| Songbei District of Harbin is located on the north bank of the Songhuajiang River in Harbin reaches. The water level for Harbin reaches will be kept on 116 m after Dadingzishan Navigation Reservoir, which is under construction at downstream, begins to store water. The level will be 6 m higher than that in dry season in the past, resulting in the backing up of groundwater in Songbei District and immersion problem in Songbei District .So it is necessary to investigate the hydrogeological conditions, calculate the back-up water level, find the critical depth, determinate the immersion scope, conduct immersion assessment and put forward the control measures for immersion hazard in order to provide scientific base for construction design in the district.The overall tendency of the district is high northern part and low southern part, with an alluvial topographic origin type and the shape types of Songhuanjiang River high and low flood plains.The Prequaternary stratum in the area is grey or grey-black mudstone and fine siltstone of Nenjiang Group, Cretaceous System, overlaid by Quaternary grey or grey- white sand, gravelly sand and silt clay. Surface layer is yellow-brown silt clay underlying grey or grey-yellow fine sand , middle-coarse sand gravel intercalated thin silt clay in the high flood plain;1~4 m uncontinuous silt clay underlying yellow, grey or grey-brown middle-sized sand, gravel in the low flood plain.Groundwater type in the area is friable rock porous underground water stored in the aquifer forming by sand, gavel of Sheli Group, Epipleistocene and middle-coarse sand, middle sand of Holocene. The aquifer is characteristic by coarse granule, big capacity, open top and close bottom with 20m-40m thickness and 15d/m- 56d/m permeability coefficient. The bottom of the area is Cretaceous System mudstone as confining bed.The area is divided into three water abundance areas:1. Very abundance area (individual well yield >5000m~3/d)2. Abundance area (individual well yield 3000 m~3/d -5000m~3/d)3. Relative abundance area (individual well yield 1000 m~3/d -3000m~3/d)Water circulation zoning is very clear in the area e.g., river water setup-stagnation zone along the river bank;discharge zone with flood filtration and rainfall penetration in the flooded area;evaporation-discharge zone with rainfall penetration within 2km in normal and rainy years.Groundwater level is shallow and water table fluctuates with seasons in the area. Along with the rainfall, groundwater table goes up and reaches highest level in rainy season;then the water table goes down gradually since rainfall decreases and river water level declines after rainy season. Until rainy season of next year, groundwater fluctuation enters next circulation.Based on the existing geological data and drill data conducted in this investigation in the area, by using SOLID module of GMS software and interpolating method between the drills, three dimensional modal is established. Considering the regional uncontinuity of alluvial layers and the uniformity of geological sedimentation, part of strata is generalized.According to groundwater fluctuation, surface soil structure, dispersion, permeability and their relations with capillary action, groundwater critical depth is calculated by following formula:Hcr=Hk+AHWhere: Her......immersion groundwater critical depth (m);Hi;......height of capillary rise (m );AH......safe superelevation. For farmland the value and root depth of vegetationadopt 0.5m by experience;for construction adopt 2.0m depending on foundation depth and soil quality.Height of capillary rise is determined by on-the-spot investigation, indoor experiment and empirical values. They are 50m for silt clay, 1.0m for silt fine sand. Therefore the critical depth in the area is as follows: Farmland area, silt clay Her=2.50m, silt and fine sand Hcr= 1.50m,-Construction area ,siltclay Hcr=3.50m, silt and fine sand Hcr=3.00m,> In accordance with the lithologic and distribution character of surface soils, and groundwater level depth, the area can be divided into immersion area, transitional area and no-immersion area.Immersion scope for existing extraction quantity (in the year 2010) ,short-term extraction quantity (in the year 2015) and long-term extraction quantity (in the year 2020) is delineated according to the groundwater level after backing up, groundwater critical depth, lithologic character in aeration zone and consistently considered as construction land.In immersion area the following influence will occur:1. The surface soils will appear swampiness due to too wet or become saline soil because of strong evaporation and concentration. In this case not only original mechanical property of the soils will change but also chemical composition of groundwater will be influenced, resulting in a high degree of mineralization and stronger groundwater corrosive ability to construction.2. Water content of rock body above the groundwater table increases and the rock body under the groundwater table is hydrolyzed and softened. Soluble materials in the rock body are solved and structure of rock body is destroyed, giving a rise to unequal subsidence, decreased bearing capacity and causing construction inclination, cracking and tension rupture for ground and underground pipes.3. Due to groundwater table is raised and foundation of construction will be floated up construction will be out of balance acted by floating force.4. The area is a seasonal frozen area with 1.95m highest frozen depth. Influenced by immersion, water content increases and water in the rock body will move and be concentrated forming intercalated ice layer or ice cone and making the foundation frost upheaval and ground surface uplift .Rock body in frozen situation has a high strength and low compressibility. Once it is defrosting shear and compressive strength will decrease greatly, leading to foundation subsidence and construction cracking, out of balance. Road thawing damage will happen for road foundation after defrosting and destroy road surface.Overall, some countermeasures are put forward to control immersion harms.1. Combing with the first phase design for Songbei Qianjin Area and the second phase design for Qianjin liman, original ground surface should be raised. According to predicting results of existing groundwater flow conditions, when ground surface is raised to the elevation of 117.5m, the whole area will become transitional area. Within the groundwater contour of 115.5m, ground surface needs only to be filled to the elevation of 117.0—117.5m.For road foundation sand should be filled under the bottom so as to stop capillary rise.2. It is suggested that pipe foundation should be used for high buildings and water proof treatment should be taken for underground construction.3. Combing with surface water and urban drainage, some ditches should be dug in order to drain water and at the some time lower groundwater level and beautify urban landscape.
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