# Effect of Micropiles on Clean Sand Liquefaction Risk Based on CPT and SPT
## 1 Introduction
A micropile is a type of drilled shaft with a diameter in the range of 75 to 300 mm. Micropile elements can be installed in areas with limited geometrical conditions, and with various length or installation angles [24]. The installation of micropile elements requires relatively light equipment, compared to the traditional pile installation methods which accelerates the construction rate [25]. High-strength steel bars, or steel tubes, increase the capacity of the micropile element against buckling, and enhance the punching shear resistance of the connected concrete slab [26]. The bond between the micropile and the soil develops using the high-velocity jet grouting method. The FHWA-NHI guideline classified two cases regarding the design and implementation of micropiles. In Case 1, the structural capacity of the micropile is used to bear the axial or lateral load applications. Case 1 micropiles are designed individually, and are commonly employed in almost all of the micropile projects in North America [23]. Many research investigations have been conducted on the structural responses of the micropile elements [27; 28]. Case 2 micropiles are designed as a reticulated network. In this case, both the structural and soil reinforcing capacities of the group of micropiles are used to increase the soil-bearing resistance and control the settlement [29]. Few research studies have investigated the use of micropiles to enhance the mechanical properties of the soil together with the high-pressure grouting method, to reduce the risk of liquefaction [30]. Methods of evaluating liquefaction risk are based on the semi-empirical results following case-history data [31; 32]. These methods have been developed through the years, using the latest available seismic data, and are commonly used in the design and construction process of infrastructures. The first simplified procedure for measuring the liquefaction potential was developed by [PERSON] and [PERSON] [33], and has been widely adopted in practices of geotechnical engineering. However, recent studies show that such simplified liquefaction assessment methods are only reliable for soil profiles that mainly contain clean sands, with fines contents of less than 5%. The outcomes of the simplified method are not well founded for problematic soil [34]. Problematic soil layers mostly include interbedded layers of sand and gravel with high percentages of fines contents and plasticity variations. Silty sand, clayey sand, silty gravel, and clayey gravel with more than 12% of fines are categorized as problematic soils [35]. Many researchers have investigated the potential of the liquefaction following the results of cone penetration test (CPT) and the standard penetration test (SPT) [36; 37]. These studies were conducted based on the case history database of the major earthquakes throughout the world. However, to the knowledge of the authors, there has been a lack of investigation on the effects of using jet-grouted micropiles on mitigating the risk of liquefaction. Moreover, simplified soil liquefaction assessment methods are only reliable for clean sands. The existence of a saturated soil profile with the majority of clean sand layers creates a unique opportunity to use the simplified liquefaction assessment method to further evaluate the effectiveness of soil remediation practices on decreasing the risk of soil liquefaction. This study presents a novel approach to reduce the risk of soil liquefaction using jet-grouted micropiles in a case study research. The investigated project is located in a long coastal area in the southwest of Iran, and adjacent to the Persian Gulf. The soil profile mainly includes interbedded layers of clean sands. Since the majority of the soil is clean sand, the simplified liquefaction evaluation method was used to assess the liquefaction potential. Improvement of the soil after using jet-grouted micropiles was evaluated and compared to the native existing soil. CPT and SPT are the most reliable indices for assessing the liquefaction specifications of soils. [31].