School of Civil Engineering URBAN ROAD DESIGN Lecture 11- Subgrade and Pavement Design School of Civil Engineering 孙仁娟 2012.5.15

Objectives Concepts of subgrade and pavement; Design of the subgrade; Type and composition of rigid and flexible pavement; Variables involved in and be able to calculate required thickness of rigid and flexible pavements.

Concepts Subgrade is one of the most crucial part of embankment fills or natural surface just below the sub-base or lower sub-base of road pavement and shoulder. The surface above the subgrade is known as the formation level or finishing level. Subgrade is the in situ material upon which the pavement structure is placed or constructed at selected location.

Concepts Pavement: is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway. In the past cobblestones and granite setts were extensively used, but these surfaces have mostly been replaced by asphalt or concrete. Such surfaces are frequently marked to guide traffic.

Subgrade Although there is a tendency by looking at the pavement performance in terms of pavement structure and mix design alone, the subgrade can often be the overriding factor in pavement performance. Unsuitable soil materials for subgrade (or embankment fills) are as follows: Clay soil which contains the value of Liquid Limit more than 80% and/or Plasticity Index more than 55%; Having the value of Lost On Ignition (LOI) more than 2.5%; It is flammable materials (oily), and organically clay soil; Contain lots of rotten roots, grass and other vegetation; Considered as unstainable soil or toxic and categorized as peat soil; Soil which is soft and unstable because it is too wet or dry which makes it difficult to compact properly.

Testing for Subgrade There are several testing method that were used to test the subgrade layer. The notable and most recommended test (among others) to be carryout are as follows: California Bearing Ratio (CBR), as accordance to: BS 1377: Part 4 1990, ASTM D1883-05 or AASHTO T-193 Compaction Test, as accordance to: BS 1377: Part 4 1990, ASTM D-698 or AASHTO T-99 Liquid Limit (LL) and Plastic Limit (PL) test, as accordance to: BS 1377: Part 2 1990, ASTM D-4318 or AASHTO T-89 Lost On Ignition (LOI) test, as accordance to: BS 1377: Part 3 1990 or AASHTO T-267

Performance of Subgrade The subgrade’s performance generally depends on two interrelated characteristics: Load Bearing Capacity The subgrade must be able to sustain loads transmitted from the pavement structure. The load bearing capacity is frequently affected by the types of soil, moisture content, and degree of compaction. A subgrade that can sustain a highly sum of loading without an excessive deformation was considered good quality. The types of soil especially from gravel type considered the best and from peat type considered as the worst material. Moisture content of soil is also important and determine by conducting the soil compaction test at lab as to find out which type contains more water. The degree of compaction normally reflect to the method of compaction used at construction site, by means of machinery and the numbers of passes.

Performance of Subgrade Changing in Volumes In most cases, soils will undergo some amount of changes in volume when exposed to excessive moisture, rise in temperature or in freezing conditions. For instance, some clay soils would shrink and swell depending upon its moisture content, whereas soils with excessive fines may be susceptible to frost heave in freezing areas. As a conclusion, the subgrade must be form properly to prevent any possible damage to the road pavement. Factors of choosing the right or suitable materials, affecting the strength, materials specification, materials classification, and method of testing is vital for the road construction especially in earthworks stage.

Subgrade design 一、路基设计应满足如下要求： 1、路基路面应根据道路功能、道路类型、交通量，结合沿线气候、地形、地质及路用材料等自然条件进行设计，保证其具有足够的强度、稳定性和耐久性。同时，路面面层应满足平整和抗滑的要求。 2、路基必须密实、均匀，应具有足够的强度、稳定性、抗变形能力和耐久性，应结合当地气候、水文和地质条件，采取防护措施，以保证路基稳定。 3、路基工程应遵循节约用地、保护环境的原则，减少对自然、生态环境的影响。 4、路基断面形式应与沿线自然环境和城市环境相协调，避免深挖、高填；同时应因地制宜，合理利用当地材料和工业废料修筑路基。 5、路基设计应重视排水系统、防排水设施和防护设施的设计。 6、对于特殊地质和水文条件的路基，应查明情况，分析危害，结合当地成功经验，采取综合措施，增强工程可靠性

