Road Damage Due to Expansive Soils : Survey of the Phenomenon and Measures
for Improvement
Dafalla, M. A .1, Shamrani, M.A.2,
1
Asst. Professor, 2 Professor,
Bugshan Research Chair in Expansive Soil (BRCES), Civil Engineering Department, King Saud University, Riyadh, Saudi Arabia, mdafalla@https://www.doczj.com/doc/04111469.html,.sa
ABSTRACT
This research work involved surveying major road damages attributed to expansive soils with particular focus on Saudi Arabia. Roads and light structures in various parts of this country experienced damage as a result of moisture increase and environment changes to subgrade soils. Strains in asphalt pavement due to volume change in soils can lead to cracking and attract water seepage to lower formations. An attempt was made to link the road damage to the soil conditions, road geometry and road drainage arrangements. The parameters influencing the swelling process including thickness of pavement, slope of the shoulder, adjacent topography and drainage setup are discussed. Recommendations to achieve better road performance are presented. Applications of cut–off walls and dressing layers are suggested for certain conditions.
INTRODUCTION
Expansive soils hit hard in arid and semi-arid region where drying and wetting are common and frequent. Swelling problems associated with buildings attracted wide attention in research during the last four decades while very little work was done for roads and pavements. Nelson and Miller (1997) surveyed the problems of expansive soils that are related to pavements. In general, the asphalt pavements perform better than rigid concrete pavements response to expansive soils due to the flexibility and tolerance of asphalt to some displacements. However, asphalt pavements were still found to suffer seriously due to expansive soils. Cracks develop in the asphalt surface as a result of frequent up and down movement and vehicles tire pressures. Once hair cracks are initiated, moisture and water seepage propagate more serious problems. Forms of damage and mechanisms of failure can be explained based on the possible subsurface and pavement conditions. Asphalt institute listed different kinds of distress
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and suggested the possible causes of each type. Among these they listed upheaval/swell distress and mentioned the possible cause is expansive soil or frost heave. Edge cracks due to shrinking or drying soil can also be attributed to expansive soils.
Maintenance work which vary from filling up cracks with asphalt emulsion slurry or emulsified asphalt to patch repair and overlays, cost multi million dollars worldwide. Maintenance work affects the integrity of the asphalt pavement, results in less intact pavement, and paves the way for further deterioration and damage. Many roads were abandoned or replaced due to the exercise of frequent maintenance. Saudi Arabian roads within North and North-west parts of the country are seriously affected. Al Ghatt and Tabuk cities are examples for most severe damage. Al-Shamrani and Dhowian (2003) highlighted the expansiveness of the clay in the region. The road connecting Tayma to Tabuk (now abandoned) is shown in FIG.1. and King Abdulaziz access road in AlGhat (still under periodic maintenance) is shown in FIG. 2.
FORMS OF CRACKS LINKED TO EXPANSIVE SOIL
Expansive soils can result in many types of asphalt cracking. These can be categorized in two phases: the swelling phase and the shrinkage phase. The following explanations show how they can occur. Transverse cracks occur when the subgrade of the road is of medium to high swelling potential and the heave is beyond the tolerance of the asphalt mix. This type occurs when a subsurface flow of moisture takes a transverse direction and a mound/depression feature is formed. Position of cracks is either on the top of the mount or the bottom of the depression. Longitudinal cracks take place either parallel to the edge or along the asphalt joint between lanes. Block cracks occur when the underneath of a large patch is subjected to moisture changes. Three new forms of damage are introduced. The first type is "yield" cracks and they occur when the asphalt is subjected to frequent up and down movements due to swelling and repeated heavy tire pressure. This is mostly taking the form of multiple parallel longitudinal cracks. The second type introduced is the "spot ridge" cracks and defined as those having a clearly defined center where damage is excessive. The third
FIG.1. Tayma–Tabuk Road (Abandoned)
FIG.2. King Abdulaziz Road- AlGhatt
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is the "green zone" cracks and these are related to the vicinity of landscaping and green areas. Figure 3 presents typical features of six types of crack associated with expansive soils. Longitudinal Crack Transverse Crack
Block Crack Yield Cracks
Spot Ridge Cracks
Green Zone Cracks
FIG.3. Crack types associated with expansive clays.
The frequency of occurrence of these crack types in Al Ghatt city was found to consist mostly of the green zone type, followed by the block type. Spot ridge, transverse and longitudinal types were found less frequent. Poor drainage system near the edge of the asphalt and watering the plants are serious factors causing frequent damage.
