1.1 Landslides: An Overview

1.1.1 What Is a Landslide?

We can define a landslide as the movement of rock, detritus, or soils caused by the action

of gravity. To distinguish landslides from other forms of gravity mass flows, we require in the

definition that the bulk of the moving material should have density at least 10% greater that the

density of water. Most landslides are very small: Every year mountain roads need removal of

blocks fallen from the flanks. Larger collapses may affect local watercourses and influence the

activity of local communities; greater slides may provoke disasters and change the

geomorphologic setting of several square kilometers of land. Some landslides evolve very

slowly, and special instruments may be necessary to become aware that they are in fact moving.

Others may travel faster than 100 km/h. And several ones start with a creeping, imperceptible

movement, to suddenly accelerate and degenerate in a catastrophic debris avalanche. Some

landslides travel similar to a fluid, resembling the flow of water. Others are akin to granular

flows. Many landslides come to a halt without affecting vast areas beyond the immediate

surroundings; others plunge into the sea and cause damage hundreds of kilometers away.

In this book, the denomination of gravity mass flow is also adopted. It stands for

a somehow broader class comprising any catastrophic movement due to the action of gravity,

irrespective of the material involved and density. Thus, snow avalanches, catastrophic flood

water waves, hyperpycnal flows (caused by variation in salinity or temperature that affect the

density of water), as well as suspension flows (due to suspension of solid material in air or water,

such as pyroclastic flows and turbidity currents) are gravity mass flows but not landslides. The

division seems at first artificial, and indeed it is partly, but it is justified by the different physics,

means of investigation, and scientists involved in the analysis.

1.1.2 Landslides as a Geological Hazard

The basic motivation behind landslide studies is the prevention and mitigation of

disasters and reduction of risk (Fig. 1.1). Much has been published on this theme, so very little

will be added in the present book. Most landslides are small and their killing potential is limited.

However, large landslides may be very catastrophic. Table 1.1 reports some of the deadliest

landslides in the twentieth century.

Delayed consequences to landslides may also tragically contribute to the death toll. In

1786, a strong earthquake in the province of Sichuan in China released landslide that dammed a

local river for 10 days. As many as 100,000 people were drowned when the dam failed

inundating an area 1,400 km downstream. The Vaiont tragedy (Northern Italy, 1963) was the

result of water spilling over a dam, thrust by the body of a large landslide that invaded the

artificial reservoir. Submarine landslides may generate devastating tsunamis. Accounting for

both the small but frequent and for the large and rarer catastrophic landslides, it has been

calculated that during one average year, landslides kill about 5–7 people in Norway, 18 in Italy,

25–50 in USA, 186 in Nepal, 170 in Japan, and 140–150 in China (Sidle and Ochiai 2006).

Fortunately, this is not an extremely heavy death toll compared to other natural disasters

or car accidents. However, the negative consequences of landslides are not limited to loss of life,

but include the destruction of houses and infrastructures (Fig. 1.1), loss of productivity in the

area affected, unpredictable changes in the local watercourse, and reduction of arable or

habitable land. Estimated costs of landslide damage (including both the direct costs caused by

destruction, and

Fig. 1.1 Top: In addition to the death toll, landslides often interrupt railways and roads and

devastate infrastructures. (a) A boulder has interrupted the railway in Maccagno (Varese, Italy).

It is part of a larger landslide occurred in 2004 in Varenna (Northern Italy). (Photograph courtesy

of G. B. Crosta.) (b) The village of La Conchita in California is frequently affected by killing

landslides. (Photograph USGS of public domain.) Bottom (c) A small landslide in Cortenova

(northern Italy). (Photograph courtesy of G. B. Crosta.)

1.1.3 Landslides as a Geomorphic Driving Force

Landslides contribute significantly to the geomorphic evolution of the natural

environment. As soon as a slope is steepened by tectonic uplift or by river and glacial

erosion, gravity will tend to redistribute the rock or soil and smooth out the terrain. The

instability that frequently ends with the observable, catastrophic event (a rock fall, a rock

slide, topple, or soil movement) is thus a normal result of natural phenomena. For

example, a glacial valley appears as “U” shaped also as a consequence of rock falls and

slope adjustments that after glacier retreat have smoothed the steep valley walls. It is

difficult to assess the amount of landslide material and the total volume of rock and soil

derived from landslides and its distribution, because ancient landslide deposits may have

been covered by more recent sediments and go unnoticed or misinterpreted, for example,

as moraines. Landslides may affect the geomorphology of vast areas, creating new local

topography. Landslide bodies often interrupt the river

networks and give origin to new lakes, like the Fernpass Lake in the Austrian Alps or the

Molveno Lake in the Italian Alps. Normally, lakes dammed by rock avalanches may last for

thousands of years, while those created by debris flows are ephemeral. By reducing ablation,

landslides falling on glaciers act as insulating layers changing positively the mass balance of the

glacier, as documented for example for the Sherman landslide in Alaska (!Box 7.3).

1.2 Types of Landslides

1.2.1 Geometrical Characteristics of a Landslide

Figure 1.4 reproduces an idealized block diagram of a landslide. The failed material starts from a

zone of depletion and deposits in the accumulation zone. The crown of the landslide identifies

the region adjacent to the highest parts of the failed mass. The scarp is the steep rupture surface

between the failed body and the terrain. Several minor scarps due to internal shearing may also

punctuate

Fig. 1.4 Main geometrical elements of a landslide (From Varnes 1978, redrawn and simplified)

the main landslide body. The surface of rupture identifies the interface at the base of the

landslide where the material has slid. In this example the surface appears curved while in other

cases it may be planar of complex. The foot is the material deposited in the accumulation zone,

beyond the surface of rupture. The landslide deposit ends with a toe, which is the line (usually

bent) between the accumulated material and the untouched terrain. The tip is the point of the

landslide deposit farthest from the crown. Some landslides exhibit transverse tension cracks in

the region of the foot closest to the scarp. Closer to the toe, transverse ridges are sometimes

formed. The right and left flanks of the landslide are identified by standing with shoulders to the

crown.