Subgrade design 二、路基设计回弹模量应满足如下要求： 1、路床顶面土基设计回弹模量值，对快速路和主干路应不小于 30MPa；对次干路和支路应不小于 20MPa。不满足上述要求时，应采取措施提高土基的回弹模量。 2、路基设计中，应充分考虑道路运行中的各种不利因素，采取措施减小土基设计回弹模量的变异性，保证其持久性。 3、道路路基应处于干燥或中湿状态，对于潮湿或过湿路基，必须采取措施改善其湿度状况或适当提高土基回弹模量。

Subgrade design 三、路基设计高度应满足如下要求： 1、路基高度设计应使路肩边缘的路基相对高度不低于路基土的毛细水上升高度，并考虑冰冻的影响。 2、河堤顶及浸水路段的路基边缘标高， 应不低于路基设计洪水频率的水位加雍水高、波浪侵袭高度和 0.5m的安全高度。

Subgrade design 四、土质路基压实度应不低于表 12.2.4 规定，否则应采取处治或换填等措施。符合以下工程条件时，可通过试验路检验或综合论证，在保证路基强度和稳定性要求的前提下，适当降低标准。 1、 特殊干旱或特殊潮湿地区； 2 、为保护管线，沟槽回填压实确有困难； 3 、明确禁止大型车和拖挂车等重载车型进入的主干路； 4 、专用非机动车道、人行道。

Subgrade design Degree of compaction- Highway

Degree of compaction Degree of compaction-Highway- Urban road

Subgrade design 五、深挖、高填路基边坡路段，必须针对其工程特性进行路基防护设计。对存在稳定性隐患的边坡，必须进行稳定性分析，采取防护措施。

Pavement type Basically, all hard surfaced pavement types can be categorized into two groups.  Flexible pavements are those which are surfaced with asphalt materials.  These can be either in the form of pavement surface treatments ( generally found on lower volume roads) or, HMA surface courses (generally used on higher volume roads such as the Interstate highway network). These types of pavements are called "flexible" since the total pavement structure "bends" or "deflects" due to traffic loads.  A flexible pavement structure is generally composed of several layers of materials which can accommodate this "flexing".  Rigid pavements are composed of a PCC surface course.  Such pavements are substantially "stiffer" than flexible pavements due to the high modulus of elasticity of the PCC material.  Further, these pavements can have reinforcing steel, which is generally used to reduce or eliminate joints.

Distribute load Each of these pavement types distributes load over the subgrade in a different fashion.  Rigid pavement, because of PCC's high elastic modulus (stiffness), tends to distribute the load over a relatively wide area of subgrade .  The concrete slab itself supplies most of a rigid pavement's structural capacity.  Flexible pavement uses more flexible surface course and distributes loads over a smaller area.  It relies on a combination of layers for transmitting load to the subgrade. 

Distribute load

Maitenence Flexible pavements generally require some sort of maintenance or rehabilitation every 10 to 15 years.  Rigid pavements, on the other hand, can often serve 20 to 40 years with little or no maintenance or rehabilitation. 

Rigid Pavement Typical Applications High volume traffic lanes Freeway to freeway connections Exit ramps with heavy traffic

Advantages of Rigid Pavement Good durability Long service life Withstand repeated flooding and subsurface water without deterioration

Disadvantages of Rigid Pavement May lose non-skid surface with time Needs even sub-grade with uniform settling May fault at transverse joints Requires frequent joint maintenance

Flexible Pavement Typical Applications Traffic lanes Auxiliary lanes Ramps Parking areas Frontage roads Shoulders

Advantages to Flexible Pavement Adjusts to limited differential settlement Easily repaired Additional thickness added any time Non-skid properties do not deteriorate Quieter and smoother Tolerates a greater range of temperatures

