Effectiveness of horizontal drains for slope stability

H. Rahardjoa,*, K.J. Hritzuka, E.C. Leongb,1, R.B. Rezaurc,2

aSchool of Civil and Environmental Engineering, Nanyang Technological University, Blk N1, #1A-02, Nanyang Avenue,

Singapore 639798, SingaporebSchool of Civil and Environmental Engineering, Nanyang Technological University, Blk N1, #1C-80, Nanyang Avenue,

Singapore 639798, SingaporecSchool of Civil and Environmental Engineering, Nanyang Technological University, Blk N1, #B4-06, Nanyang Avenue,

Singapore 639798, Singapore

Received 29 August 2002; accepted 11 December 2002

Abstract

Horizontal drains have been commonly used in stabilising unsaturated residual soil slopes. This study examines the

effectiveness of horizontal drains in stabilising residual soil slopes against rainfall-induced slope failures under a tropical

climate. The study includes field instrumentation at two residual soil slopes complemented with a parametric study relating to

drain position. Field monitoring results indicate that rainfall infiltration is limited to a certain depth below which infiltration

becomes insignificant. This zone tends to be unsuitable for horizontal drains. Horizontal drains were found to be most effective

when located at the base of a slope. The parametric study indicated conditions under which horizontal drains are effective or

ineffective in improving the stability of a slope. It was also found that horizontal drains have little role in minimising infiltration

in an unsaturated residual soil slope. Benefits of using horizontal drains can be obtained through the lowering of the water table.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Rainfall-induced slope failure; Horizontal drain; Residual soils; Pore–water pressure; Tensiometer

1. Introduction

Singapore is located in the tropics where heavy

rainfalls and high temperatures provide conditions for

rapid and thorough in-situ chemical and mechanical

weathering that result in the development of deep

residual soil profiles. Because of the frequent high

intensity tropical rainfalls, slope failures are common

in this region.

Pierson et al. (1992) observed that landslides in

Hawaii coincided with or followed an extremely heavy

rainfall. Studies of slope failures in Hong Kong

(Brand, 1992) and in Singapore (Pitts, 1985; Tan et

al., 1987; Toll et al., 1999; Rahardjo et al., 2001) also

showed the destabilising effects of short duration, high

intensity rainfalls. Some studies have also shown the

effects of rainfall on residual soil slopes by pore–water

pressure measurements using piezometric and matric

suction (Fredlund and Rahardjo, 1993) measuring

0013-7952/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0013-7952(02)00288-0

* Corresponding author. Tel.: +65-6790-4104; fax: +65-6791-

0676.

E-mail addresses: chrahardjo@ntu.edu.sg (H. Rahardjo),

cecleong@ntu.edu.sg (E.C. Leong), crezaur@ntu.edu.sg

(R.B. Rezaur).

URL: http://www.ntu.edu.sg/cse/.1 Tel.: +65-6790-4774; fax: +65-6791-0676.2 Tel.: +65-6790-6199; fax: +65-6791-0676.

www.elsevier.com/locate/enggeo

Engineering Geology 69 (2003) 295–308

devices (e.g., Pitts, 1985; Lim et al., 1996; Rahardjo

et al., 1998, 2000). In a study in Singapore, Lim et al.

(1996) showed that, in general, matric suction in the

soil increased during dry periods when evaporation

was predominant, resulting in increased shear

strength. Matric suction and shear strength decreased

during wet periods when infiltration was predomi-

nant. Maximum changes were found to occur near

the ground surface. Short duration rainfalls were

shown to leave a considerable amount of matric

suction in the soil, but prolonged heavy rainfalls

destroyed matric suction in the soil zone near the

ground surface even to the extent that a perched

water table would be formed.

