Study Landslides in The Field

O: 5/XI-4/GTK/11

A. Field Survey

The purpose of the survey field (site reconnaissance) is to observe, notice and record things that are important in the surface that may affect stability of slopes. Field survey is used for additional information and verify information obtained from studies in the office and aerial photography. This - matters related to the field study is the delivery of tools, field mapping, and field observations, which will be discussed below. modified, if necessary

B. Equipment

Equipment for field survey includes a camera, field log books, pens, pocket penetrometer, shear fan (torvane), level slope, geology hammer, compass (GPS), measuring tape meter long (30 to 60 meters), and others. Photos taken on the existing natural conditions. In addition, photographs were also taken in buildings that have a relationship with an investigation location. Unconfined compressive strength (unconfined compression strength) at the site can be measured with a pocket penetrometer. Saturated clay soil shear strength can be measured with a fan-shear. The degree of hardness of rock weathered in place, can be roughly determined by the geological hammer or more accurately with the hammer Schmidt (Schmidt hammer). Slope is measured with a slope level. Dip angle and dip direction of the connection (joint) can be determined with a compass rock geology.

Strike and dip show the structure of flat (planar structure), such as the shear field (shear surface), fractures (faults), ground plane (bedding plane) and so forth. Strike is the compass direction of a line drawn on the structure height field. Dip angle is the angle formed by the structure of the field and horizontal surfaces.

C. Field Mapping

Mapping the field (field mapping) is the most important thing from the investigation site. Field mapping based on topographic maps, so that field notes can be recorded directly on a copy of the map field. Sometimes, in remote areas, topographic maps are not available. In this case, geologist or engineer must be returned to the field after the initial map is prepared to find things that are important. Based field maps, topographic maps can be changed, if necessary

Important things that are observed during field mapping of rock outcrops in the study area and surrounding areas. For example: the basic rock structure, lithology, unit thickness, or other geological data to be recorded. Associated with areas that have been landslides, important things to note are:

1. The head scarp, scarp lateral, secondary scarp, slippery layer orientation (slickenside).
2. Freshness scarp as seen from kecuramannya, plants cover the slopes, width of cracks and others.
3. The condition of the existing material in the area of landslide toe, is it still moving above the original ground surface, or avalanches still intact.

Slickenside is the surface of a land mass that has been made slippery by the result of shear deformation on the surface. On slippery surfaces, the general has been visible streaking due to a shift between two blocks of rock.

During field mapping, the photographs must be taken in the avalanche area. This is due to an appearance may be changed by the continuous movement of avalanches that could damage data. Photos should be documented with dates and weather conditions, so the photo comparison can be made at the same location, but on different dates. Highly recommended the use of cameras that include the shooting date to avoid confusion at the time to do the analysis later.

D. Observation Location

The purpose of the observation site (site observation) is to convince the patterns or model geological conditions, drainage area, as well as research sites, to prove the concept of three-dimensional region learned from literature. In addition, the observation location is also useful to identify the fault (faults), such as scarp and canyon, and other geological weaknesses, such as a thin layer of clay and a layer of slippery areas (slickenside). In addition, the study of patterns of topography and drainage area can provide valuable information about the possibility of building underground at the site. This information will be used to develop the program further investigation.

The amount of time required at each investigation is determined by the topographic slope, the geological environment and the number of anticipated excavation or construction. Observation of detailed investigation areas, will be combined with the results obtained from the image. Predicted unstable area must be investigated in detail.

According to Abramson et al. (1996), effective method of field observation consists of two things:

1. Doing quick observations for all locations with special attention to the position of the slope toward the creek, drainage, etc., and the overall relationship between the slope of one another. This stage can be done by walking or cycling, combined with aerial photo interpretation. Quick observation was also conducted, both in area and slopes are unstable.
2. Conducting a detailed investigation of an unstable slope, followed by a journey to every slope, and observe the things that give an indication about the stability or instability

Data collected from these two steps are the basis for planning an underground investigation at the site slopes.

During field visits, there may be important matters relating to planning or design of slopes in the future. Here are some things that are generally observed during the observation in the field.

1. Topographic forms that are not experienced often indicate an artificial slope, such as ravines and ridges that cut, and flat slopes. Changes to the side of the hill can be formed by excavation on the one hand, and accumulation on the other side. The effect on the overall topography may not be very visible by this change. 
2. Certain types of plants can often result mengkarateristikkan sediment avalanches. Certain plants often live in wet environments, and therefore may indicate activity springs, standing water or poor drainage in this location. Certain types of plants often live in loose soils, which may have been damaged by a landslide movement.
3. The orientation of the slope of the tree to inform the type of movement and can indicate the possibility of a new land movement. Land that moves to encourage the tree, and can cause barren hill. If the movement lasts long, the lower trunk becomes curved back and straight across the top. In the avalanche type ratasional, a tree that fell in the area at the foot of the scarp or avalanches, tend to tilt towards the top of the hill (Zaruba and kissed, 1982).