Disadvantages of Flexible Pavement Loses some flexibility and cohesion with time Needs resurfacing sooner than PC concrete Not normally chosen where water is expected

Design Code 城镇道路路面设计规范》(CJJ169-2012)(Code for pavement design of urban road)

关于发布行业标准《城镇道路路面设计规范》的公告 现批准《城镇道路路面设计规范》为行业标准，编号为CJJ169-2012，自2012年7月1日起实施。其中，第6.2.5条为强制性条文，必须严格执行。 本规范由我部标准定额研究所组织中国建筑工业出版社出版发行。 中华人民共和国住房和城乡建设部 　　　　　　 二〇一一年十二月十九日

Design parameters

Selection of pavement

Design of flexible pavement 1、沥青混凝土路面可分为面层、基层和垫层三个主要层次。 2、沥青混凝土面层可分为单层、双层或三层。采用三层铺筑的沥青面层，自上而下称为上面层（也称表面层） 、中面层和下面层。面层应具有足够的结构强度、高温稳定性、低温抗裂性、抗疲劳、抗水损害及耐磨、平整、抗滑、低噪音等良好的表面特性。 3、沥青混凝土路面设计的内容应包括：面层类型选择和结构层组合设计；路面内部排水设计；各结构层材料组成设计；材料设计参数确定；结构层厚度计算以及经济评价和最终方案的选择等。

Design of flexible pavement 沥青混凝土路面设计方法与指标的选用应满足如下规定： 1、沥青路面设计应选用多种损坏模式作为临界状态和选用多项设计指标。 2、沥青路面设计宜采用双圆垂直均布荷载作用下的多层弹性连续体系理论，按设计荷载所产生的应力、应变和位移量不超过路面任一结构中材料所容许的范围来选择确定路面结构层的组合及其尺寸。

Design of flexible pavement 沥青混凝土路面结构层组合应满足以下要求： 1、按道路的等级及交通繁重程度选择面层的类型和厚度； 2、满足对各结构层（面层、基层和垫层）的相关功能要求 3、适应各结构层的荷载应力分布特性； 4、要考虑当地水文状况的不利影响； 5、结构层数和各层厚度要适当，以便利施工。

Design of rigid pavement 1. 水泥混凝土路面结构应具有足够强度，良好的耐久性，其表面应满足平整、抗滑和排水的要求。 2. 水泥混凝土路面设计的内容应包括：面层类型选择，结构层组合、接缝构造、配筋和排水设计，各结构层材料组成设计，路面表面特性设计以及工程技术经济评价等。 3. 水泥混凝土路面结构应以行车荷载和温度梯度综合作用产生的疲劳断裂作为设计标准。 4. 水泥混凝土路面通常由面层、基层、垫层组成。 5. 水泥混凝土面层应具有足够的强度、耐久性、表面抗滑、耐磨、平整。面层通常选用设接缝的普通水泥混凝土，其他面层类型可根据适用条件按表选用。 6. 选用基层应具有足够的刚度、水稳定性和耐冲刷性。 7. 当路面总厚度小于最小防冻厚度，或路基水文条件不佳时，需设置垫层。 8.水泥混凝土路面通常需设置纵、横向接缝。纵向接缝与路线中线平行，并应设置拉杆。拉杆应采用螺纹钢筋，设在板厚中央。横向接缝一般分为横向缩缝、胀缝和横向施工缝，快速路、主干路的横向缩缝应加设传力杆；在邻近桥梁或其它固定构筑物处、板厚改变处、小半径平曲线等处，应设置胀缝。