The use of horizontal drains as a preventive meas-

ure is a common practice in the region. Horizontal

drains are defined as holes drilled into a cut slope or

embankment and cased with a perforated metal or

slotted plastic liner (Royster, 1980). The purpose of

using horizontal drains as part of landslide control

work is to drain away groundwater, thus keeping the

soil dry. The design and success of these measures are

often governed by local experience alone. The effec-

tiveness of the horizontal drainage system is a func-

tion of many factors including the drain location,

length and spacing, as well as soil properties and

slope geometry. Typically, effectiveness is described

in terms of increase in slope factor of safety as

compared to factor of safety for the case without

horizontal drains. There have been a few studies

(Royster, 1980; Lau and Kenney, 1984; Martin et

al., 1994), which attempted to describe in part the

many parameters controlling the horizontal drainage

design or evaluate the feasibility of using a system of

horizontal drains to lower groundwater levels in hill-

sides (e.g., Craig and Gray, 1985). Martin et al. (1994)

suggested that a small number of drains installed at

appropriate locations in accordance with a well-con-

ceived conceptual groundwater model may be more

effective than a large number of drains installed at

uniform spacing over the slope. Presently, there seems

to be limited information available in the literature in

the area of horizontal drainage.

Negative pore–water pressure or matric suction

(when referenced to pore–air pressure) plays a crucial

role in the stability of earthworks. Although its

importance has been identified, it is not often under-

stood and therefore ignored in many geotechnical

designs. Avoidance of principles of unsaturated soil

mechanics can be attributed to two main reasons. The

first reason is the development of unsaturated soil

mechanics has lagged in comparison to saturated soil

mechanics. Secondly, those who are aware of it will

often question whether or not the negative pore–water

pressure can be maintained throughout the lifetime of

a design. It is for this second reason that this study

was undertaken by attempting to show that dewatering

methods can be used to aid in the control of negative

pore–water pressures in a slope. Thus, it is expected

that the negative pore–water pressure or matric suc-

tion will not be destroyed and can therefore be in-

cluded in design.

2. Field study

Since the destabilisation effects of rainfall infiltra-

tion on slopes are clear, a study was undertaken to

investigate the influence of horizontal drains on

slope stability (Hritzuk, 1997). Two different sites

(NTU-CSE slope and Nanyang Heights slope),

located on the Nanyang Technological University

(NTU) campus in Singapore, were used in this

investigation. The locations of the slopes are shown

in Fig. 1. Both slopes were instrumented to monitor

the effectiveness of the drainage system as related to

rainfall.

The geology of Singapore consists of three for-

mations: (a) igneous rocks of granitic or similar

composition (Bukit Timah Granite) in the center and

northwest region, (b) sedimentary rocks (Jurong For-

mation) occupying the west region and (c) a semi-

hardened alluvium (Old Alluvium), which masks older

rocks beneath in the eastern region (PWD, 1976). The

oldest rock in Singapore, Bukit Timah Granite, prob-

ably comes from the Plaeozoic era, which ended about

225 million years ago. Granite occurs in two separate

masses. The larger one is found in the central and

northern areas; the smaller one in parts of northern

Singapore. According to radioactive age determina-

tion, the granite in Singapore is more than 200 million

years old (Leong et al., 2002). The sedimentary rocks

of the Jurong Formation skirt the central granite and

form extensive areas in the southwest, southwestern

and western Singapore. The Jurong formation is

composed of a series of sedimentary rocks such as

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308296

sandstone, siltsone, mudstone, shale, tuff, conglom-

erate and limestone. The formation has been severely

folded and faulted in the past as a result of tectonic

movements. The semi-hardened Old Alluvium was

deposited by an ancient river system, probably in the

Pleistocene epoch, during a low stand of the sea.

Residual soils of the granitic and sedimentary rocks

occupy about two-thirds of the land area of Singapore.

The present day configuration and much of the

morphology of the low-lying areas of Singapore is a

result of erosion and deposition during the period of

fluctuating sea levels in the late Tertiary and Quater-

nary. As sea level rose after the end of the last cold

stage about 11,000 years ago, it formed Singapore a

group of islands, separated from the Malay Peninsular

by the straits of Johor. Extensive marine clay, beach

deposits and associate terrestrial sediments were

deposited around the rocky flanks of the island

(Leong et al., 2002).

The relief of Singapore is relatively gentle. Only

10% of the land area is over 30 m high and more than

60% of the land is less than 15 m high (Pitts, 1984a).

