Peace in 1945 did not bring international stability. By 1948, as Turnbull and the Waterways Experiment Station played an important role at the Second International Conference, the global strategic balance had shifted dramatically. Gone were the threats from Nazi Germany, Fascist Italy, and Imperial Japan which had stirred the world to war from 1941 to 1945: all lay in ruins. America and its partners in the Grand Alliance Britain and the Soviet Union were totally triumphant. Yet the euphoria of victory quickly began to give way to apprehension as the unlikely coalition between the Western democracies and Communist Russia collapsed. Old enemies were replaced by new. The Soviet occupation of Eastern Europe, the establishment of Communist regimes there, and subsequent clumsy attempts to force the Western powers out of Berlin marked the onset of the Cold War. Within three years of the defeat of the Axis powers, the United States and the Soviet Union faced off as antagonists in a bipolar world. China's fall to Communism in 1949 further intensified the confrontation between East and West.
The Cold War dominated strategic considerations for the next generation. American efforts were concentrated in an attempt to "contain" what was visualized as an aggressive and expansive Soviet state, along with China as its perceived ally. A vital area of competition involved military technology and engineering, and it was there that WES emerged as a major participant. Rather than encountering a decrease in military research in the post-World War II world, the Station's role in that area expanded immensely.
Whereas military research had only begun at WES in earnest in 1943 with the establishment of the Flexible Pavement Branch for airfield construction studies, the WES Soils Division by the 1950s dedicated the majority of its activities to military purposes. Airfield design and trafficability research initiatives started during the war were extended dramatically, while WES also became the military's premier institution in the evaluation of existing airfields around the globe. Related areas such as mobility and military environmental studies enlarged the functions of the Station, and the new realms of nuclear cratering research, soil and rock dynamics, and geological investigations for military purposes emerged as significant entities.
Fittingly, one of the first confrontations of the Cold War had an unexpected WES connection. On 24 June 1948, three days after the opening of the Rotterdam conference, the Soviet Union initiated the Berlin Blockade. Deep inside Soviet-occupied East Germany, Berlin was a divided city with the Western powers holding tenuously to their zones of occupation. The blockade, which prohibited all rail, highway, or canal traffic between the West and Berlin, was designed to squeeze the Western bloc out of the city, leaving it entirely in Soviet hands. Western leaders refused to budge and instead determined to supply the beleaguered city by air. By fall thousands of aircraft American, British, and French winged their way through narrow air corridors across the Soviet zone each day, bringing provisions for the near-starving metropolis. The Berlin Airlift escalated into the first open showdown between the erstwhile World War II allies.
Supplying Berlin was an engineering nightmare, as runways in the Western zones were woefully inadequate. During the recent war the Germans had paved few local airstrips, and the heavy cargo planes of the airlift required hard surfaces. Tempelhof, a former municipal field and later Luftwaffe base in the American zone, had one surfaced runway and subsequently became the primary landing facility for the entire airlift operation. It quickly proved insufficient. Army Engineers endeavored to add two additional runways but did not have access to conventional base materials such as sand, slag, and gravel. The only readily available substance was rubble from bombed-out portions of the city, of which there was a surfeit. However, major runways had never been built of rubble before, and the effort was understandably tentative.
Engineers in Berlin appealed to the Chief of Engineers for advice and expertise, and OCE referred the problem to WES. Turnbull then dispatched Charles R. Foster, Assistant Chief of the Flexible Pavement Branch, Soils Division, to Berlin, with Foster supposedly replacing thirteen sacks of coal on an incoming transport. Foster made recommendations concerning the density of aggregate and the amount of asphalt needed for the strips. Using WES instruments to test weights, stresses, and strains that such ersatz facilities could withstand, he contributed materially to their design. Engineers constructed the desperately needed landing facilities, eventually employing over a million cubic yards of aggregate as subgrades. Foiled by Western technology and resolve partly contributed by WES the Soviet blockade was lifted in May 1949.1
Even before the advent of the Cold War, WES had earned a reputation in the area of evaluating existing airfields, especially those with flexible pavements or more primitive surfacing. In September 1945 the Army Air Corps directed all Theater Commanders to provide soil samples from certain bases in their zones along with data on runways: description of pavements and base courses, types of planes in use, frequency of operation, and other pertinent features. Many sites were in recently hostile nations such as Germany and Italy, or in formerly occupied France. Others ranged the globe. A second directive provided details for collecting and forwarding samples and information. This was to provide the raw materials for a comprehensive review of the air arm's overseas facilities. At first, project planners designated the Aviation Engineer School in Geiger Field, Washington, to analyze the collected materials, but this was precluded by the reduction of personnel and facilities there in the post-War demobilization. Consequently, in February 1946, at the request of the Army Air Corps, OCE shifted the task to WES.
WES received, identified, and tested a total of 551 samples from 222 overseas air bases. Correspondence indicated that samples were shipped from 20 additional locations, but these were either lost en route or had insufficient markings to be identified. Sixty-four fields in Europe, 77 in Africa, 42 in the Near East, India, Burma, and China, and 39 in the Pacific Islands were included. Large, well-established facilities at bases such as Rhein-Main, Germany, contrasted sharply with expedient PSP-covered strips in Algeria and water-bound gravel runways in China.
