Zhou Lei

(Hunan Xiangtong Engineering Consulting Co.,LTD,Changsha,Hunan 410000)

Abstract：The roadbed will encounter many disease problems during the expansion process,including uneven settlement,lateral displacement and cracks of the roadbed.It is necessary to take effective measures to control it in time.This paper studies and analyzes the main diseases and causes of highway widening roadbed,and proposes specific control methods for reference.

Keywords：expressway;widening;subgrade;main disease;control method

Expressway engineering is a complex,cumbersome and systematic project.The expressway not only promotes the rapid development of the national economy,but also strengthens the economic,political and cultural exchanges between regions,thereby improving people's living standards[1].As an important part of the entire highway structure,to strengthen the quality control of roadbed can improve the stability of the roadbed,and avoid quality problems.However,during the construction of the roadbed,some diseases will often be encountered,and the relevant staff are required to carefully analyze the reasons and adopt certain control methods to improve the quality.

I.Main Roads and Causes of Highway Widening Roadbed

(i)Disease Form

1.Differential Settlement between New and Old Roadbeds

With the continuous advancement of the project,the old roadbed has basically completed the consolidation work,while the new roadbed consolidation is in an ongoing state.At this time,the settlement of the new roadbed is significantly higher than that of the old roadbed.If this special form is maintained all the time,the problem of the slippage of the new roadbed and the old roadbed will inevitably occur.At the same time,the new roadbed will be unstable and eventually develop into differential settlement.In addition,differential settlement can cause deformation of the surface of the subgrade,causing structural damage to the surface of the road.Highways of different materials also have different ways of disease manifestation.For example,asphalt pavement will have many longitudinal cracks at the junction,when the situation is serious,the surface layer will even break.The cement concrete pavement will have extensibility,longitudinaldislocation and crack at the junction,and with the continuous development of settlement,the phenomenon of floor cavitation will occur[2].In addition,the differential settlement between the old and new subgrade is proportional to the height of the subgrade,see figure 1 for details.

Figure 1 Differences between the New and Old Subgrade Settlement and Height

2.Pavement Crack

In the process of continuous development of subgrade differential settlement,the structural performance of expressways will continue to decline.If the road surface condition index of the road and the bearing capacity of the structure cannot meet the relevant standards,cracks will occur,which will lay a huge safety hazard for people’s driving safety.The basic principle of generating longitudinal cracks is that due to the continuous strengthening of the differential settlement of the subgrade,cracks will first occur around the joint at the top of the subgrade,and then redistributed under the action of tensile stress,while the bottom of the longitudinal crack is the weakest.At the location,and in the process of continuousdevelopmentofsubgradesettlement problems,the cracks will also become longer and deeper.At the same time,because the bottom of the base layer above the subgrade joint is in the tension zone,if the roadbed is cracked,the stress conditions are not good,and the base layer will be cracked,showing a trend from the bottom to the top,until the road surface appears obvious cracks[3].The manifestations of pavement cracks are mainly in the following aspects:First,the cracks are frequently in the widened section of the high embankment;second,the widened parts are mostly in special areas such as low-lying and soft soil layers,so the overall stability deviation;Mainly concentrated in the old roadbed,and the specifications of the cracks are also different;the fourth is that the cracks will generally appear three months after the end of construction,so there will be differences in the stability time.

(ii)Reasons for Formation

In general,the influencing factors of the roadbed traits of expressway widening project mainly include the difference of roadbed anti-deformation ability,the uncoordinated deformation between new and old roadbed and the stability of roadbed,as well as the hydrological conditions and geological conditions around the road.Wait.In other words,broadening the roadbed disease is determined by a combination of factors,including the following aspects.

1.Differential Settlement Between New and Old Roadbeds

From the perspective of the newer roadbed,the old roadbed has basically completed the settlement work after a long period of high-load operation.After the completion of the new roadbed,it will take some time and construction operation to settle.This will lead to uneven settlement in the concentrated area.Cracks and displacements will occur at the junction of the old and new roadbeds,which will eventually affect performance of the whole highway structure.

2.Differences in Carrying Capacity between New and Old Roadbeds

In the construction process,the new and old roadbeds face different basic conditions and the processing methods are also complicated.Under this circumstance,the load stress of the new roadbed will be significantly different from that of the old road-bed.After the completion of the widening project,the expressway will increase significantly compared with the previous load,and the difference between the bearing capacity of the subgrade will become more apparent.At the same time,the expressions of the differences are different at different degrees.The slight difference will cause the road structure to undergo cumulative deformation,and the serious difference will cause the roadbed to collapse and slip.

