• Stochastic Analysis Of Response Functions Of Nitrogen In Stream Water

      Park, Hyun Ae; Tumeo, Mark A. (1992)
      In the present study, a stochastic model of nitrogen in streams is created using a new mathematical technique, Probability Density Function/Moment technique (PDF/M). The model is based on a set of four highly non-linear second order equations for nitrogen species in streams (NH$\sb3,$ NO$\sb2,$ NO$\sb3,$ and organic-N). The purpose of the PDF/M technique is to include occurrence of natural variability. The first step is to separate the stochastic terms from the non-stochastic terms and solve the resulting set of equations simultaneously. The moments of the output variables then are obtained using expectation mathematics. The moments are used in a solution of the Fokker-Planck equation to produce an analytical solution for the probability density functions of the dependent variables. Comparison of the present study to the results of the Monte Carlo method showed the application of PDF/M technique to nitrogen cycle simulation valid. <p>
    • A Mechanical Evaluation Of Alaskan White Spruce

      Syta, Dean Edward; Curtis, Kevin; Gasbarro, Tony; Raad, Luffi (1993)
      This project serves to demonstrate the usefulness of Alaskan White Spruce as a construction material. This is done through the development of allowable strength values for design purposes. Such values allow engineers to design structures using Alaskan White Spruce, increasing the usefulness of the wood species.<p> The mechanical properties of Alaskan White Spruce are investigated. Summaries of the mechanical properties and of subsequently developed allowable structural design values are given. Included are discussions of the Alaskan White Spruce species, general wood behavior, mechanical testing of wood, statistical data analysis, and allowable property development. Results are compared against the work of other researchers. Appendices of test data are given.<p> Test results and subsequent data analysis indicate Alaskan White Spruce possesses strength similar to Douglas-Fir/Larch lumber and higher strength than Spruce/Pine/Fir type lumbers. This indicates that Alaskan White Spruce may have considerable worth as a construction material. <p>
    • Tests and analysis of geogrids as base-reinforcing materials

      Fu, Xuemin (1998)
      A quantitative assessment of geogrids as base reinforcing material in paved roads is clearly necessary when a design is needed and decisions are to be made as a consequence. Two full scale single wheel load tests were conducted to determine the performance of geogrids as base reinforcing materials in paved roads. These two full scale tests were set up with different base thicknesses, material properties, loading conditions and geogrids. Load, speed, and direction of a test cart were controlled with a computer. Although many types of instruments were installed, measurements of vertical deformation of the pavement surface proved to be the most useful. The Traffic Benefit Ratio (TBR), defined as the ratio of the life of a reinforced section to the life of a similar unreinforced section, was used as a primary design parameter. Comparisons between reinforced and unreinforced bases are presented. The parameters used for comparison were permanent vertical deformation, number of repetitions to failure, tire load, and thickness of base course. Test results showed that the maximum TBR for a Tensar BR2 geogrid was 10. This TBR was obtained at a design deformation of 1.0 inch with 2 inches of asphalt over 10 inches of base over a CBR 3 clay subgrade. TBR's for other conditions ranged between 1 and 10. A design reference chart is presented for using Tensar BR1 and BR2 Geogrids.
    • Fracture and shakedown of pavements under repeated traffic loads

