evaluation of infiltration capacity and water retention ... · capacities of soil using bamboo...

Journal of Earth Science and Engineering 6 (2016) 150-163 doi: 10.17265/2159-581X/2016.03.002

Evaluation of Infiltration Capacity and Water Retention

Potential of Amended Soil Using Bamboo Charcoal and

Humus for Urban Flood Prevention

Rei Itsukushima1, Kazufumi Ideta2, Yuki Iwanaga3, Tatsuro Sato1 and Yukihiro Shimatani4

1. Department of Decision Science for Sustainable Society, Kyushu University, 744, Motooka, Nishi-ku, Fukuoka 819-0395, Japan

2. River Basin & Infrastructure Division, Dam & Hydraulic Power Department, NIPPON KOEI CO., LTD., 1-14-6, Kudan-Kita,

Chiyoda-ku, Toky 102-8539, Japan

3. Department of Urban and Environmental Engineering, Graduate School of Engineering, Kyushu University, 744, Motooka,

Nishi-ku, Fukuoka 819-0395, Japan

4. Department of Urban and Environmental Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

Abstract: In Japan, floods occur frequently in urban areas because non-infiltrating areas are seeing increased urbanization. To prevent floods, urban basins must improve the infiltration capacity and water retention of the whole basin. There are several basic technologies for river basin management, such as infiltration trenches or rainwater storage. However, a method of soil amendment that prevents flood disasters has not been established. This study aims to evaluate the infiltration capacity of soil amendments using bamboo charcoal and humus. A constant-head infiltration test and rainfall simulation were conducted to evaluate the properties of the soil amendments. The constant-head infiltration test’s results showed that soils mixed with 30% humus had the greatest potential for influencing initial and final infiltration rates, and the more the mixing rates of bamboo charcoal and humus were increased, the higher the water retention capacity. The results of the rainfall simulation showed that soils mixed with 30% humus had the highest final infiltration rates and lowest multiplication spillage. To reduce the runoff volume using soil amendment technology, it is important to delay overland flow, and the hydraulic properties of the soils mixed with bamboo charcoal and humus were as effective as those of granite soils. Key words: Soil amendment, infiltration capacity, urban flood prevention, constant-head infiltration test, watering experiment.

1. Introduction

Urbanization changes the runoff mechanism and

causes various problems related to flood-control,

water utilization, and river environment, such as

increasing the flow discharge rate, decreasing the flow

discharge in the normal state [1], or causing water

quality deterioration by CSO (combined sewer

overflow) [2]. Recently, flood disasters caused by

localized torrential rain in urban areas have occurred

frequently in Japan [3]. This phenomenon is

considered to have become more serious because of

Corresponding author: Rei Itsukushima, Ph.D. in

engineering, assistant professor, research field: river engineering.

global warming. The Intergovernmental Panel on

Climate Change predicted that localized torrential rain

will occur more frequently in the mid-latitude region

[4]. In addition, CSO causes the serious problem of

water quality deterioration by heavy rain in cities

where combined sewers have been adopted.

The concept of “integrated watershed management”

has strengthened the infiltration, retention, and storage

capacities of the whole basin, which is effective for

reducing the impact of urbanization on flood control,

water utilization, and river environment [5]. Legal

systems or policies were established to mitigate

flooding in urban areas, such as the Integrated Urban

Flood Prevention Plan for Severe Rainfall, or the Act

D DAVID PUBLISHING

Evaluation of Infiltration Capacity and Water Retention Potential of Amended Soil Using Bamboo Charcoal and Humus for Urban Flood Prevention

151

on Countermeasures against Flood Damage of

Specified Rivers Running Across Cities in Japan.

However, damage by urban flooding has not been

reduced owing to the lack of element technology

development for mitigating urban flooding and a

quantitative rating of these technologies.

