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4.1 Summary

In the process called open pumping, water can be allowed to flow into the excavation as it is advanced. The water is collected in ditches and sumps and pumped away. Open pumping from sumps and ditches is usually the least expensive method from the standpoint of direct dewatering cost. Under favorable conditions, it is a satisfactory procedure. But if conditions are not conducive, attempts to handle the water by open pumping can result in delays, cost overruns, and occasionally catastrophic failure. The key is to identify those conditions that are or are not favorable for open pumping, and to recognize which conditions predominate in a given job situation. Generally, the main sump is placed in the middle of the excavation, with the result that the entire subgrade was turned into a quagmire because the water had to travel across subgrade to enter the sump. The condition is exacerbated by the presence of stratified or fine-grained soils at or near excavation subgrade that inhibit vertical drainage. This method suits in dense sands, coarse sands, graded sands, hard fissured rock and clay with surface runoff drainage. But in loose sand, soft soil or rock, problems of slope stability and boiling of the bottom must be anticipated. Tables 4.1 and 4.2 tabulate the conditions that, in authors’ experience, may affect whether open pumping is viable on a given project.

Table 4.1 Conditions favorable to open pumping
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Table 4.2 Conditions unfavorable to open pumping (predrainage or cutoff usually advisable)
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4.2 Open Pumping Methods

4.2.1 Open Ditches and Sump Pumps

4.2.1.1 Stage Excavation Drainage

Shown as Fig. 4.1, the final sump must be deep enough so that when it is pumped out the entire excavation will be drained. This is an obvious point but surprisingly it is often violated. Digging the sump down that extra several feet, or meters, is difficult and sometimes risky; there is a tendency to give up too soon. If necessary, a temporary sump at a shallower level should be constructed and pumped long enough to improve conditions so that the final sump can be safely constructed to the proper depth. Generally, the ditches are stratified dug at one/two sides or in the middle of foundation pit. And the sumps are placed at each 20.00–30.00 m distance for the water to be collected and pumped out. The depth of ditches and sumps can be deepened as the excavation advances. The bottom of ditches should be always kept 0.30–0.60 m lower than the pit bottom elevation. Usually in small excavations, depth of ditches can be 0.30–0.6 m with the width of 0.40 m and slope ratio of 1:1–1:1.5. And small slope of 0.2–0.5 % can be set in the ditches bottom for the drainage. The sectional area of sumps should be 0.60 × 0.60–0.80 × 0.80 m. And the bottom elevation should be kept 0.40–1.00 m lower than the ditches. The sump walls can be reinforced by bamboo cages and wood plates. The pumping should be continuously conducted until the backfill is completed.

Fig. 4.1
figure 1

Stage excavation drainage

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4.2.1.2 Double Well-Point Drainage

Shown as in Fig. 4.2, the cement concrete pipes with diameter around 80–100 cm are driven into the earth section by section. The water table outside or in the bottom of foundation pit is lowered by a centrifugal pump. Usually, single well is sufficient for construction requirement. Double system is just for the very deep drawdown.

Fig. 4.2
figure 2

Double well-point drainage

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The last section of the well is the filter, which is drilled as quincunx holes in 15–20 cm space for better inflowing of water. The diameter of the quincunx hole is set large out and small in, which is filled by sackcloth. Sand filter material is employed in the filter to block the soil particle flowing through with water.

4.2.1.3 Main Central Sump Pumps

Shown as Fig. 4.3, in the condition that there are no sheet piles surrounding, or slope excavation and no drilled-in supporting, could not meet the construction requirement; some failures would happen, such as slope collapse. Thus, a main seepage well is established for the sump-pump system in the center of foundation pit. This system can be set during the whole construction period. Until the foundation is completed, it is sealed to prevent water seepage.

Fig. 4.3
figure 3

Main central sump pumps

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4.2.1.4 Range of Application

The above three methods are generally applicable for the water drainage in the common foundation, medium area group foundations, or building foundation pit. Easily constructed, simple equipments, low costs, they are mostly used.

4.2.2 Multilayer Open Pumping from Ditches and Sumps

4.2.2.1 Method

Shown as Fig. 4.4, along the slope of foundation pit, 2–3 ditches and sumps system is set to collect the groundwater and to block the water out of excavation area. The distribution and specific sizes of ditches and sumps are almost the same with those in the above common ditches and sumps. It should be paid attentions that to prevent the water in the upper ditches flowing down to the lower ditches. If so it is probable that the slope of foundation pit may collapse by the water seepage.

Fig. 4.4
figure 4

Multilayer open pumping from ditches and sumps

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4.2.2.2 Range for Application

This method is used in the very deep foundation pit project, in which the initial groundwater table is relatively high and multilayer permeable soils. Establishing multilayer ditches and sumps can effectively prevent the slope collapse when the groundwater in the upper layers scours the underlying layers. The single pumping head and slope height can be shortened but the excavation area and earthwork volume are both increased.

