# Study on the Increase of the Supporting Capacity of a Cement Milk Pile with Expansive Additives
## 1 Introduction  
As recent buildings have become larger and taller, pile foundations have seen wider use [1, 2]. The driven pile method, a technology introduced earlier in history, inserts piles by applying percussion or vibration [3]. Although it is advantageous in terms of support and material economy, the method also causes social problems due to severe noise and vibration [3, 4, 5]. With the enforcement of Korea's Noise and Vibration Control Act in the 1990's, the low-noise and low-vibration bored pile method has attracted attention [6, 7, 8]. The bored pile method drills the ground in advance, as deep as the length of a precast pile, before pile insertion [9]. The method started in Japan during the early 1980's, and it has been modified for use in Korea's construction conditions [10]. The most common variations used in Korea are the soil-cement injected precast pile method, special auger and injected precast pile method, separated doughnut auger method, percussion rotary drill method, and Cortex method [11]. According to the Korea Expressway Corporation, the 3-year span of 2008-2010 saw 92.5% of steel pipe piles for highway bridges constructed using the bored pile method [12]. In particular, the separated doughnut auger method (55.6%) and the special auger and injected precast pile method (36.8%) are mainly used [12] among the variations. General construction using the bored pile method involves drilling the ground 100 mm larger than the diameter of the precast pile. Injecting a thin cement paste called \"cement milk\" before and after inserting the pile improves the stability and bearing capacity of the excavation wall [9]. The bearing capacity of the bored pile includes the end bearing capacity supported by the pile end's bottom and skin friction force (a frictionalforce acting along the pile surface). According to previous studies conducting static load tests, the share of a pile's skin friction force in relation to its total bearing capacity accounts for 66-97% of the capacity, making skin friction force one of the important considerations in structural foundation design [13].  
Generally, ordinary Portland cement has been used without admixture as a filling material for each bored pile. Occasionally, blast furnace slag cement is used as a filling material to improve long-term strength, chemical resistance, and seawater resistance along with cost reduction. However, the use of blast furnace slag reduces effectiveness due to a decrease in early age strength and an increase in shrinkage [14]. In addition, to effectively use construction waste, recycled solidified soil from sludge is used as a filling material and thickener is used to prevent soil loss around buried piles by increasing the viscosity, thereby reducing the consumption of cement [15]. However, applying these techniques is not useful and is mostly considered from an environmental point of view. Therefore, this study focused on cost reduction and ensured the rigidity of the bored pile by using the expansive additive as an admixture to increase the skin friction force. Meanwhile, expansive additives mainly produce calcium hydroxide or ettrinigite during the early stages of hydration reactions [16; 17]. This production expands the matrix and reduces matrix cracking due to compressive force [16; 18]. If expansive additives are added to cement milk used in bored pile construction, knowing the addictive characteristics can ensure that the compressive force applied to the principal plane of the piles is expansive while also ensuring that pores are sealed during hardening [9]. It is thought that this can improve pile bearing capacity.  
Thus, in this study, the applicability of using expansive additives to increase bored pile bearing capacity was examined. Segregation resistance, compressive strength, frictional resistance stress, scanning electron microscope (SEM) images, porosity, and economic feasibility were analyzed by adding expansive additives to cement milk.