Samples and data proved inadequate in an alarming number of cases. Instructions from the Air Corps to Theater Commanders to ship 50-pound soil samples were either totally ignored or misconstrued, as half the samples weighed less than 30 pounds and one-fourth less than 15 pounds. In numerous instances base personnel provided little or no information as to the location and depth of samples from airfields or whether the sample was representative of paving aggregate, base course, subgrade, or fill. WES reports noted that samples had been taken by "non-engineering types of army units, who were not familiar with details of the construction of the fields represented," and who had not acquired the "techniques and experience necessary to secure representative soil samples." Facility descriptions, usage patterns, and other conditions requested were also more often than not missing or incomplete.2
Still, WES compiled a confidential report utilizing the data available. Lab crews prepared soil descriptions and categorized specimens according to the Casagrande Airfield Classification system whenever possible. Further lab tests provided grain size analyses, specific gravity, Atterberg limits, swell and compaction values, and CBR ratings.3 Inevitably, the project was far from satisfactory due to the aforementioned obstacles. Future airfield evaluations would rely on direct, on-site investigations by teams of experts rather than lab technicians at a central location.
In March 1951 the U.S. Air Force, no longer under the auspices of the Army, requested that OCE execute an evaluation program of overseas airfields, especially in but not limited to NATO countries. The following month OCE assigned responsibility to WES, and by June the Flexible Pavement Branch had produced a "Manual for Evaluation of Overseas Airfield Pavements."4 During the next four years WES personnel conducted over 125 airfield investigations in such disparate locales as Iceland, Greenland, Newfoundland, England, Scotland, France, Germany, Italy, Spain, Norway, Denmark, Greece, Crete, Turkey, Libya, Morocco, Saudi Arabia, Pakistan, and Liberia.5 Supplementing the individual airfield studies, WES researchers produced summary reports on the geological and soil conditions of French Morocco and West Germany, especially as they might apply to airfield construction.6
Turnbull and Foster, now Flexible Pavement Branch Chief, were general directors of the OCE program, but the field work fell to teams of globetrotting WES specialists. These normally consisted of five to seven individuals. Each was headed by a Corps liaison officer, who provided assistance to a WES engineer. The engineer in turn supervised a WES geologist and two or three WES lab and field technicians. Local authorities furnished drivers, mechanics, and laborers as needed.
Groups departed from Vicksburg and flew to Wiesbaden, Germany, where soils testing equipment was stored. After outfitting themselves with the necessary accoutrements, they proceeded to airfields selected by the Air Force for evaluation. Realizing that a lengthy period of time would be required to evaluate the large number and variety of bases designated, planners attempted to limit crews to a maximum of one week at each site. Moments of leisure were rare. The groups traveled for three to four months, performing their tasks at several fields, then returned to WES for three months to prepare reports. Foreign accommodations ranged from hotels and barracks to boarding houses and tents; food varied routinely from bad to worse.7
A study of Tempelhof Air Base, where Foster had participated in the design of some paved sections during the Berlin Airlift, was typical of many overseas ventures. In September 1952 a team composed of Captain William R. Cordova of the Corps of Engineers, WES engineer Kenneth Jensen, WES geologist Charles R. Kolb, and WES soils technicians O.J. Woodrow, R.B. Wilson, and J.W. Loviza arrived at the former Luftwaffe base. Kolb prepared a historical review of the construction, maintenance, and usage patterns of the airstrips. Ranging further afield, the WES geologist evaluated geological conditions and resources in the whole vicinity, paying particular attention to materials and conditions which might affect future construction. In addition to his on-the-scene observations, Kolb utilized a profusion of other resources to assemble a geological overview of the region. These included local oral sources and documentary records, U.S. Army service topographic maps, German Department of Agriculture soil and terrain studies, and other primary and secondary works. He further supervised the acquisition and publication of aerial photographs.
Under Jensen's supervision, the technicians thoroughly evaluated the pavement, base course, and subbase of the airfield itself. Ten test pits and thirteen auger holes dug through the runways furnished data on pavement thicknesses, damage and wear, and base course materials and conditions. Workers also took numerous pavement samples for laboratory investigation and performed CBR tests on the surface of the base course, subbase, and subgrade. Moisture and density determinations were made on the subgrade, and Atterberg limits and mechanical analysis tests were conducted on typical subgrade soils. Larger pits excavated at the runway borders, one nearly twenty feet deep, yielded an in-depth profile of soil composition. Other research generated data on drainage, frost conditions, and related factors.
Derived from the mass of information collected, the 1953 WES report on Tempelhof furnished a highly detailed evaluation of conditions at the airfield. Officials could then make projections as to the anticipated life and condition of the airfield under current usage and could develop strategies for future repairs or construction.8
WES teams found various other locations less hospitable, although this was not reflected in official reports. Beer was not available in Saudi Arabia or Pakistan. C-rations in North Africa added to the discomfort of tent accommodations, and mutton soup in Pakistan may have been even less palatable. Geologist John H. Shamburger needed an armed guard for his excursions in Pakistan, as tribal wars were still in progress.9
A June 1952 expedition to Roberts Field, Liberia, was even more eventful for a WES team that included engineer Z.B. Fry, geologist William B. Steinriede, and technicians J.G. Kennedy, W.J. Harper, and G. Britt Mitchell. The group was not encouraged by the knowledge that a previous commercial flight had crashed on the same air route from Germany and the wreckage had not been found for a year. Encountering a severe storm, the pilot for the WES crew was unable to land at Roberts Field. Around midnight, low on fuel, he sent out a "mayday" and prepared to ditch at sea. Fortunately, a native of neighboring Sierra Leone answered the distress call and gave directions to a local airfield. Because it had no electric lights, the field was lit only by flare-pots around the runway. According to Steinriede, the flares appeared to die out several times as the plane circled the strip in pitch darkness. Brakes locked, the plane landed safely and the crew spent the rest of the sleepless night in thatched huts. Daylight revealed that the plane had been circling a volcanic spire that had made the flare-pots only appear to extinguish.10
While some WES teams were evaluating overseas airfields, others were engaged on a smaller scale in testing facilities within the United States. Unlike overseas bases, which the Air Force designated for testing, WES suggested domestic fields for evaluation. Field units on the two projects often overlapped. The group that performed the first evaluation at Sheppard Air Force Base, Texas, for instance, included WES soils and pavement engineer Alfred H. "Red" Joseph and WES soils technician Wilson, both of whom were involved in overseas projects. Stateside teams also tended to be smaller because domestic evaluations did not require a geological report.
