URBAN ROAD DESIGN Lecture 11 Subgrade and Pavement

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 In the past cobblestones and granite setts were extensively used, but these surfaces have mostly been replaced by asphalt or concrete.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: Ø Clay soil which contains the value of Liquid Limit more than 80% and/or Plasticity Index more than 55%; Ø Having the value of Lost On Ignition (LOI) more than 2. 5%; Ø It is flammable materials (oily), and organically clay soil; Ø Contain lots of rotten roots, grass and other vegetation; Ø Considered as unstainable soil or toxic and categorized as peat soil; Ø 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: Ø California Bearing Ratio (CBR), as accordance to: BS 1377: Part 4 1990, ASTM D 1883 -05 or AASHTO T-193 Ø Compaction Test, as accordance to: BS 1377: Part 4 1990, ASTM D -698 or AASHTO T-99 Ø Liquid Limit (LL) and Plastic Limit (PL) test, as accordance to: BS 1377: Part 2 1990, ASTM D-4318 or AASHTO T-89 Ø 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 Ø 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. Ø 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 Ø 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. Ø 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 • Degree of compaction- Highway

Degree of compaction • Degree of compaction-Highway- Urban road

Pavement type • Basically, all hard surfaced pavement types can be categorized into two groups. Ø 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". Ø 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. Ø 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. Ø 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》(CJJ 169 -2012)(Code for pavement design of urban road)

Design parameters

Selection of pavement

Design of rigid pavement

Questions?