Archive for the ‘ Civil Engineering ’ Category

Base Isolation System

Gempa menjadi hal yang tak asing beberapa tahun belakangan ini (dari gempa di Jogja, gempa di Padang, sampai gempa di Jepang). Masalah gempa merupakan masalah yang terkait erat dengan bangunan. Banyak kerugian baik harta maupun nyawa ketika bangunan tidak mampu menjalankan fungsinya untuk melindungi. Oleh karena itu, diperlukan metode/rekayasa bangunan dalam mengatasi masalah gempa. Metode sebelumnya, orang mengurangi potensi bahaya gempa dengan membuat bangunan menjadi lebih kaku (rigid) dengan menambahkan dinding geser (shear walls). Ada pendekatan yang berbeda dalam menangani masalah gempa yaitu dengan mengisolasi struktur atas bangunan dari tanah untuk mengurangi kejutan rambatan gempa. Kita mengenal metode ini dengan sebutan base isolation system. Ide dasar dari base isolation system adalah dengan mengisolasi bangunan dari tanah.Base Isolation system sendiri harus memenuhi persyaratan sebagai berikut :

  1. Mampu mengisolasi bangunan dari tanah.
  2. Mampu mendukung berat bangunan.
  3. Mampu meredam ayunan bangunan ketika gempa.
  4. Mampu mengembalikan bangunan ke posisi semula setelah gempa.

Berikut adalah perbandingan perilaku struktur bangunan antara struktur konvensional (conventional structure) dengan struktur yang terisolasi (base-isolated structure) :Pada gambar nampak struktur konvensional akan mengayun (terdeformasi) ketika terjadi gempa. Hal ini berbeda dengan struktur yang terisolasi dimana struktur bangunan tidak mengayun (relatif tidak terjadi perubahan bentuk). Walaupun metode ini sudah lama muncul, namun di Indonesia sendiri masih sangat jarang bangunan yang menggunakan metode ini. Berikut adalah contoh-contoh gedung yang menggunakan metode ini :

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Bridge

The bridge is an infrastructure that connects two sections separated by an obstacle (a river or a railroad or other obstacles). Based on the type of the structure, the bridge can be classified into several types, namely :

  1. Arch Bridge
  2. Suspension Bridge
  3. Cable Stayed Bridge
  4. Truss Bridge
  5. Cantilever Bridge
  6. Girder Bridge

Location and workload are the major considerations in determining the structure type.

Road Construction on Peat Soil

Road construction on peat soil may have difficulty. It is because the bearing capacity of peat soil is low and the settlement is high. Therefore we need a method to solve this problem. What is peat soil? Peat soil is a type of soil formed from the accumulation of the remnants of decompose vegetation which has high organic matter content. The characteristics are high organic content, high water content, large void ratio, high compressibility, and low bearing capacity.

This is the distribution map of soft and peat soil in Indonesia :

Distribution Map of Soft and Peat Soil in Indonesia (GeoGuide Departemen Pekerjaan Umum, 2002)

Black printed are areas of soft and peat soil. It is important to be known because there are many areas of soft and peat soil in Indonesia.

These are the actual condition of peat soil :

  1. Soil Bearing Capacity. Peat soil bearing capacity is very low (extremely low bearing capacity). Consequently, there are many problems of construction that must be built on peat soils. Damage is relatively fast from the age of the plan so the cost of  maintenance is high.
  2. Soil Settlement. Peat soil has a highly compressible nature in which the lining will have a very large potential settlement when loaded on it. The thicker the peat layer, the larger the settlement that may occur.

In road construction on peat soil (with thickness no more than 2 meters), I recommend to cut the layers of the peat soil and replace it with good soil. The other alternative, I recommend to use a combination of geotextile non-woven and geogrid triax. In this occasion, I want to explain about the improvement with a combination of geotextile non-woven and geogrid triax. Technically, geotextile non-woven will prevent mud got into the pavement system. Whereas geogrid triax will lock the aggregate in the pavement system (so that the layer of geogrid triax is more rigid and easier to compact). The rigid nature of geogrid triax perform a function as a reinforcement.

Why I choose this material in conducting this improvement? It is because the use of this material has many advantages. The following are the advantages of using this material :

  1. Has high durability of chemical compounds, weathering, napless, ultra violet rays, and microorganisms.
  2. More economic than using conventional methods (slab on pile).
  3. Faster in processing than using conventional methods (slab on pile).
  4. It has been recognized internationally through ASTM and ISO.
The following are the work sequence of the road construction using this improvement :
The work sequence is devided into 3 main sections (engineering design works, subgrade works, and pavement works). At engineering design works, we specify the design of road by determining road function, traffic load, type of pavement, pavement thickness, road width, and road length. At subgrade works, we prepare the subgrade before it is used as a pad for the pavement. It starts from land clearing, cut and fill until request level, installing geotextile non-woven, installing geogrid triax, subgrade filling, and the last is subgrade compaction. At pavement works, it starts from subgrade course, base course, and the last is surface course.

About the analysis of soil stability, when we add geotextile non-woven to the soil layer, the stability will increase. It is because of the tensile strength of geotextile non-woven. Rigid nature of geogrid triax will reinforce the soil. It improves the soil stability.