Introduction
Housing is an integral part of human life, including one of the basic physical infrastructures that is very much needed in meeting the needs of human life, in addition to food and clothing. The human need for houses tends to increase through time, while the availability of land is relatively very limited, and even tends to decrease in area from year to year. Therefore, the need of land for housing tends not to be fulfilled as a whole, both in terms of quantity (area) and quality (readiness and ease of construction and maintenance). This has resulted in the community, both individuals and developers, trying to get a location for land for housing at lower prices despite its poor quality. This condition causes several settlement locations to be located in areas that are not suitable for residence, such as on relatively steep slopes, unstable locations, and locations prone to flooding.
Selection of prospective land locations for good housing/residence is not only depending on road infrastructure, electrical network, and water facility. It also needs to pay attention to the quality of the location, which includes soil stability, morphology and slope, and the possibility of ground movement. For this reason, it is very necessary to have knowledge of the geological conditions at the prospective location that will be used as a housing location. Moreover, in the location of many houses, it is usually done by housing developers. Errors or lack of accuracy in determining the location of housing, can result in large losses and last for a long time.
Building in a stable land, both when there is vibration (earthquake, heavy vehicles passing) or during the rainy season, will stand firm and not easily cracked. Hence, the occupants will feel comfortable, safe, and not worrying about costs in case of the damage to the building.
Environmental Issue in Land Development
Semarang City, as a metropolitan city, continues to improve, providing lots of housing to meet the housing needs of city residents. A number of housing locations are widely scattered in various places, both in the lower city of Semarang, and in the upper city of Semarang. Unfortunately, several locations that were chosen as housing locations had several indications related to geological problems, such as the presence of clay which expands easily around the fault location.
Geologically, in the city of Semarang, there are rock formations/rock types in the form of blackish-gray clay layers, known as the Kalibiuk Formation. This type of clay layer found in parts of Semarang, including around Sekip village, Jangli sub-district, Tembalang, and around the Bukit Manyaran Permai housing complex, Gunungpati, has properties that swell easily when exposed to water. When this soil expands, then its volume will become larger. Therefore, it will require more space and will press the surrounding soil. As a result of this soil pressure, the surrounding buildings will be pushed aside, so that the foundations and walls of the house will tend to shift. Meanwhile, the body of the road which is located on the clay soil of the Kalibiuk Formation will tend to be bumpy. This happens a lot during the rainy season, due to clay soil that is often exposed to rain. Such conditions certainly result in high costs to repair roads or parts of houses that are raised/damaged, so that many houses are eventually abandoned by their owners and then look for a safer place.
Road damage in Bukit Manyaran Permai, Gunungpati
House condition as the result of soil instability at Bukit Manyaran Permai
While the damage to the building which is suspected to be due to the presence of a geological structure, in the form of a fault. It was found at the location of one of the housing estates in the Sekaran area, Gunungpati. A location that is passed by a rock fracture structure will tend to become unstable or move easily. Especially if there are motion triggers, such as earthquake vibrations or vibrations from passing vehicles.
How Should It Be?
Early prevention of potential greater physical and material losses can actually be done. This is done from the start of the search for the location of the land to be used, namely by looking at the topography and geological conditions in general, by looking at the geological maps that are already available. In the geological map, information is available about the rock formation and the existing geological structure. Then the candidate locations that have been “scanned” from the geological map, are reviewed in the field, by observing the physical conditions on the ground surface. Are there any signs of cracked soil, what is the slope of the soil layer (if any), are there any signs of landslides, are there any small horseshoe-like landslides, is there any black clay which is sticky when exposed to water, are there any trees or electricity or telephone poles that are tilted, and so on. It would be even better if it was followed up with a subsurface condition observation survey, such as to determine the depth/thickness of the soil layer, determine the position of the soil slip plane (if any), determine the depth of the groundwater zone, and so on. This activity can be done for example by geoelectric survey. Of course, this subsurface survey has cost consequences.
Tilted electric pole as indicator of soil instability at Sekip village, Jangli
Recognizing these symptoms is very important for anticipating or early handling, about what to do, if a location candidate becomes the main alternative for a choice. So that the impact of physical and material losses can be avoided or minimized as much as possible.
