Through the knowledge collation of the architectural knowledge column of this website, Zhong Da Consulting has a basic overview of foundation pit dewatering and recharge as follows:
Reducing groundwater in foundation pit engineering is also called groundwater control, that is, in the process of foundation pit engineering construction, groundwater should meet the requirements of supporting structure and excavation construction, and will not bring harm to the environment and facilities around the foundation pit because of the change of groundwater level.
Through sorting out the relevant contents, Zhong Da Consulting formulated the relevant contents of foundation pit dewatering and recharge:
It is not enough to consider only the dewatering problem in deep foundation pit construction on the site with dense buildings, but also the uneven settlement and displacement of the surrounding original building foundation caused by excessive dewatering, which leads to the settlement, cracking or inclination of the original building. In addition, if a large amount of groundwater is not recharged, it will cause a lot of waste of water resources. Therefore, it is the inevitable trend of dewatering construction to recharge the groundwater.
At present, in the construction of deep foundation pit, only precipitation is generally paid attention to, and groundwater recharge is rarely paid attention to. Domestic research in this field is not mature, and it is difficult to find relevant information because there are no national and local norms. Scholars also have different understandings of the pumping and recharging mechanism of groundwater. Combined with the project of Anxi Jinlong Modern Plaza, this paper mainly introduces the design and construction precautions of dewatering and recharge of deep foundation pit, which can provide reference for similar projects in the future.
Basic principles of dewatering and recharge of foundation pit;
Well-point recharge means that when the well point is dewatering, the pumped groundwater is pumped into the foundation soil layer through the recharge well point, and the water seeps out from the soil layer around the well point, forming an inverted funnel in the soil layer opposite to the dewatering well point, so that the influence radius of the dewatering well point does not exceed the range of the recharge well point. In this way, the water-proof curtain is used to prevent groundwater loss under buildings outside the recharge well point, so that the groundwater level remains basically unchanged and the earth pressure is still in the original equilibrium state, thus effectively preventing the influence of the dewatering well point on surrounding buildings.
Basic design of foundation pit dewatering and recharge;
(1) design data
Jinlong. Modern Square is located on the west side of He Bin North Road, Anxi County, Fujian Province. The north side of the basement foundation pit is only 15m away from the private houses, with an area of 5420.8m2 and a general excavation depth of 5.30m The main soil layers from top to bottom are: (1), and the miscellaneous fill is 3.55m； in average thickness. (2) Sand filling: the average thickness is 2.25m; ; (3) Plain fill: the average thickness is 2.6m；; (4) Mixed sand of medium sand and pebbles: the average thickness is 9.35 meters; (5) Residual sandy clayey soil: the average thickness is 4.60m, and other strata have nothing to do with calculation, so it will not be considered.
The groundwater in this project is mainly phreatic water, which occurs in the pores of mixed sand of medium sand and pebbles. The stable groundwater level is about 4.5m The permeability coefficient of groundwater in mixed sand of medium sand and pebbles is 3.8× 10-2cm/s, and that in residual sandy cohesive soil is 5.0×10-5 cm/s.
It is planned to build a recharge well near the houses on the north side of the foundation pit, and recharge the groundwater at the same time of foundation pit excavation and dewatering to control the excessive change of groundwater level near the houses.
(2), the foundation pit water inflow q calculation
According to the geological survey report, the water inflow of foundation pit can be considered as a complete well with homogeneous aquifer. The nearest place of the foundation pit is about 35 meters away from Dalong Lake, the influence radius of precipitation is R=54.229m, and the distance from the center of the foundation pit to the river bank is b = 76.54 > 0.5r = 27.114m. Therefore, the water inflow of foundation pit is calculated as the foundation pit is far from the boundary.
(1) Equivalent radius of foundation pit r0 = (a/π) 0.5 = (5420.8/3.1415926) 0.5 = 41.54m;
(2) Water level drop of foundation pit S = 5.3–4.5+0.5 =1.3m.
(3) Thickness of confined aquifer H: H = 3.55+2.25+2.6+9.35-4.5 =13.25m.
(4) Influence radius of precipitation: r = 2×s×(k×h)0.5 = 54.229 m.
(5)k is the permeability coefficient, and k = 3.8×10-2 ㎝/s = 32.832m/day.
According to the formula F.0. 1- 1 in Technical Specification for Building Foundation Pit Support, the calculation is as follows: q =1.366× k× (2h-s )× s/LG (1+r/r0.
(3), single tube well water ask:
1, calculated according to formula 8.3.4 in Technical Specification for Building Foundation Pit Support (JGJ 120-99):
q = 120×π×RS×l×(k) 1/3 = 1646.924 m3/d
Where: RS-filtering radius, which is 0.175m; Here;
L—— length of water inlet part of filter, l =13.25-1.3-41.54× i = 7.796m, where i is hydraulic gradient and i = 0.1;
K is the permeability coefficient, k = 32.832/day.
2. According to the Technical Specification for Building and Municipal Dewatering Engineering (JGJ/T11-98) 6.4.5-1:
q = L '×d×24/α' = 1309.728 m3/d
Where: l'-length of water inlet part of filter l =13.25-1.3-41.54× I = 7.796m, where I is hydraulic gradient and I = 0.1;
D- filter outer diameter (㎜), d = 350 ㎜;
α'-empirical coefficient related to the permeability coefficient of aquifer. Look-up table α'=50.
3. Calculate the water yield of a single well according to complete diving (no interference)
q =π×k×(h02-hw2)/ln(R/rw)= 2064 m3/d
Where: h0—— the distance from the well level to the bottom of the aquifer without pumping, H0 =13.25m;
Hw—— the distance from the water level in the well to the bottom of the aquifer during pumping, with HW = 7.796 m;
K- permeability coefficient, k = 32.832/day;
R—— influence radius of precipitation: r = 2×s×(k×h)0.5 = 54.229 m.
Rw-radius of dewatering well, rw = 0.175m.
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