Between 1952 and 1956 WES published eight major project reports under its original domestic program, involving bases in Arizona, Florida, Kentucky, North Carolina, Oklahoma, and Texas.11 Then, after completion of the overseas \evaluations in 1957, OCE authorized a more thorough program of investigations in the United States. By 1960 WES experts had tested and reported on a further eighteen bases in the Zone of the Interior, adding jaunts to Alabama, California, Colorado, Georgia, Louisiana, Utah, Virginia, and Washington to an already impressive travel résumé.12
While WES teams circled the globe to evaluate existing airfields, the Flexible Pavement Branch continued work at the Station in various areas of airfield design. Arguably, WES had its most profound effect in the 1950s in that field. Foster directed activities as Branch Chief from 1949 until 1957, then was succeeded by Audley A. Maxwell until 1964. Formation of a prestigious Flexible Pavement Board of Consultants that included Casagrande, Rutledge, Taylor, O.J. Porter, and G.W. Pickett reflected the magnitude of the Station's endeavors.
An extraordinary effort had been required during World War II simply to keep design procedures current ircraft. This trend continued, then intensified, in the post-war period. Whereas the heaviest wartime aircraft weighed well under 200,000 pounds fully loaded, by 1957 the B-52 bomber reached nearly 500,000 pounds. The challenge of dealing with aircraft weight increases was compounded by related factors: augmented compaction requirements, field moisture content effects, use of high-pressure tires, adoption of complex multiple-wheel landing gears, and difficulties in calculating pressures and deflections on and under pavements. Engineers were forced to reevaluate CBR indexes and bituminous mix standards constantly. In addition, new jet aircraft presented an unexplored range of difficulties such as the effects of channelized traffic, jet blast and fuel spillage on pavements, and the occurrence of "porpoising."
Three Corps of Engineers research facilities specialized in airfield design criteria: the Flexible Pavement Branch at WES, the Rigid Pavement Laboratory of the Ohio River Division Laboratories, and a smaller Frost Effects Laboratory at the New England Division Laboratory in Boston. ORDL, under the leadership of Philippe and F. M. Mellinger, had emerged as the Corps' primary facility in rigid pavement research during the war, while WES had taken a similar role with flexible pavement (see Chapter 3). Now WES maintained its position as an international leader in conventional airfield design with bituminous surfacing, and at the same time continued to test and develop landing mats and membranes for expedient landing surfaces.
Airfield designers had long recognized the importance of compaction of base and subbase materials prior to paving, especially where flexible pavements were to be used. However, no systematic analysis of compaction requirements had been performed prior to the end of World War II. In May 1945 OCE authorized a comprehensive soil compaction investigation and in June assigned testing to WES. The first of ten major WES compaction studies soon commenced. Its purpose was to determine compaction characteristics of a particular soil clayey sand in the field with existing equipment then readily available, and to determine the proper methods of using such equipment for maximum compaction results. The original study provided a model for later investigations.
Project D. Burns first attempted to locate sources of the requisite clayey sand in the immediate vicinity of Vicksburg. Laboratory tests of samples from a number of potential sites determined their compaction and CBR characteristics, but none were satisfactory for the OCE initiative. Further afield, borings finally indicated that an open pit near Clinton, Mississippi, about forty miles east of Vicksburg, met specifications for the type and quantity of material needed for field tests. Construction units then cleared an adjoining area of brush, built a system of drainage ditches, and transformed the site into a major test facility. Further site preparation involved stripping subgrades of all root-bearing topsoil and scarifying the underlying soil with a disk and plow. A mixing plant, built on location, blended soils from the pit in huge quantities, and these were spread by bins on draglines onto 200-feet-long test lanes. Drivers then passed various compaction machines repeatedly over the lanes, a technique developed in earlier flexible pavement and trafficability tests. Equipment tested included a sheepsfoot roller loaded to produce 250, 450, and 750 psi; a 19,500-pound wobble-wheel roller; and 10,000-, 20,000-, and 40,000-pound rubber-tired wheel loads.
Researchers varied the water content of the soils tested, the number of passes, the types of machinery n a complex series of tests. Field and laboratory examinations determined the compaction and CBR data produced by each set of variables. From this, specific compaction expectations and methodology in field conditions could be predicted for future construction.13 Later WES examinations under the original OCE mandate involved different soil types, other equipment such as vibratory rollers, and various subgrades.14
In another effort to improve engineering knowledge concerning base courses and subbases, WES from 1945 to 1959 conducted studies of soil moisture contents beneath paved surfaces. Previous experience had indicated that the moisture contents of soils under pavements increased with time, often over a period of years, until a point of near-saturation was reached. This had a profound effect on CBR values and, consequently, on airfield pavement performance. Investigations attempted to ascertain the extent to which soils became wet, the effect of drainage systems on moisture conditions under pavements, the influence of surface course leakage on changes in base course and subgrade moisture contents, the extent and rate of capillary moisture movement under pavements, and the effect of climate on moisture contents.
