Project sector: Operation and Maintenance (O&M)

Project status: Completed

Project location: Lar Dam, Tehran City, Tehran and Mazandaran Provinces

Project objective:

  • Finding the unknown leakage points in the body and reservoir of the dam
  • Finding remediation measures for solving the problems

Project starting Date: November 2016

Project completion Date: November 2017

Project duration: One year

Abstract

Lar Dam, situated in the northeast of Tehran with access to Haraz road (Tehran- Caspina Sea road) and 75 Km distance from Tehran is the main drinking water source of the capital city of Tehran which has the largest water reservoir around Tehran. The main purpose of construction of Lar dam is to supply drinking water of the metropolitan city of Tehran as well as irrigation of agricultural lands of the northern province of Iran.

The study area is a tectonically active area and bed and walls of the reservoir include seven geological structures among which Lar limestone is the most challenging one due to high rate of dissolution.

The current karstic structure of the Lar and Tiz-kuh has extended to deep levels. Its ultimate depth has still remained unknown. This issue was observed in the first case of injection cement to fill kasrtic caves in the early years of impoundment of the reservoir. Significantly, even in one case 100000 tons cement were injected to fill the holes, which had no effect on the impoundment of reservoir.

Saman Ab Sarzamin Consulting Engineering Company (S.A.S) took several measures for investigating and study on this area, including:

  • Reanalysis the results and outputs of reservoir researches

According to inspections and dying tests, most of injected substances had been existed through the Galugah and Haraz Springs and this fact indicates the relation between the mentioned springs and the reservoir. Moreover, it has shown that few injections did not exist and remained in the reservoir or the exact location of their outflow was unknown. The primary focus of this study was on the available geological reports and reanalyzing the data that were obtained by Terra Company to identify potentially leakage of the area. During the studies of S.A.S, in addition of Tiz-Kuh structure, two new regions were introduced as suspicious points that needed more investigation, including: (1) Right bank of entrance to Emamanak Valley (SLP02 area), and (2) 1.5 Km upstream of Emamanak Valley (SLP01 area).

  • Terra company studies

 S.A.S. analyzed the outputs of Terra Company’s studies. The data were analyzed and converted into the visual output for better understanding of the status of reservoir. This issue was conducted by codifying and programming in MATLAB Software, drawing 2D diagram of velocity magnitude and the diagrams of flow vectors. One of the changes analyses of velocity is presented in a diagram. The weak point of these measures is related to the bad timing for surveying and surveying. At the end of the September and early October, the water level was at 2474 m. Since many outlet points were identified above this level, it was preferable to measure the velocity at the time of maximum level of the reservoir and more accurate information from outlet points could have obtained.

 

lar 1

Identified probable leakage points for field monitoring

During this period, the water level raised from 2488.7 m to 2490.9 m. In addition, Tizkuh structure and the body of dam at the place of sinkholes were monitored in the second site visit,new cases were observed.

lar 2

The first case was hearing loud sound of stream flow from the inside of the rock in Tiz-Kuh structure (P1 point). Injections at this point indicated definitive water leakage in the whole range of this level and the location of injection and exiting injected substance for future monitoring was determined. The large karstic holes and cracks that has not been identified in previous monitoring (like C1), were investigated with more details.

lar 3

  • Field visits

On 2017-Aug-05, a huge hole was observed at Joint LP, but due to the high water level, it was not possible to go inside it. In field monitoring on 2017-Aug-17 by decreasing water level, it became possible to inspect the size of the cave, which called C1. Numerous corrosions and cracks on the wall showed the water leakage in C1. Injections showed the water leakage from the western cracks and holes on the cave and the body of dam.

lar 4The results of investigations and dye test in field visit in July

In right abutment and the structure of Tiz-Kuh besides dye injection in cracks, investigations were carried out with more details on the dissolution cavities. Above the level of LP04 a new crack observed. The scouring erosion in eastward and inside of the caves, existence of wood and debris within a few meters inside of the cave were evidence of water leakage. A series of injections were carried out from 2491 m level to the lowest level 2482 m.

lar 5The results of field monitoring and dye injection in September 2017

  • Field monitoring

Due to the decreasing of water level and appearing suspicious water leakage points, besides common inspections, detected leakage points in previous field visits were investigated again to measure size and dimensions of the holes accurately. In general, three types of signs for water leakage were evident at these points, (1) one was changes in color of the rock masses and darken them, (2) existence of dried algae and accumulation of them in holes and crack and (3) decreasing the sharpening of the edges of rock masses which were exposure of outflows. 

lar 6Evidences of water leakage

During field visit on 12 October, dye test was performed at suspicious areas. Also sampling from he surface of the rock with corrosion problem were conducted to make assessment of efficiency of past operations. Some sinkholes were visited such as S1 sinkhole during this series of site inspection

lar 7

On 28th of November, the last field visit was conducted to investigate the suspicious leakage points which had not been visited before. The diversion Tunnel 01 is one of the major leakage points. At high water levels, when the tunnel were 3 meters below the water surface, vertices were formed which indicated suction of water into the tunnel and depth of lake. During several visits, by entering in to inside of tunnel and observing cracks on the walls of the tunnel and debris inside the holes clearly revealed that water leakage from the body of the tunnel was happened. Since the tunnel was drilled by explosion, the crack and holes became wider and the corrosion of stones has increased during these years.

lar 8Vertices formed on above the Tunnel 01, visiting inside of the tunnel

Project outcome:

The diversion tunnel 01 as the main leakage point were monitored several times by S.A.S. The results of site visits indicated that presence of numerous cracks on the walls of tunnel and using explosion procedure to create this tunnel led to widening and increasing these holes and water leakage at this tunnel. Presence of caverns and boreholes on the way of this tunnel to the Joint LP showed that digging of this tunnel could have a large impact on the intensification of the problem.

llar 10Schematic location of diversion tunnel 01 and its connection to Joint LP 

S.A.S decided to conduct dye injections at levels under the water surface. Otherwise all calculations of the outflow velocity would be unreliable because of neglecting the head pressure and velocity in the holes under the water surface. There are 4 leakage points along with D-3 line which is surveyed by Terra Company.

lar 11Velocity magnitude of flow in D-3 line (Terra Company) (m3/s)

During several site visits, S.A.S Company found out that there is a need for more attention to small leakage points, which could accumulatively produce a significant leakage flow rate. They also found out a loud voice, coming out from inside the mountain at outlet points and under the lateral facilities at the right abutment, Emamanak Valley and its upstream that neglected before as probable leakage points. After the last field monitoring, by using the obtained data of water surface elevation in each monitoring period, the changes of water elevation during total periods of monitoring investigated

lar 12Changes of surface water elevation during monitoring periods

The investigations made by S.A.S revealed that:

  • Two points are more sensitive: the first one is diversion tunnel 01 and the second one is upstream of Emamanak Valley.
  • The tunnel has no lining and there are several long cracks and holes on the wall of this tunnel. Therefore, providing surface lining and grouting cement for the Tiz-Kuh structure without sealing lining will not so effective to solve the problem because this tunnel transfer water flow by cracks and hole into the surface water and this problem will be intensified under the hydrostatic pressure when the water level is high.
  • The upstream site of Emamanak valley is recommended to be covered by sealing lining. There is a need to investigate the stability and vulnerability of dam by consideri-ng the fractures, determination the capacity of dam for current status which is able to store water up to 2531 m level.
  • Investigation of changes in water level at Galugah and Haraz springs as outlet points should be carried out by tracing inspection and dye injections. Investigating the required strength to withstand the double hydraulic pressure at the level of 2531 m should be addressed in future.

lar 13Prioritized treatment plan