1.2.2 Description of the Seven Types of Movements

Various systems of landslide classification have been proposed, such as the system by Varnes

(1978), Hungr et al. (2001), and Hutchinson (1988). Various authors have contributed to the

EPOCH classification system, which stems from the Hutchinson system (1998). The EPOCH

system recognized seven classes and three material types, for a total of 21 possibilities. These are

listed in Table 1.2.

1.2.2.1 Fall

A fall is the movement of material from a stiff headwall or cliff. It generally involves limited

volumes of material, most usually rock. The material falls en

masse, moving freely in the gravity field. The contact with the terrain occurs especially in the

last part of the trajectory, where the material becomes frequently shattered (Fig. 1.5a).

1.2.2.2 Topple

A topple is the rotation of a vertical slab about a pivoting point located at the base (Fig. 1.5b).

Topple is typical of compact vertical slabs (usually but not exclusively rock) lying on soft,

unconsolidated terrain. The movement may be extremely slow for long periods, culminating with

a catastrophic fall of the slab.

1.2.2.3 Translational Slide

A slide is defined as the movement of material along a shear surface. For a translational slide,

this surface is planar (Fig. 1.5c). The identity of the shear surface is somehow preserved and

distinguishes a slide from a flow.

1.2.2.4 Rotational Slide

In a rotational slide, the detachment surface is roughly circular, spoon-like. In contrast with

translational slides, where the planar surface often originates from a weakness zone, the circular

shape of a rotational slide is created by the failure itself and derives from the geometrical

distribution of the shear stress (Fig. 1.5d).

1.2.2.5 Flow

According to Dikau et al. (1996, p. 149), a flow is “a landslide in which the individual particles

travel separately within a moving mass. They involve whatever material is available to them and

may therefore be highly fractured rock, clastic debris in a fine matrix or a simple, usually fine,

grain size. Flow in its physical sense is defined as the continuous, irreversible deformation of a

material that occurs in response to applied stress.” A flow is thus characterized by a fluid-like

movement, in which the information on the detachment surface has been lost. A slide may

evolve into a flow if the energy and/or the run-out are sufficient to rework completely the

material (Fig. 1.5e). In this work, the word “rock avalanche” will be preferred to “rock flow” to

denote a catastrophic landslide mostly composed of rock, usually very fast and mobile. Rock

avalanches, however, fall into the type “Complex” in the EPOCH classification scheme.

1.2.2.6 Lateral Spreading

It consists of a lateral movement of rock or soil, often of large extension (Fig. 1.5f ). In the case

of rock spreading, the rate is often slow (from one tenth of mm to 10 cm per year) and is

generally caused by deep-seated viscoplastic material underneath the rocky slabs. Soil spreads,

like the ones involving quick clays, can move extremely fast, with speed in the range of several

meters per second.

1.2.2.7 Complex

It is a generic name used when a landslide changes behavior during the movement. In addition to

the rock avalanche (rock slab turned into a granular flow) the EPOCH system considers the flow

slide as a member of this class (Dikau et al. 1996). A flow slide consists of a portion of soil

loosing cohesion during the flow, to the point of becoming a completely fluidized mass.

Fig. 1.5 Six landslide types: (a) fall, (b) topple, (c) translational slide, (d) rotational slide,

(e) flow, and (f) lateral spreading (Some drawings inspired from Dikau et al. 1996)

2.5 Prinsip Kerja Ground Penetrating Radar

a radar system comprises a signal generator, transmitting and receiver ing antennae, and a

receiver that may or may not have recording facilities or hardcopy graphical output. Some

advanced systems have an on board computer that facilities data processing both while acquiring

data in the field, and post recording. The basic constituents of radar system are shown in figure.

The radar system causes the transmitter antenna (Tx) to generate a wavetrain of radiowaves

which propagates away in a broad beam. As radiowaves travel at high speeds (in air 300.00 km/s

or 0.3 m/s

Gambar 2.2 Prinsip Kerja Ground Penetrating Radar ( Butler et al. (1991) and Daniels et

al. (1988).

The travel time of a radiowave from instant of transmission through to its subsequent

return to the receiving antenna (Rx) is of the order of a few tens to several thousand nanoseconds

(ns; 10-9 seconds). This requires very accurate instrumentation to measure the transmit instant

precisely enough for the final accuracy of the system to be reasonable with respect to the travel

times in question. The antennae are used in either a monostatic mode is when one antenna device

is used as both transmitter and receiver, whereas bistatic mode is when two separate antennae are

used with one serving as a transmitter and the other as a receiver. There are specific cases (such

as in wide-angel reflection and refraction (WARR) measurements) when the bistatic mode is

advantageous over the monostatic mode. The pulseEKKO system uses only bistatic antennae.

For the majority of this chapter it can be assumed that any antennae are deployed in monostatic

mode unless indicated other-wise.

The transmitter generates a pulse of radiowaves at a frequency determined by the

characteristics of the antenna being use at a repetition rate of typically 50.000 time per second.

The receiver is set to scan at a fixed rate, normally up to 32 scans per second, depending upon

the system being used. Each scan lasts as long as the total two-way travel time range, which can

be set from a few tens to several thousand nanoseconds. Each scan is displayed on either a video

screen or a graphic recorder or both. As the antenna is moved over the ground, the received

signals are displayed as a function of their two-way travel time of detection by the receiver, in

the form of a radargram. This display is analogous to a seismic section (seismogram).