A WES review of published accounts first led researchers to adopt the Bouyoucos resistance block as a tool for measuring moisture in soils under pavement. With J.F. Redus as project engineer, WES technicians installed Bouyoucos blocks under airstrips at Albuquerque, Clovis, and Santa Fé, New Mexico, and checked their readings periodically for eighteen months. Results were disheartening, because readings from the devices did not match results from direct sampling. As a result, future tests rejected the Bouyoucos blocks and researchers depended on more reliable field samples until better apparatuses were developed.15
In a second moisture content test series from 1945 to 1952, WES took samples and conducted tests at eleven locations. A mixture of arid, semiarid, and humid sites included airfields in Alabama, Mississippi, New Mexico, Tennessee, and Texas. Redus remained as project engineer, while R.E. Manning oversaw field testing. Two further investigations in overlapping programs from 1952 to 1956 and from 1955 to 1959 involved additional sites and provided data for a lengthy WES report on field moisture phenomena. The following conclusions continue to have major influence in airfield construction and evaluation:
All aircraft in use before the final year of World War II relied on single-wheel landing gear. This concentrated all of a plane's weight to a small area of a landing surface. With the introduction of the much heavier B-29, however, engineers realized that new gear designs must be developed to better distribute the aircraft's weight over pavements. Thus the B-29 and the later B-50 had dual-wheel assemblies, and the even more ponderous B-36 led engineers to develop a larger dual-tandem system. These multiple-wheel designs successfully enabled existing airports to accommodate heavier planes and reduced the strength requirements of new landing strips.
The use of multiple-wheel gear dictated development of new design curves for flexible pavement thicknesses. Researchers had, in fact, already developed theoretical methods of extrapolating existing single-wheel curves into curves for multiple-wheel gears, but these had not been tested by actual traffic.17 OCE in 1948 inaugurated an investigation to that end, and WES performed pilot tests in 1949 and 1950. The Flexible Pavement Branch laid two test lanes, each 120 feet long, inside a huge new hangar on the Station grounds and subdivided each lane into three 40-feet units of differing pavement thicknesses. One unit represented the thickness indicated by theoretical Corps CBR criteria for multiple-wheel loads, while the other two represented under-designs and overdesigns of approximately thirty percent. A Tournapull rig powered gear assemblies from B-29, B-36, and B-50 aircraft with various tire pressures and with weights added to reproduce aircraft burdens. Drivers applied passes to the test lanes, with 2,000 passes as the norm. The underdesigned sections failed, but the design and overdesign units showed little or no distress. Observations, measurements, and data taken from deflection gages led to the conclusion that multiple-wheel design criteria developed from single-wheel curves were reasonably correct.18
In 1953 the Board of Consultants recommended a second study to reevaluate pavement design for multiple-wheel gear, and OCE forthwith authorized a new project. This represented a different approach in that WES suggested the program to OCE in substantial detail, rather than the other way around. The Station thereafter regularly initiated proposals and projects.
In the renewed study, under the direction of Richard G. Ahlvin, engineers reviewed and reinterpreted existing data from the first WES multiple-wheel investigations and from traffic tests performed at Stockton Air Force Base, California. Efforts relied heavily on new stress-strain and deflection concepts pioneered by Ahlvin and S.M. Fergus (see discussion later in this chapter). Their observations indicated that individual wheels in multiple-wheel arrangements acted essentially independently at and near the surface of a landing facility, distributing loads over their total contact areas. However, at substantial depths within the pavement or base course, the overlapping of wheel-load effects resulted in much the same stress and deflection as would be induced by a load on a single wheel. The new study further indicated that the depth at which a multiple-wheel load acted as single-wheel load was half the length of the spacing between the edges of tire prints of a dual-wheel gear, with the same factor applied to each dual unit of a dual tandem. The total strut load of a dual tandem gear acted as one wheel load at a depth established as twice the center-to-center distance between diagonally opposite wheels on the rig. These findings led to further design improvements and a better theoretical understanding of stress strain and deflection relationships.19
By 1953 the B-47 had replaced the B-36 as the primary Air Force strategic bomber, and by early 1955 a number of flexible pavement airfields carrying B-47 traffic were experiencing unexpected distress. A common problem was a grooving of sections of taxiways along their center lines. WES observations determined that several factors combined to increase the rate of coverages in those critical areas. Unlike previous craft, the B-47 had bicycle gear that actually applied two gear passes each time the airplane passed over pavement. The practice of painting taxi-stripes for pilots to follow down the center of lanes narrowed the lateral wander of bombers and greatly increased coverages over small areas. Also, B-47s flew many more missions per aircraft because of the ease of their preflight preparation.20
In 1954 WES began a full-scale study of channelization, ultimately collecting data from twenty-three Air Force bases with 116 bituminous-surfaced facilities. Corps division and districts, assisted by base commanders, compiled reports on usage and other factors and made visual observations that they forwarded to WES. Local Corps personnel or WES field specialists also took pavement and soil samples that were analyzed on location or sent to Vicksburg for evaluation. Findings reinforced the original observations of increased coverages. In fact, project directors calculated that B-47 load repetitions were sometimes applied at a rate of about six times that planned for in existing design criteria. Air Force and Corps planners first responded by increasing pavement thickness requirements in channelized areas by twenty-five percent. Further analysis, however, proved that such increases were not required for normal usage. Channelization was more a product of densification of pavement and insufficient compaction than of pavement thickness. Thus, subsequent recommendations provided improved asphalt mix and compaction specifications.21
B-47 gear designs also led to the new phenomenon of "porpoising": longitudinal rocking of aircraft after passing over bumps or changes in grade. A bump encountered successively by the front and rear gear of a B-47 at relatively high speeds sometimes resulted in severe longitudinal rocking. Two or more bumps or grade changes encountered at critical spacing points rocked the aircraft even more intensely. Pilots complained of shaking, inability to read instruments, damage to instruments, loss of control of the aircraft, and even of aircraft becoming airborne prematurely.
