the Hand Type Connector does not use sand, it does not require human and additional time for sand filling work, which must be done manually.
PHD with this Hand Type Connector system is a PHD that does not use sand as a media for water drainage.The connection between the PVD pipes and PHD pipes is connected by Hand Type Connector made of polymer, so that the PVD and PHD can be tightly connected.PHD Hand Type Connector Method is suitable for projects that have limited availability of sand material and difficulty accessing the delivery of sand material from outside the site area.Therefore, this method is suitable for Project A. PHD Hand Type Connector for Project A can be seen in Fig. 6 and installation of the connector can be seen in Fig. 7.

Sand ditch system installation
During the initial contract period of Project A, soil improvement was carried out using PHD Sand Ditch System due to time constraints on the project period and a limited budget.Installation of PHD Sand Ditch System was done manually by connecting PVD material on the PHD pipe.The installation of PVD to PHD is shown in Fig. 8. Then the PHD pipe was installed into the drainage channel with dimensions of 20x20 cm 2 .PHD pipe installation was done with a grid of 1x6 m 2 .The installation of PHD Sand Ditch System on Project A can be seen in Fig. 9. Fig. 9. Installation PHD sand ditch system on project A.
After the installation, the drainage channel was backfilled using sand material which was used as a media for draining water.Sand filling can be seen in Fig. 10.

Cost and time analysis on PHD sand ditch system
The availability of sand quarry in the area around the site was very limited.In addition, the limited access to site due to low sea depth and land access made it more difficult to access material shipments, both by sea using barges and by land using dump trucks.This caused the need for a re-evaluation of costs and time when using PHD Sand Ditch System Method.Calculation of the sand's unit price can be seen in Table 1.In addition to the high cost of sand material, the purchase of sand from outside the site using barges will also affect the work schedule.The work schedule for additional soil improvement areas using PHD Sand Ditch System Method can be seen in Fig. 11.Based on Fig. 10, additional soil improvement area work using PHD Sand Ditch System Method had a high cost and a long time in procuring sand material.Procurement of sand material must use the sea route so that the arrival of the material must adjust to the tides of the sea.The procurement of sand material was carried out long before the PHD work was carried out so the planned schedule can be achieved.Since the arrival of sand material must also be adjusted to the tides of the sea, it must be planned carefully because using this method can affect the overall project schedule.Therefore, it is necessary to find an alternative method that is more efficient and effective in terms of cost and time for this additional soil improvement area.

Hand type connector installation
PHD Hand Type Connector Method was other option implemented on Project A since PHD Sand Ditch Systems was considered ineffective and inefficient in terms of cost and time.Hand Type Connector Method does not require sand material as a media, so there is no time and cost inefficiency caused by sand material.In addition, this method does not require additional labourer and time for sand filling work.This was the reason why PHD Hand Type Connector could be considered as the best option to solve the condition of limited sources of sand material in Project A. PHD pipe installation was done with a grid of 2 m x 20 m (the distance between PVD 1,0 m x 1,0 m), so that the use of material became more economical and efficient.The installation of PHD Hand Type Connector is shown in Fig. 12. Fig. 12. PHD hand type connector installation.

Cost and time analysis on PHD hand type connector
Installation of PHD Hand Type Connector as shown in Fig. 10 did not use sand but used a different horizontal drain material and also a connector.Because of this, PHD Hand Type Connector might be having a higher unit price than the PHD Sand Ditch System.The installation of PHD Hand Type Connector also needed to be done by trained worker or subcontractors to maintain the quality of the work.The cost of installation PHD Hand Type Connector by experienced subcontractors on additional soil improvement work is obtained as shown in Table 3.In addition, the productivity of PHD using the Hand Type Connector Method is lower when compared to PHD Sand Ditch System, because the work of PHD Hand Type Connector Method used worker to connect, tie, and tighten the PVD and PHD.The work schedule for additional soil improvement areas using PHD Hand Type Connector Method can be seen in Fig. 13.