A green infrastructure approach to manage storm

water was first adopted in New York City [6]. For

instance, storm water is infiltrated or impounded by

rain gardens [7], green roofs [8], permeable pavement

[9], or rain storage tanks [10, 11] to avoid flooding or

effluence of untreated sewage into the river. Runoff

reduction by these green infrastructure element

technologies has been verified and implemented for

practical use. In addition, the water retention capacity

and infiltration of agricultural land was noticed in the

EU [12], and a conservation policy was established to

mitigate flooding.

It is obvious that most soils have high infiltration

and retention capacities. In the agricultural sector,

there are advanced studies on enhancing infiltration

and water retention capacities using Shirasu soil and

seashells, respectively [13]. However, there are few

studies on improving the infiltration capacity of soil

for flood mitigation.

In this study, we focused on bamboo charcoal and

humus as the materials for improving the water

infiltration capacity of soil. We also conducted a

constant-head infiltration test and rainfall simulation

to evaluate the properties of soil amendments.

2. Methodology

2.1 Concept of Runoff Prevention Using Soil

Amendments

This study aims to establish outflow prevention

technology by enhancing the infiltration and retention

capacities of soil using bamboo charcoal and humus.

In this section, we describe the concept of

strengthening infiltration capacity by soil amendment

and its physical mechanism.

Infiltration is a phenomenon in which water on the

ground surface penetrates into the soil. At the

beginning of rainfall, effective rain is spent in

moistening the ground, and surface runoff does not

occur. Surface runoff occurs when the effective rain

exceeds subsurface percolation. This subsurface

percolation is described by the Horton infiltration

equation (Eq. (1)):

ktcc effff )( 0 (1)

where f is the infiltration amount, f0 is the initial

infiltration amount, fc is the final infiltration ratio, t is

time, and k is a constant.

Fig. 1 presents a conceptual diagram of soil

moisture fluctuation and surface runoff emergence.

When rainfall (r) occurs, rainwater reaches the ground

surface, and the amount of soil water (θ) increases. In

the case of r < fc, all rainwater infiltrates into the soil,

and surface runoff does not occur. In the case of fc < r,

surface runoff occurs at the time when rainfall (r)

exceeds the infiltration amount (f) and the amount of

soil water reaches θ0. From this point forward, surface

runoff occurs, which is indicated by the colored area.

Fig. 1 Conceptual diagram of soil moisture fluctuation and surface runoff emergence.

土壌

深度

z

θ

θi

r

f

tt'ptpθ'0θ0

Dep

th z


152

It is essential to strengthen the infiltration capacity

of the soil in order to achieve urban flood mitigation.

The following two methods are considered to reduce

the runoff amount:

(1) Delay the time to reach critical saturation and

the occurrence of saturation (tp→tp’);

(2) Enhance the final infiltration ratio (fc→fc’).

2.2 Experimental Setup for the Constant-Head

Infiltration Test

We measured the infiltration characteristics of

amended soil by measuring the soil water content

using a profile probe. We implanted a cylinder, which

was filled with amended soil, into the soil and

supplied water to the cylinder. Cylinder intake rate

tests were used as a reference for constructing the

experimental setup [14].

Fig. 2 shows the configuration of the experimental

system. A cylinder (PVC pipe VU, inside diameter: 20

cm, length: 60 cm) was vertically introduced into the

soil to a depth of 50 cm and filled with amended soil.

Waterproofing material (bentonite) was coated onto

the outer bottom edge to prevent soil water entering

from outside. Amended soil was added at 10-cm

intervals and tamped using a ram (weight: 10.5 kg).

We used seven types of soil in this experiment. The

mixing ratios of the improved soil by volume ratio

were: (1) 100% decomposed granite; (2) 90%

decomposed granite and 10% bamboo charcoal; (3)

80% decomposed granite and 20% bamboo charcoal;

(4) 70% decomposed granite and 30% bamboo

charcoal; (5) 90% decomposed granite and 10%

humus; (6) 80% decomposed granite and 20% humus;

(7) 70% decomposed granite and 30% humus. The

mixed bamboo charcoal was 2-4 mm granular

charcoal. Bamboo charcoal and humus were

sufficiently mixed with the surface-dried decomposed

granite so as to be equitable.