4.2.3 Deep Ditches Pumping

4.2.3.1 Method

In appropriate locations or upstream of groundwater in the construction site, a longitudinal deep ditch is dug as a main collector, in which the groundwater flows away or is pumped out (Fig. 4.5). Sub-ditches are connected to the main ditch and equipped all round to induce the water directions. The main ditch should be deepest and the depth is lower than the bottom of the foundation pit 1.00–2.00 m. Sub-ditches must be set to be shallower than the main ditch by 0.50–0.70 m. At the locations through the foundation, blind ditches should be set by gravels and sands. Before foundation pit backfilling, they are blocked by clays to prevent the groundwater flowing in the ditches to cause the failure of the subgrade. The deep ditches can also be set in the permanent drainage places in or surrounding the buildings.

Fig. 4.5
figure 5

Deep ditches pumping

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4.2.3.2 Range for Application

This method is suitable for dewatering of large-area deep basement, caisson foundation, and group foundations.

4.2.4 Combined Pumping

4.2.4.1 Method (Fig. 4.6)

Based on deep ditches pumping, combined the multilayer ditches and sumps pumping, or light well-point dewatering in the upper soil layers, this combined pumping method is employed to drain large amount of underground water.

Fig. 4.6
figure 6

Combined pumping

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4.2.4.2 Range for Application

This method is used in the very uniform soil condition and deep foundation pit, or large amount of water discharge in large-area foundation excavation. The effectiveness is very good by this method but the cost is relatively high.

4.2.5 Dewatering by Infrastructure

4.2.5.1 Method (Fig. 4.7)

In this method, the deep foundation of the plant is constructed firstly, which is set to be the total water collecting site; or the surrounding drainage and sewer system is built previously, so that open sump pumps or blind seepage ditches are established in one/two sides along the foundation pit to induce the water into the main drainage and sewer system.

Fig. 4.7
figure 7

Dewatering by infrastructure

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4.2.5.2 Range for Application

It is specially employed in group foundation dewatering of the large scale infrastructure construction (such as underground garage, oil depot).

4.2.6 Open Pumping in Sheet Pile Supporting System

Shown as Fig. 4.8, when sheet piles are constructed for the support of foundation pit excavation, small scale side ditches are set in the foundation pit edge beside the sheet piles, which is also called collecting ditches. Groundwater flows into the ditches and is pumped away immediately. Gravels and sands are filled in the ditches as filter. The depth of the ditch depends on the water amount. Generally, it is 0.60–1.00 m. Sometimes it can be set outside the foundation pit just beside the outer edge for convenient manipulation.

Fig. 4.8
figure 8

Open pumping in sheet pile supporting system

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4.3 Calculation on Open Pumping Amount

4.3.1 Formulas

In industrial and civil engineering construction, very high groundwater table is usually encountered, which is much adverse to the excavation of foundation pit. Thus it is necessary to take some dewatering measures to depress the groundwater table. The dewatering mode and size can be hardly unified. Generally, some simplification is employed to estimate the rate of groundwater flow.

4.3.1.1 Long and Narrow Foundation Pit

Long and narrow foundation pit is defined as the ratio of foundation pit length B to the width C is larger than 10:

$$ \frac{B}{C} > 10 $$
(4.1)

When groundwater flows into a long and narrow foundation pit, it can be regarded that the groundwater laterally infiltrates in from two sides. According to Dupuit’s equation:

Unconfined aquifer:

$$Q = KB\frac{{H_{0}^{2} - H_{\rm w}^{2} }}{R} $$
(4.2)

More specifically, the flow rate of groundwater in two ends along width should be considered. So the calculation mode is divided into two parts (Fig. 4.9). The lateral flow rate can be estimated just by Eq. (4.2). As for the two ends, each can be approximate as a half of well with radius of C/2, which is sum up as an entire dewatering well. Thus,

Unconfined aquifer:

$$Q = KB\frac{{H_{0}^{2} - H_{\rm w}^{2} }}{R} + \frac{{\pi \cdot K(H_{0}^{2} - H_{\rm w}^{2} )}}{{\ln R - \ln \frac{C}{2}}} $$
(4.3)

Confined aquifer:

$$Q = 2KBM\frac{{H_{0} - H_{\rm w} }}{R} + \frac{{2\pi \cdot K \cdot M(H_{0} - H_{\rm w} )}}{{\ln R - \ln \frac{C}{2}}} $$
(4.4)

where C is the width of the foundation pit, m; B is the length of the foundation pit, m; Q it the flow rate, m3/d; K is the hydraulic conductivity, m/d; H 0 is the initial groundwater table, m; H w is the water table in the well, m; R is influence radius, m; M is the thickness of confined aquifer, m.