A WES study begun in 1955 ascertained that porpoising actually did not relate to the magnitude of roughness of runways, but rather correlated to the spacing of deviations from grade. This led OCE and the Air Force to tighten requirements for runway smoothness, but by 1957 other developments had essentially corrected the problem. In particular, Boeing had revised shock and strut design for the B-47, and the newer B-52 did not experience significant porpoising at all.22
Jet aircraft, unlike propeller-driven craft, generated high exhaust temperatures and ejected exhaust blasts directly onto pavements. Damage sometimes appeared severe, especially to the surfaces of flexible pavements or the joints of rigid pavements. Some engineers in the early 1950s even questioned whether current pavements of either type could withstand the deleterious effects of jet-engine exhaust.23 In 1951, at the request of the Air Force, OCE assigned investigations to WES and to the Rigid Pavement Laboratory at ORDL. The Air Force furnished and operated all aircraft involved and assisted in tests and observations at various airfields. Project consultants included W.J. Emmons, J.L. Land, F.S. Gilmore, and H.J. Skidmore, a former WES director.24
A WES investigation at Eglin Air Force Base, Florida, typified studies. Turnbull and Foster personally visited the site, as did O.B. Ray; however, E.C. Meredith conducted most on-site activities. Mellinger from ORDL was also active in the project. Because both flexible and rigid pavements were involved, contractors constructed test sections of each on an abandoned runway. Base and subgrade preparation was not necessary because test materials were laid directly on top of an existing paved surface. Thermocouples installed in each section were to obtain accurate heat measurements from jet blast.
Plans called for simulating the normal jet blast operations of each type of aircraft currently used by the Air Force. Consequently, the Air Force provided and operated the latest models of the F-80, F-84, F-86, F-89, and F-94 fighters; the T-33 trainer; and B-45 and B-47 bombers. Detailed time-movement studies of each established norms for pre-takeoff run-ups, takeoffs, and maintenance cycles. These indicated that almost no blast damage was inflicted on runways during takeoffs, but that damage tended to occur during pre-takeoff checks or maintenance testing. Jets required time periods of from 1.5 to 2.5 minutes for pre-takeoff tests, during which the stationary aircraft expelled blasts directly onto a relatively small paved area. Maintenance tests lasted an average of fourteen minutes, again localizing blast, often on the aprons of airstrips. The Eglin investigations also established critical temperatures those at which damage would occur for different types of bituminous pavements.25
Supplemental projects at Presque Isle Air Force Base, Maine; Davis-Mothan Air Force Base, Arizona; and Hunter Air Force Base, Georgia, contributed further data and incorporated the effects of afterburners.26 At the Station, personnel also constructed five test sections of different flexible pavements and subjected them to more severe than normal blasts from an in-place F-80B. In a noteworthy study, engineers used wet pavement sections to determine the patterns of jet blast as the pavements dried.27 All observations indicated that blast effects resulted in unsightly patterns and occasional erosion, but that damage was not sufficient to warrant unusual maintenance.
Spillage of jet fuel onto flexible pavements created another problem. Jet fuel did not evaporate quickly and, when left on flexible pavement surfaces long enough, acted as a solvent and dissolved the asphalt. This resulted in a temporary softening of the surface. Jet blast and fuel spillage in combination were clearly detrimental and, because the two problems were interrelated, researchers often examined them simultaneously. WES investigations at three Air Force bases in Florida and at the Station determined the normal amounts of fuel spilled, frequency of spillage, and typical results of spillage. Findings showed that pavements softened by fuel spillage usually returned to normal hardness after the fuel evaporated. In addition, researchers noted that rubberized-tar concretes were more resistant to blast and spillage than normal asphalt mixtures. These studies formed the basis for specifications for rubberized-tar cement and modifications to Corps mix design procedures. Even without modifications, data showed that spillage caused little or no real distress under normal conditions.28
Since its introduction to WES during World War II, the Corps had accepted the Marshall method of testing flexible pavement density as its standard, leading to its ultimate approval by the paving industry in general (see Chapter 3). However, by the mid-1950s engineers observed that some density values produced by field tests of pavements exceeded those obtained in the laboratory by Marshall compaction tests. This was particularly true where pavements were subjected to heavy-load, channelized, high-pressure tire traffic. Further investigations showed that it was unfeasible, if not impossible, to attain pavement densities encountered in those field conditions using the Marshall method. Whereas the original Marshall test employed fifty hammer blows of a confined pavement sample to produce desired densities, even seventy-five blows could not approximate compactions manifested by the new heavy-load traffic. More blows simply cracked the aggregate in the sample and produced unreliable test results. WES researchers then sought an improved compaction testing apparatus that could yield the high densities that developed under traffic such as B-47 and B-52 aircraft.
About 1940 the Texas State Highway Department produced a gyratory compaction machine that became the model for future development. Unlike the Marshall apparatus, the Texas machine did not rely on dynamic compaction the application of blows of a standard weight but rather used a gyratory kneading motion. Technicians dubbed it the "Body Beautiful" because of the physical dexterity required in hand operating it with cranks. Texas engineers felt that the device more closely approximated the effects of heavy-load traffic. John L. McRae, whom Turnbull had sent to Texas to teach CBR methods, saw a demonstration of the machine at Galveston and was duly impressed. He and A.R. McDaniel then instituted a developmental program at WES that culminated in the production of improved models, with design engineer Joseph P. Wislocki fashioning the newer devices to run on electrical rather than human power. Laboratory and field tests proved the principle of the gyratory machine to be sound and its predictions more accurate for heavy-load traffic than those of any previously established methods. The apparatus, which continued to be refined at WES, found wide acceptance in both military and civilian applications.29
In 1941, shortly after the adoption of the CBR method for design of flexible pavements, the Corps initiated concurrent theoretical studies directed toward developing a less empirical design method. Stress, strain, and deflection phenomena within pavements were of particular concern. Engineers could predict all for linear elastic media by theoretical methods, but supporting test data were unavailable. By 1946 it was apparent that data from tests on typical pavement structures were too complex for direct analysis. OCE then enlarged and reoriented the research program, assigning responsibilities to WES in that same year.
