George M. Filz

DETERMINING HYDRAULIC CONDUCTIVITY OF SOIL-BENTONITE USING THE API FILTER PRESS

The American Petroleum Institute (API) filter press is commonly used to measure the hydraulic conductivity of soil-bentonite, both during mix design and as a part of construction quality assurance and quality control. However, interpretation of the test results is complicated by the fact that, during the test, the soil-bentonite specimen is consolidated by seepage forces, which produce a variation in effective consolidation stress from the top of the specimen to the bottom. This paper presents the results of a laboratory investigation undertaken to illustrate that seepage consolidation theory provides a logical way to interpret filter press test results. A description of seepage consolidation theory, which relates stress, compressibility, and hydraulic conductivity in soil consolidated by seepage forces, is presented. Following this is a description of the laboratory testing program that involved comparing the hydraulic conductivity of two soil-bentonite mixes, as measured in rigid wall consolidometer permeameter cells, a flexible wall permeameter, and an API filter press. Using seepage consolidation theory to interpret the filter press tests produces hydraulic conductivities consistent with those obtained from the other tests. The filter press test results are used to illustrate a method for estimating the hydraulic conductivity of soil-bentonite in a cutoff wall by taking backfill consolidation pressures into account.

Analysis of a Column-Supported Test Embankment at the I-95/Route 1 Interchange

Deep mixing stabilization supports new embankments over deposits of soft organic silts and clays at the I-95/US Route 1 interchange reconstruction in Alexandria, Virginia. During design, an instrumented test embankment was constructed on dry mix columns to assess performance of this technology at the interchange site. Numerical modeling disclosed that the most significant material properties affecting stress concentrations in the soil above the columns are the strength and stiffness of the columns while the shear modulus of the untreated clay is most important for lateral deformations at the end of construction. As the column strength and stiffness increase, the stress concentration ratio increases. As the soil shear modulus increases, the lateral deflections decrease. The values of stress concentration ratio from the calibrated numerical model are 4.2 and 2.1 for area replacement ratios of 6.4% and 17.9% respectively.

Earth Pressures Due to Compaction: Comparison of Theory with Laboratory and Field Behavior

Compaction of backfill adjacent to stiff and unyielding structures induces earth pressures in the compacted fill that exceed normal at-rest earth pressures. A numerical method that can be used to calculate compaction-induced lateral earth pressures has been proposed by Duncan and Seed. The purpose of the study described in this paper is to evaluate the theory by comparing calculated and measured compaction-induced lateral earth pressures. The data for the comparisons is from values measured in backfills behind three stiff, unyielding walls: the instrumented retaining wall in the Transport and Road Research Laboratory in Crawthorne, England; the instrumented retaining wall in the Virginia Tech Geotechnical Laboratory in Blacksburg, Virginia; and the lock walls at Eisenhower and Snell Locks in New York state. The lock walls were found to be cracked, apparently by high earth pressures induced by compaction, and an extensive rehabilitation program was required. The measurements from all three walls confirm th...

Column-Supported Embankments: Settlement and Load Transfer

Column-supported embankments (CSEs) can reduce settlements, improve stability, and prevent damage to adjacent facilities when embankments are constructed on ground that would otherwise be too weak or compressible to support the new load. Geosynthetic reinforcement is often used to help transfer the embankment loads to the columns in CSEs. This paper addresses three important design issues for CSEs: (1) the critical height above which differential settlements at the base of the embankment do not produce measurable differential settlements at the embankment surface, (2) the net vertical load on the geosynthetic reinforcement in the load transfer platform at the base of the embankment, and (3) the tension that develops in the geosynthetic reinforcement. Based on bench-scale tests, field-scale tests, and case history data, the critical height was found to be a linear function of the column spacing and the column diameter. The net vertical load that acts down on the geosynthetic reinforcement can be determined using the load-displacement compatibility method, including determination of the limiting stress distribution at the base of the embankment by a generalized form of the Adapted Terzaghi Method, which accommodates any column or pile cap shape, any repetitive column arrangement, and different soil types in the load transfer platform and the overlying embankment fill. The tension in the geosynthetic can be calculated using a generalized form of the parabolic method, which incorporates stress-strain compatibility and which accommodates rectangular and triangular column arrangements and biaxial and radially isotropic geogrids.

Rapid Chemical Stabilization of Soft Clay Soils

Since World War II, the military has sought methods for rapid stabilization of weak soils for support of its missions worldwide. Over the past 60 years, cement and lime have been the most effective stabilizers for road and airfield applications, although many nontraditional stabilizers also have been developed and used. The most effective stabilizer to increase the strength of two soft clay soils within 72 h for contingency airfields to support C-17 and C-130 aircraft traffic needed to be determined. The treatment of one clay with cement resulted in relatively high unconfined compressive strengths (UCS), whereas treating the same clay with quicklime and calcium carbide resulted in lower UCS. The treatment of another clay with higher plasticity resulted in similar UCS for cement, quicklime, and calcium carbide. Secondary stabilizers, including sodium silicate, superabsorbent polymers, a superplasticizer, and an accelerator, were ineffective in increasing the UCS of a soil treated with cement, quicklime, or calcium carbide.