The area of granitic and other igneous rocks in the

center of the island forms a landscape of rounded

hills and gentle spurs and valleys. The coast is flat,

but in few places cliffs can be found. Considerable

stretches of the coastline have been markedly modi-

fied by reclamation work, building of embankments

and swamp clearances.

The NTU campus is located on the sedimentary

rocks of Jurong formation. The geological formations

at the NTU campus are very complex as the area has

been subjected to intrusions, isoclinical overfolds and

faults in the past (Pitts, 1984a,b). The general strike of

Fig. 1. Location of NTU-CSE and Nanyang Heights slopes.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308 297

the Jurong formation is northwest/southeast and the

dips of the formation may vary over short distance from

a few degrees to vertical or overturned (Leong et al.,

2002). There are a number of major and minor faults in

the Jurong formation with displacement ranging from

unknown distances to a few decimeters. The major

faults are normally infilled with clay gouge, which is

extremely soft when wet. The upper portions of the

Jurong Formation have been heavily weathered into

residual soils of clayey silts to silty clays. Both instru-

mented slopes on the NTU campus consist of residual

soils from the Jurong sedimentary formation overlying

siltstone and sandstone as the bedrock.

2.1. Site description, instrumentation and monitoring

results

2.1.1. NTU-CSE slope

The slope surface of the NTU-CSE site measured

approximately 10 m wide by 24 m long rising at 2:1

Fig. 2. Layout of instrumentation and soil profile characteristics at NTU-CSE slope: (a) Plan view, (b) Section A-A.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308298

(H:V) to a height of about 12 m. Twelve horizontal

drains made from 6-m long and 89-mm diameter

perforated PVC pipes were wrapped in a geofilter

and installed in four rows in the study area (Fig. 2a

and b). Each row contained three drains, which were

spaced at 2-m intervals and inclined at a 10% gra-

dient. Thirty tensiometers were installed to measure

matric suctions at varying depths amongst the drains

(Area A, Fig. 2a). Another 35 tensiometers were

installed approximately 7 m away from the drained

area (Area B, Fig. 2a) for another study (Gasmo,

1997), but serves as a point of comparison between

drained and undrained conditions. Five piezometers

and a rain gauge were installed within the instru-

mented area. All instruments were connected to a

computer data acquisition system located in the Geo-

technic Laboratory near the site. A schematic diagram

of the instrument layout and soil profile characteristics

are shown in Fig. 2a and b.

Based on the borehole profiles through this site, it

was determined that the soil zone being studied was

typically a tropical residual soil of one soil type: a

dark purple hard and brittle silty clay, which in some

locations contained fine streaks of white clay. This

purple silty clay was overlain by a shallow layer of

light brownish orange silty clay mottled with some

white clay at the crest of the slope. The soil proper-

ties are given in Table 1, and the soil–water char-

acteristic curves and permeability functions for the

orange and purple soils are shown in Figs. 3 and 4,

respectively.

A 4-month data set was compiled from the

instrumentation of this site. Tensiometer data from

most of this zone showed that the slope remained

unsaturated during this period. On some occasions,

pore–water pressures would go slightly positive in a

few specific locations but the average pore–water

pressure at each depth remained negative (Fig. 5). A

similar depth of infiltration and pore–water pressure

response was found for the undrained section. Fig. 5

demonstrates that pore–water pressures were very

dynamic (i.e., changed frequently in response to

climatic conditions) at the 0.3-m depth due to rainfall

events and dry periods where the infiltration and

evaporation took place, respectively. During rainfalls,

Table 1

Properties of two soil types found at NTU-CSE slope

Properties (1) Purple soil (2) Orange soil (3)

Effective cohesion, cV 90 kPa 20 kPa

Effective angle of internal

friction, /V35.0j 26.5j

/b angle (angle indicating

the rate of a change in

shear strength relative to

a change in matric suction)

35.0j 23.0j

Saturated coefficient of

permeability, ks

2.8� 10� 9 m/s 7.8� 10� 7 m/s

Saturated density, qsat 2.30 Mg/m3 2.14 Mg/m3

Saturated water content, wsat 14.0% 28.8%

Specific gravity, Gs 2.72 2.67

Liquid limit, LL 34.0% 47.0%

Plasticity index, PI 13.5% 21.0%

Sand 15% 23%

Silt 42% 26%

Clay 43% 51%

USCS group symbol CL CL

USCS: Unified Soil Classification System.

