Editing Underpinning (section)

==Chemical grout==

One of the uses of the [[Soil improvement#Soil Stabilization|soil improvement]] technique of [[pressure grouting]] is foundation underpinning especially during excavations like subway constructions.<ref name=":01">{{cite book |last1=Chu |first1=Jian |last2=Indraratna |first2=Buddhima |title=Ground Improvement: Case Histories |date=2005 |publisher=Elsevier |isbn=0080446337 |pages=357–358}}</ref> This technique has been used in municipal development projects of significant scope in the United States. One of the largest chemical grouting projects was the extension of the Pittsburgh Light Rail Transit subway when the foundations of six large buildings needed to be protected from ground movement and related building settlement.<ref>{{cite web |title=Underpinning with Chemical Grout |website=Transportation Research Board (National Academics of Sciences Engineering and Medicine |url=https://trid.trb.org/view/202523}}</ref>

Pressure grouting can be low pressure or high pressure. "Jet grouting" is a general term used for high pressure grouting where the high pressure air, water and cementing grout are injected into the ground at high velocity. This can be doing with single tube systems to mix the grout with in situ soil to form a grouted "column" in the ground. Double tube systems and triple tube systems using air and water remove some of the soil to create larger grouted bulbs or columns.<ref name=":01"/>

Low pressure chemical injection grouting is used to underpin structures in sandy soils.<ref>{{cite web |last1=Hayward Baker |first1=Wallace |title=Design and Control of Chemical Grouting |url=https://rosap.ntl.bts.gov/view/dot/34976 |website=Bureau of Transportation Statistics |publisher=Department of Transportation |date=1983 |quote=One of the fundamental questions that must be asked when grouting is first considered is whether is ground is groutable...For very low permeability sands, the injection rate at permissible pressures may be so slow that grouting becomes unfeasible. Thus, chemical grouting is recommended only in predominantly sandy materials with less than 25% silts and clays.}}</ref>