Design of rigid pavement

Design of rigid pavement 9. 混凝土面层自由边缘，承受繁重交通的胀缝、施工缝，小于 90°的面层角隅，下穿市政管线路段，以及雨水口和地下设施的检查井周围，面层应配筋补强。 10. 钢筋混凝土面层的配筋量，连续配筋混凝土面层的横向钢筋配筋量，按钢筋允许拉力不小于基层摩阻所产生的拉力为准则。 连续配筋混凝土面层纵向钢筋配筋率的设计，根据允许裂缝间距和最大缝隙宽度，以及钢筋拉应力小于其屈服强度三个设计指标确定。 11. 面层水泥混凝土的抗弯拉强度不得低于 4.5MPa，快速道、主干道和重交通的其他道路的抗弯拉强度不得低于 5.0MPa。水泥用量不得小于 300kg/m3（非冰冻地区）或320kg/m3（冰冻地区） 。冰冻地区的混凝土中必须掺加引气剂。钢纤维混凝土的集料最大粒径不大于钢纤维长度的 2/3。钢纤维的抗拉强度不宜小于 600 级。水泥用量不得低于 360kg/m3（非冰冻地区）或 380kg/m3（冰冻地区） 。混凝土预制块的抗压强度不宜低于 50MPa（非冰冻地区）或 60MPa（冰冻地区） 。

Strengthen and reconstruction of pavement 当路面的平整度、抗滑能力、结构损坏和承载能力等使用性能退化到某一规定的限值，或者其承载能力不能满足未来交通的需求时，应采取结构补强或改建以恢复或提高其使用性能。 旧路面结构补强和改建方案，应充分把握旧路面的结构性状、使用历史，以及路面环境条件，并依据路面今后的交通需求，以及材料、施工技术、实践经验以及环境保护要求等，通过技术经济分析论证确定。 2. 旧路面使用性能的评定指标有路面平整度、抗滑能力、路面结构损坏状况、剩余承载力等。路面平整度宜采用车载式颠簸累积仪检测；小区域时可采用连续式平整度仪或三米直尺检测。路面抗滑能力采用横向力系数（SFC）测定仪测定，次干路和支路也可用摆式仪测定。路面结构状况宜采用先进快速设备调查，次干路和支路可以目测为主调查。路面弯沉采用落锤式弯沉仪或贝克曼梁测定。 3. 当路面平整度不佳，抗滑能力不足，但路面结构强度足够，结构损坏轻微完好时，沥青路面宜采用稀浆封层、薄层罩面等措施；水泥混凝土路面则采用刻槽（抗滑能力不足） 、板底灌浆和磨平错台等措施恢复路面表面使用性能。 路面结构破损较为严重或路面承载能力不能满足未来交通的需求时， 路面结构需采用加罩结构层予以补强。加罩结构层一般可采用水泥混凝土或沥青混合料两类。 路面结构破损严重，或路面纵、横坡需作较大调整时，宜采用改建方案，即将旧路面作为新路面结构层的垫层或下基层。

4. 沥青路面在加铺层铺筑前，局部松散龟裂处需挖去并修补，铣刨车辙隆起部分，清 除裂缝内杂物并予封填。 水泥混凝土路面在加铺层铺筑前，应更换断板；压浆填封板底脱空；清洗旧混凝土 表面；磨平错台，清除剥落碎块及接缝中的杂物，并重新封缝。水泥混凝土路面作 下基层或垫层时，旧混凝土碎块平面尺寸不大于 400mm，并用重型轮胎压路机碾 压使之固定地落在地基上。 5. 沥青路面的加罩层一般采用沥青混合料。 沥青加罩层厚度按补足路面结构层总承载 能力要求确定，新旧路面之间必须有很好的粘结。沥青路面的加罩水泥混凝土面层 属路面改建，旧沥青路面可作为水泥混凝土面路面结构层的基层或下基层。 6. 水泥混凝土路面的断板率较低、接缝传荷能力良好时，且路面纵、横坡基本符合要 求、板的平面尺寸和接缝布置合理，可选用直接式混凝土加铺层；否则，应采用分 离式水泥混凝土加铺层。 水泥混凝土路面足够强度且断板和错台病害少时，可选择直接加罩沥青面层的方 案，并根据道路所在地区的气候特点、交通荷载、旧路面的性状等情况，选择经济 有效的防反措施。

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