Important hydrological phenomena in a landslide is the number of lakes (puddles) which arise on the surface of the landslides. An example of this occurs in avalanches in the Manti Utah (Fleming et al, 1988; William, 1988), where a lot of lakes with varying dimensions occur within a period of 4 years. Lakes formed after landslides are generally more irregular, while the lake from previous avalanches somewhat irregular and rounded. Measuring emigration of the lake due to avalanches can be done using aerial photographs taken from time to time. Measuring emigration of the lake can be done in the following manner:

1. The object remained domiciled in stable location outside the landslide, one line with the direction of landslide movement, or 
2. A base line was drawn in a fixed position between two objects that are not changed, and described the direction perpendicular to avalanches.

Locations that have experienced landslides can be identified from aerial photo interpretation. However, many fine details can not be identified, either from aerial photographs and maps of small scale, and can only be detected from observations and surveys in the field.
In a field visit, it is important to identify the previous motion. Often some early signs of soil movement is of significant deterioration in the road surface, or from the cembungan in road pavement and the outbreak of the gutters are made of masonry or concrete

Other signs of land movement on the highway is (FHWA, 1988):

1. The existence of cracks in the pavement
2. Small avalanches on the embankment
3. Falling debris or impeachment material on the road from the slopes above
4. Gradual collapse of the area under the embankment which may trigger large avalanches which endanger highway.

E. Instructions on The Observation

1. Looking at the ground motion. Sign ground motion is the existence of tensile cracks, the wrinkles on the surface of the slope, broken pipes and electricity poles, trees tilt, the chip (spalling) or damage to equipment such as road safety fencing, drainage channel breakdown on the slopes of the brand, closed-plate connection in bridge expansion or rigid pavement, loss of straightness of the building foundation. Base of the bridge that seems skewed toward the end of girder bridges indicated that the soil near the bridge to move towards the bridge. Decrease in bridge near the plate pavement, indicating accumulation oprit will decrease or landslides. Retaining wall is usually used to hold the bottom of the slope. If the soil moves continuously, the wall will tilt and probably will be overturned. Cracks in the retaining wall may be an indication of ground movement behind the retaining wall. Similarly, buildings located in areas prone to landslide generally will experience cracks in walls, floors and foundations. Total building elevation may rise or fall depending on the location of areas prone to landslide. 
2. Identify the pattern of surface cracks. Crack surface is not always the direction perpendicular to ground movements. For example, a crack near the crown, generally perpendicular to the direction of motion, but cracked along the wing is generally close to parallel to the direction of motion. Before the landslide, small cracks develop generally stratified in the soil surface before other signs appear. Cracks parallel to the slope indicates a landslide blocks, are cracks with a horseshoe pattern indicates the type lorotan landslide (slump). 
3. Noting the hydrogeological disturbance or land formations. If there is a change in the formation of weak layers (soft / less dense) and strong soil, then the avalanche may occur in the weak layer. In certain locations there is land that is affected by liquefaction, for example: solid sandy soil is not saturated water in California. At some locations that are generally affected by erosion, for example: in the creek bank, toe of the hill, pile toe, and sides of steep hills. Erosion is happening in these places remove support for the toe embankment or slope.
4. Noting the existence of springs. Naturally, the spring that lies at the foot or top of the slope soften the soil reduces soil shear strength that can cause the collapse of the slope. Location springs can often be obtained in areas with dense vegetation. Embankment of the river, a natural steep slope, where the decision of the pile and excavation for highway or rail, or seepage springs, can reveal the presence of ground water flow in a particular location. An unstable pile can be caused by things the following:
   a) Built in a small river in the valley, where the material on the valley floor consists of material that has been weathered. b) Built in the hillside area where potential landslide areas in italics. 
5. Noting that the existing drainage pattern. Drainage at the site is one of the important things related to slope stability. Underground water to make a saturated and soften the soil embankment, soil foundation and the original soil. This often resulted in avalanches. Surface water, if not out of the surface slope didrain alsosaturate the soil and cause erosion. Therefore, it needed special attention to the flow of drainage which has the potential to slope stability problems. As observed in the field, the entire flow of small rivers, ditches, canals, rivers and the culvert should be mapped. Detail, size and condition should be plotted on site plan the research. This information will be used to assess the characteristics of the existing surface drainage at the site, what will be repaired or modified for the purposes of slope stability in the future. Impaired stability of slopes along the highway is sometimes caused by not doing maintenance on the drainage channel. Therefore, the entire drainage equipment should be checked against the possibility of damage, clog, leakage and others. Seep water into the road embankment from leaking drains can cause a rise in water levels of the road embankment, which may be can trigger avalanches.
6. Always record the building of natural or man-made buildings in the vicinity of the location of the study. Buildings natural and artificial, such as land excavation, embankment and retaining walls often give clues about: a) The possibility of how the design, construction and repair of the slope. b) Possible problems that will occur after construction. c) the type of improvements should be done so that slope instability. 
7. Use simple terms to explain matters related to soil movement and to determine the cause of the movement.Ground motion will occur, if the land had "something" that damage the stability. "Something" is the bias ofnatural causes, such as weathering, heavy rain, the soft layer, or other causes related to human activities, such as: excavation at the foot of the slope, development on the surface slope and others. All observations should be recorded in writing and photographs, so these results can be viewed again at a later date in the office. The observation that does not seem important, often become important in the future.