Fiber Reinforcement for Rapid Stabilization of Soft Clay Soils

Since World War II, the military has sought methods for rapid stabilization of weak soils for support of its missions worldwide. Over the past 60 years, cement and lime have been the most effective stabilizers for road and airfield applications, although recent developments show promise from nontraditional stabilizers, such as reinforcing fibers. The benefits derived from fibers may depend on whether they are used alone or in combination with chemical stabilizers. The ability of stabilizers to increase the strength of two soft clay soils within 72 h to support C-17 and C-130 aircraft traffic on contingency airfields was investigated. Laboratory test results showed that longer fibers increased the strength and toughness the most for a clay treated only with fibers. For a clay treated with fibers in addition to a chemical stabilizer, shorter fibers increased toughness the most, but the fibers had little effect on strength. Higher dosage rates of fibers had increasing effectiveness, but mixing became difficult for fiber contents above 1%. Poly(vinyl) alcohol fibers were anticipated to perform better than other inert fibers because of hydrogen bonding between the fibers and clay minerals, but these fibers performed similarly to other fibers.

Sand-to-Concrete Interface Response to Complex Load Paths in a Large Displacement Shear Box

The large displacement shear box (LDSB) allows testing of interfaces as large as 711 by 406 mm (28 by 16 in.) with maximum interface displacements of 305 mm (12 in.). This device has been used to investigate the response of a variety of interfaces, including clay-geomembrane interfaces for which large displacements are important. The most recent application of the LDSB was to study the response of several sand-to-concrete interfaces under complex loading paths. In this application, the relevant characteristics of the LDSB are its geometry, which reduces the significance of end effects, its ability to apply monotonic and cyclic loading, and its ability to apply simultaneous changes in shear and normal stresses so that complex loading paths can be followed. This paper describes the main features of the LDSB, as well as the testing procedures and results of the sand-to-concrete interface tests that were performed. A procedure for normalizing the interface shear test data is also presented. This procedure facilitates comparative evaluations of interface response to different types of loading. The test results formed the basis for development of an extended hyperbolic model for interfaces that has been implemented in finite element analyses of soil-structure interaction problems.

AXIAL AND LATERAL LOAD PERFORMANCE OF TWO COMPOSITE PILES AND ONE PRESTRESSED CONCRETE PILE

Composite piles use fiber-reinforced polymers (FRPs), plastics, and other materials to replace or protect steel or concrete, with the intent being to produce piles that have lower maintenance costs and longer service lives than those of conventional piles, especially in marine applications and other corrosive environments. Well-documented field loading tests of composite piles are scarce, and this lack of a reliable database may be one reason that composite piles are not in widespread use for load-bearing applications. The purpose of this research is to compare the axial and lateral load behavior of two different types of composite test piles and a conventional prestressed concrete test pile at a bridge construction site in Hampton, Virginia. One of the composite piles is an FRP shell filled with concrete and reinforced with steel bars. The other composite pile consists of a polyethylene plastic matrix surrounding a steel reinforcing cage. The axial structural stiffnesses of the prestressed concrete pile and the FRP pile are similar, and they are both much stiffer than the plastic pile. The flexurel stiffness of the prestressed concrete pile is greater than that of the FRP pile, which is greater than the flexural stiffness of the plastic pile. The axial geotechnical capacities of the test piles decreased in order from the prestressed concrete pile to the FRP pile to the plastic pile. The prestressed concrete pile and the FRP pile exhibited a similar response for lateral load versus deflection, and the plastic pile was much less stiff in lateral loading.

Field-Scale Column-Supported Embankment Test Facility

Column-supported embankments (CSEs) have seen increasing use in recent years where new embankments or widening of existing embankments is required over soft ground. The primary advantages of CSEs are more rapid single-phase construction and protection of adjacent facilities and embankments, but their use is limited because of lack of consensus on CSE design procedures. Investigations into CSE performance in the literature include bench-scale testing, centrifuge testing, case histories with instrumented test sections, numerical modeling, and a few examples of field-scale research test sections. This paper describes a field-scale CSE test facility that employs a unique approach where expanded polystyrene geofoam and its subsequent dissolution are used to model the soft soil beneath the CSE. The properties of the geofoam are similar to those of soft clay, although the geofoam layer is thin. The geofoam is dissolved after embankment construction using an environmentally friendly solvent to represent a worst-case scenario where there is zero soft soil support. Some field applications can approximate this condition. Based on an extensive literature review, the authors believe that this is the first published use of geofoam for temporary support of a column-supported test embankment, where the geofoam is later dissolved to remove support between columns. In addition to investigation of CSE behavior, the facility, instrumentation, and test concept have broader applications for investigation of other arching phenomenon including: localized loss of support beneath portions of other geotechnical structures such as mechanically stabilized earth walls, sinkhole mitigation, tunneling, and mining engineering. This paper focuses on description of the CSE test facility, materials, test procedures, and instrumentation. The results from one of the CSE tests are presented to illustrate the capabilities of the facility and data that can be collected. Future publications will present results from the entire CSE test program and make recommendations for CSE design.

Design of Deep Mixing for Support of Levees and Floodwalls

The deep mixing method increases the strength and decreases the compressibility of soft ground, and thereby improves stability and reduces settlement of embankments and levees. Continuous shear panels oriented perpendicular to the levee or floodwall centerline are more efficient for stability than isolated columns because shear panels are not subject to the same type of bending failure that isolated columns can experience. Even when continuous shear panels are used, stability analyses must consider multiple modes of failure, such as composite shearing, rotation of the deep-mixed zone, shearing on vertical planes along column overlaps, extrusion between shear panels, crushing of the deep-mixed ground at the toe of the deep-mixed zone, and global instability. Furthermore, the strength of deep mixed ground is more variable than the strength of naturally occurring clay deposits. Multiple failure modes and high strength variability must be considered to develop economical and reliable designs of deep-mixed support systems for levees and floodwalls. This paper presents three examples of flood protection facilities in Louisiana for which the deep mixing method was applied after Hurricane Katrina. In addition, simplified analysis methods for stability and settlement, as well as consideration of other design and construction issues, are discussed in the context of a consistent overall design approach.