Fig. 3. Soil–water characteristic curve and permeability function for

orange soil at NTU-CSE slope: (a) soil –water characteristic curve,

(b) permeability function.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308 299

the pore–water pressures rose very quickly at the

shallow depths. At these depths, evaporation was

also very significant creating high negative pore–

water pressures. Tensiometers often cavitated during

prolonged dry periods. Cavitation in the tensiometers

is evident in the months of July and August and can

be seen as the sudden loss of negative pore–water

pressure measurements (Fig. 5). The influence of

rainfall and evaporation became less significant at

deeper depths into the slope. At the 3.2-m depth,

pore–water pressure response to individual rainfall

events was not really apparent. Pore–water pressures

gradually rose or fell with extended periods of rain

or dry weather, respectively. It therefore appears that

the depth of rainfall influence is within approxi-

mately the first 4-m depth, beyond which the

pore–water pressure remains relatively unaffected

by rainfall.

2.1.2. Nanyang Heights slope

In December 1992, a large shallow slip occurred

on part of the Nanyang Heights slope after a spell of

heavy rainfalls. The slope stood at approximately 28

m in height with a gradient of about 1:1.75 (V:H). The

only slope improvement measure that was imple-

mented prior to the failure was geotextile wrapped,

500-mm-thick gravel drainage blanket placed approx-

imately 2.5 m below the slope surface (see Fig. 6b).

This drainage blanket was examined after the failure

and found to be non-functional due to plugging with

fine-grained soil materials.

The soil in the slip zone was found to be soft to

medium stiff clayey silt, silty clay and sandy clay fill

with a unit weight ct of 20.2 kN/m3, an effective

cohesion cVof 3.5 kN/m2 and an effective angle of

internal friction /Vof 33j. The slope was repaired

with a combination of regrading and horizontal drain-

age. For this design, 6-m length, geofilter wrapped,

75-mm diameter perforated PVC pipes were installed

in a 3.0� 3.0-m grid on the slope surface. Upon

visiting the site in early 1995, it was noted that the

drains might still be active, as there was evidence of

scouring at the drain outlets and some standing water

in the pipes. These effects were possibly attributed to

rainfalls. For this reason, a small instrumentation

program was undertaken to monitor the effectiveness

of the drainage system as related to rainfall.

A total of 16 tensiometers were installed at depths

of 0.3, 0.8, 1.4, 2.3 and 3.2 m. Five tensiometers were

installed in the upper area of the slope without any

horizontal drains (undrained) and 11 tensiometers were

installed in the lower area of the slope with horizontal

drains (drained). The layout showing the locations of

the instrumentation for Nanyang Heights slope is

shown in Fig. 6a and b. All tensiometer readings were

obtained manually from the attached Bourdon gauges.

The location of the groundwater table obtained from

previous observations (23 February 1990, prior to the

shallow slip in 1992) in observation wells is indicated

in Fig. 6b, but, later during the instrumentation pro-

gram of this slope (after the shallow slip), the pre-

existing observation wells could not be located. There-

fore, it was not possible to monitor the water table

readings after the instrumentation of the slope.

Tensiometer measurements for isolated periods of

rainy days (between April to June 1996) show the

dynamics of the pore–water pressure distribution in

Fig. 4. Soil–water characteristic curve and permeability function for

purple soil at NTU-CSE slope: (a) soil –water characteristic curve,

(b) permeability function.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308300

the drained and undrained areas (Fig. 7a and b) at

Nanyang Heights over the course of a few days of

heavy rainfall. An extended period of dry weather

existed prior to each of these events. The measured

results from these events are summarized as pore–

water pressure profiles in Fig. 7a and b. Generally,

Fig. 5. Average pore–water pressure readings at various depths of the NTU-CSE slope: (a) monitoring period: April– June 1996, (b) monitoring

period: July–September 1996.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308 301