A WES pilot study in 1947 and 1948 presented limited results of an investigation of the stresses and deflections produced within a homogeneous soil mass by static loads applied to its surface. Researchers found existing instrumentation, notably two-gage pressure cells, to be inadequate.30 A second study concentrated on development and testing of improved instruments such as the four-gage cell, which was found to be satisfactory.31
Fergus and Ahlvin conducted a much more intensive testing program in 1950 and 1951. They designed a unique test section placed in a rectangular excavation 48 feet long, 32 feet wide, and 6 feet deep. The section itself was 9 feet thick, extending 3 feet above the original grade. A fill of air-dried, homogeneous sand provided a consistent and simple test material. Technicians installed a variety of test instruments within the section, including pressure cells, shear cells, strain cells, and deflection gages. Placed at different depths and at differing angles horizontal, vertical, and diagonal these instruments measured changes produced by static loads. Circular loads best produced stress distributions approximating that of an aircraft tire and also lent themselves readily to mathematical manipulation. WES consequently designed circular loading plates of various dimensions up to 1,000 square inches, which were placed on the surface of the test section. A jacking device applied loads of up to 200,000 pounds to the plates, and electrical connectors from the embedded test instruments furnished data for comparison to theoretical projections. Conclusions indicated that the test methods and new instrumentation provided accurate measurements of stresses and deflections within homogenous materials and that theoretical forecasts could indeed approximate empirical readings.32
Resumption of an acrimonious dispute in the mid-1950s between patrons of flexible or rigid pavements was of vital importance to the Flexible Pavement Branch. Each had inherent advantages. Rigid pavements, primarily portland-cement concrete, were without question capable of supporting the heavy loads of new aircraft. Flexible bituminous pavements were cheaper, sometimes costing as much as fifty percent less, than their rigid counterparts. Flexible surfaces, however, required more extensive base, subbase, and subgrade preparation, and fully acceptable design criteria for heavy loads were not yet available.
Army Air Corps designers at the outset of World War II had shown a preference for rigid pavements. After much debate, Army policy makers required wartime airfield designers to use more economical flexible pavements whenever feasible (see Chapter 3). By the early 1950s, however, Air Force engineers were again inclined toward "luxury" rigid surfaces. Nearly eighty percent of Air Force expenditures after 1951 for "primary" facilities, essentially runways, were for rigid pavements. Indeed, by the mid-1950s many Air Force engineers were convinced that flexible pavement design for the B-47 bomber and its anticipated replacement, the B-52, was impossible. In addition to revolutionary increments in weight, the new craft exhibited new phenomena such as channelization and porpoising and relied on high-pressure tires, all of which were detrimental to flexible surfaces.33
In March 1956, at the insistence of the Air Force, OCE issued a "Criteria for Selection of Pavement Type at Air Force Installations" bulletin that effectively halted the use of flexible pavements in any new construction. The directive ordered that all "primary use pavement" for the Air Force was to be of portland-cement concrete. The definition of "primary use pavement" was expanded to include "all paved areas on which aircraft are regularly parked, serviced, maintained, or preflight checked," plus "all runways...with sustained use by combat or combat-support aircraft," and all taxiways connecting the above areas.34 Only non-primary areas such as shoulders, runway overruns, and erosion-control surfaces could be paved with either rigid or flexible pavements. This aroused a storm of controversy that ultimately involved the Air Force, the Asphalt Institute, the Portland Cement Association, OCE, the Corps' Rigid Pavement Laboratory at ORDL, the Flexible Pavement Branch at WES, and the U.S. Congress.
The Flexible Pavement Branch was already pushing the limits of research to deal with new airfield demands. Pilot investigations at the Station in 1956 used twin-tandem loads up to 325,000 pounds and tire pressures up to 325 psi on bituminous test sections, producing strains well beyond the requirements at hand. Trials indicated that such loads would dictate a reduction in asphalt content from laboratory optimums (as determined by the Marshall method) and that a pavement with a greater resistance to densification was necessary.35 Concurrent WES studies proved that the effects of jet blast, fuel spillage, and high-pressure tires on flexible pavements were not nearly as critical as previously feared (see previous discussion in this chapter).
In 1955, even before the Air Force policy of "rigid pavements only" was established, OCE had ordered its two primary pavement labs WES and ORDL to conduct field comparisons of the materials under consideration. The Air Force designated Kelly Air Force Base, San Antonio, Texas, as the site for testing. Turnbull, Foster, Ray, and Burns represented WES on location, while Mellinger, James P. Sale, Ronald L. Hutchinson, and others represented ORDL. In the spring of 1956 contractors built rigid and flexible pavement test sections, each 200 feet long, according to current Corps specifications. In May and June drivers passed a test cart with a 100,000 pound dual-wheel load over both sections, simulating B-47 traffic.
