The study area is located in Kaiyuan village, Fanzhen, Ruichang city, Jiangxi Province, China. It is situated along the Henggang River, a tributary of the Yangtze River system, approximately 6.8 km from Fanzhen. The Maoshan Reservoir was constructed in 1976, and its hydraulic structure mainly consists of a dam, a spillway, and water conveyance culverts. The dam is a clay-core rockfill dam with a crest length of 90 m and an elevation of 151.40 m. The catchment area above the dam site is 1.71 km2 and the river channel is 1.29 km long. The total storage capacity is 259,300 m3. The Maoshan Reservoir is a small reservoir primarily used for irrigation, with additional benefits such as flood control and aquaculture (see Fig. 1).
The dam is constructed with compacted layers of crushed stone soil. Despite multiple extensions and raisings of the dam, there has been significant seepage in the dam area, posing a safety hazard. Two risk mitigation and reinforcement projects were carried out between September 2012 and 2022. The main measures preformed included sealing culverts below the dam, constructing a new water release tunnel on the right bank, and grouting along the dam axis to prevent seepage. After completion in July 2023, a site survey revealed that downstream seepage at the dam base remained severe. This was mainly attributed to incomplete understanding of the seepage scope and causes, incomplete design plans, and the inability of construction to fully address the seepage issue. As a result, with the increase in the reservoir water level, the seepage flow has significantly increased.
2.1. Geological Settings
The study area is located in the central and low mountainous area of Mufu Mountain in the northern part of Jiangxi Province. The elevation ranges from 11 m to 923.1 m, with the lowest point adjacent to the Yangtze River and the highest point of the summit of Qingshan Mountain. The terrain is primarily characterized by hilly and low mountainous structures, with some areas exhibiting eroded and accumulate of landforms.
The orientation of the Maoshan Reservoir dam is southeast to northwest, and the valley has a "V" shape, with the river channel being approximately 100 m wide. The banks of the riverbed consist of natural slopes, covered with vegetation. The dam material consists of blocky gravelly soil, with a dam crest width of 4.0 m. The maximum dam height is 22.95 m, and the slope ratios for the upstream and downstream slopes are both 1:2.25. From the dam base to the design flood level, C15 concrete prefabricated block revetment is used, while from the design flood level to the dam crest and downstream slope, grass turf revetment is employed. The dam slopes are equipped with longitudinal and transverse drainage ditches, and downstream, at the base of the dam, a slope-attached filter and drainage system are established.
The study area depicted in Fig. 2 is generally situated in the Silurian sandstone formation of the Fanjiapu area. It is composed of the Xikeng Formation and Xiajiaqiao Formation, and is characterized by thick layers of gray-yellow sandstone. The original lithology of the Xikeng Formation and Xiajiaqiao Formation can be divided into upper and lower sections. The upper section consists mainly of thin to moderately-thick layers of gray-yellow and gray-green sandstone and sandy shale, with intermittent purple-red sandy shale. The lower section is composed of purple-gray sandy mudstone, silty mudstone, and thin layers of bioclastic rock.
The lower Silurian muddy-silty sandstone (S3) is characterized by gray-yellow muddy and silty structures, with well-developed joint fractures, and yellow slickenside patches on the fracture surfaces. The thickness of this formation is approximately 125-281.1 m. The upper layers of sandstone and shale appear yellow-green and purple-red, while the lower layer consists of fine-grained quartz sandstone interbedded with yellow-green and gray-green sandstone. There is a pseudo-conformable contact between the upper and lower layers[32].
To understand the geology of the study area, drilling was conducted on the reservoir dam during the operation of the reservoir. Figures 3 and 4 respectively show the stratigraphic profiles at the base of the dam (at borehole BH1) and the midpoint of the dam crest (at borehole BH3), respectively. The lithologies at various drilling locations in the study area are essentially the same. The subtle differences lie in the presence of alluvial deposits in the dam foundation at the midpoint. Additionally, there are variations in the thickness and burial depth of the different strata.
The overlying layers exposed in the study area mainly consist of dam fill material, Holocene floodplain deposits, Holocene upper renewal floodplain deposits and Silurian lower muddy-silty sandston.
