道路工程畢業(yè)設計外文翻譯---公路線形設計

1、<p><b>　　公路線形設計</b></p><p><b>　　A.平面設計</b></p><p>　　道路的線形反映在平面圖上是由一系列的直線和與直線相連的圓曲線構成的?，F(xiàn)代設計時常在直線與圓曲線之間插入緩和曲線。</p><p>　　線形應是連續(xù)的，應避免平緩線形到小半徑曲線的突變或者長直線末端與小半

2、徑曲線相連接的突然變化，否則會發(fā)生交通事故。同樣，不同半徑的圓弧首尾相接（復曲線）或在兩半徑不同的圓弧之間插入短直線都是不良的線形，除非在圓弧之間插入緩和曲線。長而平緩的曲線總是良好的線形，因為這種曲線線形優(yōu)美，將來也不會廢棄。然而，雙向道路線形全由曲線構成也是不理想的，因為一些駕駛員通過曲線路段時總是猶豫。長而緩的曲線應用在拐角較小的地方。如果采用短曲線，則會出現(xiàn)“扭結(jié)”。另外，線路的平，縱斷面設計應綜合考慮，而不應只顧其一，不顧其二

3、，例如，當平曲線的起點位于豎曲線的頂點附近時將會產(chǎn)生嚴重的交通事故。</p><p>　　行駛在曲線路段上的車輛受到離心力的作用，就需要一個大小相同方向相反的由超高和側(cè)向摩擦提供的力抵消它，從公路設計的角度看，超高或橫向摩擦力都不能超過某一最大值，這些控制值對于某一規(guī)定設計車速可能采用曲線的曲率作了限制。通常情況下，某一圓曲線的曲率是由其半徑來體現(xiàn)的。而對于線形設計而言，曲率常常通過曲度來描述，即100ft長的曲

4、線所對應的中心角，曲度與曲線的半徑成反比。</p><p>　　公路的直線地段設置正常的路拱，而曲線地段則設置超高，在正常斷面與超高斷面之間必須設置過度漸變路段。通常的做法是維持道路每一條中線設計標高不變，通過抬高外側(cè)邊緣，降低內(nèi)側(cè)邊緣以形成超高，對于直線與圓曲線直接相連的線形，超高應從未到達曲線之前的直線上開始，在曲線頂點另一端一定距離以外達到全部超高。</p><p>　　如果車輛以高

5、速度行駛在一段受限制的路段，如直線與小半徑的圓曲線相連，行車會極不舒服。汽車駛進曲線路段時，超高開始，車輛向內(nèi)側(cè)傾斜，但乘客須維持身體的垂直狀態(tài)，因為此時未受到離心力的作用。當汽車到達曲線路段時，離心力突然產(chǎn)生，迫使乘客須作進一步的姿勢調(diào)整。當汽車離開曲線時，上述過程剛好相反。插入緩和曲線后，半徑從無窮大逐漸過渡到圓曲線上的某一固定值，離心力逐漸增大，沿緩和曲線精心設置超高，離心力平緩逐漸增加，避免了行車顛簸。</p>&

6、lt;p>　　緩和曲線在鐵路上已經(jīng)使用多年，但在公路上最近才得以應用，這可以理解。火車必須遵循精確的運行軌道，只有采用緩和曲線后，上述那種不舒服的感覺才能消除。然而，汽車司機在公路上可以隨意改變側(cè)向位置，通過迂回進入圓曲線來為自己提供緩和曲線。但是在一個車道上（有時在其他車道上）做這種迂回行駛是非常危險的。設計合理的緩和曲線使得上述迂回沒有必要。多叢安全為計，公路廣泛采用緩和曲線。</p><p>　　對于