Fig. 2 Experiment system of constant-head test.

PVC pipe

Profile Prove

amendment soil

mariotte’s bottle

wind shield

Bentonite

50 cm

10 cm

weight scale

sensor① （depth 5 cm）sensor② （depth 15 cm）sensor③ （depth 25 cm）sensor④ （depth 35 cm）


153

We measured the decrement of water in a Mariotte’s

bottle and electric permittivity in the amended soil

using a profile probe (Delta-T Devices, PR2/4) at

one-minute intervals. The sensors of the profile probe

were installed at four depths from the soil surface: 5,

15, 25, and 35 cm. Soil water content was measured on

the basis of the ADR (amplitude domain reflectometry)

method [15]. We calibrated the relationship between

the volumetric water content and electric permittivity

for each experimental case because this relationship

was different between mixing ratios.

The water supply to the cylinder was stopped when

the electric permittivity of the lowest censor (35 cm

from the soil surface) became a constant value: at this

point, it was considered that the amended soil was

saturated. After the soil was saturated, we tracked the

drainage process by measuring electric permittivity.

The electric permittivity measurements were

completed when the value of the lowest sensor

became constant.

2.3 Experimental Setup for the Watering Experiment by Rainfall Simulation

In the watering experiment, the method was to apply

a coarse water spray from above to imitate actual

rainfall. A characteristic of this method is that hydraulic

pressure at submerged depths cannot be provided, but

a water supply close to the actual precipitation is possible.

In an actual rainfall, water droplets with radii greater

than 0.1 mm, called raindrops, fall in a state in which

the downward force of gravity and upward force of air

resistance are balanced. However, a large space and

artificial rainfall equipment with a height greater than

20 m are necessary to reproduce the actual rain

conditions of a particle diameter, dropping velocity,

and rainfall amount. In this study, we established an

artificial rainfall device to reproduce only the rainfall

amount. Fig. 3 illustrates the configuration of the

experimental system. The artificial rainfall device

could reproduce rainfall rates from 48 to 200 mm/h

using a centrifugal humidifying device (NAKATOMI

Fig. 3 Experiment system of watering experiment.

amendment soil

humidifying device

soil moisture meter

acrylic cylindrical tube

raingage

spreading filter


154

Ltd., AHF 10). A diffusion filter was installed to

spread and up-size the droplets.

The acrylic cylindrical tube (inside diameter: 30 cm;

length: 70 cm) was filled with amended soil and

supplied with water from an atomizer placed above it.

We measured the amount of supply water and surface

runoff volume. In addition, the volumetric water content

was measured using a soil moisture meter (Campbell

Ltd., CS616 TDR) at ten-second intervals. The sensors

of the soil moisture meter were installed at four depths

measured from the soil surface: 5, 15, 25, and 35 cm.

We examined seven types of amended soil ((1) 100%

decomposed granite; (2) 90% decomposed granite and

10% bamboo charcoal; (3) 80% decomposed granite

and 20% bamboo charcoal; (4) 70% decomposed

granite and 30% bamboo charcoal; (5) 90% decomposed

granite and 10% humus; (6) 80% decomposed granite

and 20% humus; and (7) 70% decomposed granite and

30% humus), which were identical to those used in the

constant-head infiltration test. The rainfall rate was set

in two patterns at 50 and 100 mm/h.

3. Results

3.1 Results of the Constant-Head Infiltration Test

As the result of the constant-head infiltration test,

the volumetric water content increased soon after the

starting the water supply, became stable at the

saturation point, and gradually decreased in all cases.

In this section, we describe the time change of the

volumetric water content of the amended soil at each

depth, using three cases ((1) 100% decomposed

granite; (4) 70% decomposed granite and 30%

bamboo charcoal; and (7) 70% decomposed granite

and 30% humus) as examples.

In the experimental case of (1) 100% decomposed

granite (Fig. 4), the maximum volumetric water

content was 41.8% at a depth of 15 cm and became

constant 230 minutes after starting the water supply.

