Contents preface (VII) introduction 1—37

11.9. UPLIFT PRESSURE ON CULVERT FLOOR
In siphon aqueducts, the culvert floor is subjected to uplift pressure due to: (i) the subsoil water (below the stream bed), and (ii) the water that seeps from the canal to the stream bed through the embankment (Fig. 11.12).

			Impervious
			stream bed
		Stream	floor begins
Impervious canal	B	here
floor starts
from here
Canal	0
A

C

F.S.L.
Canal bed

Piers

Seepage path

Abutment pier
Culvert floor
Fig. 11.12 Assumed path of seepage from canal to stream
For some part of a year, the water table below the stream bed may rise up to the bed level itself. This will exert uplift pressure on the floor of the culvert. The maximum uplift will occur when the stream is dry and will be equal to the difference of levels of the stream bed and bottom of the culvert floor.
As shown in Fig. 11.12, the canal water seeps to the stream bed and reappears at B and C on both sides of the culvert floor. This seepage is, obviously, due to the difference of head between the canal water level and stream water level. The maximum value of the uplift pressure on the culvert floor due to this seepage will occur when the canal is running at FSL and the stream is dry. This seepage flow is three-dimensional in nature and can only be approximately solved by numerical methods or model studies. However, for relatively small structures, the following simple method based on Bligh’s creep theory can be used for the design (3). The method can also be used in the preliminary design of major important structures which should, however, be checked by model studies.
Measure the length of the seepage path AOB or AOC which, in fact, will be equal to the length of the seepage path, shown with dotted lines in elevation, and the distance OB (or OC). But, for further simplifying the calculation, the seepage length between A and O is taken as

398 IRRIGATION AND WATER RESOURCES ENGINEERING
equal to the length of impervious concrete floor on the canal bed up to the flumed end and one-half of the barrel span. If the seepage head, i.e., the difference between the canal FSL and level of the bottom of the culvert floor is H, and the total seepage length is l then the residual head at O, H₀ is equal to H((l-l_l)/l) in which, l₁ is the length of seepage path between A and O, i.e., from the start of the impervious canal floor to the centre of the bottom of the floor of the first barrel. This residual head H₀ and the uplift due to subsoil water are to be counterbalanced by the weight of the culvert floor and, hence, the thickness of the culvert floor can be calculated. In these calculations, it is assumed that the barrels are dry and free of any sediment deposit so that the structure is safe even under the worst condition. However, the resulting floor thickness will be excessive and uneconomical. Besides, higher floor thickness further increases the static uplift pressures. Therefore, the uplift pressure is transferred to the piers by utilising the bending strength of the floor. With such a provision, a floor of reasonable thickness (say, around 0.50 m) would be adequate. Sometimes, inverted arches are also used.
When uplift pressures on the culvert floor are excessive, some additional measures are taken to reduce the uplift. A simple measure would be to extend the impervious floor of the canal on both sides of the trough. Alternatively, the seepage water is made to emerge into the barrel by providing a number of openings (known as relief holes) in the culvert floor. Relief holes are in the form of pipes embedded in the barrel floor. However, the seeping water may bring with it the soil particles also resulting in subsidence of the floor in course of time. This can be prevented by providing inverted filter just below the culvert floor. This inverted filter would consist of one or more pervious layers of soil (inside the embedded pipes serving as relief holes) such that the permeability increases in the upward direction. However, the voids of these pervious layers are not large enough to permit the movement of subsoil particles with the seepage water. Terzaghi’s criteria are used for the design of different layers of the inverted filter. These criteria are as follows:

	D15 of filter layer	≥ 4
	D		of protected soil
	15
and	D15 of filter layer	≤ 4
D	of protected soil
	85

When the canal is dry, there exists a possibility of sediment-laden stream water entering the filter layers through the holes in the culvert floor and, thus, filling the filter voids with sediment. This can be prevented by providing upward-opening flap valves on the top of the relief holes.