7、半徑相同的圓曲線來說，在末端加上緩和曲線就會改變曲線和直線的相關位置，因此，應在最終定線勘測之前應決定是否采用緩和曲線。一般曲線的起點標為PC或BC，終點標為PT或EC。對含有緩和曲線的曲線，通常的標記配置為：TC，SC，CS和ST。</p><p>　　對于雙向道路，急彎處應增加路面寬度，這主要基于以下因素：1.駕駛員害怕駛出路面邊緣；2.由于車輛前輪和后輪的行駛軌跡不同，車輛有效橫向?qū)挾燃哟螅?.車輛前方相對

8、于公路中線傾斜而增加的寬度。對于寬度為24ft的道路，增加的寬度很小，可以忽略。只有當設計車速為30mil/h，且曲度可達2ft然而，對于較窄的路面，即便是在較平緩的曲線路段上，加寬也是很重要的，推薦加寬值及加寬設計見《公路線形設計》。</p><p><b>　　B.縱坡線</b></p><p>　　公路的豎向線形及其對車連運行的安全性和經(jīng)濟性的影響構成了公路設計

9、中最重要的要素之一。豎向線形由直線和豎向拋物線或圓曲線組成，稱為縱坡線?？v坡線從水平線逐漸上升時稱為上坡，反之，則稱為下坡。在分析坡度與坡度控制中，設計人員通常要研究中線縱斷面上坡度變化的影響。</p><p>　　在確定坡度時，最理想的情況是挖方和填方平衡，沒有大量的借方和棄方。所有的運土都盡可能下坡運并且距離不長，坡度應隨地形而變，并且與既有排水系統(tǒng)的升，降方向一致。在山區(qū)，坡度要使得挖填平衡以使總成本最低。

10、在平原或草原地區(qū)，坡度與地表近似平行，但高于地表足夠的高度，以利于路面排水，若有必要，可利用風力來清除表面積雪。如公路接近或沿河流走行，縱坡現(xiàn)的高度由預期洪水位來決定。無論在何種情況下，平緩的坡度現(xiàn)要比由短直線段連接短豎曲線構成的不斷變向的坡度線好得多。</p><p>　　由上坡向下坡變化的路段應設在挖方路段，而由下坡向上坡變化的路段應設在填方路段。這樣的線形設計較好往往可以避免形成與現(xiàn)狀地貌相反的土堆或是凹地

11、。在挖填方平衡相比，在確定縱坡線時，其他考慮則重要得多。</p><p>　　城市項目往往比農(nóng)村項目要求對控制要素進行更詳盡的研究，對高程進行更細致的調(diào)整。一般來說，設計與現(xiàn)有條件相符的坡度較好，這樣可避免一些不必要的花費。</p><p>　　在坡度的分析和控制中，坡度對機動車運行費用的影響是最重要的考慮因素之一。坡度增大，油耗顯然增大，車速就要減慢。一個較為經(jīng)濟的方案則可使坡度減小而增

12、加的年度成本與坡度不減而增加的車輛運行年度成本之間相平衡。這個問題的準確解決方法取決于對交通流量和交通類型的了解，這只有通過交通調(diào)查方能獲知。</p><p>　　在不同的州，最大縱坡也相差懸殊，AASHTO建議由時間車速和地形來選擇最大縱坡。現(xiàn)行設計以設計車速為70mil/h時最大縱坡為5%，設計車速30mil/h時，根據(jù)地形不同，最大縱坡一般為7%---12%。</p><p>　　當

13、采用較長的持續(xù)爬坡時，在沒有為慢行車輛提供爬坡道時，坡長不能夠超過臨界坡長。臨界坡長可從3%縱坡的1700ft變化至8%縱坡的500ft。</p><p>　　持續(xù)長坡的坡度必須小于公路任何一個端面的最大坡度，通常將長的持續(xù)單一縱坡斷開，設計成低部為一陡坡，而接近坡頂則讓坡度減小。同時要避免由于縱斷面傾斜而造成的視野受阻。</p><p>　　高速公路的最大縱坡為9%，只有當路面排水成問題