About seven hours after the water supply stopped, no

significant change in volumetric water content was

observed. Subsequently, the volumetric water content

declined drastically and became stable 20 hours after

stopping the water supply. The volumetric water

content at each depth was reduced to 21.8% (5 cm),

30.4% (15 cm), 28.9% (25 cm), and 27.2% (35 cm) 49

hours after stopping the water supply.


granite and 30% bamboo charcoal (Fig. 5), the

maximum volumetric water content was 45.3% at a

depth of 5 cm. We determined that the amended soil

was saturated 210 minutes after starting the water

supply because the volumetric water content at the

deepest point became constant. The volumetric water

content at a depth of 5 cm began to decrease one hour

after stopping the water supply and became stable at

40%. Subsequently, it declined rapidly and then

gradually decreased. Similar behavior in the time

change of volumetric water content was also observed

at other depths. The volumetric water content at each

depth was reduced to 26.4% (5 cm), 38.2% (15 cm),

38.2% (25 cm), and 37.0% (35 cm) 73 hours after

stopping the water supply.


granite and 30% humus (Fig. 6), the maximum

volumetric water content was 40.0% at a depth of 5

cm. We determined that the amended soil was

saturated 200 minutes after starting the water supply

because the volumetric water content at the deepest

point became constant. The volumetric water content

at a depth of 5 cm began to decrease rapidly seven

hours after stopping the water supply and returned to a

moderate decrease 17 hours after stopping the water

supply. Similar behavior in the time change of

volumetric water content was also observed at depths

of 15 and 25 cm. The volumetric water content at the

deepest point (35 cm) increased after stopping the

water supply. After the volumetric water content

increased to 41%, it began decreasing 17 hours after

stopping the water supply. The volumetric water

content at each depth was reduced to 22.6% (5 cm),

30.2% (15 cm), 32.2% (25 cm), and 31.8 % (35 cm)

105 hours after stopping the water supply.


155

Fig. 4 Result of constant head infiltration test (1).


Fig. 7 shows the relationship between infiltration

capacity and elapsed time in each of the experimental

cases. The infiltration capacity (mm/h) was calculated

from the decrement of water in the Mariotte’s bottle

using the Horton infiltration equation (Eq. (1)). We

defined the first measured infiltration ratio as the

initial infiltration amount (f0) and the last measured

infiltration ratio, at the end of water supply, as the

final infiltration ratio (fc). The damping constant (k)

was calculated by the least-square method as the

actual measured value. The initial infiltration amount

(f0) for 100% decomposed granite was the lowest, and

the infiltration capacity increased with increasing

mixing ratios of bamboo charcoal and humus.

3.2 Results of the Watering Experiment

As an overall trend, we identified the increasing

volumetric water content as time proceeded in common

0

5

10

15

20

25

30

35

40

45

9:00 13:00 17:00 21:00 1:00 5:00 9:00 13:00 17:00 21:00 1:00 5:00 9:00 13:00 17:00 21:00

体積

含水率(%

)

時刻

センサー①

センサー②

センサー③

センサー④

11/10 11/11 11/12

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60

elapsed time (hr)

sensor①

sensor②

sensor③

sensor④

0

5

10

15

20

25

3030

35

40

45

volu

met

ric

wat

er c

onte

nt (

%)

20

25

30

35

40

45

50

12:00 18:00 0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00

体積

含水率(%

)

時刻

センサー①

センサー②

センサー③

センサー④

12/7 12/8 12/9 12/10

0

elapsed time (hr)

6 12 18 24 30 36 42 48 54 60 66 72 78 84020

25

30

35

40

45

50

volu

met

ric

wat

er c

onte

nt (

%)

sensor①

sensor②

sensor③

sensor④


156


Fig. 7 Time change of infiltration capacity.

with the constant-head infiltration test. In this section,

we describe the time change of the volumetric water

content in amended soil at each depth, using three

cases at 50 mm/h as examples: (1) 100% decomposed

granite; (4) 70% decomposed granite and 30%

bamboo charcoal; (7) 70% decomposed granite and

30% humus.