      Zhang, Tinggang; Raad, Lutfi; Lee, Jonah H.; Hulsey, J. Leroy; Gislason, Gary A.; Covey, David (1998)
      Under repeated external loads, engineering structures or objects may fail by large plastic deformation or fatigue. Shakedown will occur when the accumulation of plastic deformation ceases under repeated loads; the response of the system is then purely elastic. Fatigue and shakedown have been individually studied for decades and no attempt has been made to couple these two mechanisms in the mechanics analysis. In this study, an attempt is made to couple shakedown and fatigue in pavement mechanics analysis using numerical simulation. The study covers three main areas: fatigue, static shakedown, and kinematic shakedown analysis. A numerical approach to fatigue analysis is proposed based on elastic-plastic fracture mechanics. The amount of the crack growth during each load cycle is determined by using the J-integral curve and $\rm R\sb{-}curve.$ Crack propagation is simulated by shifting the $\rm R\sb{-}curve$ along the crack growth direction. Fatigue life is predicted based on numerically estabiished fatigue equation. The numerical results indicate that the algorithm can be applied to fatigue analyses of different materials. A numerical algorithm based on the finite element method coupled with the nonlinear programming is proposed in static shakedown analysis. In this algorithm, both the inequality and equality constraints are included in the pseudo-objective function. These constraints are normalized by the material yield stress and the reference load, respectively. A multidirectional search algorithm is used in the optimization process. The influence of finite element mesh on shakedown loads is investigated. An algorithm that utilizes eigen-mode to construct the arbitrary admissible plastic deformation path is proposed in kinematic shakedown analysis. This algorithm converts the shakedown theorem into a convex optimization problem and can be solved by using a multidirectional search algorithm. Fatigue behavior of a two-layer full-depth pavement system of asphalt concrete is analyzed using the proposed numerical algorithm. Fatigue crack growth rate is estimated and fatigue life is predicted for the system. Shakedown analyses are also carried out for the same pavement system. The comparison between the shakedown load and the fatigue failure load with respect to the same crack length indicates that the shakedown dominates the response of the pavement system under traffic load.
    • Fatigue behavior of conventional and rubberized asphalt mixes

      Saboundjian, Stephan K.; Raad, Lutfi; Lee, Jonah H.; Hulsey, J. Leroy; Succarieh, Mohamed; Gislason, Gary (1999)
      One of the main distress modes of flexible pavements is the fatigue cracking of the asphalt concrete surface layer. The addition of crumb-rubber modifier (CRM), obtained from scrap tires, to asphalt-aggregate mixtures has shown promise in enhancing their fatigue behavior. In this study, conventional unmodified and CRM modified asphalt-aggregate mixtures are evaluated in terms of their fatigue behavior. Controlled-strain flexural beam fatigue tests are conducted in the laboratory over a wide range of temperatures. Experimental results are compared in terms of flexural, tensile and compressive stiffnesses, phase angle, fatigue life and cumulative dissipated energy. Results showed that CRM mixes are more flexible than unmodified mixes, and that mix fatigue resistance is enhanced by the addition of CRM. Furthermore, a method of converting controlled-strain test data into equivalent controlled-stress behavior is presented. Experimental results revealed the existence of two types of controlled-strain stiffness-ratio variations. For each type of variation, an equivalent controlled-stress stiffness-ratio variation with cycles is derived. Using the predicted variations, fatigue lives for both modes of loading are determined. Predictions showed that, at a given temperature, controlled-stress mode of loading yields, as expected, shorter fatigue lives than its controlled-strain counterpart. An implicit validation of the proposed conversions revealed that fatigue equation parameters K and n for the different mixes fit within the range of values obtained from the literature for controlled-stress conditions. In addition, a fatigue life model, applicable to the haversine pattern of loading used in this study, is presented. The model takes into account the cumulative dissipated energy to failure, mode-of-loading, and initial phase angle, strain and stiffness of the mix. Analogy with the traditional strain-based fatigue equation revealed that K is a temperature-dependent parameter, whereas n and m are independent of mix temperature. A decrease in K is associated with an increase in temperature. The newly developed model is then used to predict fatigue lives of conventional and CRM mixes in typical pavement structures. For this purpose, a finite element-based mechanistic analysis is used. Results revealed the enhanced fatigue resistance of CRM mixes in comparison to unmodified conventional mixes.
    • Phytoremediation strategies for recalcitrant chlorinated organics

