60 CHAPTER 2: HIGHWAY ENGINEERING EARTHQUAKE DAMAGE TO ROAD SUBGRADES AND PAVEMENT IN THE TANGSHAN AREA Mingtian Hu1 and Fengyun Xu2 1. General Description In the Tangshan area there are 9 trunk highways amounting to 936 km, 28 county roads accounting for 1,083 km and 103 main commune roads totaling 2,085 km. The total length of highways in this area is 4,104 km. Eleven trunk highways and county roads with a cumulative length of 228 km suffered earthquake damage. Damage to highways was classified as severe (capital repair required) and slight (only routine repair needed). The respective lengths of highways damaged to various degrees of severity are listed in Table 1. It is noted from Table 1 that earthquake damage to road subgrade and pavement in the Tangshan area occurred mainly in the southern part, namely in Laoting County, the Bogezhuang Reclamation Area and Luannan County where sand and water were heavily ejected and in the high seismic intensity zone near Tangshan City. Typical earthquake damage to road subgrade and pavement with site conditions are summarized in Table 2. II. Types of Earthquake Damage 1. Longitudinal cracks Longitudinal cracks on subgrades and pavement were the most common phenomenon of earthquake damage. The width of the cracks were generally 10-30 cm and the length ranged from dozens of meters to over one thousand meters with a depth of 1-2 m (see Photo 1). Longitudinal cracks were usually found on the shoulders of the roads. Severely damaged sections of the Tangshan-Guye, Hugezhuang-Bogezhuang, and Hugezhuang-Laoting highways had many longitudinal cracks, which occurred on the shoulders and pavement with a width up to 40-50 cm (see Photo 2). Longitudinal cracks were apt to occur at the joints of old and new subgrades. 1 Highway Planning and Design Institute, Ministry of Communications 2 Highway Research Institute, Ministry of Communications 61 2. Transverse cracks Severe transverse cracks occurred on the subgrade and pavement near the Daodi Bridge in the intensity XI zone. They extended the entire width of the embankment and cut off the subgrade and pavement with a width up to 50-60 cm. 3. Settlement of pavement Settlement of the pavement and subgrade occurred on the Hugezhuang-Bogezhuang, Hugezhuang-Laoting and Lengda highways. The subgrade near Laoting on Lengda Highway settled 40-60 cm and trees on the roadside tilted inward over a length of 13 km. 4. Cracks, settlement and slide of subgrade at bridgehead Damage to the embankment at the bridgehead was more serious than at ordinary subgrade due to the high fill and the sliding of the riverbank. The subgrade at the bridgehead in the intensity zones VIII and above suffered various degrees of damage, which usually consisted of the embankment sliding toward the river and toward the side slopes, settlement, and longitudinal and transverse cracks. Settlement of the embankment at the bridgehead of the Daodi Bridge amounted to 1.0 m. Damage was most severe on embankments at bridgeheads filled with silty soils. The 5.0 m high embankment at the bridgehead of the Huopao Bridge in the intensity VIII zone was filled with silty soil, and was deformed and slumped after the earthquake settling about 1.0 m. The embankment at the bridgehead of the Tuozitou Bridge in the intensity XI zone was also filled with silty soil with a 1:2 side slope which cracked and settled unevenly after the quake. Four to five cracks occurred at the same section with a crack width of 5-50 cm. The difference in elevation between the two sides of the cracks was 10-30 cm. The length of road damaged was 70 m and traffic was suspended. 5. The upheaval of the embankment at the bridgehead Photo 3 shows the upheaval of pavement on the embankment, which was caused by the thrust of the bridge span at the bridgehead at the Luanhe River Bridge III. Earthquake Damage to Culverts Passing Beneath Roads In the Tangshan area there were a total of 1,852 culverts of various types, mainly of RC box type, with a total length of 17,126 m of which 80 of them with a cumulative length of 767.81 m suffered various degrees of damage. According to the severity of damage to the culverts there were 4 types of damage. (1) Type I: The main damage of this type was the fracture and collapse of the side wall or middle pier of the RC culverts resulting in fracture of the top slab of the culvert and fall of the arch, see Photos 4 and 5. Eighteen culverts totaling 195.20 m in length suffered this type of damage. 