3.Difference in Bonding Strength between New and Old Roadbeds

In the construction process of roadbed widening,in order to ensure good coordination between the old and new roadbeds,geotechnical materials are usually used for reinforcement at the junction,but in terms of actual construction,there will be factors such as construction and geological differences.The effect of the reinforcement is not ideal.At the same time,the difference in strength between the junctions will cause the weaker body to be weaker due to load stress,and local uneven settlement will occur,which will affect the stability and robustness of the entire road.

II.Control Method of Highway Widening Roadbed Diseases

In view of the current application of roadway disease control methods for highways at home and abroad,the author believes that the following methods can effectively reduce the differential settlement of old and new roadbed:strengthening the quality control of raw materials,excavation of slope steps,laying of geogrids,ensuring degree of compaction,handling of new and old roadbed junctions,etc.

(i) Strengthening the Quality Control of Raw Materials

As an important part of the whole roadbed widening project,raw material management plays a decisive role in the quality of expressways and people's travel safety.In this regard,it is necessary for the relevant departments of the enterprise to strengthen the quality control of raw materials,adopt the method of bidding,select the best material supplier,and at the same time,for the selection of materials,it is necessary to go deep into the processing factory to check the supplier's business license and material qualification certificate.The raw materials specified in the design drawings ensure the high degree of material quantity,specification,quality and shape.In addition,regarding the storage of raw materials,it should be stored reasonably according to the precautions of the supplier,and should be placed in a ventilated and dry warehouse as much as possible.At the same time,the classification and entry and exit records should be done.Also,before entering the market,you need to conduct another inspection to ensure that the quality is passed before you can use it.Only in this way can we effectively improve the strength of the roadbed and ensure the quality of the road.

(ii)Excavation of Slope Steps

In the construction process of road widening,the humus soil and soft soil at the original roadbed slope should be treated to ensure the firmness of the weak joint surface.For the treatment method of the joint surface,the steps can be excavated to compact the steps,and at the same time ensure the width within the design standard to achieve an effective combination of the old and new roadbed.Also note that the step excavation should be carried out immediately after the excavation of the slope to avoid damage to the slope caused by external factors.By excavating the steps on the slope,the humus soil and the soft soil at the original roadbed slope can be disposed in time,and the reinforcement can be carried out by filling and compacting.And it can make the joint part of the new and old roadbed have more contact area and enhance the shearing ability of the joint surface[4].In addition,the lateral step surface facilitates the effective use of the geogrid,and provides a length step shape and an anchoring platform to improve the splicing quality of the roadbed.There are also many types of steps,such as inclined type,internal excavation type and standard type,which need to be selected and determined according to the actual conditions of the project during the construction process.

(iii) Laying of Geogrid

The layering of geogrids in the splicing of new and old roadbeds is also an important means to improve the integrity and stress characteristics of the roadbed.By laying the geogrid,the shear stress and tensile stress of the subgrade can be effectively reduced,and the lateral settlement difference generated by the top surface of the subgrade can be significantly reduced,and the lateral displacement of the subgrade can be avoided.

(iv) Ensuring Degree of Compaction

Compaction is a key element in the quality control of highway widening disease.It requires construction workers to use layered filling and compaction methods in the construction process.At the same time,combined with strong eccentricity and eccentric vibration,the whole road surface is compacted.Especially for the junction of new and old roadbeds,it is necessary to further increase the degree of compaction and avoid the problem of widening roadbed diseases.

(v) Handling of New and Old Roadbed Junctions

Geogrids and geocells are commonly used civil materials in the widening of highway subgrades.Among them,the geogrid has strong friction,can generate a large friction with the contact surface,and control the vertical stress of the widened soil,thereby improving the bearing capacity of the soil and reducing the occurrence of settlement;The geogrid has the function of locking the soil,enhancing the shear resistance of the soil and avoiding the problem of lateral deformation of the soil[5].At the same time,it is also possible to adopt the method of lengthening and encrypting powder jet piles or jet grouting piles to give priority to the disease problems occurring during the construction of highway subgrade widening project,and also has a very significant effect on the treatment of differential settlement of new and old roadbeds.

III.Conclusion

In summary,the highway as the'lifeline'of the country promotes economic,cultural and political exchanges between various regions and plays a vital role in the development of the national economy.Quality control of construction.Sometimes,under some special requirements,the roadbed of the expressway needs to be widened.However,in the specific construction process,various factors are often encountered to cause the road to widen the roadbed,including differential settlement and cracks,which require attention from relevant departments.At this time,measures such as strengthening the quality control of raw materials,excavation of slope steps,laying of geogrids,ensuring the degree of compaction,and handling of new and old roadbed junctions can be taken to effectively solve the problem of widening roadbed diseases and further improving highways.The overall quality makes people travel safely.