Underpinning a Residential Structure on Uncontrolled Fill With Helical Screw-Piles

Underpinning of a private residence using square-shaft helical Screw-Piles is described. A two story wood frame single-family residence constructed in 1996 in a small subdivision started to experience differential settlement not long after construction. The settlement continued for several years, leading to excessive cracking in the basement walls and floor, severe misalignment of doors and windows and cracking of interior walls. It was discovered that the area of the housing development had previously been used as a commercial sand and gravel pit which had subsequently been used as a local dumping area for miscellaneous refuse and which had then been covered by a layer of sand and gravel. In order to stop additional movement, a series of square-shaft helical Screw-Piles was installed around the perimeter of the structure extending through the fill to the underlying dense sand and gravel. Foundation brackets were attached to the existing concrete footings for transferring load to the Screw-Piles. The site conditions are described and the results of the test borings are presented to show the composition and variability of the underlying materials with focus on the fill. A description of the underpinning work is presented to illustrate successful use of Screw-Piles for underpinning lightly loaded structures

Historical Application of Screw-Piles and Screw-Cylinder Foundations for 19th Century Ocean Piers

Iron Screw-Pile foundations were first introduced by Alexander Mitchell in the middle of the 19th Century for support of offshore lighthouses in shallow water ocean environments. Large diameter Screw-Piles with helical blades ranging from 2.5 ft. to 4 ft. in diameter were routinely used during this period and were constructed with either a solid iron central shaft (Screw-Pile) or a large diameter hollow cylindrical pipe shaft (Screw-Cylinder). Soon after their introduction, Screw-Piles and Screw-Cylinders were being used in pier construction, to allow ocean front piers to be constructed quickly and economically in many parts of the world, while at the same time providing adequate support for loads. A brief summary of this technology is presented and several examples of the variety of Screw-Piles and Screw-Cylinders that were used to construct ocean front piers are presented. Several examples of the application of Screw-Piles and Screw-Cylinders at specific sites are described using available historic records demonstrating that this foundation technology was well developed and well accepted by engineers

Bearing Capacity of Footings on Compacted Sand

This paper presents the results of footing load tests conducted on compacted sand beds to evaluate the bearing capacity and load-displacement characteristics of shallow foundations. Tests were conducted on square concrete footings with widths of 0.30, 0.61, 0.91, and 1.22 m and with embedment ratios (D/B) of 0, 0.5, and 1.0 to investigate the influence of footing size and embedment on the load-displacement behavior and ultimate bearing capacity. A description of the soil and test procedures used is given and the results of the footing load tests are presented. A discussion of the definition of ultimate bearing capacity and the use of normalized curves to describe the footing behavior is presented. A simple model is presented that may prove useful for the design of shallow foundations on sands

Tension Tests on Drilled Micropiles in a Stiff Clay

A series of 20 small diameter drilled and grouted micropiles were installed at three different depths in a stiff surficial clay crust at the National Geotechnical Experimentation Site in Amherst, Massachusetts. A detailed site characterization program was performed to evaluate· the soil characteristics in the crust. Three different sizes of micropiles ranging in diameter from 76 mm to 152 mm and having lengths from 1.52 m to 4.57 m were installed vertically at the site using both continuous flight augers and hand auger techniques. Concrete was placed in the open holes using gravity free-tall. After allowing the concrete to cure for a period of 30 days, tension tests to failure were conducted on each of the micropiles. Following initial tests, some of the micropiles were retested after a resting period of one year to evaluate the recovery in tension capacity. This paper presents a description of the soil characteristics at the site including both laboratory and field test results and a description of the methods used to construct and test the micropiles. A comparison is made of the ultimate capacity obtained from the tests. The influence of drilling technique and the effect of reloading on the measured capacity are discussed

Analysis of Small Pipe Piles Using the Field Vane

Results of axial compressive load tests on three small diameter pipe piles driven in a varved clay deposit are presented. Predictions of the axial pile capacity were made using the a-method originally proposed by Tomlinson (1957, 1971) and incorporating undrained strength profiles determined with the field vane. Predicted and measured capacities are compared and discussed in light of the various factors which can affect the outcome such as vane geometry, vane testing procedure and interpretation, pile load testing conditions, and empirical relationships incorporated in predictions

Behavior of Laterally Loaded Drilled Shafts in Stiff Soil

Results of lateral load tests on four drilled shafts installed in stiff cohesive soil are presented. Predictions of the load/displacement behavior were made using p-y curves generated from the results of Prebored Pressuremeter and Dilatometer Tests. A new method to develop p-y curves from the DMT is presented and discussed

Passive Earth Pressure Tests on an Integral Bridge Abutment

Passive loading tests were conducted on a rigid concrete retaining wall to study the effect of wingwall orientation on lateral earth pressure development. Loads were applied at the top of the wall to produce a rotational wall movement. Six tests were conducted (three of which are described herein), two with the wingwalls oriented parallel (0°) to the main wall, and two with the wingwalls oriented at an angle of 45° to the main wall, and two with the wingwalls oriented at an angle of 90° to the main wall. Based on these tests the distribution of passive earth pressure at the centerline of the main wall for different wall displacements and the displacement of the wingwalls for different wall orientations were determined. Results from these tests indicate that passive earth pressures show a triangular distribution, reaching a maximum passive condition in the upper 1/3 portion of the wall after which they decrease to near zero at the base of the wall. This maximum value of earth pressure is dependent on wingwall orientation for the same relative wall movement

Settlement Performance of a Mat Foundation on Unsaturated Loess

Previously reported settlements of structures founded on loess soils in the United States have been limited to cases of rapid deformation: the result of wetting and structural collapse. While the possibility of subsidence is usually a consideration in design, when the potential for wetting does not exist, economics generally govern the use of a shallow foundation system; in which case an accurate prediction of settlement is required. Since most loess deposits occur as unsaturated sediments, the use of one-dimensional consolidation theory is not applicable to the problem of settlement prediction. In 1977, a circular mat foundation was constructed on typical subsident loess to support a water tank for a small town in eastern Iowa. Between 1977 and 1983, settlement observations were made to monitor the magnitude of deformation. Laboratory one-dimensional compression tests and triaxial stress path tests were conducted to provide a means to predict settlements. Unlike many structures which are monitored for settlement, a water tank allows an accurate measure of load, thus eliminating an important unknown. This paper compares the results of settlement measurements with a number of techniques to predict settlement. The results indicate a time-dependent behavior of deformation which must be considered creep in light of the current understanding of loess behavior