The soil investigation for the construction of minor bridges and culverts and the hydraulic parameters are discussed.
Soil Investigation for Construction of Bridges and CulvertThe construction of culvert employs a simple soil investigation. Based on the type of foundation proposed, the depth to which the investigation is carried out is decided by the engineer. It involves the planning about the level up to which the foundation is to be taken and the anticipated velocity of the flowing water during floods. The anticipation of any sort of deep scour will result in the contradiction for pipe and the box culverts. Areas where a pier or an abutment is proposed, a trial pit is made at the location. This pit is extended up to the rock or any form of hard strata that is available. If the case is that the soil cannot be considered as hard strata to a depth of 2 to 3m, the investigation has to be extended. This extended depth has to be taken below the proposed bottom level of the structure in the case of box or pipe culverts. The soil investigation is adequate if the soil obtained is uniform and not soft. Mostly, the practice is to conduct the investigation up to a depth that is equal to one and half the width of the proposed foundation. This depth is taken below the proposed foundation. The normal auguring method is mainly employed for this.
Table:1. Bearing Capacities of Different Soils
|Sl. No||Type of Rocks or the soil||Safe Bearing Capacity kg/sq.m|
|1||Hard Rocks free from lamination and other defects. Examples are granite, basalt, and diorite||3200|
|2||Rocks that are laminated. Examples are limestone and sandstone in sound condition||1600|
|3||The residual deposits of broken bed rocks cemented materials, hard sales||850|
|4||Soft rock and weathered rock||425|
|5||The sand, gravel that is compact which offers higher resistance to penetration during the excavation||425|
|6||Compact and dry coarse sand||425|
|7||Compact and dry medium sand||250|
|8||Sand gravel mixture or loose gravel||250|
|9||Hard or stiff clay, soft shale- that are deep in bed and dry||425|
|10||Medium clay that is indented readily with a thumbnail||200|
Hydraulic Parameters in Investigation for Construction of Bridges and CulvertProper inquiry with the residents who were residing in the proposed location for long is carried out to understand the condition of flooding and the highest possible flooding levels. These are verified with the help of any tell-tale marks that will be available in the nearby building, trees, banks of stream etc. A suitable margin should be provided even if there is no doubt on the collected data. For already defined channels, the following is considered:
- For 300m upstream and 150m downstream, a longitudinal section of the channel is taken
- Minimum three cross sections, one over the alignment, the second one 150m upstream and downstream must take.