The load-bearing capacity of the beam and whether it can be used normally will not depend on the original design load standard and integrity of the superstructure. As an important part of the substructure, the piers and foundations will directly bear the load of the upper structure (including dead load and live load) and transfer the load to the foundation. Therefore, the quality of the substructure of the bridge directly affects its bearing capacity and normal use, and the reduction of the bearing capacity of some old bridges and the occurrence of major diseases are caused by the diseases of the substructure. 

Therefore, in the reinforcement and reconstruction work, the reinforcement and reconstruction of the substructure is very important, otherwise, the normal purpose of restoring and improving the overall bearing capacity of the bridge will not be achieved. This paper mainly analyzes the countermeasures for strengthening the substructure of the bridge (pier, abutment, foundation).

Analysis of Countermeasures for Bridge Pier Reinforcement

The commonly used methods and countermeasures for strengthening bridge piers are as follows:

Reinforcement method with shroud: Longitudinal penetration cracks occur in the pier, which can be reinforced with reinforced concrete or steel hoops. If bottom-up cracks are caused by uneven foundation sinking, the foundation should be reinforced first, and then the method of grouting or hooping should be used for reinforcement.

Reinforced concrete hoop reinforcement method: When the pier and abutment are seriously damaged (such as large-area cracks, damage, weathering, peeling) or the old pier and abutment with rough masonry and masonry, generally can be reinforced with reinforced concrete "hoop sleeve". Its size should be able to meet the needs of transmitting all or most of the load through the hoop. At the same time, the top of the pier was re-engineered, and new and powerful reinforced concrete slabs were poured and supported on the hoop to replace the old supporting pad stones, so that the hoop could take part in the work.

Underwater repair and reinforcement of bridge piers: When there are defects in the surface of the masonry or reinforced concrete pier and the pier is under normal water level, the following reinforcement methods can be adopted according to different situations:

a. When the water depth is below 3m, the grass bag cofferdam can be housed, and then the water can be drained. When the water is difficult to drain, the concrete can be poured under the water and then pumped. After pumping, stone or concrete can be used to fill the vacated part. In this case, it is also possible to sink the reinforced concrete thin-walled box cofferdam to the bottom of the river near the damage without pumping water, and pour underwater concrete between the box and the pier to wrap the damaged or empty part.

b. When the water depth is more than 3m, dry hard concrete is packed in sack, and then the sacked concrete is filled into the empty part in layers through diving operations, and it should be 0.4~0.6m wider than the original foundation.

Analysis of countermeasures for bridge abutment reinforcement

The commonly used methods and countermeasures for the reinforcement of bridge abutment diseases are as follows:

Load reduction and reinforcement method: This countermeasure is used when the pressure on the abutment is large and the abutment is displaced in the direction of the bridge hole. After excavating the back of the abutment and filling, replace it with lightweight materials to reduce the load on the back of the abutment to stabilize the abutment.

Pile reinforcement method: This countermeasure is adopted when the original bridge's vertical bearing capacity is insufficient. Generally, a row of piles can be added in front of the platform, and the cover beam can be poured to share the pressure from the superstructure. When piling or drilling piles, the original bridge deck can be used as a scaffold to make holes and insert piles on the bridge deck. The cover beam can be stressed separately, and can be connected to the old cover beam and the old pile to bear the force together.

Thickening abutment reinforcement method: This countermeasure is used when the back pressure of the beam abutment is large and the abutment is displaced in the direction of the bridge hole. The back of the platform can be dug out and filled with soil, the platform body (bridge abutment breast wall) can be thickened, and attention should be paid to the firm combination of the new and the old concrete.

Supported lintel reinforcement method: This countermeasure is mainly applied to single-bridge small-span girder bridges. A supporting lintel can be established between the foundations of the two abutments to prevent the abutment from shifting to the middle of the span. For example, reinforced concrete support beams or mortar stone support plates are used for reinforcement, and the support is not higher than the river bed.

Retaining wall support rod or block reinforcement method: This countermeasure is suitable for embedded abutments that are inclined or slipped in the direction of the bridge hole due to insufficient size and difficult to withstand the pressure on the back of the abutment. It can be reinforced in the form of retaining walls, support rods or blocks. Soil bags can also be used for temporary repairs to stabilize the bridge abutment.