Fig. 6. Layout of instrumentation and soil profile characteristics at Nanyang Heights slope: (a) Plan view, (b) Section A-A.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308302

the pore–water pressures at all depths followed the

same trend, rising and falling at the same time with

respect to rainfall and evaporation. For the depths at

0.3, 0.8 and 1.4 m, very little distinction could be

made between the pore–water pressures in the

undrained and drained areas. At the 2.3- and 3.2-m

depths, there was a difference in pore–water pres-

sures between the undrained and drained area, which

was maintained at about 15 kPa. The validity of the

results obtained at 2.3 and 3.2 m in the undrained

area is questionable. Proper contact between soil and

the ceramic cup of the tensiometer may not have been

achieved and this may explain why these tensiometer

readings seemed to be relatively unresponsive when

compared with those in the drained area (see Fig. 7a

and b).

Based on site observations, a couple of drains were

found to be active after a series of heavy rainfalls

ending early June 1996. Fig. 7 does show that the soil

became saturated during rainfall. The resulting drain-

Fig. 7. Pore–water pressure profiles for drained and undrained area at Nanyang Heights slope: (a) drained area, (b) undrained area.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308 303

age only appeared as small water droplets forming at

the drain outlet, which accumulated at a very slow

rate. Pore–water pressures at the surface tended to be

more negative than at deeper depths, but extended

rainfalls caused the negative pore–water pressures to

be eliminated. Fig. 7 also shows that full saturation

occurred down to a depth of about 2 m, which was

when the above mentioned drains started showing

activity. It indicates that the soils were still capable

of becoming saturated even though the drainage

system was in place. Therefore, these shallow drains

seemed to have little significance in removing infil-

trating rainwater.

Using the field monitoring results from NTU-CSE

and Nanyang Heights slopes, some general observa-

tions can be made. At NTU-CSE slope, it appears that

the negative pore–water pressure zone was not cre-

ated by the drainage system. If that were true, pore–

water pressures would have been positive for a sig-

nificant period of time (as in the case at Nanyang

Heights slope) and there would have been evidence of

seepage from the drain outlets. The negative pore–

water pressure zone existed simply because the water

table was located significantly deeper than the zone

with horizontal drainage. The maximum depth of the

piezometers was more than 10 m below the slope

surface and yet the piezometers remained empty. At

Nanyang Heights slope, drainage develops well after

the rainfall suggesting that the soil were drained too

slowly. Pore–water pressures would therefore have

built up and become most critical just at the end of the

rainfall. It seems that, whether the drains were present

or not, would be of no consequence to the pore–water

pressure condition within the slope.

An understanding of relationship between drainage

and infiltration in the upper few meters of a slope was

gained from NTU-CSE and Nanyang Heights slopes,

but data were inadequate to form a concept of the

function of horizontal drainage at deeper depths in the

vicinity of the main water table. As such, numerical

analysis was adopted to supplement the understanding

of the importance of drain location.

3. Parametric study

A parametric study was performed to investigate

the significance of drain location within a slope. The

seepage analysis was performed using the finite ele-

ment seepage software Seep/W (Geo-Slope, 1998a)

and the stability analysis was performed using the

limit equilibrium slope stability software Slope/W

(Geo-Slope, 1998b).

A homogeneous slope rising at 45j to a height of

30 m was used as the model for this parametric study.

The slope configuration, horizontal drainage location

and finite element mesh configuration for the para-

metric study are shown in Fig. 8a and b. Five

scenarios were analyzed based on different drainage

configurations. Three evenly spaced drain locations

were considered as shown in Fig. 8a. The first

scenario involved the slope configuration without

drainage. The next three scenarios involved the slope

configuration with each drain on its own and the fifth

scenario incorporated all three drains. Soil properties

used in this model were as follows: ct of 21.0 kN/m3,

an effective cohesion cVof 20 kPa and an effective

angle of internal friction /V of 26.5j, the angle

indicating the rate of a change in shear strength

relative to a change in matric suction (Fredlund and

Rahardjo, 1993) /b of 23j. The soil–water character-istic curve and permeability functions shown in Fig.

3a and b, respectively, were also used in conjunction

with these soil properties for the analysis.

