The history of micropiles stems over seven decades ago from Europe though widespread acceptance and standardization of the technology is more recent and has not matured for as long as its first documented use in 1950s Italy. Micropiles were largely introduced as a solution to provide additional support to unstable structures. Present use cases still reside primarily within the context of deep foundation design engineering. With more documented usage of micropiles in broader applications, the body of knowledge in this niche of foundation design accumulates. This paper aims to provide students with interest in geotechnical and/or foundation engineering basic understanding to the use of micropiles in deep foundation applications. It introduces the reader to the basic use of micropiles, provides theoretical background knowledge on bearing capacity calculations, and explains the functionality and benefit of micropiles within the context of expansive and karstic ground conditions. Though beyond the scope of the paper, micropiles are also a viable option for other geotechnical engineering design challenges, namely with regards to slope stabilization, earth retention, and ground strengthening. In addition to bearing capacity considerations, the practical micropile design process also accounts for settlement requirements, but the theoretical calculations for this design consideration is not covered in this paper.
Micropiles are deep foundation elements used for structural reinforcement between above grade structure and below grade soil structure. Micropiles (or minipiles) are small diameter, cast in place grouted columns used as load transfer elements from the supported structure above grade to the sub-soil skeleton. The Federal Highway Administration (FHWA) categorizes the use of micropiles between structural support, encompassing all usage of micropiles in the extent of foundation reinforcement and seismic retrofitting, and geotechnical support, encompassing the smaller portion of use for in-situ ground improvement relating to slope stability and differential settlement (Sabatini et. al 2005). Publications by the FHWA in the late 1990s detailing the practical use of micropiles largely contributed to the legitimization of the use of micropiles for addressing design challenges as it relates to structural support or in-situ ground improvement techniques.
Soil Mechanics And Foundation Engineering By Vns Murthy Pdf Free 12
Whether micropiles are utilized for structural or slope stabilization purposes, there are several key considerations that would favor the use of micropiles. Some of these considerations are based on site conditions (e.g., pile driving can induce liquefaction), machinery access (e.g., remote environments, small spaces, and existing obstructions), and cost (e.g., determining if the cost per kilonewton of force provided by the micropile installation is financially acceptable). Though micropile installation can vary due to drilling techniques and the considerations previously stated, the installation process can be generalized to three steps: The first step is drilling a hole wherein the support element will be casted in. Depending on the site conditions, the most cost-efficient and least disturbing drilling technique can be employed along with drilling liquid (slurry) or outer drill casing to prevent internal collapse while drilling to the desired depth of a competent soil stratum. A steel rod is inserted securely along the centerline of the bored hole (step 2) and the opening is encased with grout from the top of the competent layer to the desired length of the micropile element (step 3). Variations of the reinforcement material and grout used are integrated in the design process ensuring the load is sufficiently supported either by the skin friction between the grout-soil interface, the end bearing capacity, or a combination of both. From previous studies and experimentation, we know that as deep foundation elements, micropiles will largely derive its bearing load from friction between the soil-grout interface. An additional layer of complexity in creating a micropile element is introduced in the installation process of the grout itself. Figure 4 visually highlights four types of micropiles with differing grout installations ranging from simple grouting with only gravity head (Type A) to a two-step process using pressurized grouting method (Type D), which reveals a modified profile of the end product of the micropile (Perkins 2015, Shong and Fong 2003). See appendix for Details of Micropile Classification Based on Type of Grouting (after Pearlman and Wolosick, 1992). Different type grouting is a function of cost, availability of materials, and the intended design capacity of the piles.
The bearing capacity of a material is a measure of the amount of load it can sustain and transfer to subsequent elements, in this case the load transfer to the surrounding soil skeleton. The ultimate bearing capacity is a fundamental concept in solid mechanics indicating the maximum contact pressure a support element can sustain from other load bearing elements before failure. In soil mechanics, bearing capacity of soil is the load it can sustain without shear failure. It is a function of the soil shear strength and the shape for the footing. The ultimate bearing capacity largely dictates the allowable bearing capacity an element will be designed to sustain to ensure an acceptable design factor of safety for the overall structure.
The Sanita factory located in Zouk-Mosbeh, Lebanon was subject to the same constraints having been designed on a one-meter thick high plasticity silt cover on top of a karstic rock formation (Ballouz 2012). Although access to a hard rock stratum merits a shallow foundation design to transfer structural loads to the bearing soil, unstable and unpredictable conditions from the weakened geography needs critical design consideration. The original designs included seven strip footing foundations to be excavated at bedrock (Figure 7), but it proved to be time and cost inefficient. Ultimately, a combined micropile and footing foundation (CMFF) was utilized for this project where the end bearing capacity of the footing and the frictional capacity of the micropiles both contribute to the ultimate bearing capacity (Figure 8). Across all the spread footings supporting the structural elements of the Sanita factory, a total of 386 Type B piles, grouped beneath each footing, were used. Pressure grout injection pushed the cohesive material along the full length of the pile as well as the preexisting fissures in the rock to help ensure a strong bonding zone between the pile and the surrounding soil and rock. On-site axial compression static load tests were conducted to determine the load deflection characteristics of the pile-soil foundation and ultimately learn a safe bearing capacity that can be applied to the piles. Figure 9 describes the load transfer relationship between the applied load and the depth of the pile, which again reaffirms that end bearing capacity of piles contribute a minor difference to the ultimate bearing capacity. The use of micropiles in this project proved to be cost and time efficient: reducing the labor and material costs for more excavations and optimizing the project schedule to locally excavate at multiple footings at the same time.
Soil mechanics is the branch of engineering mechanics that studies the properties and behavior of soil at a site and its potential effects on any man made structure built upon it. It is a part of geotechnical engineering and helps engineers in determining the suitability of a particular place for the structure they intend on building.
Textbook Of Soil Mechanics And Foundation Engineering thoroughly covers soil properties in relation to construction engineering. It begins with the study of the formation and structure of soil and goes on to discuss the various soil properties that are relevant when studying a site for construction projects.
Textbook Of Soil Mechanics And Foundation Engineering covers the latest developments in the field of soil mechanics and foundation engineering. This includes the concepts of mechanically stabilized earth retaining walls and drilled pier foundations.
The book also explains new methods of predicting the behavior of laterally loaded long vertical and batter piles. It also explores the various concepts and methods involved in foundation engineering like braced cuts and drainage, and Caisson foundations.
The book presents a comprehensive coverage of the theory and practice of soil mechanics, and also contains detailed illustrative examples. These help clarify the practical applications of the concepts and gives the students a solid foundation in the understanding of foundation engineering and soil mechanics. It contains many problems that gives the student practice in working out solutions based on the concepts learnt.
\r \tSoil mechanics is the branch of engineering mechanics that studies the properties and behavior of soil at a site and its potential effects on any man made structure built upon it. It is a part of geotechnical engineering and helps engineers in determining the suitability of a particular place for the structure they intend on building.
\r \tTextbook Of Soil Mechanics And Foundation Engineering thoroughly covers soil properties in relation to construction engineering. It begins with the study of the formation and structure of soil and goes on to discuss the various soil properties that are relevant when studying a site for construction projects.
\r \tTextbook Of Soil Mechanics And Foundation Engineering covers the latest developments in the field of soil mechanics and foundation engineering. This includes the concepts of mechanically stabilized earth retaining walls and drilled pier foundations.
\r \tThe book also explains new methods of predicting the behavior of laterally loaded long vertical and batter piles. It also explores the various concepts and methods involved in foundation engineering like braced cuts and drainage, and Caisson foundations. 2ff7e9595c
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