Subgrade Pressures Research Articles

Environmental impact on critical responses of lightweight cellular concrete subbase flexible pavements

Previous analytical studies have demonstrated that low-density lightweight cellular concrete (LCC) subbase pavements can support up to 20 times more traffic loads than unbound granular subbase pavements while protecting the pavement subgrade from adverse freeze–thaw effects in cold regions. This study examines the possibility of providing better-performing pavements on the field through the construction, instrumentation, and monitoring of flexible pavement sections incorporating three LCC densities (400, 475, and 600 kg/m³) as subbase material compared with unbound granular material in Canada. The effects of daily and seasonal temperatures on pavement critical responses to stress and strains were evaluated. The findings showed that these LCC pavements reduced asphalt concrete tensile strain by over two times compared with unbound granular pavements, and that strain increased with a daily temperature increase. Daily subgrade pressure (stress) change was reduced by up to 68%. The study concluded that longer life pavements could be achieved with LCC subbase thicknesses ≥250 mm.

Multi-directional Falling Weight Deflectometer (FWD) testing and quantification of the effective modulus of subgrade reaction for concrete roads

ABSTRACT Falling Weight Deflectometry (FWD) tests are performed around the centres of two rectangular concrete plates, with geophones measuring vertical deflections in eight directions. Experimental results allow for quantifying asymmetries regarding the structural behaviour. Significant asymmetries are found for a 22-year-old plate scheduled for replacement. A new plate, tested a few weeks after production, is found to behave in a virtually double-symmetric fashion. Structural analysis of the new plate is based on Kirchhoff–Love plate theory, using free-edge boundary conditions. The support of the plate is provided by a Winkler foundation. Performing a static analysis, the uniform modulus of subgrade reaction is optimised to reproduce the measured deflections. The result is not convincing. The model is extended towards consideration of a second optimisation variable: a uniform auxiliary surface load. This allows for reproducing the measured deflections. The auxiliary load is superimposed with the pressure resulting from the Winkler foundation. This yields a realistic distribution of subgrade pressure. Dividing it by the deflections results in the distribution of the effective modulus of subgrade reaction. Finally, the analysis is extended towards the consideration of inertia forces. They increase the effective moduli of subgrade reaction determined by means of static analysis by less than 3.5%.

Evaluation of Chemically Stabilized Quarry Byproduct Applications in Base and Subbase Layers through Accelerated Pavement Testing

Research conducted at the Illinois Center for Transportation evaluated sustainable applications of quarry byproducts (QB) or QB blended with coarse recycled aggregates in chemically stabilized base and subbase layers in flexible pavements. In total, eight full-scale test sections, including one conventional flexible pavement with no QB as the control section, were constructed over a subgrade with an engineered strength of 6% California bearing ratio. The test sections were stabilized with either 3% Type I Portland cement or 10% Class C fly ash by dry weight. Fractionated reclaimed asphalt pavements and fractionated recycled concrete aggregates were also used as the recycled coarse aggregates. Based on laboratory tests conducted to determine strength properties of the chemically stabilized samples, QB and recycled aggregates were blended in a ratio of 70% to30% by weight, respectively. A lightweight deflectometer was used to evaluate the quality of the construction and the curing of the test sections. The constructed test sections were then evaluated for performance through accelerated pavement testing (APT) and frequent measurement of surface deformations. The results of APT showed quite a satisfactory rutting performance of all the evaluated QB applications with no observed surface cracking after 135,000 cycles. Four of the test sections were also instrumented with soil pressure cells to measure the wheel load deviator stress on top of the subgrade. The pressure measurements indicated subgrade pressures 3−5 times lower for the stabilized QB sections compared with that of the conventional flexible pavement control section.

