Safety factor investigation based on finite element analysis in basement design using bamboo soil nailing

The use of retaining walls for basements in buildings generally uses the retaining wall that utilizes the weight of the wall itself as reinforcement because the reinforcement is carried out only on the soil’s surface. Using steel in soil nailing construction that is too large can result in the availability of steel running out, considering that steel is a non-renewable material. The soil nailing method in this paper uses bamboo as nails. The method used in this paper is the analytical method. Data bamboo material from the tests carried out in the Laboratory Building Materials Universitas Sebelas Maret Surakarta. While material data soil obtained from tests carried out at the Soil Mechanics Laboratory Sebelas Maret University, Surakarta. The analytical model is a basement excavation 10 m deep and 20 m wide. Finite element analysis calculations using the Plaxis 8.2 program on retaining walls. The analysis stages at each excavation depth of 2 m. The model obtained that the safety factor value is 4.271; this value is greater than the required safety factor value, namely FS 2, which indicates that soil stability achieve. The analysis results also obtained the achievement of soil stability at each stage of the research. There was no collapse of the bamboo during the analysis stage. The discussion results found that bamboo substitutes steel in soil nailing reinforcement as a retaining wall in basement construction.


Introduction
The use of retaining walls in basement construction generally uses the retaining wall that utilizes the weight of the wall itself as reinforcement [1,2].However, Increasing support means increasing the dimensions of the wall, which will result in high construction costs [3,4].A retaining wall, commonly called a retaining wall, is a building structure that functions to withstand lateral soil pressure caused by a backfill or original soil due to an excavation [5,6].
Soil nailing is one type of soil reinforcement used in retaining walls and slope stability construction [7,8].Reinforcement with soil nailing is considered more profitable than other soil reinforcement [9,10].It utilizes passive pressure that will deploy in movement [11].Therefore, It can maintain excavations [12] and stabilize natural slopes [13] (original soil) by creating a reinforced retaining structure .Soil nailing is rigid reinforcement that can carry regular, transverse, and moment forces [14].If the component of gravity that occurs is large enough, it can cause landslides on the slope [15].However, it is preventive the driving force does not exceed the resistance force originating from the shear strength of the soil along the landslide plane [16].Soil nailing can be used as an alternative option to overcome this problem [17,18].However, Using steel in soil nailing construction that is too large can result in the availability of steel depleted, considering that steel is a non-renewable material [19].Therefore, it is necessary to find alternative materials to replace steel, one of which is bamboo [20].
Bamboo is a type of plant in the Bamboidae group, a sub-family of grasses with fast growth [21].In Indonesia, about 12 species of bamboo use building structures.Still, only four species (Petung, Tali, Duri, and Wulung bamboo) are generally [22].
Bamboo has a relatively high tensile strength [23].For example, Based on research from 1994 to 1999 conducted by Morisco showed that the tensile strength of Petung bamboo reached 3100 kg/cm 2 .Bamboo has good material properties; one of The stress-strain diagrams of several types of bamboo compared to structural steel with a yield strength of 2400 kg/cm 2 can be seen in Figure 1.
In terms of strength, bamboo has an excellent tensile strength value.Based on research conducted by Morisco, the magnitude of the tensile strength of bamboo can reach twice the tensile strength of reinforcing steel.In comparison, reinforced concrete steel had yield stress of about 240 MPa, representing the steel-concrete widely available on the market.Furthermore, the research found that the tensile strength of Ori bamboo bark is relatively high, which is 151.54-402.18MPa [24], while the average tensile strength of putting bamboo is also higher than the yield stress of steel.Meanwhile, based on the tensile strength test of bamboo conducted by Efa Suriani,etc [24,25], the yield stress of Petung Bamboo or Dendrocalamus asper was more significant than the yield stress value of plain reinforcing steel.

Figure 1. Stress-strain of bamboo and steel
Its high tensile strength value can reach twice the tensile strength of reinforcing steel [20].However, bamboo has a low adhesive strength value [26], so special treatment is needed to increase its adhesive strength [27,28] .One of them is to make twisted bamboo.The discussion of this paper is to determine the magnitude of the safety factor obtained from strengthening soil nailing in basement buildings using bamboo as nails.

