The soil investigation for the construction of minor bridges and culverts and the hydraulic parameters are discussed.
Soil Investigation for Construction of Bridges and Culvert
The 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.
Confronting soft soils will call for the different type of structure and foundation. This will make it necessary to ascertain the type of soil at various depths with the use of trial bore holes.
In some situations, the drilling of bore hole will result in the collapse of the surrounding soil. These happen due to the soft texture of the soil material. Here the bore hole will not stand supported.
During such situations, a casing pipe is driven into the ground for some depth and later the hand auguring is carried out.
For investigation depth greater than 5m or in the case of pile or shallow foundation, the method of wash boring by using a casing pipe will be employed.
In the case of culverts and minor bridges, there is only the need for a qualitative idea of the various soil layers. If there is no rock present, the representative samples are taken. These are obtained from the borings at 1 to 1.5m intervals. This interval decision depends on the continuity of the soil type or a change in the type of soil.
The table-1 shows the safe load for the foundation that is proposed for culverts and bridges.
The final design of the foundation involves the design of piles or well that will be based on the properties of the soil that are determined from laboratory tests. These tests are conducted on the undisturbed soil which is collected at intervals.
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 Culvert
Proper 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.
The HFL (Highest Known Flood Levels) are marked on each of the cross section taken. The rugosity coefficient is judged based on the type of bed and the bank material.
The rugosity coefficient will hence enable the calculation of the velocity of flow. The various method that is available is employed to determine the design flood discharge.
These details will provides the waterway required for the bridge and the arrangement of the span.