Application of Geocell Track Substructure Support System to Correct Surface Degradation Problems Under High-Speed Passenger Railroad Operations

Ballast fouling and associated degradation of track geometry is a serious problem for railway systems in general and high-speed passenger rail systems in particular. This paper presents the results of a field test on Amtrak’s Northeast Corridor where a long-term problem area existed near Oakington Road, Havre de Grace, Maryland, near MP 63.7 between Philadelphia and Washington DC. This test looked at the application of a new generation of three-dimensional cellular confinement systems (geocells) in reducing the rate of track geometry degradation, particularly in poor subgrade and ballast locations which require frequent, expensive, track surface maintenance. The field test compared two distinct sets of rebuilt track conditions, to include zones with and without a layer of geocell material. Both zones were rebuilt with improved drainage and a good, well-defined track structure and substructure (to include a well-defined depth of clean ballast). The test measurements included pre-maintenance and post-maintenance track geometry measurements together with comparative subgrade pressure measurements inside and outside the geocell cell zones. The pressure cell measurements, which looked at subgrade pressure under left and right rails in both the geocell zone and the control (non-geocell) zones, included measurements under both Acela high-speed trains and lower speed regional trains. In all cases, the subgrade pressures in the geocell zone were approximately half of those for the cells in the control zone (no geocell). Track geometry measurements were made using Amtrak’s track geometry vehicle which measures key track geometry parameters at 1-ft intervals along the track. There were several well-defined locations in the overall test zone that experienced significant track geometry degradation; these were all corrected during reconstruction. In the zones with no geocell material, these geometry variations reappeared within 6 to 7 months with the same if not greater amplitudes. By contract, in the geocell zone, the “after” geometry variations were significantly smaller than the pre-reconstruction geometry variations. Furthermore, the rate of geometry degradation was signficantly less for the geocell zones compared to the pre-geocell time periods for the exact same track. This indicated the effectiveness of the geocell material in reducing the rate of track geometry degradation and extending the surfacing maintenance cycles. Analysis of the rate of degradation showed that the effect of installing the geocell material was to significantly reduce the rate of degradation (and thus increase the surfacing cycle) by a factor of 6.7 times the pre-geocell installation surfacing cycle.

Performance of Granitic, Shale, and Limestone Forest Road Aggregates Subjected to Repeated Loading

AbstractThis study compared the performance of three aggregate layers, commonly used in the construction of unbound forest roads in Ireland, when they were subjected to repeated loading in a new large-scale test rig. These layers comprised: (1) a layer of uncrushed, granitic, sandy gravel-a good quality road aggregate, (2) a layer of shale-a poor quality aggregate, and (3) a layer of crushed limestone–an excellent quality aggregate with a wet mix macadam (WMM) grading–on top of a poor quality shale subbase layer. The repeated load testing rig was designed and constructed to test different surface or completion layering thicknesses of the aggregates over a common formation or subgrade material of silty sandy soil. This testing was achieved by surface loading the aggregates through a 200-mm-diameter rubber pad-attached to a hydraulic actuator on the test rig-for up to 150,000 load applications. The subgrade pressures and surface deflections were measured at applied stresses of 500, 750, and 1,000 kPa. The g...

Direct Measurement of the Impact of Heavy Loads on Thin Membrane Pavements

This paper presents the results of a field study of the loads imposed by heavy oilfield cranes (with hydraulic suspensions and super single tires) on thin membrane asphalt pavements in Alberta, Canada. Three 150-m test road sections (thin asphalt wearing course, bituminous surface treatment, and granular surface) were built and instrumented for strain at the bottom of the asphalt layer, surface deflection, and subgrade pressures. Temperature and moisture profiles were also measured. Field testing involved controlled speed experiments of standard axle configurations and heavy-axle (12,000-kg) vehicles with and without hydraulic suspensions. Focusing on the hot-mix asphalt section, this paper presents a description of the test road design, instrumentation, and testing plan, followed by some results and findings from two seasons (spring and fall 2005) of testing. Vertical stress in the subgrade, longitudinal interfacial strain, and surface deflection are compared for three vehicle types used in the test. Results from tests show that subgrade stress and interfacial strain are very similar for the standard axle configuration during spring compared with those of the cranes without the dolly during the fall season. It could be argued that on the basis of the pavement response, the cranes could operate during the winter season without the dolly (and thereby increase road safety by removing a long combination vehicle from the traffic stream) without causing substantial long-term deterioration.