Comparison of PHD sand ditch system and phd hand type connector
Based on the data from the Sand Ditch System and Hand Type Connector, it is necessary to do a comparison of cost and time schedule for horizontal drain type to be applied as the most suitable method for the additional soil improvement work.The comparison of the cost for PHD work using Sand Ditch System and Hand Type Connector can be seen in Fig. 14.
It can be concluded from Fig. 14 that PHD Hand Type Connector Method has a more efficient cost than using PHD Sand Ditch System, which was Rp 18.845,00 per linear meter of PHD installation.The cause of the high unit price for Sand Ditch System was due to the difficulty in accessing sand materials to the site.
Apart from cost, it was necessary to compare the time between using Sand Ditch System and Hand Type Connector.Comparison of the duration between Sand Ditch System and Hand Type Connector can be seen in Fig. 15.
Based on Fig. 15, it was obtained that PHD Hand Type Connector had a shorter work duration of 11 days when compared to PHD Sand Ditch System.The procurement of sand material in PHD Sand Ditch System required quite a long time to be delivered due to tidal conditions.The comparison summary of additional soil improvement areas using PHD Sand Ditch System and PHD Hand Type Connector can be seen in Fig. 16.
Based on the Fig. 16, it can be concluded that the work of additional soil improvement areas at the Project A using Hand Type Connector was more efficient and effective when compared to Sand Ditch System in terms of cost and time.Manpower on the Sand Ditch System and Hand Type Connector had a difference of 10 people.However, with manpower that was more employed in the Hand Type Connector, this method still produced a cost and duration of work that was much more effective and efficient than the Sand Ditch System.At the same time, using Hand Type Connector Method can be advantageous in time by saving the project duration up to 11 days.Apart from cost and time advantageous, by using Hand Type Connector can reduce the risk of transferring material using water transportation which affected by weather and tidal.      .The analysis of cost and work schedule on Project A showed that PHD using Hand Type Connector Method was the most effective and efficient method to be applied in additional area soil improvement work at Project A in Kalimantan due to the limited sources of sand material for Sand Ditch System Method.These results showed that in choosing the best methods used in project, not only do we consider only for the unit rate, but also the material resources and the material handling.
In this research on alternative method for Sand Ditch System, it has been concluded that the Sand Ditch System can be replaced with the Hand Type Connector in Project A. This study can be a reference for other project, but the evaluation for overall project is necessary, not only for material and labour but also the access and material handling.However, there are several variables that were not taken into account in this study.Thus, for further research, it is necessary to carry out an overall analysis, in terms of the quality, safety of the workers, and the calculation for geotechnical engineering such as material properties in carrying out a comparison Sand Ditch System and Hand Type Connector Methods.

Fig. 11 .
Fig. 11.Work schedule of additional soil improvement areas using PHD sand ditch system method.

Fig. 13 .
Fig. 13.Work schedule of additional soil improvement areas using PHD hand type connector method.

Fig. 14 .
Fig. 14.Cost comparison of sand ditch system and hand type connector.

3. 6
Conclusion and suggestion PHD using Sand Ditch System Method costs Rp 56.095,00/m 2 while PHD using Hand Type Connector costs Rp 37.250,00/m 2 .With this result of calculation, it can be concluded that using PHD Hand Type Connector is more efficient than PHD Sand Ditch System with the difference of approximately Rp 18.845,00/m 2 .By percentage, using PHD Hand Type Connector could save the cost by 33,59%.In terms of work schedule or time, PHD Sand Ditch System Method took 194 days while PHD Hand Type Connector took 183 days.The calculation result showed that PHD Hand Type Connector had a shorter time by 11 days compared to PHD Sand Ditch System.The installation of PHD Hand Type Connector could save time by approximately 5,67%

Table 1 .
Unit price of sand supply.Based on Table1, it was found that the unit price of sand is Rp 493.354,00 per 1 m 3 .However, after carrying out additional CPT tests, data was obtained that there was a need to have soil improvement for an additional area of approximately 8,688 Ha.The unit price of PHD Sand Ditch System for additional area can be seen in

Table 2 .Table 2 .
Breakdown unit price PHD sand ditch system.

Table 3 .
Breakdown unit price of additional area using PHD hand type connector.