Each horizontal drain was specified internally

along the edges of elements as a line of zero flux

and reviewed by maximum pressure. During the

iterative process, drain nodes were checked for instan-

ces where pressures were calculated to be greater than

zero. If such was the case, the nodal value was

adjusted back down to zero pressure or, in other

words, the total head was equated to the elevation

head. In the instance where pressures of the drain

nodes were less than the specified zero value, the

nodal values remain as such and were treated like any

other node. Therefore, there was no ‘backflow’ into

the slope model which would be the case if the drain

nodes were fixed at a constant zero pressure without

review and the surrounding region was unsaturated.

This review by maximum pressure allowed the model

to be representative of the case where one would

expect drains to be active only under positive pore-

water pressure conditions or in the saturated part of

the slope. If drains were in the unsaturated part of the

slope, the groundwater would move throughout the

slope as if the drains were not present at all.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308304

Steady-state conditions were established for all

different drainage scenarios. The steady-state water

table was developed below the drained zones so that

each initial condition was identical. In other words,

the drainage configurations had no influence on the

initial conditions. Subsequently, a rainfall rate equal to

the saturated permeability was applied to the slope

and the transient process was monitored. Data from

each time step were saved over the course of the

transient analysis. The transient analysis was termi-

nated at a time, which showed no significant differ-

ence in seepage results from the previous time step.

The Seep/W program has a ‘flux line’ feature to

measure flow across any cross-section drawn in the

model prior to the analysis. Flux lines were used in

this model around each drain location and on the slope

surface. Flux lines drawn around drains incorporated a

zone of soil into which seepage flow was monitored

and compiled as flow per unit length across each flux

section. Flux values calculated for the slope surface

were not easily interpreted because there was a

countering effect of positive inflow across the boun-

dary in the form of rainfall and a negative outflow due

to the establishment of the seepage face on the slope.

Flux values in the vicinity of the drain locations

tended to be more reliable as there was no significant

multidirectional flow across the boundaries. Flux

values obtained for seepage flow through the bottom

zone for different drainage configuration are presented

in Fig. 9.

It was found that a totally unrealistic rainfall

condition and time frame were required to create a

Fig. 8. Finite element model for the parametric study: (a) slope configuration and horizontal drain location, (b) finite element mesh.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308 305

significant change in the soil pore–water pressure. A

continuous rainfall rate equivalent to ks was applied to

the slope for all five drainage configurations until

equilibrium was achieved. It took more than 20,000

consecutive days of continuous rainfall in all cases in

order to achieve equilibrium. This meant that satura-

tion would never occur in this type of soil and there-

fore installing horizontal drains in such material

would be pointless. However, the model still served

the purpose of illustrating the relative benefits of drain

positioning.

A factor of safety was calculated for each drainage

scenario at every time step by importing the pore–

water pressure head files into Slope/W model. The

results of the slope stability analyses are shown in Fig.

10. The general trend of Fig. 10 shows that the least

amount of benefit was derived from the drains located

in the upper region of the slope. The most benefit was

derived from the drain located at the bottom of the

slope and a very small additional benefit was obtained

by combining the bottom drain with other drains in

the upper region. This is shown by comparing the

factor of safety obtained from the case with three

drains as opposed to the case with one drain at the

bottom of the slope. The condition with the three

drains only gives a 3% additional factor of safety as

compared to the condition with the bottom drain. The

reason for the slight difference in factor of safety is

that the bottom drain attracted the most water. It can

be seen in Fig. 9 that the seepage flow in the bottom

zone was relatively the same when either a single

drain was present in this bottom zone or if all three

drains were present in the slope. When the three

drains were present, the top and middle zones showed

approximately two orders of magnitude less flow than

the bottom drain. This again shows the bottom drain

as most efficient in draining out the water and main-

taining the stability of the slope.

4. Conclusions

Both the field study and parametric study show the

importance of locating a horizontal drain as low as

possible in the slope. It attracts the majority of

groundwater and has the largest effect on lowering

the water table. This confirms the findings of Lau and

Kenney (1984) and Martin et al. (1994). The para-

metric study has also shown that drains located in the

upper region of a slope are of no real significance as

long as a drain is already located in the lower part of a

slope. The water table will eventually be reduced to

the lowest drain level and any drains above the bottom

drain will no longer serve any significant use. It

appears that rainfall does not cause the water table

to rise much above the drainage zone once the water

table has been established at the lowest level. Martin

et al. (1994) has also noted similar behaviour.