The rigid pavement endured 30,000 coverages with no significant damage. The flexible section, however, sustained so much damage after 9,098 passes that tests were discontinued. WES and ORDL then issued a joint report which concluded that asphaltic-concrete mixtures designed in accordance with existing Corps criteria were not adequate to carry 30,000 passes by B-47s.36 This seemed to verify the Air Force's rigid pavement endorsement.
The Kelly report, however, aroused a howl of protest from the Asphalt Institute and other flexible pavement advocates. Political pressure led to congressional hearings in July 1957. There, Air Force engineers vehemently defended their rigid pavement preference, citing the Kelly tests as primary supporting evidence. In opposition, J.E. Buchanan, president of the Asphalt Institute, launched a spirited attack on the Air Force position. Buchanan argued that the Kelly tests were inconclusive, if not outright bogus, because the Air Force had specified a Corps asphalt mix which was not designed for 30,000 passes and that 30,000 passes was far in excess of any potential usage requirements. Turnbull and Foster also testified on behalf of flexible pavements, noting that the pavement at Kelly was not allowed to season properly, that repeated coverages 500 a day did not allow the pavement to cool or restore itself, that asphaltic surfaces had performed well in all reasonable cases, and that improved pavement design was available. H.K. Griffith, Executive Director of the National Bituminous Concrete Association, calculated that the entire planned Strategic Air Command (SAC) bomber force would have to land two and a half times a day on every SAC base in existence to duplicate the passes of the Kelly tests in the time allotted.Congress called for more tests.37
Spurred by Congress, in July 1957 the Air Force requested further investigations of flexible pavements with improved design mixes at Columbus Air Force Base, Mississippi. There, Foster and his assistants designed a 200-feet flexible pavement test section laid end to end with a rigid section 50 feet long. Since rigid surfaces had already proven worthy, engineers were primarily interested in proof-testing the flexible strip and its juncture with the rigid area. Rather than simulate B-47 traffic as in the Kelly tests, planners used a test cart with loads and tires to match those of the aft gear of a B-52, as all new strategic facilities would be built for the latter. Through September, drivers applied 5,004 passes, working day and night, seven days a week. Intermittent cracks appeared in the asphaltic surface after about 1,000 coverages, as did some expected settlement. Analysts found neither to be of particular importance.38
OCE conclusions emphasized that the Columbus tests "demonstrated the validity of the design and construction procedures developed by the Corps of Engineers for heavy-load flexible runway interior pavements" and that "it is possible to construct heavy-load flexible runway interior pavement that will provide adequate service for normal B-52 operations."39 As a result of Corps research, Air Force planners thereafter were required to take competitive bids from both flexible and rigid pavement construction agents, although center runway strips of rigid surfacing were still preferred. Thus flexible pavements remained an integral part of military airfield construction, largely through the efforts of WES.
Continued development of expedient surfaces at WES paralleled advances in flexible pavement design. In the immediate post-war period, expedient surfacing investigations slowed markedly after the hurried experiments of 1944 and 1945 (see Chapter 3). Limited Corps efforts concentrated on developing a pierced steel plank landing mat capable of carrying B-29 class traffic. OCE placed responsibility for mat design with the Corps' Engineer Research and Development Laboratories (ERDL) at Fort Belvoir, Virginia. ERDL then submitted its products to WES for testing.
By 1947 ERDL had designed a heavy, deep-ribbed PSP mat with the designation M7. WES field tests led ERDL to modify M7 in order to produce improved M8 steel and M9 aluminum versions.40 In 1950 WES conducted lengthy examinations of both. The primary testing objective was to determine if single layers of M8 and M9 could sustain for periods of one year and one-half year, respectively, normal operations of military aircraft having a dual-wheel load of 80,000 pounds. One thousand coverages was accepted as a standard yearly average. Secondary objectives involved determining the effects of 50,000-pound single-wheel loads on both types and determining the suitability of rubber sheeting for dustproofing and waterproofing soil bases under the mats.41
Unlike previous outdoor tests, the M8 and M9 test sections were conducted in the hangar on the Station grounds. Three prepared lanes with predetermined CBR values (24, 17, and 12) provided a range of subgrade strengths; loads applied replicated aircraft traffic of the prescribed weights and repetitions. Data indicated that the M8 could sustain the desired 1,000 coverages of an 80,000-pound dual-wheel load when placed on a subgrade having a CBR index of about 15. The M9 also successfully carried 500 passes of that load with similar CBR subgrades. However, damage to both from 50,000-pound single-wheel loads was substantial. Rubber sheeting proved unsatisfactory as a waterproofing agent.42
Succeeding investigations in 1951 compared the effectiveness of 10-, 11-, and 12gage M8 mats. Standard M8, as tested the previous year, was constructed of 10-gage steel sheets (thicker than sheets of higher numbered gages). Although 10-gage mat proved effective in bearing traffic, its heavy weight presented transportation and installation problems. Air Force planners consequently reduced service life criteria for steel mats to six months, with a desired coverage life of 700 passes (down from one year and 1,000 passes), in hopes that lighter weight mats could resolve logistical difficulties, yet carry projected traffic. ERDL then developed thinner 11-and 12-gage M8 mats. Continued tests indicated that only standard 10-gage mat would sustain 700 passes of both 80,000pound dual-wheel and 50,000-pound single-wheel loads, thus meeting the new standards. The 11-and 12-gage models failed after only about 280 coverages of 50,000-pound single-wheel traffic and were judged inadequate as replacements.43 The Air Force, largely as a result of these tests, adopted M8 10-gage steel as the standard heavy landing mat. It saw extensive use in the Korean conflict.