      Schnabel, William Edwin; White, Daniel (2000)
      The purpose of the research was to investigate novel strategies for the phytoremediation of recalcitrant chlorinated organic soil contaminants. The recalcitrance of many chlorinated organics is related to chemical stability and bioavailability. Mycorrhizal fungi have the potential to enhance the degradation of such compounds through the action of lignolytic enzyme systems, and to increase the bioavailability of such compounds through increased root surface area and reach. Furthermore, the addition of surfactants has the potential to increase compound bioavailability via increased solubility. The organochlorine pesticide aldrin, and the polychlorinated biphenyl 3,3'4,4 '-tetrachlorobiphenyl (TCB) were chosen as representative recalcitrant contaminants. Feltleaf willow (Salix alaxensis) and balsain poplar (Populus balsamifera) were chosen as vegetative species likely to be useful for phytoremediation in sub-arctic ecosystems. Mixed-culture mycorrhizal fungi were first shown to be capable of taking up the hydrophobic contaminants in vitro. In the same experiments, surfactant addition increased the level of contaminant uptake. In subsequent vegetative uptake studies, mycorrhizal infection was highly correlated with the uptake of aldrin and TCB in the willow systems. In the poplar systems, this correlation was not as strong. Once taken up into the vegetative matrix of either species, most of the carbon originating from the chlorinated compounds existed as bound transformation products. Additionally, water-soluble transformation products of aldrin were formed in all of the soils tested, and such transformations were enhanced in the presence of vegetation. TCB transformation products were not detected in any of the soils tested. Surfactant addition did not impact the fate of either contaminant in the vegetative uptake studies. The surfactants, in the concentrations added, did not sufficiently solubilize the contaminants into the soil solution. The results of these studies indicated that the phytoremediation of recalcitrant chlorinated organics such as aldrin and TCB could be enhanced through the action of mycorrhizal fungi, and that surfactant addition has the potential to increase mycorrhizal uptake. Field studies were recommended, involving the use of specific degradative fungal species and effective surfactants.
    • Fuel penetration rates in frozen and unfrozen soils: Bethel, Alaska

      McCauley, Craig Alan (2000-05)
      Alaska fuel-storage facilities are required by law to provide secondary containment for their largest tank-volume. Secondary containment commonly includes berms, catchment basins and ditches. Fuel-penetration rates on site soils are needed to predict potential fuel-penetration depths. Field and lab tests quantified fuel-penetration rates for three site soils in Bethel, Alaska. In-situ tests following ASTM D 5093-90 allowed measurement of fuel infiltration rates using a falling-head permeameter allowed quantification of hydraulic conductivities, permeabilities and infiltration rates in frozen and unfrozen soils prepared at various moisture contents. Unfrozen samples were tested at room temperature, 19.8-24.0C̊ (67.6-75.2F̊). Fuel-penetration rates were similar regardless of moisture contents. Tests for frozen samples occurred at -4C̊ (25F̊). Fuel-penetration rates decreased as ice-saturation increased. The permeant used for each test was a Diesel #2/Jet A-50 fuel mixture (heating fuel) consisting of predominately Jet A-50.
    • Disinfection by-product experiences in Alaskan village drinking water systems and the Caribou-Poker Creek watershed

      Narr, Jasprit (2001-08)
      The purpose of this research was to study the disinfection by-product formation potential (DBPFP) in small drinking water systems in Alaska. As per the US. E.P.A's disinfectants/disinfection by-products (D/DBP) rule, the maximum contaminant limit (MCL) for the two major DBPs namely, total trihalomethanes (TTHMs) is 80 micrograms (ug) per liter (L) and 60 micrograms/liter for the five 5 halloacetic acids (HAA5). It was decided to conduct research on the total trihalomethane formation potential (TTHMFP) and the 5- haloacetic acid formation potential (HAA5PF) of the 17 Alaskan village drinking water systems with reportedly high TTHM and HAA5 values. It was found that specific UV absorbance (SUVA) had excellent correlations with TTHM/DOC and HAA5/DOC. These correlations were used to aid in drinking water source selection in a sub-arctic watershed named the Caribou-Poker Creek Research Watershed (CPCRW).
    • Ice, bedload transport, and channel morphology on the upper Kuparuk River

      Oatley, Jeffrey Albert (2002-12)
      The objective of this study was to quantify the impact of bottom ice on sedimentation processes at a study site on the Upper Kuparuk River, in Northern Alaska. The approach taken was to use the Meyer-Peter and Mueller (1948) and Parker (1990) equations to determine bedload rating curves at four cross sections within the study reach, and to apply these rating curves to the ten year flow history of the study site to determine the total potential bedload transport that was suppressed during snowmelt runoff. In conjunction with this analysis, a tracer rock study was performed at the study site. During the first two years of the project, the field study yielded little bedload transport information, as there were no competent flows during this time. However, the storm of record occurred in August 02 2002, which provided an opportunity to observe the geomorphic response to a major event, to estimate an average bedload transport rate based on the virtual velocity of the recovered tracer rocks, and to compare the predictive methods to the tracer data based calculations. The results suggest that the potential bedload transport (500 m³) over the ten-year flow history is comparable to the amount of transport that occorred during the extreme event of August 2002 (870 m³), and that the suppression of bedload transport, due to an ice covered bed surfaces, likely affects the morphology and sediment supply of the river.
    • Behavior Of Granular Materials Under Cyclic And Repeated Loading