Installing drains in the upper region of a slope for

the purpose of intercepting perched water tables and

infiltrating rainwater does not appear to be effective as

was evident from NTU-CSE and Nanyang Heights

slopes. This is consistent with observations made by

Martin et al. (1994). On very rare occasions, one or

two drains from the NTU-CSE and Nanyang HeightsFig. 9. Theoretical flux through bottom zone for various drainage

configurations.

Fig. 10. Theoretical factor of safety with respect to drain position for

a constant rainfall rate.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308306

sites showed activity but only in the form of water

droplets slowly forming at the drain outlet. This

indicates that a perched water table developed in the

vicinity of these drains. Drainage took place well after

the rainfall suggesting that the soil was draining too

slowly as compared to the infiltration rate. Pore–

water pressures would therefore have built up and

become most critical just at the end of the rainfall

regardless whether the drains were present or not.

One notable observation that can be made with the

data collected from the CSE site is how the influence

of rainfall becomes less significant with depth. The

tensiometers at the 0.3-m level were highly responsive

to rainfall events but the tensiometers located at 3.2-m

depths showed little more than a gradual rise or fall in

pore–water pressure over an extended period of time.

This suggests that there exists a zone of influence,

perhaps not much deeper than 3.2 m, where rainfall

effects have no more influence. Brand (1984) stated

that the wetting band thickness calculated for soils in

Hong Kong was around 2 m in a wet season with a

return period of about 10 years. Lau and Kenney

(1984) stated that, for clay soils with small values of

coefficient of consolidation (cvc 0.1 m2/day), depths

of soil greater than about 7 m below the ground

surface are stable and insensitive to changes of

hydraulic conditions at the ground surface. The impli-

cation of such a feature is that a water table could

potentially be lowered below this zone of rainfall

influence. Since it appears that horizontal drains can

maintain the water table after drawdown, the soil that

exists above this water table and below the zone of

influence should have a negative pore–water pres-

sure. This negative pore–water pressure should not

fluctuate and therefore an engineer should have the

confidence to incorporate this unchanging part of the

unsaturated zone in design.

5. Recommendations

When designing horizontal drainage systems, more

emphasis should be placed on lowering of the main

water table and not so much for preventing the

development of perched water tables. Lowering the

water table should be done using horizontal drains

that are placed as low as possible in the slope.

Horizontal drains located in the upper regions of the

slope are unnecessary in the long term.

Shallow drains do not effectively remove ground-

water near the surface. Problems with perched water

tables should be mitigated with measures that do not

permit water to infiltrate into the slope such as

chunam (a thin layer of low permeability, cement–

Fig. 11. Conceptual drain design.

H. Rahardjo et al. / Engineering Geology 69 (2003) 295–308 307

lime–soil mixture for reducing infiltration (Fredlund

and Rahardjo, 1993)) or other soil covers, vegetative

or geotextile covers, and proper surface drainage. The

depth of zone of influence that rainfall has on the soil,

which may range from 2 to 7 m, should be established

for local slopes. If the soil is intact, some tension

cracks may be expected to develop resulting in an

increase of the depth of this influence zone. The

region above the already lowered groundwater table

and below the zone of influence due to rainfall could

potentially be considered to maintain a constant

matric suction profile that can provide an additional

factor of safety to the slope via the gain in shear

strength. This constant matric suction zone can be

incorporated in the design as conceptualized in Fig. 11

that considers the above criteria.

Acknowledgements

This work was funded by a research grant from

Nanyang Technological University, Singapore (Grant:

RG22/97). The authors gratefully acknowledge the

field assistance of the Geotechnics Laboratory staff,

School of Civil and Environmental Engineering,

Nanyang Technological University, Singapore, during

the field instrumentation, trouble shooting and data

collection for this study. The second author gratefully

acknowledges the research scholarship made available

by NTU for this study.

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