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Frequently Asked Questions - ground anchoring

What is a ground anchor?
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A ground anchor is a metal bar that is inserted into a pre-drilled hole in the slope. The metal bar is encased in cement grout which fixes the anchor in place as it hardens. One end of the anchor goes into the slope behind the retaining wall, and the other end is set into the retaining wall.

How are ground anchors installed?
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A hole (approximately 185mm in diameter) is drilled into the slope using a purpose built drilling rig. The ground anchor is inserted into the centre of the hole. Cement grout is pumped into the hole around the anchor and left to set. The other end of the anchor is set into the concrete block of the retaining wall with grout. Exact measurements for the diameter of the hole drilled, the anchor length, the anchor depth and the method to secure the anchor to the wall may change from site to site.

Why are ground anchors needed?
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Ground anchors are needed to transfer the horizontal load acting on a retaining wall back into the ground or rock where the anchor is fixed. In simple terms the anchors will hold the retaining wall in place in the event of an earthquake.

Why is the use of ground anchors the best option for many retaining walls?
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Ground anchors have been chosen as the design solution for some retaining walls in Christchurch for three reasons:

  1. They are an optimal design solution for retaining walls under the post-earthquake design codes.
  2. They provide increased resiliency for future earthquake events.
  3. They can be used successfully on high retaining walls.
Will ground anchors affect my property?
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The anchor installation will produce a small amount of vibration which is unlikely to be felt in the properties above due to the depth of the installation. The anchors are below ground level and generally will not interfere with the property. In locations where the ground anchors go onto private property, Council will contact the owners regarding easement agreements for the anchors.

Where do ground anchors sit and how many do you have to use?
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The location and number of ground anchors varies according to the location and height of the retaining wall, and the soil/ground conditions.

Can the anchors cause any damage to our land in the event of another earthquake?
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Ground anchors allow a retaining wall to be more resilient to earthquakes, and therefore less likely to fail. The ground anchors and retaining wall will not 'pull away' taking private land with it in the event of another earthquake.

How deep do you need to drill ground anchors?
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The depth of the ground anchors varies from site to site. Generally drilling is undertaken until rock or solid ground is reached.

Are ground anchors used anywhere else in New Zealand?
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Ground anchors were used in the construction of a number of retaining walls in the Central Motorway Junction project in Auckland. Locally, ground anchors have been used to repair retaining walls at both Dublin Street and Sutton Quay in Lyttelton. These repairs took place before the Canterbury earthquakes, and both sections of wall where the ground anchors were used did not fail in the recent earthquakes.

Who is liable for any damage to my land/property if the retaining wall and the ground anchors fail?
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A public retaining wall, including any ground anchors, remains the property of the Christchurch City Council. Any future maintenance or repairs are the responsibility of Council.

Do ground anchors need any maintenance?
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Once the anchors are installed they require no maintenance. Should there be any future large earthquakes, Council will inspect the retaining walls to make sure they are safe and functioning well.

Can I plant trees over the anchors on my property?
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This will depend on the depth and number of anchors on your property, which also varies from property to property. Generally you will be able to plant trees over the anchors as they are at some depth below the ground (i.e. they are not at the surface). We will be able to discuss this with you once we know where and how many anchors are proposed for your property.

Can I build over ground anchors on my property?
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This will depend on the depth and number of anchors on your property, which also varies from property to property. It will also depend on the foundation design for any structure you are proposing. Generally you will be able to build over the ground anchors as they are at some depth below the ground; however any foundation design may need to take the anchors into account. We will be able to discuss this with you once we know where and how many anchors are proposed for your property.

Will it affect any resource or building consents in the future when I have ground anchors?
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Any future resource or building consents will be subject to standard Council processes. A Certificate of Title is required to be submitted with either a resource or building consent and when assessing any applications Council will consider the effects of the proposal on the retaining wall.

What is a ground anchor easement?
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A ground anchor easement is a right for Council to use your land to locate ground anchors where they are necessary for the repair or replacement of the retaining walls.

See also What is an easement?

What is a ground anchor easement agreement?
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This is the agreement between you and Council that records your agreement to grant Council the right to use your land to locate the ground anchors. The easement document will be registered on the certificate of title (now called the computer register) for your property.

Do I have to seek legal advice, or can I just sign the easement agreement?
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We strongly recommend you take legal advice from your lawyer on the ground anchor easement agreement. You will also need a lawyer when it is time to register the easement on your title.

Will you pay the legal costs that I incur in signing a ground anchor easement agreement?
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Council will meet reasonable legal costs. This is generally not more than $1,000.

Will you pay compensation for me to sign a ground anchor easement agreement?
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Council is not offering compensation in return for the easement as these retaining walls are either protecting or providing access to your property.

What if I don't sign a ground anchor easement agreement?
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Generally construction cannot begin until all easement agreements are in place. In some places preparatory work can begin in areas where easements are not required.

What if I sign the ground anchor easement agreement for my property and the neighbours don't sign theirs; will you still repair the retaining wall outside my property?
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Generally construction cannot begin until all easement agreements are in place. In some places preparatory work can begin in areas where easements are not required.

I can't get my house repaired until the retaining wall is fixed and my neighbours won't sign their easement agreement; can't you just fix the wall anyway?
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The construction methodology for repairing the retaining wall requires us to build the retaining wall from the bottom up along the entire length of the wall, so all easements must be in place.

I don't want a ground anchor easement; will Council buy my house?
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Council is not proposing to purchase any properties in respect of repairing damaged retaining walls.

I only part own the property. Do you need all the other owners to sign the ground anchor easement agreement?
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Yes, if the property is in a multiple (or joint) ownership then all owners of the property need to sign the agreement.

Will having a ground anchor easement on my property affect its valuation?
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It is considered that there is no significant effect on the value of the property. Owners should take some comfort in knowing the remedial work on the retaining wall is being undertaken.

What do I do if I need a private retaining wall repaired or replaced on my property but I have a ground anchor easement and ground anchor in the way?
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We will work with individual owners in these circumstances to ensure the repair of the public retaining wall does not adversely impact on the repair of the private retaining wall.

Will you start construction immediately after all ground anchor easement agreements are signed?
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This will be influenced by the length of time it takes to secure all the necessary agreements. We have allowed approximately 2 months for this to occur. If it takes substantially longer to gain the necessary easements, construction may be delayed due to work crews and resources being diverted to other high priority projects.

If I sell my house do I have to tell the new owner about the ground anchor easement?
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We would recommend that you do as it demonstrates openness and honesty, and may avoid any potential delays or issues with the sale if you were not to disclose that information. Their lawyer will see the easement document when they search the certificate of title for the property once the easement is registered.

What impact will the retaining wall rebuild have on my property?
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The design and construction methodology for each wall will take into account proximity of properties. As this will involve heavy engineering equipment the construction may affect residents, depending largely on how close property is to the retaining wall being rebuilt.

The details of how construction may impact on your property will be discussed with you by the contractors in detail before works begin in your area. In a number of instances it will be necessary to extend ground anchors into an adjoining private property to provide the structural strength and support to the retaining wall. The number and depth of ground anchors will vary depending on the height and location of the retaining wall. The ground anchors will generally be 2 metres or more below ground level. We will make contact with the owners to get approval before starting this work. Council will be in contact to put easements in place for these properties.