      Minassian, George H.; Raad, Lutfi (2003)
      Granular layers are essential contributors to the structural integrity of the pavement system, their premature deformation radically decrease support of the asphalt concrete surface layer, thus leading to the early deterioration of the overall pavement structure. This research was conducted to better understand the behavior of granular materials when subjected to the complex nature of traffic loading. Long-term triaxial tests were conducted on typical Alaskan base course material using both repeated as well as cyclic loading to also account for the shear reversal effects induced by wheel load. Results show that the shear reversal component of the traffic loads, which have been ignored so far, induces considerable damage to the granular layers. Models were presented to predict the different soil moduli while also accounting the effect of strain hardening or densification due to the repetitive nature of the loads applied. Moreover, a simple yet powerful model was presented to predict accumulated permanent strains as function of the stress state, number of load repetitions and the strength level applied. The results obtained in this study also show a clear indication of the existence of given stress level limit beyond which incremental collapse of the system takes place. Furthermore, regions of instability of granular layers subjected to dynamic loading have been defined using a simple response parameter and monotonic shear strength of the soil. An effort was made to explain the instability zones identified in this research by the shakedown theory.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office.
    • Hydrologic comparisons and model simulations of subarctic watersheds containing continuous and discontinuous permafrost, Seward Peninsula, Alaska

      Carr, Anne T. (2003-05)
      Mauze Gulch (4.9 km²), and Niagara Creek (6.5 km²), located near Zougarok, Alaska and Melsing Creek (80.6 km²), and Clyde's Gulch (2 km²), located near Council, Alaska were studied to quantify the hydrologic response of these watersheds. They have been studied extensively as part of the NSF Arctic Transitions in the Land-Atmosphere System (ATLAS) study. These watersheds demonstrate a progression from a continental subarctic to a marine moderated subarctic environment. Each contains varying proportions of permafrost. Mauze Gulch and Niagara Creek are underlain by continuous, warm, thin permafrost (1̃5-50m thick). Melsing Creek and Clyde's Gulch are underlain by discontinuous permafrost. The Swedish HBV-96 model was applied to these watersheds. The model requires minimal input of meteorological data (temperature and precipitation) to generate hydrographs. Examining the basic stream hydrology showed that differences in hydrologic response based on permafrost distribution can be identified. By changing parameters, such as field capacity and the percolation to the groundwater, it is possible to reflect physical differences between watersheds using the model. Results of this study were compared with previously studied basins in Alaska. Comparisons showed that model parameters can be used to distinguish differences in hydrologic response in basins with varying amounts of permafrost.
    • A geohydrologic analysis of an upland-bedrock aquifer system: applications to interior Alaska

      Youcha, Emily K. (2003-05)
      Ester Dome, an upland-dome bedrock aquifer system, located nearby Fairbanks, Alaska, was studied to identify important geohydrologic processes occurring in Interior upland aquifer systems. The ground-water dynamics at Ester Dome are complex due to the fractured nature of the aquifer system. The geology at Ester Dome consists of metamorphic and igneous rocks. Valley bottom deposits include gravels and loess. The flow pattern of the dome aquifer system is radial. Ground-water flows from a central high elevation recharge area and discharges into lakes, streams, and wetlands in the valley bottoms. The primary form of recharge to the bedrock aquifer is from spring snowmelt. Snow water equivalent and snow depth increases with elevation. Ground-water levels were observed at fifty sites on Ester Dome for two years. Water levels in wells at high elevations or locations with no silt or permafrost coverage show seasonal fluctuations. However, ground-water levels in the valley bottoms show little seasonal fluctuations, except wells that penetrate gravel deposits and have no overburden. A ground-water flow model was developed to aid in the understanding of these geohydrologic processes. The ground-water flow model shows recharge and bedrock hydraulic conductivity as the most sensitive parameters.
    • Estimation of operation time for soil vapor extraction systems