Fig. 4.9
figure 9

Long and narrow foundation pit. a Cross sectional profile. b Plane view

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When the lateral recharge conditions in two ends of the foundation pit are different, the calculations should be correspondingly various. Then the total flow rate must be summation of the two parts, i.e., \( Q = Q_{ 1} + Q_{ 2} \). This circumstance mostly occurs in unconfined aquifer, shown as Fig. 4.10.

Unconfined aquifer:

$$Q_{1} = KB\frac{{H_{1}^{2} - H_{\rm w1}^{2} }}{{2 \cdot l_{1} }} $$
(4.5)
$$ Q_{2} = KB\frac{{H_{2}^{2} - H_{\rm w2}^{2} }}{{2 \cdot l_{2} }} $$
(4.6)

where l 1, l 2 are the distances from the recharge boundaries to the foundation pit, m; H w1 and H w2 are the water tables on the lateral walls of the foundation pit. Others are same as previous equations.

Fig. 4.10
figure 10

Different lateral recharge boundaries to the foundation pit

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In the case of two paralleling fully drainage channels (Fig. 4.11), the calculation can be considered as the combination of Channel I and Channel II.

Channel I:

$$Q_{\text{I}} = KB\frac{{H_{1}^{2} - H_{\rm w}^{2} }}{{2l_{1} }} $$
(4.7)

Channel II:

$$Q_{\text{II}} = KB\frac{{H_{2}^{2} - H_{\rm w}^{2} }}{{2l_{2} }} $$
(4.8)

where B is the length of the foundation pit. H w is much smaller than the thickness of aquifer, then it can be neglected, so the calculation can be largely simplified.

Fig. 4.11
figure 11

Two paralleling fully penetrated drainage channels

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$$ \frac{B}{C} < 10 $$
(4.9)

The flow rate can be estimated as large well method regardless of the shape is rectangle, square, or some others. The reference radius of the hypothesized large well can be calculated as follows.

In the case of square foundation pit, it is

$$ R_{0} = \eta \frac{C + B}{4} $$
(4.10)

where the value of \( \eta \) can be selected from Table 4.3 based on the ratio of width over length of the foundation pit.

Table 4.3 The value of \( \eta \)
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In the case of irregular shape foundation pit, the reference radius can be estimated by Eq. (4.11).

$$ R_{0} = \sqrt {\frac{F}{\pi }} $$
(4.11)

where F is the area of the foundation pit, m2; R 0 is the reference radius in the calculation of large well method, m.

4.3.1.2 The Fully Penetrated Large Well Method in Horizontal Impermeable Base

In the case of the foundation pit fully penetrating an unconfined aquifer, shown as Fig. 4.12a, the calculation formula is as follows as Eq. (4.12).

Fig. 4.12
figure 12

A foundation pit fully penetrating an aquifer. a Unconfined aquifer, b Confined aquifer

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$$ Q = \frac{{\pi \cdot K(H_{0}^{2} - H_{\rm w}^{2} )}}{{\ln \frac{{R + R_{0} }}{{R_{0} }}}} $$
(4.12)

In the case of the foundation pit fully penetrating a confined aquifer, shown as Fig. 4.12b, the dewatering of groundwater must depress the water table down into the confined aquifer. The groundwater farer than the distance of a is the confined groundwater, while it is the free-surface flow within the range of a. According to the principle of continuity, under the condition of steady flow, it has the relationship of Q unconfined = Q confined.

$$ Q_{\text{unconfined}} = \frac{{\pi \cdot K(M^{2} - H_{\rm w}^{2} )}}{{\ln \frac{a}{{R_{0} }}}} $$
(4.13)
$$ Q_{\text{confined}} = \frac{{2\pi \cdot K \cdot M(H_{0} - M)}}{{\ln \frac{{R + R_{0} }}{a}}} $$
(4.14)

In conjunction with Eq. (4.13) and (4.14), eliminating lna, it has:

$$ Q = \frac{{\pi \cdot K(2MH_{0} - M^{2} - H_{\rm w}^{2} )}}{{\ln \frac{{R + R_{0} }}{{R_{0} }}}} $$
(4.15)

Assuming H w = 0, s = H 0 when dewatering for the foundation pit, the flow rate can be deduced as Eq. (4.16).

$$ Q = \frac{\pi \cdot K \cdot M(2s - M)}{{\ln \frac{{R + R_{0} }}{{R_{0} }}}} $$
(4.16)

where s is the groundwater drawdown, m; others in the equation are the same as above.