Investigation of thin flexible pavement response between traffic and the falling weight deflectometer (FWD)

The Falling Weight Deflectometer (FWD) is commonly used to evaluate structural characteristics of pavements. The FWD simulates vehicular loads at typical highway speeds and the data is used to back-calculate layer moduli and design overlays (among other functions). In routine testing, the only data obtained is on the surface of the pavement, which leaves potentially significant behaviors within the pavement undetected. The focus of this paper is to present the analysis of a heavily instrumented, full-scale flexible pavement that was subjected to vehicular traffic and FWD loads under controlled testing conditions. Statistical analysis and finite element modeling separately demonstrated that the responses produced by the FWD were, in general, lower than those from corresponding vehicular traffic, and this behavior was more pronounced with depth. Finite element modeling resulted in traffic asphalt strain, base pressure, and subgrade pressure responses exceeding FWD responses by factors of 1.27, 1.76, and 2.43, respectively. Similarly, a statistical analysis resulted in factors of 1.36, 1.66, and 2.39, respectively. Additional analysis showed the deflection from FWD loading would have to be increased between 1.3 to 1.6 times to produce corresponding vehicular traffic stress states.

Subgrade Contact Pressures under Rigid Pavements

Subgrade contact pressures under rigid pavements were analyzed using the finite element method for concrete slabs on liquid and elastic subgrades. It was found that when the maximum bending stress in the slab is made in agreement in the two analyses, the deflections and subgrade contact pressures are much greater for elastic than for liquid subgrades. Although initial bending stresses in the concrete slab are well below the concrete strengths, excessive subgrade pressures undoubtedly cause large permanent deformations in the subgrade soil, possibly increasing the stresses in the concrete slab rapidly and eventually leading to early failure of the concrete pavement. The computation of large subgrade pressures at slab edges only in pavements with weak subgrade soil supports the Corps of Engineers design practice of reduction of pavement thickness for pavements with high subgrade k values, although bending stresses in the concrete slab are only slightly affected by variations in k values. Once initial cracki...

Pavement Performance As A Basis For Design

The assumptions in the Westergaard and Burmister theories for the design of concrete pavements, and the use of some empirical equations in the design are discussed. The results of load tests on seven concrete slabs with different foundations are presented. These tests were performed 1 yr after the construction of the pavement, i.e., when the separation (or break in continuity) between the slab and its foundation is likely to have taken place. The data on subgrade moisture content, warping and temperature distribution in the concrete slabs, pavement deflection and stresses and subgrade pressures are presented. Revised equations are given for: (1) Modulus of subgrade reaction, (2) interior, corner and edge load deflection, (3) corner and edge load stress, and (4) edge stress when cracking takes place parallel to the edge (for thin slabs). The equations show that the deflections and stresses due to corner and edge loading are not a direct function of load P but P0.67 and P0.8, respectively.

Tests Of Concrete Pavements On Crushed Stone

Static load tests were performed on 8-in. thick concrete slabs on densegraded and open-graded stone subbases in 5-in., 10-in., and 15-in. thicknesses. Both types of crushed stone subbases and all thicknesses contributed to the load capacity of the slabs by reducing slab strains and deflections and subgrade pressures. The first 5 in. of subbases were more effective in reducing strains and deflections than the next 10 in. In areas without frost or soil swell problems, a subbase of 5 in. of 6 in, is ordinarily optimum, and it was shown that thin additions to the slab thickness will meet load capacity requirements as readily as thick subbases. Agreement between experiment and theory was good with respect to deflections, but measured strains were more sensitive to foundation bearing value than theory. For each 5-in. thickness of stone subbase beneath the slab, the subgrade pressures were reduced approximately 1 psi at free corners, ½ psi at edges, and ¼ psi at interiors. Incidental studies were made to observe response for many load positions, to study the effect of plate size, and to evaluate joint effectiveness.