In the case of 100% decomposed granite (Fig. 8),

the maximum volumetric water content was 0.29 at a

depth of 5 cm, and the experiment time was 150 minutes.

5

10

15

20

25

30

35

40

45

0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00

体積

含水率(%

)

時刻

センサー①

センサー②

センサー③

センサー④

1/24 1/25 1/26 1/27 1/28elapsed time (hr)

12 24 36 48 60 72 840 96 108 1205

10

15

20

25

30

35

40

45

volu

met

ric

wat

er c

ont

ent

(%)

sensor①

sensor②

sensor③

sensor④

0

100

200

300

400

500

600

700

800

900

1000

0 10 20 30 40 50 60

浸透

能(m

m/hr)

経過時間(min)

0 10 20 30 40 50 60

elapsed time (hr)

infi

ltra

tion

cap

acit

y (m

m/h

r)

0

100

200

300

400

500

600

700

800

900

1000 case(1)case(2)case(3)case(4)case(5)case(6)case(7)

(1)decomposed grantie 100%(2)mixing bamboo charcpoal 10%(3)mixing bamboo charcpoal 20%(4)mixing bamboo charcpoal 30%(5)mixing humus10%(6)mixing humus20%(7)mixing humus30%


157

Both curves were S-shaped, and the volumetric water

content values were comparable except for the depth

of 35 cm. In the case of decomposed 70% granite and

30% bamboo charcoal (Fig. 9), the maximum

volumetric water content was 0.34 at a depth of 35 cm,

and the experiment time was 150 minutes. The

behavior of volumetric water content gradually settled

down to a stable declining value after rising was

observed at depths of 5, 15, and 25 cm. In the case of

70% decomposed granite and 30% humus (Fig. 10),

the maximum volumetric water content was 0.36 at a

depth of 5 cm, and the experiment time was 149

minutes. The volumetric water content settled down to

a stable value except for the depth of 5 cm.

We calculated the infiltration amounts as the

difference between the watering amount and surface

runoff. The amount of infiltration and runoff for each

of the experimental cases are indicated in Fig. 11 (50

mm/h) and Fig. 12 (100 mm/h), respectively. In the 50

mm/h case, surface runoff occurred earliest in the

100% decomposed granite, approximately 20 minutes

after the start of the experiment. As for the bamboo

charcoal amendment soil, surface runoff occurred

earliest in the 30% mixing ratio, approximately 60

minutes after the start of the experiment. In contrast,

surface runoff did not occur in the cases of (6) 80%

decomposed granite and 20% humus and (7) 70%

decomposed granite and 30%

Fig. 8 Result of watering test (1).


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0:00 0:30 1:00 1:30 2:00 2:30 3:00

体積

含水

率

経過時間

センサー①

センサー②

センサー③

センサー④

00 0.5 1.0 1.5 2.0 2.5 3.0

elapsed time (hr)

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

volu

met

ric

wat

er c

onte

nt

sensor①

sensor②

sensor③

sensor④

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0:00 0:30 1:00 1:30 2:00 2:30 3:00

体積

含水

率

経過時間

センサー①

センサー②

センサー③

センサー④

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

volu

met

ric

wat

er c

onte

nt

0 0.5 1.0 1.5 2.0 2.5 3.0

elapsed time (hr)

sensor①

sensor②

sensor③

sensor④


158


Fig. 11 Amount of runoff and infiltration (50 mm/h).

humus. In the 100 mm/h case, surface runoff occurred

in all of the amended soils. It occurred earliest in the

(2) 90% decomposed granite and 10% bamboo

charcoal at about 10 minutes after the start of the

experiment. The occurrence of surface runoff was

delayed in the cases of: (4) 70% decomposed granite

and 30% bamboo charcoal; (6) 80% decomposed

granite and 20% humus; and (7) 70% decomposed

granite and 30% humus.

