Continuous Flight Auger Research Articles

Loss of Ground during CFA Pile Installation in Inner Urban Areas

Observations and comments are made related to continuous flight auger (CFA) pile installation during construction of the deep foundation system for a 52-story apartment building in the city of New York. The study found that the CFA Piles are an effective technology for low-vibration and low-noise applications in inner urban areas. Settlement of existing structures adjacent to the construction site may be controlled by controlling the volume of soil being removed druing CFA pile installation. Pile installation by installers experienced with CFA piles, and inspection by engineers knowledgeable about the potential problems that could occur and how to avoid them is essential for successful installation of this system.

A case history of pre-extraction site investigation and quarry design, Cliffe Hill Quarry, Leicestershire

Abstract A brief history of the reasons leading up to the initiation of this project is followed by a description of the geographical location and geological significance of the chosen site. An initial phase of test pitting produced evidence which warranted a follow-up phase of continuous flight auger drilling along existing tracks. This work in turn indicated that serious exploration at great expense was warranted and a relatively powerful, brand new, drilling rig, capable of both coring and drilling through substantial thicknesses of overburden, was purchased. Several years of drilling, much of it on a 50m grid, subsequently proved the existence of a viable quantity of resource beneath an acceptable depth of overburden. Simultaneous testing of the core samples for a range of physical properties (as per BS 812) indicated the materials to be of acceptable quality for the anticipated markets, including rail ballast for high speed tracks. The drilling information was supplemented by a limited amount of geophysical work, mainly employing a shallow seismic refraction technique. Initial quarry planning studies indicated that a viable reserve could be generated by stripping overburden to a depth of 40 m around the periphery, and a planning application for areas of quarrying and overburden disposal was submitted. After considerable discussion and some modification, permission was granted, subject to a large number of conditions. Phased overburden stripping will ultimately involve several million cubic metres. This site will be subject to a full investigation as required by Section 9 of the Mines & Quarries (Tips) Regulations and this will determine the angles of safety for both the cut slopes and the deposited overburden. Preliminary configurations for the quarry, in terms of face heights, batter angles and bench widths were determined from geotechnical logging of the cores, the results of which were subsequently augmented by detailed mapping of the early exposures created. Full site investigations were also carried out for the location of load bearing structures.

Bearing Capacity of Auger‐Cast Piles in Sand

Auger‐cast piles are formed by drilling a continuous flight auger into the ground and, on reaching the required depth, pumping grout or concrete down the hollow stem as the auger is steadily withdrawn. The sides of the hole are supported by the soil‐filled auger, eliminating the need for temporary casing or bentonite slurry. This paper outlines important features of auger‐cast pile installation that influence the structural integrity and geotechnical capacity of completed piles. These include soil decompression, correlation between the rotational and vertical speeds of the auger, and precise coordination of auger extraction and grout supply. The results of 66 loading tests on auger‐cast piles in sand are presented. Shaft resistance is found to be independent of the relative density of the sand, while point resistance can be directly correlated with results of standard penetration and cone penetrometer tests. Empirical design methods for bored piles were found to underestimate the failure loads of auger‐cast piles. New correlations, based on pile length and standard penetration resistance, are proposed.

Ground losses during bored piling through weathered Keuper Sandstone in Bristol

Abstract The redevelopment of Brunei House in Bristol required that the 1820 facade be retained. The facade was badly cracked having been constructed on only a shallow masonry strip footing in thick historical fill and up to 5 m of soft Keuper clays and-sands. The groundwater table was at approximately 3 m depth. The Keuper Sandstone, at typically 7 to 10 m depth had weathered to a loose sand and the site investigation had encountered sub-artesian water pressures in this stratum. The new building required piled foundations, which had to be installed alongside the facade. The inevitable vibration during driven piling meant that bored piles were the preferred solution. The integrity of continuous flight auger piling had not been adequately proven in these ground conditions, and such piles were also rejected. It was appreciated that with conventional bored piling, ‘boiling sand’ would prove to be a problem in the saturated Keuper Sandstone, which had weathered to a sand. The paper describes how the volume of ground loss was reduced to an acceptable amount by maintaining satisfactory hydraulic head. Twin telescopic casings were also experimented with in an attempt to seal the pipe casings into the underlying Carboniferous sandstone bedrock, thus reducing the ground loss. The facade was supported with a large concrete counter balance, and a scaffolding grillage was designed to allow the facade to articulate during the piling. Details are given of the thorough programme of monitoring which was undertaken. The ground loss into the pile bores was successfully controlled and the facade moved in the manner predicted.

Finding ones bearings in piling

The author examines recent developments in driven and bored piling. Of the three basic types of driven pile - completely preformed, partially preformed and cast-in-situ -, the advantage of the preformed pile is that it can be inspected and tested prior to driving, and the possible disadvantage is that it may possibly be damaged during. Partially preformed piles combine the advantages of the totally preformed pile with the flexibility of cast-in-situ piles as regards uniformity and finish. The piling systems of three manufacturers are discussed, piles which are extendable, and a preformed core reinforced with a sand cement grout. Driven in situ pile improvements include a method where very dry concrete with a dense finish is used. Anticipated negative friction can be countered by a proprietory bitumen compound. The author also considers bored or replacement piles, both percussion bored - a simple inexpensive method - or rotary bored, including the use of a continuous flight auger which elminates the need for lining tubes. The author concludes with the reminder that any method of piling is relevant only to particular applications, and that this brief appreciation covers only a sample of those piles available. /TRRL/