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SCALE OF DAMAGE Roads and highways are normally designed for periods of 20 to 30 years. The problems arising from expansive soils reduced the life time to less than half of this period. In some cases severe damage took place in the first few years. Example of this is the main access road to Al Ghatt city extending over a two kilometer distance from the express Qassim Highway. Every two years major maintenance is carried out. The total cost for preserving the road over 30 years is enough to construct a bridge over a two kilometer distance. The authorities tried different treatments including soil replacement to three meter depth and introduced gravel subgrade. All the trials to restore the road in a stable condition failed and the problem of heave continued to appear shortly after maintenance. The lesson learned from this site confirmed that the initial investment in protection works and appropriate design is the better way to have good functioning structure. The damage to main and secondary roads in Al Ghatt city is found very severe in the older parts of the city. Many new constructed main streets showed cracks and deterioration. The surface drainage of roads direct rainwater to the asphalt edges. Cracks due to expansive soils were found more close to edges. Proper drainage shall be taken care of for the road networks in Al Ghatt. The cycle of overlays and scraping the asphalt will continue unless special attention is paid to the expansive nature of the ground. The city authorities are very keen to have a green look and breeze of their area and resisted strongly the calls for avoiding introducing landscaping that would introduce water close to structure and road foundations. When realized that palm trees and plantations will cause problems for King Abdulaziz access road they utilized large pots to accommodate big trees. Figure 2 shows an alternative for green dual road islands. The damage to some parts of Al Ghatt city as reported by the Municipality of Al Ghatt officials is equal to 100% initial investment in road networks. The damage to roads due to expansive soils across the country of Saudi Arabia is in hundreds of million United States dollars (Dafalla & Al-Shamrani 2008). This is estimated based on annual maintenance and asphalt pavement replacement. Tabuk region contributes more to these costs.
GEOTECHNICAL PROPERTIES FOR THE STUDY AREA
The study area and survey in this publication covered the region of Al Ghatt in Saudi Arabia. Al Ghatt is a populated small city located 270 km to the Northwest of Riyadh at latitude 26o 32′ 42′′ N and longitude 43o 45′ 42′′ E. The city is situated within a large valley with surrounding high hills to the south and west. The subsurface formation at Al Ghatt is generally overlain by quaternary silt, sand and gravel layers followed by greyish brown to olive green silty to clayey Shale. The Shale belongs to Durma formation which is made of accumulation of mud deposits (Dhowian et al., 1990). The thickness of the overburden layer is variable across the town being very thin close to the hilly zone and increasing towards the valley (wadi ) centre. The geotechnical properties of the silty shale and clayey shale are investigated by several researchers and geotechnical firms in Saudi Arabia. Generalized soil profile for the area is shown in Fig. 4. Table 1 tabulates results of laboratory tests.
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Table 1.Geotechnical properties
of Al Ghatt clay (Dafalla et al 2008)
FIG.4. Generalized soil profile - Al Ghatt
The light structures including asphalt pavements, boundary walls are the most affected by upheaval forces and shrinkage. Single and two storey buildings also
suffered from expansive soils to variable degrees. The main factors contributing to the amount of damage include but not limited to: depth of foundation, type of foundation, stress level applied by the superstructure and subsurface soil conditions. The first author led a team from King Saud University, Riyadh, Saudi Arabia and conducted a general survey for damage in the area. Failures due to expansive soil problems took many forms in the area. Uplift and twisting of boundary walls for the residential villas are very common. This type of failure is reported frequently in Al Ghatt and found associated with water leakage or waste water disposal. Crack or failure intensity was found mild to the east of the town where the thickness of the overburden top soil is in excess of 1m and very severe to the west part where foundations are placed right on the shale formation. Failures due to expansive soil problem is classified by the author as given in table 2.
Table 2. Classification of Building Failures in Al Ghatt Region
Class of failure Description Occurrence Mild Distortion in the form of partial uplifting with minor cracks seen only in close vicinity ( 1 to 2 mm)
East of AlGhat Medium Diagonal cracks in the order of 2mm to 5mm. Minor wall tilting
East of AlGhat Severe Displacement and disintegration of structural units. Cracks in the order of 5mm to 10mm. Tilt observed.