4. Discussion

4.1 Influence of Void Structures between Bamboo

Charcoal and Humus on Infiltration and Water

Retention Capacity

Table 1 indicates the initial infiltration capacity and

final infiltration ratio obtained from the constant-head

infiltration test. Initial infiltration capacity is defined

as the infiltration amount per unit of area from the

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0:00 0:30 1:00 1:30 2:00 2:30 3:00

体積

含水

率

経過時間

センサー①

センサー②

センサー③

センサー④

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

volu

met

ric

wat

er c

onte

nt

0 0.5 1.0 1.5 2.0 2.5 3.0

elapsed time (hr)

sensor①

sensor②

sensor③

sensor④

0

10

20

30

40

50

60

0 20 40 60 80 100 120 140

浸透

量・流

出量（ m

m/hr）

経過時間(min)

solid line: runoff amountdash line: infiltration amount

elapsed time (min)0 20 40 60 80 100 120 140


0

10

20

30

40

50

60

runo

ff a

mou

nt /

infi

ltra

tion

am

ount

(m

m/h

r)

case(1)case(2)case(3)case(4)case(5)case(6)case(7)


159

Fig. 12 Amount of runoff and infiltration (100 mm/h).

Table 1 Infiltration capacity obtained from constant head experiment.

start of the experiment to the inflection point of the

penetration curve. We defined final infiltration ratio as

the infiltration capacity per unit area and the time at the

end of water supply. The initial infiltration ratio was

1.7 times larger when mixing 20% bamboo charcoal,

2.9 times when mixing 30% bamboo charcoal, 1.8

times when mixing 20% humus, and 2.3 times when

mixing 30% humus, than it was for 100% decomposed

granite. Initial infiltration capacity increases with

increasing mixing ratios of bamboo charcoal and

humus. The final infiltration ratio was 1.5 times larger

when mixing 20% bamboo charcoal, 2.3 times when

mixing 30% bamboo charcoal, 2.4 times when mixing

20% humus, and 4.3 times when mixing 30% humus

than it was for 100% decomposed granite. Focusing

on the increasing characteristics of infiltration capacity

of bamboo charcoal and humus, the initial infiltration

capacity of bamboo charcoal is greater than that of

humus, whereas the final infiltration ratio of humus is

greater than that of bamboo charcoal. We consider the

cause of these differences as follows: In the case of

bamboo charcoal, the initial infiltration capacity

increases due to large gaps in the surrounding soil.

However, the degree of increase of the final infiltration

ratio is smaller than that of humus mixing soil because

the large gaps are filled by fine soil particles carried

with the water from above (Fig. 13). In contrast, in the

case of humus, fine soil grains are attached to the

aggregate structure of the humus. Its infiltration

capacity is increased by a void structure included in

the aggregate structure (Fig. 14). This void structure is

smaller than that generated by bamboo charcoal.

Therefore, the initial infiltration capacity of humus

mixing soil is smaller than that of bamboo charcoal

0

20

40

60

80

100

120

0 20 40 60 80 100

浸透

量・流出

量（m

m/hr）

経過時間(min)

solid line: runoff amountdash line: infiltration amount

8060402000

20

40

60

80

100

120

elapsed time (min)

runo

ff a

mou

nt /

infi

ltra

tion

am

ount

(m

m/h

r)


case(1)case(2)case(3)case(4)case(5)case(6)case(7)

10% 20% 30% 10% 20% 30%initial infiltration capacity (mm) 106.1 228.7 182.8 311.6 101.4 191.9 243.3final infiltration rate (mm/hr) 8.5 9.8 12.5 19.3 5.0 20.2 36.9

decomposedgrantie soil

bamboo charcoal mixing soil humus mixing soil


160

Fig. 13 Pattern diagram of void structure of bamboo charcoal mixing soil.

Fig. 14 Pattern diagram of void structure of humus mixing soil.