In the case of the foundation pit partially penetrating the unconfined aquifer, shown as Fig. 4.13a, the flow rate per unit width can be estimated as Eq. (4.17).

$$ q = {q}_{1} + {q}_{2} = \frac{{\pi \cdot K \cdot s^{2} }}{{\ln \frac{{R + R_{0} }}{{R_{0} }}}} + \frac{{2\pi KsR_{0} }}{{\frac{\pi }{2} + 2{\text{arcsh}}\frac{{R_{0} }}{{T + \sqrt {T^{2} + R_{0}^{2} } }} + 0.515\frac{{R_{0} }}{T}\ln \frac{{R + R_{0} }}{4T}}} $$
(4.17)

where q 1 is the flow rate per unit width from lateral seepage of the foundation pit, m2/d; q 2 is the flow rate per unit width from the foundation pit bottom, m2d; T is the thickness from the impermeable base to the bottom of foundation pit, m; arcsh is the inverse hyperbolic cosine function.

Fig. 4.13
figure 13

A foundation pit partially penetrating an aquifer. a Unconfined aquifer, b Confined aquifer

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So the entire flow rate of the foundation pit in this circumstance is:

$$ Q = q \cdot B = B\left( {{q}_{\text{1}} + {q}_{{\text{2}}} } \right) $$
(4.18)

where B is the width of the foundation pit, m.

In the case of the foundation pit partially penetrating the confined aquifer, shown as Fig. 4.13b, the foundation pit bottom just penetrates the upper confining bed, it has

$$ Q = \frac{{2\pi KsR_{0} }}{{\frac{\pi }{2} + 2{\text{arcsh}}\frac{{R_{0} }}{{M + \sqrt {M^{2} + R_{0}^{2} } }} + 0.515\frac{{R_{0} }}{M}\ln \frac{{R + R_{0} }}{4M}}} $$
(4.19)

4.3.2 Empirical Method

If the project scale is not large, under the moderate groundwater head, an empirical method of unit area seepage amount can be employed. Table 4.4 provides the empirical values of seepage amount under different conditions.

Table 4.4 Seepage amount on unit area in foundation pit
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4.4 The Common Section of the Ditches in Foundation Pit

The section of the drainage ditch is generally as Table 4.5.

Table 4.5 The section of the drainage ditch
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4.5 The Calculation of the Power of Pumps in Requirement

The power in requirement can be calculated by Eq. (4.20).

$$ N = \frac{K_{\rm s} \cdot Q \cdot H}{{102 \cdot \eta_{1} \cdot \eta_{2} }} $$
(4.20)

where H is the total water head, including pumping head, suction head, and head loss generated by various resistance; K s is the safe factor, generally K s = 2; \( \eta_{1} \) is the pump efficiency, 0.4–0.5; \( \eta_{2} \) is the dynamic mechanical efficiency, 0.75–0.85.

To ensure the successful construction, there are always emergency pumps in preparation in case of the accident mechanical failure.

4.6 The Performance of Common Pumps

The performance of common pumps is presented in Table 4.6.

Table 4.6 Performance of general pumps
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4.7 Case Study

Calculate the hydraulic parameter of aquifer by sensitivity analysis method

  1. 1.

    Compile the sensitivity analysis method program by any available software, adding instruction by block diagram;

  2. 2.

    Use pumping test data to calculate the parameter of aquifer by the above designed program. The pumping test data is shown in the following table.

Pumping test data

Radius of pumping well r (mm)

20

Discharge Q (m3/d)

2592

Time t (min)

Drawdown s (m)

Time t (min)

Drawdown s (m)

Time t (min)

Drawdown s (m)

1

0.160

300

0.566

930

0.617

2

0.228

330

0.569

960

0.617

3

0.285

360

0.575

990

0.619

4

0.293

390

0.580

1020

0.622

6

0.321

420

0.583

1050

0.624

8

0.341

450

0.585

1080

0.626

10

0.370

480

0.591

1110

0.627

15

0.387

510

0.595

1140

0.627

20

0.410

540

0.596

1170

0.625

25

0.422

570

0.597

1200

0.624

30

0.443

600

0.598

1230

0.625

40

0.454

630

0.598

1260

0.623

50

0.471

660

0.600

1290

0.624

60

0.484

690

0.602

1320

0.624

90

0.515

720

0.603

1350

0.625

120

0.531

750

0.605

1380

0.625

150

0.541

780

0.608

1410

0.626

180

0.547

810

0.610

1440

0.629

210

0.556

840

0.610

1470

0.629

240

0.560

870

0.613

1500

0.631

270

0.563

900

0.615

1530

0.632

4.8 Exercises

  1. 1.

    What kinds of open pumping method are commonly used? What are application conditions?

  2. 2.

    How to estimate the open pumping water discharge in foundation pit?