(1) The Quaternary strata can be classified into the following types based on their genesis:
a. Artificial fill layer of the dam: This layer is composed of the backfill material used in the construction of the Maoshan Reservoir dam. Its composition is complex and primarily consists of crushed stone soil, clay, and concrete. The crushed stone content is approximately 30–40%, with a particle size ranging from 22 to 45 mm. Larger rocks, approximately 60 mm in length, exhibit angular and sub-angular and subangular shapes and originate from the strongly weathered sandy shale of the flanking mountains.
b. Holocene floodplain deposits of the Quaternary System in the riverbed: This layer is composed of mud, gravel, and stones, with a loose to slightly dense structure. The main components of the parent rock are sandstone and sandy shale. The gravel is flat and round, with a content ranging from 50–60% and a particle size ranging from 5 to 30 mm. The filling contains a mixture of mud and sand. The permeability coefficient (K) in the water injection test for this stratum ranges from 3.45×10− 4 cm/s to 1.04×10− 3 cm/s.
c. Holocene upper renewal floodplain deposits on the slope: This stratum consists of crushed stone soil with a crushed stone content of approximately 40–50%. The particle size ranges from 2 to 30 mm, with occasional individual crushed stones exceeding 40 mm in size and exhibiting subangular shapes. The parent rock is weathered sandstone from the mountains on both sides of the dam. Residual slope deposits are present on the slope of the dam shoulder, with a maximum thickness of approximately 3.5 m. The permeability coefficient (K) in the water injection test for this stratum is approximately 1.60×10− 2 cm/s.
(2) Bedrock
d. Silurian lower muddy-silty sandstone: The strongly weathered layer has a thickness of 1.00 to 4.5 m, is grayish-yellow, and exhibits a muddy-silty sand structure with a blocky texture. Joint fractures are developed, and yellow membrane patches are visible on the fracture surfaces. The rock mass is fragmented, and the core of the rock appears as fragments. In the weakly weathered layer, the mud-silt sandstone is not exposed, and the rock is bluish-gray with a muddy-silty sand structure. The rock has a blocky texture, is dense, and exhibits poor to moderately fragmented rock integrity. Local fractures are poorly developed, and the core sample primarily exhibits a short intact columns with some fragmented block-like structures. The permeability test results for this stratum range from 3.1×10− 5 to 16.0×10− 5 cm/s.
Based on the geological and survey data in the study area, two north-northeast-striking thrust faults, labeled F1 and F2, are identified. F1 cuts through the eastern side of the study area, while F2 is located on the western side. These two faults displaced the geological strata in the study area, causing fragmentation of the shallow subsurface structures and increasing the likelihood of strata leakage(see Fig. 2).
2.2. Geophysical Properties
The composition of embankment materials for earth-rock dams is complex and varies depending on the location. Generally, the core wall of an earthen dam is primarily composed of cohesive soil, while the anti-seepage body and filter layer consist mainly of gravelly soil, dumped stones, and block stones. Dam slope surfaces may include materials such as concrete and asphalt concrete. This information is based on historical data on the material properties of earthen dam materials [33]. According to the on-site experimental results in the study area, the main body of the dam is mostly composed of crushed stone soil, which is prone to water seepage, resulting in an increase in the dam body electrical conductivity and a decrease in resistance. The resistivity of the cohesive soil in the core wall ranges from 100 to 200 Ωm. The anti-seepage body outside the core wall is mainly composed of crushed stone soil, block stones, gravelly soil, and clay layers, with a resistivity of approximately 200 to 500 Ωm, and it can reach a maximum of 1800 Ωm. The resistivity of the lower part of the dam body's silty sandstone generally ranges from 100 to 1000 Ωm, exhibiting a wide range of characteristics from low to high resistance, which is mainly related to its degree of weathering and water saturation[34].
If the tested material is above the water level and the dam material structure is dense with undeveloped fractures, the resistivity will be relatively high. As the groundwater level rises, the fractured geological materials within the structure become submerged by groundwater. The groundwater will then seep along the fractures, leading to a rapid decrease in apparent resistivity. Additionally, if the alluvial layers below the dam body are not cleaned thoroughly or if the bedrock at the dam shoulder or dam base is highly weathered and moisture-rich, the resistivity will also rapidly decrease. Therefore, it is essential to thoroughly understand the resistivity characteristics of the study area to obtain accurate geological interpretation results[35].