Materials and Methods
The method used in this paper is an analytical method with modeling using Finite Element Analysis [2,[29][30][31].Analysis data and material strength parameters from the analysis of the Mechanics Laboratory, Sebelas Maret University, Surakarta.Meanwhile, the bamboo material data from the research results at the Building Materials Laboratory, Sebelas Maret University, Surakarta.
The form of the modeling made is a basement excavation as deep as 10 m with a width of 20 m.On the excavation surface, slopes are given walls with reinforced soil nailing to provide stability to the excavation.In soil nailing reinforcement, the type of bamboo used is Dendrocalamus asper with a slope angle of 100, a diameter of 20 mm, and a length of 10 m, referring to the requirements of the FHWA (Federal Highway Administration).The bamboo stems are made into twists to increase the adhesive strength of the bamboo.The ground surface is uniform load Q of 100 kN/m2 along 10 m as an assumption of the load on the building next to it.
The calculation analysis uses the Plaxis 8.2 program to find the value of the safety factor (SF) of the soil nailing retaining wall.The analysis results will then be displayed in pictures and graphs to discuss and draw conclusions.
The stages of calculation analysis are as follows:

Results and Discussion
Based on the results of research conducted at the Soil Mechanics Laboratory and Building Materials Laboratory, Sebelas Maret University, Surakarta, the following analytical data: Soil nailing is a passive reinforcement by driving nail pieces into the ground [12].In retaining walls, the fragments of nails will provide resistance forces due to lateral earth pressure.The resistance force exerted by the nails against lateral soil pressure is in the form of a tensile strength whose magnitude by the nail's yield stress and the surface area of the nail [32].Therefore the material used as nails must have a high tensile strength value.

Soil Material
Bamboo is a material that has a high tensile strength value.Based on research conducted by Morisco, the tensile strength of bamboo can reach two times the value of the tensile strength of reinforcing steel.Based on this, bamboo can be used as an alternative material to replace steel as nails.Bamboo is also an abundantly available material in Indonesia [3] and can be renewed not to run out.
However, bamboo has drawbacks, one of which is the adhesive strength, which is small compared to steel.Therefore, special treatment is needed on bamboo not to cause pullout failure when used as nails.One way that can do is to form a twist on the bamboo to increase the value of its adhesive strength.
Based on the pullout test conducted by Building Materials Laboratory, Sebelas Maret University, Surakarta, the adhesive strength of twisted bamboo was greater than that of plain bamboo.The following table results: After analyzing the planned modeling, the results obtained show the stability of the excavation achieved at each stage.it indicates that the natural strength of the soil maintains at each step of the analysis following the excavation plan (2m).

Figure 12. Results of Each Stage of Analysis
At the final stage of the analysis, the safety factor (SF) value is 4.271.This value is greater than the FS value suggested by Bowles in B. M. DAS [5],for retaining walls in cohesive soils, FS 2; this indicates that soil nailing reinforcement with bamboo can withstand lateral soil pressure in the basement excavation according to the design load.In addition, the analysis results showed that the bamboo material used as nails did not collapse, which indicates that there was no nail tendon failure in soil nailing reinforcement.

Figure 13. Soil Deformation
The analysis results also show that the maximum soil deformation at the final stage of the analysis is 1.75.10-3m which indicates that the soil has not experienced such a significant shift.

Conclusions
When testing the stability number from the retaining wall, the value of the safety factor (SF) from the analysis carried out is 4.271.We can use bamboo as a substitute for steel in soil nailing reinforcement to hold the wall in basement construction.Before being used as reinforcement must be given special treatment to increase its adhesive strength.One way is to make twisted bamboo.It would be better to make a scale model to see the land subsidence reaction for further research.

Table 1 .
Soil Material Data

Table 3 .
Value of Plain and Twisted Bamboo Adhesive Strength