CHAPTER 6
SPECIAL CONDITIONS

Section 3601. (Previously 3052). Excavation Dewatering.

(a)    Secure and comply with the provisions of permits required for dewatering operations, including any permits from the Alameda County Environmental Health Department for exploration, construction and abandonment of dewatering wells. No water shall be discharged into existing sanitary sewers, or new sanitary sewers constructed unless a Special Discharge Permit is obtained from Castro Valley Sanitary District.

(b)    Provide all labor, materials, and equipment necessary to adequately dewater excavations so that pipe and structures that are installed in excavations are free from standing, flowing or boiling groundwater, surface water, storm water, precipitation, or wastewater; filter soil; and prevent loss of ground from dispersion or erosion.

(c)    Drawdown the groundwater level a minimum of two feet below the trench bottom and beyond excavation sidewalls where shoring is not designed to resist hydrostatic pressures.

(d)    Control the rate and effect of dewatering so as to avoid settlement, subsidence or damage to structures or facilities adjacent to areas of proposed dewatering.

(e)    Provide adequate standby equipment to ensure efficient dewatering and maintenance of dewatering operations during power failure.

(f)    Be fully responsible and liable for all damages that result from failure to adequately keep excavations dewatered, and shall repair, restore and/or replace facilities or structures damaged as a result of dewatering operations.

(g)    The contractor shall submit a plan for all excavation dewatering procedures to the Inspector. The dewatering plan shall include the following:

(1)    Location(s) where water is to be disposed.

(2)    Scale drawings showing locations of dewatering systems.

(3)    Details of dewatering systems, such as:

(A)    Drilled hole and well casing diameter, slotted and solid lengths;

(B)    Sand packer gradation;

(C)    Size and capacity of pumps.

(h)    Where structures, utilities and/or facilities exist adjacent to areas of proposed dewatering, the contractor shall establish reference points and shall survey these reference points daily to quickly detect any settlement, subsidence or damage that may develop during or following dewatering operations.

(i)    If disposal of water to the sanitary sewer is allowed, televising and cleaning of downstream sewers may be required at the Inspector’s discretion, and shall be done per the requirements of Castro Valley Sanitary District.

(j)    Dewatering, where required, may include the use of wells, well points, sump pumps, temporary pipelines for water disposal, rock or gravel placement, standby pumps and/or generators, and other means.

(k)    Work shall meet or exceed the requirements of the Code unless applicable requirements of an agency having jurisdiction (including the terms and conditions of an encroachment permit issued by Alameda County) are greater, in which case the greater requirements shall govern.

(l)    The contractor’s dewatering operations shall not interfere with vehicle or pedestrian traffic. Under no circumstances shall dewatering water be allowed to flood streets or cause hazardous conditions for traffic. Dewatering pump noise shall be mitigated, especially at night, as required by the applicable local jurisdiction (Alameda County).

(m)    Dewatering for structures and pipelines shall commence when groundwater is first encountered, and shall continue until water can be allowed to rise without affecting structures, piping, and other project features.

(n)    Site grading shall promote drainage. Surface runoff shall be diverted prior to it entering excavations to maintain the bottom of the excavation free from standing water.

(o)    Dewatering shall be conducted so as to preserve the undisturbed bearing capacity of the subgrade soils at the proposed bottom of excavation, filter soil particles and prevent loss of ground due to dispersion and erosion. Dewatering shall lower the water outside the excavation, if necessary to insure that migration of soil particles does not occur through openings in the shoring.

(p)    If subgrade soils are disturbed or loosened by the seepage or flow of water, the affected areas shall be excavated and replacement backfill placed in accordance with Section 4046 of this Code, Bedding and Backfill.

(q)    The contractor shall prevent pipeline and/or structure flotation by maintaining a positive and continuous removal of water.

(r)    If dewatering wells are used, they shall be adequately spaced to provide the required dewatering, and the contractor shall use sand packing and/or other means to prevent pumping of soil particles (e.g., fine sand) from the subsurface. The contractor shall continuously monitor the dewatering water discharge to ensure that subsurface soil is not being removed by the dewatering operation.

(s)    The demobilization of dewatering operations shall be performed so as to allow groundwater to rise to its ambient (static) level without disturbing natural foundation soils or compacted backfill, and prevent flotation or movement of structures, pipelines, and sewers.

(t)    The contractor is advised that it is possible that sand, silty sand and/or gravel strata inter-bedded with less permeable clay and silty clay materials varying in depth, thickness and location may exist in the project area. These potentially water-bearing strata may represent areas of increased trench dewatering difficulty. The contractor shall carefully consider the possibility of encountering these strata, and plan dewatering operations accordingly.

(u)    The contractor shall properly dispose of water without nuisance or damage to adjacent property, in accordance with its Storm Water Pollution Prevention Plan. Water shall be settled or filtered using an approved method to remove sand, silt and fine soil particles before disposal into any storm drainage or sanitary sewer system, including but not limited to the use of a debris trap. The contractor shall remove and dispose of any material that accumulates in the drainage systems used for water discharge as a result of dewatering operations.

(Added, §1, Ord. 35, 04-06-59)

(Amended, §1, Ord. 52, 08-05-75)

(Amended, §1, Ord. 133, 01-07-03)

(Amended, §1, Ord. 150, 07-01-09)

(Amended, §1, Ord. 162, 07-01-14)

(Amended, §1, Ord. 169, 05-03-16)

Section 3602. Additional Engineering Reports.

(a)    Soils reports will be required on hillsides and creek areas where:

(1)    Hillslopes where sewers are proposed for installation exceed 20 percent.

(2)    Sewers are proposed for installation within 50 feet of creek beds.

(3)    Sewers are proposed for installation within the range of influence of a possible landslide from adjacent hill.

(4)    Sewers are proposed for installation in historic slide locations.

A soils report shall be prepared and submitted by a registered Civil Engineer practicing in geotechnical engineering, covering the proposed project. If the project geotechnical report provided does not cover an off-road sewer alignment, the District may require a supplementary report.

(b)    The geotechnical report, at a minimum, must address the following:

(1)    Supplementary geological setting, general soils and bedrock condition along the proposed sewer alignment, and recommended setbacks from slides and creeks.

(2)    Stability or instability of selected sewer alignment.

(3)    Potential ground water problems.

(4)    Effect of trenching on slope stability (negative impacts on slope).

(5)    Special backfill, special trenching requirements, or special supports (such as cut-off walls) may be recommended.

(6)    Erosion potential of soils around sewer near water courses.

(7)    Recommended correction if instability exists or may develop.

If, in the sole determination of the District, it is unfeasible to locate sewer mains on the downside slope of future homes and maintain a safe distance from drainage ways, consideration will be given to installing the sewers in street areas which may result in the installation of a pump system. The project engineer is to provide the District with a feasibility study describing various methods which could be used to serve the proposed development including gravity (in streets and easements), publicly maintained sewage pump station and residential sewage pumps on individual homes.

(Added, §1, Ord. 169, 05-03-16)