Robert B. Gilbert

Applied Research Using a Transparent Material with Hydraulic Properties Similar to Soil

An innovative material, transparent soil, was used to study flow from a prefabricated vertical drain (PVD). Transparent soil is a mixture of mineral oil, solvent, and amorphous silica. Before using the material to study PVD flow patterns, testing was completed to verify previously published values for hydraulic conductivity and porosity. The results of the independent testing matched the published results well. Experiments using scaled-down and full-size PVDs were completed in cells with diameters ranging from 10.2 to 30.0 cm, respectively. The results of these tests compared favorably to those obtained using an electrical analog and a mathematical model. The transparent soil was an acceptable representation of soil; however, degradations in transparency can be a problem in larger samples. Some other difficulties in using transparent soil, such as finding acceptable sealants and preventing dying of the silica particles, are discussed. In addition, recommended areas of continued research are presented.

Importance of Lower-Bound Capacities in the Design of Deep Foundations

There is generally a physical limit to the smallest possible capacity for a deep foundation. However, a lower bound on the capacity has rarely been accounted for in performing reliability analyses and developing reliability-based design codes. The objectives of this paper are to investigate the effect of having a lower-bound capacity on the reliability of a geotechnical engineering system and to propose a load and resistance factor design (LRFD) checking format that includes information on the lower-bound capacity in design. It is concluded that a lower-bound capacity can cause a significant increase in the calculated reliability for a geotechnical design even if it is an uncertain estimate. Two alternative LRFD formats that incorporate lower-bound capacities and that would not require substantive changes to existing codes are proposed. Real-world examples dealing with the design of onshore and offshore foundations indicate that the incorporation of a lower-bound capacity into design is expected to provide a more realistic quantification of reliability for decision-making purposes and therefore a more rational and efficient basis for design.

ANALYTICAL MODEL OF PROGRESSIVE SLOPE FAILURE IN WASTE CONTAINMENT SYSTEMS

The potential for progressive failure in waste containment systems is an important design consideration. Many common interfaces between components in containment systems exhibit strain-softening behaviour; however, slopes are presently designed using limit equilibrium methods that do not account for these effects. An analytical model is developed to investigate the potential for progressive failure due to strain softening. Results are presented in a non-dimensional form relating the potential for strain softening to the slope geometry, the waste properties and the properties of the containment system interface. The potential for progressive failure increases as (i) the waste stiffness decreases relative to the initial stiffness of the interface resistance, (ii) the length of the slip surface increases and (iii) the rate of strain softening with displacement increases. Analysis of a case study slope failure indicates that the analytical approach produces results that are consistent with field observations and comparable to results from a more sophisticated, numerical analysis. Although simple, this analytical approach serves as a useful design guide to identify cases where it is unsafe to use the peak shear strength in a limit equilibrium analysis.

Laboratory hydraulic conductivity testing of GCLs in flexible-wall permeameters

One of the important properties of geosynthetic clay liners (GCLs) is hydraulic conductivity. In the laboratory, hydraulic conductivity of GCLs is typically measured in flexible-wall permeameters. The most important variables in hydraulic conductivity testing of GCLs are addressed, including (1) trimming the GCL specimen; (2) determining the thickness of the specimen; (3) selecting the effective stress; (4) selecting the hydraulic gradient; and (5) selecting the first wetting liquid and permeant liquid. A round-robin testing program was conducted in which 18 laboratories independently measured the hydraulic conductivity of a GCL that was permeated with water. The test specimens all came from the same GCL sample. The coefficient of variation, accounting for all sources of variability, was 42%. For experienced laboratories, this value reduced to 35%, which was identical to the variation in quality control tests performed over a 7-month period by the manufacturer on a variety of GCL samples. The round-robin test results are encouraging; there was less variability than might be expected, considering the difficulty in accurately measuring the hydraulic conductivity of relatively impermeable materials, such as GCLs.

Tilt Table Test for Interface Shear Resistance Between Flowlines and Soils

This paper describes a tilt table test method for measuring the shear resistance between flowlines and supporting soils. This shear resistance is important in considering buckling and walking in the design of flowlines. A significant challenge in measuring the shear resistance is the very low effective normal stresses that exist at the interface in field conditions. Since the measured stresses will be small, even small amounts of friction in a test device can adversely affect the results. The tilt table method overcomes this problem by using gravity to apply the normal and shear stresses to the soil-flowline interface, eliminating the need for a mechanical loading system. A set of test results is presented to demonstrate how the test method can be used to measure the resistance between the flowline and the soil. These results illustrate that the type of flowline coating and the effective normal stress affect the shear resistance.Copyright