14、時，如水必須排至邊溝或排水溝，最小坡度標準才顯示起重要性。這種情況下，AASHTO建議最小坡度為0.35%。</p><p><b>　　C.視距</b></p><p>　　為保證行車安全，公路設計必須似的駕駛員視線前方有足夠的一段距離，使他們能夠避讓以外的障礙物，或者安全地超車。視距就是車輛駕駛員前方可見的公路長度。安全視距具有兩方面含義：“停車視距”或“不超車視

15、距”或“超車視距”。</p><p>　　有時，大件物體也許會掉到路上，會對撞上去的車輛造成嚴重的危害。同樣，轎車或卡車也可能會被一溜車輛阻在車道上。無論是哪種情況發(fā)生，合理設計要求駕駛員在一段距離以外就能看見這種險情，并在撞上去之前把車剎住。此外，認為車輛通過離開所行駛的車道就可以躲避危險的想法是不安全的。因為這會導致車輛失控或是與另一輛車想撞。</p><p>　　停車視距由兩部分組成

16、：第一部分是當駕駛員發(fā)現(xiàn)障礙物而作出制動之前駛出的一段距離，在這一察覺與反應階段，車輛以其初始速度行駛；第二部分是駕駛員剎車后車輛所駛過的一段距離。第一部分停車視距取決于車速及駕駛員的察覺時間和制動時間。第二部分停車視距取決于車速，剎車，輪胎，路面的條件以及公路的線形和坡度。</p><p>　　在雙車道公路上，每間隔一定距離，就應該提供超越慢行車輛的機會。否則，公路容量將降低，事故將增多，因為急燥的駕駛員在不能

17、安全超車時冒著撞車危險強行超車，能被看清的容許安全超車的前方最小距離叫做超車視距。</p><p>　　駕駛員在做出是否超車的決定時，必須將前方的能見距離與完成超車動作所需的距離對比考慮。影響他做出決定的因素是開車的小心程度和車輛加速性能。由于人與人的顯著差別，主要是人的判斷和動作而不是力學定理決定的超車行為隨著駕駛員的不同而大不相同。為了確立超車視距值，工程人員觀察了許多駕駛員的超車行為。在1938---194

18、1年間，進行了建立超車視距標準的基本調(diào)查。假設操作條件如下：</p><p><b>　　被超車輛勻速行駛。</b></p><p>　　超車在進入超車區(qū)時減速行駛在被超車后。</p><p>　　當?shù)竭_超車區(qū)時，駕駛員需一短時間來觀察超車區(qū)，并開始超車。</p><p>　　面對相向車輛，在一個延遲的啟動和一個匆忙的拐

19、彎的動作中，完成超車。在超車過程中，超車在超車道上加速，起平均速度比被超車快10mil/h。</p><p>　　當超車返回到它原來的車道上時，在它與另一車道上的相向車輛之間必須有一定的安全距離。</p><p>　　以上五項之和就是超車視距。</p><p><b>　　附錄2</b></p><p>　　Geomet

20、ric Design of Highways</p><p>　　A. Alignment Design</p><p>　　The alignment of a road is shown on the plane view and is a series of straight lines called tangents connected by circular curves. I

21、n modern practice it is common to interpose transition or spiral curves between tangents and circular curves.</p><p>　　Alignment must be consistent. Sudden change from flat to sharp curves and long tangents

22、followed by sharp curves must be avoided; otherwise, accident hazards will be created. Likewise, placing circular curves of different radii end to end (compound curves) or having a short tangent between two curves is poo

23、r practice unless suitable transitions between them are provided. Long, flat curves are preferable at all times, as they are pleasing in appearance and decrease possibility of future obsolesce</p><p>　　A veh

24、icle traveling in a curved path is subject to centrifugal force. This is balanced by an equal and opposite force developed through superelevation and side friction. Form a highway design standpoint, both superelevation a

25、nd side friction cannot exceed certain maximums, and these controls place limits on the sharpness of curves that can be used with a design speed.</p><p>　　Usually the sharpness of a given circular curve is i