Fig. 15 Initial infiltration capacity of each case.

decomposed granite soil bamboo charcoal mixing soil bamboo charcoal mixing soil(after water supply)

aggregate

decomposed granite soil humus mixing soil

0

50

100

150

200

250

300

350

初期

浸透

量(m

m)

散水実験

冠水実験

真砂土腐葉土

30%竹炭

30%腐葉土

10%腐葉土

20%竹炭

20%竹炭

10%(1) (2) (3) (4) (5) (6) (7)


watering experiment

constant head infiltration test

0

50

100

150

200

250

300

350

initi

al i

nfilt

ratio

n ca

paci

ty (

mm

)


161

mixing soil. However, in the humus mixing soil,

washing out of fine soil particles is prevented by the

aggregate structure, and the final infiltration ratio is

greater than that of bamboo charcoal mixing soil. To

determine the ideal void structure for improving

infiltration capacity and water retention capacity,

further verification is required.

4.2 Comparison of Experiment Results of

Constant-Head Infiltration Test and Watering

Experiment by Rainfall Simulation

Fig. 15 shows the initial infiltration capacity

obtained from the constant-head infiltration test and

watering experiment. The initial infiltration capacity

in the constant-head infiltration test is greater than that

obtained via the watering experiment in all experimental

cases. Fig. 16 shows the conceptual diagram of initial

infiltration capacity obtained from the constant-head

infiltration test and watering experiment. In the

constant-head infiltration test, water submergence

occurs, and water infiltration is promoted by the

pressure of the filling water. Therefore, the initial

infiltration capacity of the constant-head infiltration

test is likely to be overestimated. In contrast, in the

watering experiment, the initial infiltration capacity is

considered to be greater when accompanied by an

increasing rainfall rate. Therefore, to reveal an

extremely accurate value of initial infiltration capacity,

the experiment should be done with a precisely set

rainfall rate. However, the rainfall rate in Japan is up

to approximately 100 mm/h in reality; therefore, it

does not make sense to assume a high degree of

rainfall to examine flood prevention. Therefore, we

decided that an initial infiltration capacity of 100

mm/h for the watering experiment would be an

appropriate value.

Fig. 17 shows the comparison of final infiltration

ratio in the constant-head infiltration test and watering

experiment. The final infiltration ratio in the

constant-head infiltration test is smaller because the

boundary of amendment soil is in contact with the

ground line, and it is difficult for air in the amendment

soil to escape. Therefore, the boundary surface condition

influences the final infiltration ratio, and there is a

need to consider the boundary surface at locations

where soil improvements are being conducted.

Fig. 16 Conceptual diagram of initial infiltration.

100 x

initial infiltration capacity obtained from watering experiment (100 mm/hr)

Extremely-accurate initial infiltration

capacity

initial infiltration capacity obtained from constant head test

S3S2S1

initi

al i

nfilt

ratio

n ca

paci

ty (m

m)

rainfall rate (mm/hr)


162

Fig. 17 Final infiltration ratio of each experiment case.

5. Conclusions

This study aimed to reveal the infiltration

characteristics of amendment soil comprising bamboo

charcoal and humus by means of a constant-head

infiltration test and watering experiment for flood

prevention technology. The acquired knowledge from

the results is summarized as follows:

(1) From the results of the constant-head infiltration

test and watering experiment, it is revealed that the

initial infiltration capacity and final infiltration ratio

became larger along with an increase in the mixing

ratio of bamboo charcoal or humus.

(2) The infiltration characteristics of amendment

soil using bamboo charcoal or humus are dependent

on its void structure. Bamboo charcoal improved the

initial infiltration capacity, whereas humus was better

at improving the final infiltration ratio.

(3) Our experimental results suggest that an

amendment soil using bamboo charcoal or humus is

an effective element technology for comprehensive

integrated watershed management.

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0

5

10

15

20

25

30

35

40

45

50

最終

浸透

能(m

m/hr)

散水実験

冠水実験

真砂土腐葉土

30%竹炭

30%腐葉土

10%腐葉土

20%竹炭

20%竹炭

10%

watering experiment

constant head infiltration test

05

10

1520253035404550

final

inf

iltra

tion

ratio

(m

m/h

r)


(1) (2) (3) (4) (5) (6) (7)


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