Incorporating Lower-Bound Capacities into LRFD Codes for Pile Foundations

There is generally a physical limit to the smallest possible capacity for a deep foundation. However, a lower bound on the capacity has rarely been accounted for in performing reliability analyses and developing reliability -based design codes. The objectives of this paper are to (1) investigate the possibility of a lower -bound capacity using pile load test databases; (2) study how a lower -bound capacity could affect the reliability for a pile foundation; and (3) propose a Load and Resistance Factor Design (LRFD) design-checking format that includes information on the lower -bound capacity in addition to the conventional design information. Databases with driven pipe piles show clear evidence for the existence of a lower -bound capacity in both clays and sands. This lower-bound capacity is a physical variable that can be calculated based on mechanics with site-specific soil properties. For driven pipe piles, the calculated lower-bound capacity typically ranges from 0.5 to 0.9 times the calculated predicted capacity. Re liability analyses illustrate that the presence of a lower-bound capacity can have a significant effect on increasing the reliability of a deep foundation. Its effect increases as the coefficient of variation for the capacity increases and as the target re liability index increases. Finally, it is shown that information about the lower-bound capacity can be incorporated into LRFD codes using two alternative formats that would not require substantive changes to existing codes. Incorporation of a lower-bound capacity into design is expected to provide a more realistic quantification of reliability for decision -making purposes and therefore a more rational basis for design.

Case Study of Offshore Pile System Failure in Hurricane Ike

The objective of this paper is to document and study the failure of a driven, steel pipe pile foundation system supporting an offshore platform. The three-pile system failed in overturning because of a pull-out failure of the most critically loaded pile in a hurricane 5 years after installation. The calculated tensile capacity of this pile using the American Petroleum Institute (API) design method is close to the estimated load at failure. This case history is significant because it represents the failure of a large-diameter pile in service when loaded by a storm. Also, this case history generally affirms the current design methods and contradicts a widely held perception that offshore pile designs are significantly conservative. This case history highlights the opportunities to improve design practice by explicitly accounting for pile flexibility when dealing with strain-softening soils and by considering the capacity of the foundation system as well as the capacity of individual piles in design.

Development of a Laboratory Procedure to Evaluate the Consolidation Potential of Soft Contaminated Sediments

Consolidation settlement of non-aqueous phase liquid (NAPL) contaminated sediments may trigger NAPL mitigation. The consolidation potential and resulting NAPL mobilization of the sediments should be evaluated in the laboratory; however, due to the highly compressible and weak nature of riverbed sediments, it is usually not possible to conduct conventional consolidation tests on sediment specimens. In this study, a triaxial setup was modified to work effectively under low stresses. Kaolinite was used to represent the soil solid phase and Soltrol 130 (a type of mineral oil) was used to represent the NAPL. Both oil-wetted and water-wetted regimes were analyzed. Hexane Extraction and moisture content tests results confirmed the final fluid amounts in the specimen obtained by measuring the effluent volume during consolidation. The results of the tests show that approximately 0.1 g of NAPL per 1 g of soil solids is unlikely to be mobilized by consolidation. The developed procedure could also be employed to define the mobile and immobile fractions of NAPL and the expected compression of contaminated sediments. The volume of NAPL in excess of the retained residual can be used to design NAPL collection systems or to size layers of NAPL sorbent materials such as organo-clays.

Design Procedures for Marine Renewable Energy Foundations

The Geotechnical Sub-Committee of the ASCE COPRI Marine Renewable Energy Committee is preparing a general guide for foundation design. The content of this paper is a brief summary of what would be included in the guide document. Existing foundation concepts include gravity bases, monopiles, jackets/tripods and more recently, floating turbines tethered to the seabed with anchor lines. At shallow water sites with suitable soils, gravity bases have proven to be successful. Monopiles, consist of a single large diameter steel driven pile, have proven to be an efficient solution in water depths up to 35m and have formed 75% of existing turbine foundations worldwide. These piles resist lateral wind and wave loadings through cantilever action. From 35m to 60m water depths, jacket structures have been used to support the wind turbine super structure. The jacket consists of a steel lattice frame founded on piles under the legs of the structure. For deeper deep waters, floating turbines moored by mooring lines attached to suction anchors or driven pile anchors may be suitable. Included in the proposed guide are procedures recommended for computing the axial and lateral capacity of driven and suction piles, the installation of driven and suction piles, and the bearing capacity and settlement of gravity base structures. Unlike offshore foundation of oil installation which are governed by soil capacity, foundations of wind turbines tend to be governed by the lateral and rotational foundation stiffness, which controls the dynamic response of the turbinetower-foundation system.

Importance of Proof-Load Tests in Foundation Reliability

Proof-load tests play an important role in verifying the validity of design methods and construction procedures. Recently, there has been considerable interest in maximizing the value of proof-load tests in foundation design within a reliability-based framework. The objectives of this paper are to (1) illustrate how static proof-load tests can affect the reliability and design of a tested foundation, (2) discuss the effect of uncertainty in the measured proof load on the calculated reliability, and (3) study how the results of proof-load tests conducted on a limited number of foundations using relatively small proof loads can be utilized to update the tail of the capacity distribution. Results indicate that proof loads can result in a significant increase in the reliability for a foundation, even when the proof load is significantly smaller than the predicted capacity.