      White, Trevor C. (2003-08)
      Since the inception of Soil Vapor Extraction (SVE) as a viable contaminated soil restoration strategy, operators of these systems have struggled with determining the best date to terminate operation of their systems. Using principles of soil-gas flow and mass transport through unsaturated soils, stochastic hydrogeology, uncertainty analysis, and a cost/risk decision model, a procedure that can determine the appropriate time to discontinue operation of SVE systems was developed. Modeling of physical characteristics and determination of mass removal of a SVE system was accomplished using statistical realizations of permeability (uncertainty model) and a 2D advective-dispersive finite element vapor transport program (VapourT). The results from subsequent Monte Carlo analysis of the mass removal simulations are then subjected to a cost/risk analysis (decision model) to determine the appropriate termination time for the system based on costs and the probability that the system will fail to reach the regulatory standard. The decision model provides information on the cost benefits associated with either the continued operation of a SVE system, or its replacement with a more economically feasible remediation system. The intention of this research is to validate a framework for the estimation of termination time for the operational phase of a selected soil vapor extraction system.
    • Water quality from rainwater catchments throughout Alaska: looking at contaminants in catchment materials

      Hart, Corianne Irene (2003-12)
      A field study which focused on linking materials used in rainwater catchments to the quality of water they produce was conducted throughout Alaska in the summer of 2003. The importance of this project stems from the fact that many families throughout Alaska depend on rainwater catchment systems to provide water for washing, cleaning, cooking and/or drinking purposes. After a core group of participants were identified, samples were periodically collected from participants' water taps and were analyzed for a suite of contaminants that included metals (e.g., Pb and Zn), organics (e.g., volatile organic compounds) and bacteria. Based on variables, such as construction materials, the frequency of rainfall, the amount of water collected and the duration of storage, we evaluated the effectiveness of various catchments for providing safe drinking water. This fieldwork, coupled with a companion document addressing best management practices for rainwater catchments, provides valuable information for owners of small systems seeking to use rainwater catchments in Alaska. The conclusions of the study were that zinc concentrations of water collected at the tap were affected by roof and tank material, lead concentrations of water collected at the tap were affected by roof material, and copper concentrations of water collected at the tap were affected by pipe material.
    • Measurement of effective diffusion on andesite rock, Amchitka Island, Alaska

      Raghupatruni, Srinivas Rao (2004-05)
      Between 1965 and 1971 three nuclear weapon tests were conducted on Amchitka Island, Alaska. Currently research is being conducted to understand the possible movement of radionuclides through the Island subsurface into the marine environment so that a monitoring system can be developed. The possibility of radionuclide diffusion into matrix rock surrounding facture pathways needs to be better understood. This thesis presents ongoing research with the goal of determining the effective diffusion coefficients for andesite rock found in the Island subsurface. These studies are being conducted in a bench scale reactor consisting of two chambers separated by a sliced rock core obtained from the Island. The increase in conservative tracer over time is measured in the receiving chamber. The effective diffusion coefficients are then determined by applying these results to a solution to Fick's Second Law. Results from these studies will be used in the development of a long term monitoring program for the island.
    • Three dimensional finite element analysis of decked precast, prestressed concrete girder bridges

      Chaudhury, Sanjay (2004-08)
      The speed of construction has become the key issue for bridges. This is especially true in cold regions like Alaska, where you face a very short summer season. The speed of construction can be expedited by connections. As there are advantages in connections there are also disadvantages. One of the primary advantages of the connectors is that it makes the load transfer mechanism very efficient. However everything comes for a price. Maintenance of connections is very difficult. Routine inspections are to be made to ensure proper functioning of the system. This becomes a very important factor in Alaska where extreme winter conditions exists. Another issue is that the current connector design is based on an empirical approach. A rational design method is needed. In this study an effort has been made to find the presence of ISD on the live load distribution factor for single lane loading. Using ABAQUS CAE, 3D finite element (FE) models have been developed and analyzed with different parameters. By varying the parameters, different distribution factors corresponding to the reaction, strains (flexure and shear) as well as the forces originating from the connectors are evaluated and compared. This paper provides a basis for future connector design.
    • Seismic performance of Alaskan structures