26、ndicated by its radius. However, for alignment design, sharpness is commonly expressed in terms of degree of curve, which is the central angle subtended by a 100-ft length of curve. Degree of curve is inversely proportio

27、nal to the radius.</p><p>　　Tangent sections of highways carry normal cross slope; curved sections are superelevated. Provision must be made for gradual change from one to the other. This usually involves ma

28、intaining the center line of each individual roadway at profile grade while raising the outer edge and lowering the inner edge to produce the desired superelevation. Where the alignment consists of tangents connected by

29、circular curves, introduction of superelevation is begun on tangent before the curve is reached, and </p><p>　　If a vehicle travels at high speed on a carefully restricted path made up of tangents connected

30、by sharp circular curve, riding is extremely uncomfortable. As the car approaches a curve, superelevation begins and the vehicle is tilted inward, but the passenger must remain vertical since there is no centrifugal forc

31、e requiring compensation. When the vehicle reaches the curve, full centrifugal force develops at once, and pulls the rider outward from his vertical position. To achieve a position of </p><p>　　Easement curv

32、es have been used by the railroads for many yeas, but their adoption by highway agencies has come only recently. This is understandable. Railroad trains must follow the precise alignment of the tracks, and the discomfort

33、 described here can be avoided only by adopting easement curves. On the other hand, the motor-vehicle operator is free to alter his lateral position on the road and can provide his own easement curve by steering into cir

34、cular curves gradually. However, this weaving w</p><p>　　For the same radius circular curve, the addition of easement curves at the ends changes the location of the curve with relation to its tangents; hence

35、 the decision regarding their use should be made before the final location survey. They point of beginning of an ordinary circular curve is usually labeled the PC (point of curve) or BC (beginning of curve). Its end is m

36、arked the PT (point of tangent) or EC (end of curve). For curves that include easements, the common notation is, as stationing inc</p><p>　　On two-lane pavements provision of a wilder roadway is advisable on

37、 sharp curves. This will allow for such factors as ⑴ the tendency for drivers to shy away from the pavement edge, ⑵ increased effective transverse vehicle width because the front and rear wheels do not track, and ⑶ added

38、 width because of the slanted position of the front of the vehicle to the roadway centerline. For 24-ft roadways, the added width is so small that it can be neglected, Only for 30mph design speeds and curves sharp</p&

39、gt;<p>　　B. Grades</p><p>　　The vertical alignment of the roadway and its effect on the safe and economical operation of the motor vehicle constitute one of the most important features of road design

40、. The vertical alignment, which consists of a series of straight lines connected by vertical parabolic or circular curves, is known as the “grade line.” When the grade line is increasing from the horizontal it is known a

41、s a “minus grade.” In analyzing grade controls, the designer usually studies the effect of change in grade on</p><p>　　In the establishment of a grade, an ideal situation is one in which the cut is balanced

42、against the fill without a great deal of borrow or an excess of cut to be wasted. All hauls should be downhill if possible and not to long. The grade should follow the general terrain and rise and fall in the direction o

43、f the existing drainage. In mountainous country the grade may be set to balance excavation against embankment as a clue toward least overall cost. In flat or prairie country it will be approxi</p><p>　　Chang

44、es of grade from plus to minus should be placed in cuts, and changes from a minus grade to a plus grade should be placed in fills. This will generally give a good design, and many times it will avoid the appearance of bu

45、ilding hills and producing depressions contrary to the generally give a good design, and many times it will avoid the appearance of building hills and producing depressions contrary to the general existing contours of th

46、e land. Other considerations for determining the grade </p><p>　　Urban projects usually require a more detailed study of the controls and finer adjustment of elevations than do rural projects. It is of best

47、to adjust to grade to meet existing conditions because of the additional expense of doing otherwise.</p><p>　　In the analysis of grade and grade control, one of the most important considerations is the effec

48、t of grades on the operating costs of the motor vehicle. An increase in gasoline consumption and a reduction in speed are apparent when grades are increased. An economical approach would be to balance the added annual co