Additional method of reinforcement of the abutment: In order to reduce the horizontal pressure of the subgrade on the abutment, it is necessary to replace the filling behind the abutment with large-size soil with a large internal friction angle or dry rubble, masonry, etc., and at the same time add a new one behind the abutment Erection of toilet beams.

Analysis of countermeasures for foundation reinforcement

The main countermeasures of commonly used reinforcement methods for bridge pier and abutment foundation diseases are as follows:

Enlarged foundation reinforcement method: This method is suitable for situations where the foundation bearing capacity is insufficient or the embedding is too shallow, and the pier and abutment are masonry or rigid solid foundations. The expansion of the foundation area should be determined by the calculation of the foundation strength. When the foundation strength meets the requirements but the defects only show as uneven settlement and deformation is too large, the reinforcement measures of expanding the foundation area can be adopted. Add stone masonry or concrete around the rigid solid foundation to expand the pressure-bearing surface of the foundation, and the old and new foundations should be firmly combined. In specific implementation, it can be carried out in the following order.

a. The sheet pile cofferdam is usually driven within the range that must be widened. If the soil quality of the pier base is not good, the necessary reinforcement should be made;

b. Fill the soil in the digging weir, and dig straight to the necessary depth (pay attention to the safety of the pier and abutment);

c. After draining the water in the weir, pave stones (mortar) or make concrete foundations;

d. The old and new foundations should be firmly combined. During construction, connecting (anchoring) steel bars or steel pins can be added to make the reinforced and enlarged foundation firmly combined into one.

Supplementary pile foundation reinforcement method: This method is suitable for pile foundation reinforcement. Supplementing pile foundation means adding bored piles or driving reinforced concrete precast piles around the pile or foundation and expanding the original cap to increase the bearing capacity of the foundation and increase the stability of the foundation. When pile driving (or bored pile) is used for reinforcement of single-row piled bridge piers, if the original pile spacing is relatively large (when the pile diameter is 4 to 5 times), piles can be inserted between the piles. If the original pile spacing is small and the navigable net span is allowed to be reduced, the number of piles can be increased on both sides of the original bent frame to form a three-row pier pile. If piles are added between the piles, the original cap beams can be cut out and new cap beams can be poured to connect the tops of the new and old piles. But at this time, it is necessary to check whether the original cap beam can withstand the bending moment opposite to the original direction at the top of the pile. When the vertical bearing capacity of the abutment is insufficient, generally a row of piles can be added in front of the abutment and the cap beam can be poured to share the pressure from the superstructure. The added cap beam can be stressed separately or connected together so that the old cap beam, the old pile and the new pile are stressed together.

Artificial foundation reinforcement method: This method is suitable for the situation where the natural soil foundation below the foundation is soft and cannot bear a large load, or the upper soil is good, but the deep soil is poor and causes the foundation to settle. Sand pile method and grouting method are commonly used in this method. When the sand pile method is used for reinforcement, steel pipes or wooden piles can be driven into the soft soil layer around the foundation. Then the pile is pulled out, poured into the dried coarse sand, and tamped to make a sand pile to achieve the purpose of improving the density. In water-saturated sandy soil or cohesive soil, because it is easy to collapse holes and it is difficult to fill sand, the sand bag casing method and vibroflotation method can also be used to reinforce the foundation. The grouting method is to drill or drive a grouting pipe diagonally to the center of the abutment under the foundation of the abutment. Through the holes and pipe holes, inject cement mortar under a certain pressure, and add the consolidation agent of hot asphalt map to improve the bearing capacity of the foundation. The reinforcement range and depth should be determined by calculation.

Rotary jet grouting reinforcement method: This method is a foundation reinforcement technology under development. Due to its wide range of uses, reliable foundation quality, and good effect, it has gradually become one of the commonly used foundation treatment methods in my country. In recent years, in highway engineering, especially in the reinforcement of old bridge foundations, certain practical applications have also been obtained, and significant economic and technical effects have been obtained.