West of AlGhat Very severe
Displacement and disintegration of structural units. Cracks in the order of centimeters. Major tilt and mis-alignment
West of AlGhat
After Dafalla & Al-Shamrani (2008)
Property General
Area Ranges
LL 40-70 PL 15-35 PI 20-40 S.G. 2.7-2.8 Free swell % 0.1-10 Swelling Pressure (kPa) 40-1000 Swell Index Cs 0.01-0.2 Percent clay 30-60
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Settlement or heave due to expansive soils causes serious distress in asphalt pavements leading to a substantial discomfort, safety hazard and vehicle damage (Xiong Zhang, 2010).
Regarding damage to pavement the situation is assessed based on the type of cracks and the area affected. These are helpful in deciding the mode and extent of maintenance needed. When hair cracks appear, further deterioration is expected within a very short time as water will have access to the underlying subgrade formation. Schemes studying frequency and count of cracks using imaging proved to be quicker and less time consuming than conventional and manual approaches. Works of Wangxin Xiao et al (2006) is an example of the current techniques in this regard.
In order to determine the root cause of distress a visual inspection is necessary and count of batches of damage areas will be useful. An example of a main street asphalt pavement survey in Al Ghatt is shown in Fig 5.
FIG.5. Abdul Rahman Bin Abdulaziz street survey of crack types ( Al Ghatt).
PARAMETERS INFLUENCING DAMAGE
There are many factors influencing damage to asphalt pavement. These include but not limited to subgrade soil type and mineralogy, road geometry, road drainage, and adjacent topography.
Subgrade Soil Mineralogy
This parameter is a significant factor and can be assessed during the geotechnical investigation phase. Index properties can be a useful guide but detailed mineralogy is necessary (Chen, 1988). Quantifying the expanding mineral content is essential together with oedometer testing in order to predict the level of risk. The works of Abduljawad (1991) is a good example of assessing mineralogy. Al Ghatt area is well known of expansive shale formation and data in this regard is presented in table 1.
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Road Geometry
The overall thickness of embankment and the pavement have a major role in controlling the hazards of expansion. It is noticed that along King Abdulaziz access road in Al Ghatt more damage is associated with less overall road thickness. Overburden soil pressure has a significant influence in the control of heave. Embankment thickness change of 50 cm can make a difference. Evidence of this is noted in a nearby market place founded on a slopping ground and also seen at Al Zulfi road. AL Zulfi road is elevated for flood and drainage reasons as it passes through low land areas. This road is elevated about 1.5 m above ground and is found trouble free except for some areas paved on thin embankment close to Al Ghatt. King Abdulaziz access road is found not affected by the expansive soil at the zone close to the intersection with the express highway connecting Riyadh to Qassim. This is due the thick embankment at this particular location.
Road Drainage
One of the most serious factors to be considered in road design when expansive soil is involved is the drainage. Allowing water to access the underneath of the asphalt pavement will result in immediate distress. Poor drainage is reported for many streets in Al Ghatt. The shoulders shall be less permeable and steep or impermeable directing water towards drains. Measures like cut off walls and slope dressing can be useful.
Adjacent Topography
The adjacent topography may affect the performance of the pavement if surface or subsurface water is directed towards the pavement. High ranges of hills close to King Abdulaziz access road are believed to boost the hydraulic head of subsurface water level and cause wetting to shale formation supporting the road. Control of subsurface flow can involve expensive measures including deep seated water proof layers or cut off walls. Treatment of the near foundation soil using lime and cement or use of soil replacement technique proved to be useful. The effectiveness of these methods is not necessarily similar or applicable for different geographical regions due to varying environmental conditions and mineral composition.
CONCLUSION
Visual examination and condition survey can help in determining the root cause of damage to asphalt pavements. Expansive soils were found to cause most of the common types of cracks and distress known in pavement constructions. This type of soil has a double distress mode on asphalt pavements when expanding and when shrinking. Three new types of distortion related to swelling and upheaval have been identified. Parameters influencing damage were listed as soil type and mineralogy, road geometry, drainage and adjacent topography. From practical field survey it was observed that the thickness of pavement has a significant influence in the level of
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distress. Efficient drainage of pavement can minimize seepage of water to the subgrade soils. Subsurface or surface water flow from neighboring lands of varying topography can cause distress. It is suggested to consider cut-off walls or shoulder dressings as appropriate to control the flow of water. Expansive soil treatment using cement and lime or partial soil replacement is inevitable in some cases with extremely high swell potential.
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