49、st of grade reduction against the added annual cost of vehicle operation without grade reduction. An accurate solution to the problem depends on the knowledge of traffic volume and ty</p><p>　　While maximum

50、grades vary a great deal in various states, AASHTO recommendations make maximum grades dependent on design speed and topography. Present practice limits grades to 5 percent of a design speed of 70 mph. For a design speed

51、 of 30 mph, maximum grades typically range from 7to 12 percent, depending on topography.</p><p>　　Wherever long sustained grades are used, the designer should not substantially exceed the critical length of

52、grade without the provision of climbing lanes for slow-moving vehicles. Critical grade lengths vary from 1700 ft for a 3 percent grade to 500 ft for an 8 percent grade.</p><p>　　Long sustained grades should

53、be less than the maximum grade used on any particular section of a highway. It is of preferred to break the long sustained uniform grade by placing steeper grades at the bottom and lightening the gr4ade near the top of t

54、he ascent. Dips in the profile grade in which vehicles may be hidden from view should also be avoided.</p><p>　　Maximum grade for highway is 9 percent. Standards setting minimum grades are of importance only

55、 when surface drainage is a problem as when water must be carried away in a gutter or roadside ditch. In such instances the AASHTO suggests a minimum of 0.35%.</p><p>　　C. Sight Distance</p><p>

56、;　　For safe vehicle operation, highway must be designed to give drivers a sufficient distance of clear vision ahead so that they can avoid unexpected obstacles and can pass slower vehicles without danger. Sight distance

57、is the length of highway visible ahead to the driver of a vehicle. The concept of safe sight distance has two facts: “stopping” (or “nonpassing”) and “passing”.</p><p>　　At times large objects may drop onto

58、a roadway and will do serious damage to a motor vehicle that strikes them. Again a car or truck may be forced to stop in the traffic lane in the path of following vehicles. In either instance, proper design requires that

59、 such hazards become visible at distances great enough that drivers can stop before hitting them. Furthermore, it is unsafe to assume that one oncoming vehicle may avoid trouble by leaving the lane in which it is traveli

60、ng, for this might resu</p><p>　　Stopping sight distance is made up of two elements. The first is the distance traveled after the obstruction comes into view but before the driver applies his brakes. During

61、this period of perception and reaction, the vehicle travels at its initial velocity. The second distance is consumed while the driver brakes the vehicle to a stop. The first of these two distances is dependent on the spe

62、ed of the vehicle and the perception time and brake-reaction time of the operator. The second distance dep</p><p>　　On two-lane highways, opportunity to pass slow-moving vehicles must be provided at interval

63、s. Otherwise capacity decreased and accidents increase as impatient drivers risk head-on collisions by passing when it is unsafe to do so. The minimum distance ahead that must be clear to permit safe passing is called th

64、e passing sight distance.</p><p>　　In deciding whether or not to pass another vehicle, the driver must weigh the clear distance available to him against the distance required to carry out the sequence of eve

65、nts that make up the passing maneuver. Among the factors that will influence his decision are the degree of caution that he exercises and the accelerating ability of his vehicle. Because humans differ markedly, passing p

66、ractices, which depend largely on human judgment and behavior rather than on the laws of mechanics, vary con</p><p>　?、盩he overtaken vehicle travels at a uniform speed.</p><p>　?、睺he passing veh

67、icle has reduced speed and trails the overtaken one as it enters the passing section.</p><p>　?、砏hen the passing section is reached, the driver requires a short period of time to perceive the clear passing s

68、ection and to react to start his maneuver.</p><p>　?、碢assing is accomplished under what may be termed a delayed a delayed start and a hurried return in the face of opposing traffic. The passing vehicle accel

69、erates during the maneuver and its average speed during occupancy of the left lane is 10 mph higher than that of the overtaken vehicle.</p><p>　?、礧hen the passing vehicle returns to its lane there is a suita