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1. Project Overview The first phase of the Beidi River Remediation project in Foshan city covers a length of 2.679 km, extending from the Environmental Sanitation Wharf to the Foshan Pile Factory Wharf. The cross-section of the project is shown in Figure 1. The geological conditions at the site of the project are as follows: from top to bottom, there is an artificial fill layer, Quaternary alluvial and colluvial deposits, Quaternary residual deposits, and Tertiary weathered bedrock. The artificial fill is present throughout the entire length of the dam, with a thickness of 0.8–5.8 meters; it consists of pure fill and mixed fill, and it is slightly compacted in most areas ； The Quaternary alluvial and colluvial deposits consist of silty soil and fine sandy layers, with a thickness of 1.6 to 15.8 m; they are gray to dark gray in color, and locally contain decayed plants or large amounts of dead wood. The retaining wall and its pile foundation are built on Quaternary alluvial and colluvial deposits. 2. Cofferdam design scheme 2.1 Original cofferdam design scheme and its characteristics The originally designed cofferdam was constructed using material excavated on-site; its crest elevation was 2.5 meters, and its crest width was 2.0 meters ； The normal water level of the outer river is 2.2 meters, the elevation of the weir foot is around –3.0 meters, and the water retention depth is between 5 and 6 meters ； The slopes on both the water-facing side and the backside have a gradient of 1:1.5; on the water-facing slope, sand-filled structures are used for protection at the base and surface. Through measurement and setting out, the fill cofferdam was placed near the center of the river, narrowing the waterway; its height for retaining water was considerable, and its foundation consisted of fine silty sand with high water permeability. The originally designed cofferdam was not safe. Following the review of the drawings, it was decided by the owner, designer, supervisor, and contractor that steel sheet pile cofferdams should be used exclusively. 2.2 Sheet Pile Scheme ① Selection of Sheet Piles Based on the characteristics of the site where the project is located, as well as the properties of sheet piles and the construction methods involved, Larsen Type III sheet piles were chosen. These sheets have an appropriate width and good bending resistance. Their key technical parameters are: W=1600 cm3, g=60 kg/m. The length of the sheet piles to be used is determined based on geological data and operating conditions; it should be between 9 and 12 meters. The depth to which the sheet piles penetrate into the ground should be at least 0.5 times their length. ②Piling equipment: 2 sheet pile driving and pulling machines were deployed for the construction work. The pile driving machine is modified from an excavator (KATO1250) by adding a vibration hammer; the vibration hammer is of the NPK-HP-7SXB type manufactured in Japan, with a vibration force of 200 kN. 3. Process of cofferdam construction The construction of the steel sheet pile cofferdam officially began in January 2003. The sequence of construction steps was as follows: preparing the construction road → driving in the steel sheet piles → excavating the foundation pit → constructing the pile foundation → building the base slab of the retaining wall → constructing the wall body of the retaining wall → backfilling behind the wall → removing the steel sheet piles. At the start of construction, 530 steel sheet piles were used to enclose a foundation pit 200 meters long. However, it was later found that the progress was not satisfactory, mainly due to an insufficient number of steel sheet piles, which prevented the creation of the necessary working area for the foundation pit; as a result, the construction of the retaining wall was often forced to wait. It facilitates doubling the input of steel sheet piles starting in February, allowing for the creation of two work fronts: construction proceeds from both ends toward the center simultaneously, with one shift dedicated to installing steel sheet piles at each work front, which reduces the time required for their use. A retaining wall 400 meters long can be completed in about 20 days, and by May all underwater embankment construction will be finished. 4. Construction method of steel sheet pile cofferdam 4.1 Method of driving steel sheet piles one by one ① First, the surveyors determine the axis of the steel sheet pile cofferdam; guide piles can be installed at regular intervals. These guide piles are themselves made of steel sheet piles, and ropes are used as guides to help control the axis of the steel sheet piles during driving. ②Preparing the pile cap and driving the piles: The piling machine lifts the steel sheet piles, which are then manually aligned and placed in position. ③Piles are driven one by one in a continuous manner; care should be taken to ensure that the elevations of the pile tops do not differ too much. Piles 12 meters long are used in areas where the foundation is deep. 4.2 Pile extraction In principle, pile extraction can be carried out after the construction of the retaining wall is completed. During construction, once approximately 100 meters of retaining wall had been built and sand was filled behind the wall to half its height, the piles were removed immediately; after repair, these piles were reused. First, use a pile driving machine to clamp the head of the steel sheet pile and vibrate it for 1 to 2 minutes, thereby loosening the soil surrounding the pile and causing \"liquefaction,\" which reduces the frictional force exerted by the soil on the pile. After that, slowly pull the pile upward. When pulling out the pile, pay attention to the load on the pile driver. If it becomes difficult to pull upward or the pile cannot be pulled out, stop the process. Vibrate the pile for 1 to 2 minutes first, then strike it downward by 0.5 to 1.0 meter before vibrating it upward again; repeating this process will help to pull the pile out. 4.3 Problems encountered during the construction of steel sheet piles and their solutions: Due to the complex geological structure of the riverbed, various difficulties often arise during the driving and extraction of steel sheet piles. The following methods are commonly used to address these issues: ① During pile driving, large boulders or other unknown obstacles may be encountered, resulting in insufficient penetration depth of the steel sheet piles; in such cases, angled piles or curved piles are used to bypass the obstacles. ②During the driving process in areas with mixed fill soil containing steel sheet piles, deviation can occur due to varying lateral compressive forces from stones and other objects. The following measures can be taken to correct this deviation: pull the steel sheet piles upward by 1.0 m to 2.0 m at the location where deviation occurs, then drive them downward again; repeating this up-and-down motion several times can break larger stones or displace them, thereby correcting the position of the steel sheet piles and reducing their inclination. ③When the inclination of the sheet piles along their axis is significant, special-shaped piles are used to correct this issue. Such piles are generally those that are wider at the top and narrower at the bottom, or whose width is greater than or less than the standard width; they can be welded and shaped according to the actual degree of inclination ； When the inclination is small, a winch or block and cable can also be used to pull the pile in the opposite direction before hammering it. ④In areas with soft soil, it sometimes happens that adjacent piles are pulled together during construction. The solution adopted is to weld several adjacent piles together, and lubricants such as butter are applied to the joints between the piles to reduce resistance. 5. Water retention efficiency of the cofferdam After the water in the foundation pit was drained, the water retention and sealing performance of the cofferdam could be clearly observed: there was virtually no water leakage on the inner surface of the steel sheet pile cofferdam; only a few of the older steel sheet piles suffered from some leakage due to loose joints ； There were no occurrences of leakage or piping within the foundation pit either. This shows that the steel sheet pile cofferdam was successful. 6. Deformation monitoring During the period when the steel sheet pile cofferdam was used to hold back water, we regularly monitored the displacement at the top of the steel sheet piles. It was found that the displacement of the pile tops toward the foundation pit remained within the range of 2–10 cm, indicating that the cofferdam was stable. 7. Experience Summary The actual construction results show that, compared to fill dikes, steel sheet pile dikes offer advantages such as faster construction progress, greater safety, and less space requirement. These advantages make them suitable for projects involving the renovation of riverbanks in urban areas, as well as for projects with deep waters, rapid currents, or soft foundations such as silt or fine sand, where the use of fill dikes is not appropriate. However, the downside is that the initial investment in steel sheet pile materials is high, which requires significant working capital. Therefore, decisions regarding whether to use steel sheet pile dikes, as well as the quantity of piles to be used and the number of times they can be reused, must be carefully analyzed; a technical and economic comparison should be conducted before making a decision.