OCE further broadened expedient surfacing research activities in 1951 through implementation of the "Criteria for Designing Runways Surfaced with Landing Mat and Membrane-Type Materials" program. A prestigious board of consultants, reflecting the importance of the project, counted Casagrande, Turnbull, Middlebrooks, Buchanan, Taylor, Rutledge, Philippe, and Porter as members. Goals included improvement of landing mat design; development of effective membranes for waterproofing, dustproofing, or ground-surfacing purposes; and establishment of CBR subgrade design curves for a range of single-wheel and multiple-wheel assembly loads on unsurfaced soils and soils under landing mats. Further objectives involved theoretical analytical studies and model tests. Recognizing the WES Flexible Pavement Branch's capabilities for design and procurement as well as testing, OCE assigned overall responsibility for the investigation to the Station.44
Field tests from 1951 to 1953 used unsurfaced soil test lanes in addition to standard M8 steel and M9 aluminum mat sections to develop CBR design curves.45 Subsequent tests involved newer mat configurations such as T8 magnesium, T11 aluminum, and T12 plastic. Among WES personnel directly involved were Maxwell, William L. McInnis, and Burns.46
In a particularly noteworthy project, WES engineers acquired and tested a T10 steel dust-alleviation-type mat. Standard M8 mat was of a pierced-type construction, with holes to limit weight. This pattern, however, led to severe dust and subgrade erosion problems, especially with jet aircraft. Because efforts to design an adequate membrane covering had so far proven unsuccessful, OCE suggested to the Air Force in 1955 that M8 mat without holes be tried as a practical solution. WES then redesigned the M8 steel mat to incorporate features requested by the Air Force, designating the new product the T10, dust-alleviation-type landing mat. Made of 0.140-inch-thick steel sheets without lightening holes and with newly-designed but bulkier end joints, T10 proved too heavy for Air Force standards. Further alterations led to production and testing of a lighter version made with 0.125-inch sheets. Studies indicated that the lighter T10 performed well above project requirements and was a more suitable surfacing than standard M8.47
In another major effort, WES designed, procured, and tested a novel T12 plastic mat. Interest in development of a plastic mat dated to 1945, when OCE ordered a preliminary study to select the most promising plastics for that purpose. ERDL conducted early investigations, testing three plastic mat designs between 1948 and 1952. None met criteria. In 1955 OCE transferred the project, which had been inactive since completion of the ERDL tests, to WES. Stiffer requirements called for a plastic mat that, when placed in a single layer on a 15-CBR subgrade, would sustain six months' traffic of military aircraft having single-wheel loads as much as 50,000 pounds and tire pressures as high as 200 psi.
Attempting to meet this challenge, WES engineers produced the revolutionary T12. Consisting of a "sandwich" design, it was made with a glass-fabric-reinforced phenolic resin honeycomb-structured core, bonded top and bottom to glass-fabric-reinforced phenolic resin facings. Tongue-and-groove side and end connectors replaced the traditional steel or aluminum bayonet types. Despite its state-of-the-art appearance and structure, in traffic tests conducted in 1958 the T12 failed under 50,000-pound single-wheel loads.48 In subsequent investigations, however, improved versions made satisfactory surfacing materials for military access roads and were capable of withstanding over 600 coverages by M48 tanks.49 Further advances in design of both T10 steel and T12 plastic mat followed slowly, being stimulated only by the escalation of the Vietnamese conflict in the 1960s.
Complementing its landing mat studies, WES continued to develop expedient surfacing membranes for use in conjunction with mat or as independent ground covers. Previously developed (World War II vintage) prefabricated bituminous surfacing was not capable of meeting new weight and logistical requirements and was susceptible to damage from jet blast and fuel spillage (see Chapter 3). OCE planners therefore by 1950 abandoned further PBS investigations and concentrated on identification of improved materials and assembly methods.
A first series of WES studies in 1951 evaluated four types of vinyl membranes: two duck cloth vinyl-impregnated and two wire fabric vinyl-impregnated designs that had been procured by OCE. None had carrying strength but were intended solely to protect subgrades from intrusion of water. For testing purposes, WES constructed 100-feet long lanes with the usual predetermined CBR and water content values, then simulated 25,000-pound single-wheel traffic on membranes placed on test lane surfaces. Both duck membranes withstood 700 passes, outperforming the wire-mesh types, but deteriorated rapidly when exposed to weather.50
ERDL in 1953 and 1954 also conducted a series of tests on nine new membrane types, including designs of vinyl-coated duck, vinyl-coated cotton, neoprene-coated cotton, neoprene-coated rayon, and neoprene-coated dacron. In general, those placed on subgrades with adequate CBR indexes satisfied project requirements, essentially waterproofing and dustproofing, but none satisfied requirements for use on or under landing mat.51 OCE then consolidated its membrane testing program, transferring all testing and design activities to the Station. WES then performed a series of investigations from 1956 to 1959 involving additional coated fabrics. Laboratory tests determined weights; tensile and tearing strengths; permeability; and resistance to flame, fuel spillage, and weathering, while field tests validated performance with and without landing mat. The T12 neoprene-coated nylon membrane proved most satisfactory for use both as a wearing surface and as a dust and erosion preventative beneath M8 and other mat.52 As in the case of landing mat, further membrane development then awaited the demands of the war in Vietnam.
Airfield paving research, begun at WES during the emergency of World War II, continued unabated during the crucial early stages of the Cold War. The technology that helped defeat Nazi Germany and Imperial Japan, in fact, was largely obsolete by the 1950s. New jet aircraft that dwarfed World War II planes presented a new and complex set of problems such as increased weights, high-pressure tires, and jet blast and fuel spillage difficulties. WES succeeded in overcoming the new obstacles, as the Station's Flexible Pavement Branch retained its position as a world leader in airfield pavement evaluation and development.