# Effect of Micropiles on Clean Sand Liquefaction Risk Based on CPT and SPT
## 2 Material and Methods  
### Study Site Description and Design  
The research case study project was located in the seismically active city of Bushehr, southwest Iran at the seaside of the Persian Gulf (Figure 1). The region has a record of high-magnitude earthquakes. For instance, in April 2013, an earthquake occurred with a moment magnitude of 6.4, and with the epicenter 90 km from Bushehr [38]. The most recent earthquake near the Persian Gulf was recorded on 15 March 2020, with a magnitude of 5.3 and a distance of 95 km from Bandar 'Abbas, on the southern coast of the Persian Gulf [39]. The soil profiles are characterized by thick layers of clean, saturated sand. Geotechnical properties of the soil in the site project were determined by performing in situ and laboratory tests in two project sites. To specify the stratigraphy of the soil, a rotary drilling technique was employed to drill 10 boreholes to approximate depths of 30 m in two sites of the project. In each site, the mechanical, physical, and chemical properties of the soil were evaluated by drilling five additional boreholes. Soil boring logs include the classification of soil layers, identification of CPT or SPT values, groundwater levels, moisture contents, and unit weights. Based on the borehole logs, most of the interbedded layers were thick layers of poorly graded sand (SP), as clean sand. In addition to the SP layers, the soil also consisted of layers of poorly graded sand along with clay and silt (SP-SC, SP-SM) clayey sand (SC), or silty sand (SM). Based on the bore log data from the boreholes in site I, almost 81% of the soil layers SP were clean sand. In the site II, 76% of soil layers were clean sand (SP). In this research, a jet-grrouting method is performed using 420 micropiles, with a length of 24 m and casing outside diameter (OD) of 140 mm (5.5 inches), to enhance the existing soil properties. The effect of jet grouting using micropiles on reducing the potential of liquefaction was assessed using the results of CPT and SPT tests at two adjacent sites, which were conducted prior to and after jet grouting using micropiles. The factor of safety against the liquefaction was measured using the results of CPT and SPT, following the procedures of Eurocode 8, NCER-97, Boulanger [PERSON] 2014, and [PERSON] 2006 [40; 41; 42]. At site I, CPT was carried out following the ASTM D3441-16 specifications [43] to delineate soil stratigraphy and measure the geotechnical properties. The CPT tests were conducted before and after the soil improvement employing jet grouted micropiles. At site II, SPT tests were performed based on the ASTM D1586/D1586M standard [44] to specify the soil layers and measure the geotechnical properties of the soil before and after using micropiles. Sites I and II had 220 and 200 micropile elements installed, respectively. These micropiles were porous steel tubes with a casing outer diameter (OD) of 140 mm (5.5 inches) and wall thickness of 12.7 mm (0.50 inch). The elements were reinforced with a single steel bar, with a diameter of 32 mm (10 inches), and an outer diameter of 36 mm (1.43 inch) over threads. The reinforcement steel was made of 520 steel grade with a yield stress of 520 MPa and an ultimate stress of 690 MPa, following A615/A615M-18e1 [46] specifications. Micropile elements were installed as an arrangement of \(2.5\times 2.5\) m, center-to-center element distances, in a plan at site I, and a \(2.3\times 2.3\) cm grid at site II. The distance between micropile elements was selected based on previous records of micropile installation in adjacent sites. In situ tests indicated factors such as leakage control around the micropile casing and, as soil hydrofracture affects the efficiency of the post grouting, using different geometrical arrangements of micropiles. The type C-C1 post grouting procedure, based on the FHWA-NHI guideline, was used in the project. In this method, the grout is placed using micropile elements in two phases. First, the neat cement grout is placed under the gravity head for 20 min.