      Cherlopalle, Dileep; Hulsey, J. Leroy; Carlson, Robert; Shur, Yuri (2004-08)
      Alaska is the most earthquake prone area in the United States. On October 23, 2002 a 6.7 magnitude earthquake occurred in interior Alaska. This was followed by a magnitude 7.9 earthquake that occurred near Denali on November 3, 2002. The Denali earthquake was the largest inland earthquake in North America in almost 150 years. Only minimal structural damage was reported. Subsequently, structural engineers should question why minimal damage. As a part of an investigation to evaluate this unusual phenomenon, two structures were selected for seismic analysis, a building and a bridge. The building is at Alyeska Pump Station # 10 (3 miles from the epicenter) and only non-structural damage occurred. A 3 span truss bridge near Tok, Alaska was selected as the second. This structure experienced only minimal damage. Seismic analysis was performed on these two structures for horizontal excitations. A corrected seismic data from Pump Station # 10 was used as the earthquake excitation record. The maximums for displacements, member forces and member stresses were studied. The results show that no structural damage should have occurred for the building and minimal damage for the bridge. These findings compare favorably with field inspection results reported by others.
    • Single lane live load distribution factor for decked precast / prestressed concrete girder bridges

      Millam, Jason Langdon (2004-08)
      The Alaska Department of Transportation (AKDOT) uses the decked precast, prestressed concrete bulb-tee girder for most of its bridge construction. The live load distribution factor (DF) equations provided by the American Association of State Highway and Transportation Officials (AASHTO) for the decked bulb-tee girder system do not differentiate between a single or multilane loaded condition. This practice results in a single lane load rating penalty for decked bulb-tee girder bridges. The research objective of this thesis is to determine DF simplified equations that accurately predict the distribution factor of the decked bulb-tee girder system when it is only subjected to single lane loading. Eight decked bulb-tee bridges were instrumented. Each bridge was loaded with a single load vehicle to simulate the single lane loaded condition. The experimental data were used to calibrate grillage models of the decked bulb-tee girder system. The calibrated grillage models were used to conduct a parametric study of the bulb-tee girder system subjected to single lane loaded condition. Eight new simplified equations that describe the single lane loaded distribution factor for both shear and moment forces of these bridges are developed in this thesis.
    • How chemical differences in dissolved organic matter relate to vegetation

      Seelen, Sarah Jean (2004-12)
      The purpose of this study was to better understand the link between dissolved organic matter (DOM) in soil leachate and different vegetation attributes. Soil cores were collected from the Caribou Poker Creeks Research Watershed (CPCRW) and subjected to a laboratory leaching procedure. The leachates were then subjected to a number of analytical tests, including pyrolysis-gas chromatography/mass spectrometry (py-GC/MS). Py-GC/MS is a 'molecular fingerprinting' technique that was used to help determine similarities and differences in organic matter leached from soils with different vegetation attributes. Numerous statistical tests were performed including Student-t, analysis of variance, principal components analysis, and partial least squares regression (PLS). Results from Student-t tests indicated that local vegetation plays an important role in the character of the DOM in soil leachate. Additionally, a principal components test revealed relationships between soil leachates and vegetation attributes. A prediction model was created using PLS to predict components of leachate DOM based on vegetation attributes. This model, while in its early development, was able to predict 70% of the total molecular fingerprint of leachate DOM based on cover vegetation.
    • Biosorption of heavy metals by citrus fruit waste materials

      Patil, Santosh Bramhadev (2004-12)
      Conventionally used processes for removing heavy metals from wastewater are usually either expensive, such as ion exchange, or inefficient, such as precipitation. An innovative technique that is both efficient and economical is biosorption, in which living and dead biomass can act as biosorbents through physical-chemical processes like ion exchange and micro-precipitation. Pectin, a structural polysaccharide present in plant cell walls, is similar to alginate, a molecule that is often responsible for the high metal uptake by algae. Based on the structural similarity between alginate and pectin, it was expected that pectin rich bio-wastes may be as good a biosorbent material as brown algae. A comparison between different pectin-rich materials showed high stability and metal binding capacity of citrus peels. Sorption isotherms for citrus peels showed higher metal uptake capacity at pH 5 compared to pH 3. Kinetic studies revealed the time required to reach equilibrium for lemon fruit waste (0.177 mm) was 20 min while for larger particles the time increased to 30 min-60 min. For lemon fruit waste, the content and pKa values of acidic groups were determined by using a pKa spectrum technique. Isotherm modeling was carried out by using Langmuir isotherms and pH sensitive modeling.