62 (2) Type II: The main damage of this type was the tilting and cracking of the side wall or middle pier, or severe cracks to the top slab of the culvert or to the arch ring. Thirteen culverts totaling 123.70 m in length suffered this type of damage. (3) Type III: The main damage of this type was the collapse of the flare wing wall at the inlet and outlet of the culvert and severe damage to the water flow appurtenances with only slight damage to the main structure. Thirty-five culverts totaling 320.19 m in length suffered this type of damage. (4) Type IV: The main damage of this type was slight damage to the flare wing wall. Fourteen culverts totaling 128 m in length suffered from this type of damage. IV. Causes of Earthquake Damage 1. Liquefaction of soil foundations and settlement of soft soil foundations Damage to the subgrade and pavement occurred mainly in the area south of Tangshan with heavy ejection of water and sand in the high intensity zone. The foundation soil here is composed of saturated silty fine sand and soft clay. Liquefaction of silty fine sand took place during the earthquake with the ejection of water and sand in a large area and side ditches were filled flush with ejected sand at severely damaged road sections. Samples of ejected sand were taken from the side ditches of the embankment and sieve analyses were made and gradation curves from samples of 4 boring holes were obtained as shown in Figures 1 and 2. Liquefaction of sandy soil or settlement of soft soil was the cause of unequal settlement of the subgrade, as well as for longitudinal and transverse cracks and sliding of the side slopes. 2. Effect of sliding of riverbanks Since an embankment is relatively high at the bridgehead and extends into the riverbank forming a high and steep free face it is more apt to slide and settle unequally. Embankments at bridgeheads filled with silty soil with low plasticity suffered the most serious damage among all types of subgrade. 3. Inadequate compaction of subgrades and poor drainage of side ditches No aseismic measures had been taken for subgrades on most roads in the Tangshan and Tianjin areas. Some new roads were inadequately compacted. Water accumulated year round in the side ditches of some sections of road. Some sections of road which were close to the river, channel, pond and soil of the subgrade were saturated and strength and stability were greatly lowered. All these aggravated the degree of damage. 63 Table 1. Statistics of lengths of damaged trunk highways and country roads in the Tangshan area. Damaged Length (km) Total Severe Slight No. Name of Route Length (km) Damage Damage Total 1 Beijing-Shenyang 90 2 3 5 2 Tianjing-Qinhuangdao 162 23 27 50 3 Pingquan-Daqinghe 97 3 12 15 4 Tangshan-Xinglong 76 2 26 28 5 Tangshan- 32 18 4 22 Bogezhuang 6 Tangshan- Baodi 71 2 28 30 7 Tangshan-Jianhe 46 11 12 23 8 Changning- 62 2 8 10 Bogezhuang 9 Fengrun-Hancheng 24 2 8 10 10 Laoting-Wangtan 18 5 10 15 11 Tangshan-Laoting 33 15 5 20 Percentage of total length 35.2% 4.21% 7.08% 11.29% 6 4 Table 2. Earthquake damage to subgrade and pavement in the Tangshan area. Earthquake Site Conditions and Damage Location Intensity Damage to Ground Surface Bridgehead of XI The height of the embankment was about 1 m, width 11 m. The Alluvial silting deposit of the Douhe River. Soil Shengli Bridge, embankment settled forming lateral cracks 2.2-3.0 m in width is a silty sand loam and clayey loam with mud at Tangshan City (direction N 40° W). The inspection station at the bridgehead the river bottom. (Douhe River) settled 50-80 cm with several cracks in the courtyard 10-25 cm in width. The river levee slid toward the river, several longitudinal cracks 10-25 cm in width and about 120 m in length occurred on the subgrade along the river in the city; curbs were damaged and pavement depressed locally. The embankment at the east bridgehead settled: width 32 cm, maximum depth 2.2-2.3 m. The subgrade and river levee along the east bank slid toward the river resulting in several longitudinal cracks along the river. Head of Daudi Bridge X The height of the embankment at the bridgehead was 2 m with a Alluvial deposit of the Douhe River. Soil is (Douhe River) side slope of 1:1.2. The river levee slid toward the river having clayey loam. Low land. Water accumulated several ground cracks in the same direction as the river. The inside ditches and on the ground. embankment at the bridgehead settled about 1 m and transverse cracks occurred 15-30 cm in width with a depth of 1.5 m.